| <?xml version="1.0" encoding="UTF-8"?> |
| <!-- |
| Copyright 2002-2004 The Apache Software Foundation |
| |
| Licensed under the Apache License, Version 2.0 (the "License"); |
| you may not use this file except in compliance with the License. |
| You may obtain a copy of the License at |
| |
| http://www.apache.org/licenses/LICENSE-2.0 |
| |
| Unless required by applicable law or agreed to in writing, software |
| distributed under the License is distributed on an "AS IS" BASIS, |
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| See the License for the specific language governing permissions and |
| limitations under the License. |
| --> |
| <!DOCTYPE article PUBLIC "-//OASIS//DTD Simplified DocBook XML V1.0//EN" |
| "http://www.oasis-open.org/docbook/xml/simple/1.0/sdocbook.dtd"> |
| <article id="bk_programmersGuide"> |
| <title>ZooKeeper Programmer's Guide</title> |
| |
| <subtitle>Developing Distributed Applications that use ZooKeeper</subtitle> |
| |
| <articleinfo> |
| <legalnotice> |
| <para>Licensed under the Apache License, Version 2.0 (the "License"); |
| you may not use this file except in compliance with the License. You may |
| obtain a copy of the License at <ulink |
| url="http://www.apache.org/licenses/LICENSE-2.0">http://www.apache.org/licenses/LICENSE-2.0</ulink>.</para> |
| |
| <para>Unless required by applicable law or agreed to in writing, |
| software distributed under the License is distributed on an "AS IS" |
| BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or |
| implied. See the License for the specific language governing permissions |
| and limitations under the License.</para> |
| </legalnotice> |
| |
| <abstract> |
| <para>This guide contains detailed information about creating |
| distributed applications that use ZooKeeper. It discusses the basic |
| operations ZooKeeper supports, and how these can be used to build |
| higher-level abstractions. It contains solutions to common tasks, a |
| troubleshooting guide, and links to other information.</para> |
| |
| <para>$Revision: 1.14 $ $Date: 2008/09/19 05:31:45 $</para> |
| </abstract> |
| </articleinfo> |
| |
| <section id="_introduction"> |
| <title>Introduction</title> |
| |
| <para>This document is a guide for developers wishing to create |
| distributed applications that take advantage of ZooKeeper's coordination |
| services. It contains conceptual and practical information.</para> |
| |
| <para>The first four sections of this guide present higher level |
| discussions of various ZooKeeper concepts. These are necessary both for an |
| understanding of how ZooKeeper works as well how to work with it. It does |
| not contain source code, but it does assume a familiarity with the |
| problems associated with distributed computing. The sections in this first |
| group are:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para><xref linkend="ch_zkDataModel" /></para> |
| </listitem> |
| |
| <listitem> |
| <para><xref linkend="ch_zkSessions" /></para> |
| </listitem> |
| |
| <listitem> |
| <para><xref linkend="ch_zkWatches" /></para> |
| </listitem> |
| |
| <listitem> |
| <para><xref linkend="ch_zkGuarantees" /></para> |
| </listitem> |
| </itemizedlist> |
| |
| <para>The next four sections provide practical programming |
| information. These are:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para><xref linkend="ch_guideToZkOperations" /></para> |
| </listitem> |
| |
| <listitem> |
| <para><xref linkend="ch_bindings" /></para> |
| </listitem> |
| |
| <listitem> |
| <para><xref linkend="ch_programStructureWithExample" /> |
| <emphasis>[tbd]</emphasis></para> |
| </listitem> |
| |
| <listitem> |
| <para><xref linkend="ch_gotchas" /></para> |
| </listitem> |
| </itemizedlist> |
| |
| <para>The book concludes with an <ulink |
| url="#apx_linksToOtherInfo">appendix</ulink> containing links to other |
| useful, ZooKeeper-related information.</para> |
| |
| <para>Most of information in this document is written to be accessible as |
| stand-alone reference material. However, before starting your first |
| ZooKeeper application, you should probably at least read the chaptes on |
| the <ulink url="#ch_zkDataModel">ZooKeeper Data Model</ulink> and <ulink |
| url="#ch_guideToZkOperations">ZooKeeper Basic Operations</ulink>. Also, |
| the <ulink url="#ch_programStructureWithExample">Simple Programmming |
| Example</ulink> <emphasis>[tbd]</emphasis> is helpful for understanding the basic |
| structure of a ZooKeeper client application.</para> |
| </section> |
| |
| <section id="ch_zkDataModel"> |
| <title>The ZooKeeper Data Model</title> |
| |
| <para>ZooKeeper has a hierarchal name space, much like a distributed file |
| system. The only difference is that each node in the namespace can have |
| data associated with it as well as children. It is like having a file |
| system that allows a file to also be a directory. Paths to nodes are |
| always expressed as canonical, absolute, slash-separated paths; there are |
| no relative reference. Any unicode character can be used in a path subject |
| to the following constraints:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para>The null character (\u0000) cannot be part of a path name. (This |
| causes problems with the C binding.)</para> |
| </listitem> |
| |
| <listitem> |
| <para>The following characters can't be used because they don't |
| display well, or render in confusing ways: \u0001 - \u001F and \u007F |
| - \u009F.</para> |
| </listitem> |
| |
| <listitem> |
| <para>The following characters are not allowed: \ud800 - uF8FF, |
| \uFFF0 - uFFFF.</para> |
| </listitem> |
| |
| <listitem> |
| <para>The "." character can be used as part of another name, but "." |
| and ".." cannot alone be used to indicate a node along a path, |
| because ZooKeeper doesn't use relative paths. The following would be |
| invalid: "/a/b/./c" or "/a/b/../c".</para> |
| </listitem> |
| |
| <listitem> |
| <para>The token "zookeeper" is reserved.</para> |
| </listitem> |
| </itemizedlist> |
| |
| <section id="sc_zkDataModel_znodes"> |
| <title>ZNodes</title> |
| |
| <para>Every node in a ZooKeeper tree is referred to as a |
| <emphasis>znode</emphasis>. Znodes maintain a stat structure that |
| includes version numbers for data changes, acl changes. The stat |
| structure also has timestamps. The version number, together with the |
| timestamp, allows ZooKeeper to validate the cache and to coordinate |
| updates. Each time a znode's data changes, the version number increases. |
| For instance, whenever a client retrieves data, it also receives the |
| version of the data. And when a client performs an update or a delete, |
| it must supply the version of the data of the znode it is changing. If |
| the version it supplies doesn't match the actual version of the data, |
| the update will fail. (This behavior can be overridden. For more |
| information see... )<emphasis>[tbd...]</emphasis></para> |
| |
| <note> |
| <para>In distributed application engineering, the word |
| <emphasis>node</emphasis> can refer to a generic host machine, a |
| server, a member of an ensemble, a client process, etc. In the ZooKeeper |
| documentation, <emphasis>znodes</emphasis> refer to the data nodes. |
| <emphasis>Servers</emphasis> refer to machines that make up the |
| ZooKeeper service; <emphasis>quorum peers</emphasis> refer to the |
| servers that make up an ensemble; client refers to any host or process |
| which uses a ZooKeeper service.</para> |
| </note> |
| |
| <para>Znodes are the main enitity that a programmer access. They have |
| several characteristics that are worth mentioning here.</para> |
| |
| <section id="sc_zkDataMode_watches"> |
| <title>Watches</title> |
| |
| <para>Clients can set watches on znodes. Changes to that znode trigger |
| the watch and then clear the watch. When a watch triggers, ZooKeeper |
| sends the client a notification. More information about watches can be |
| found in the section |
| <ulink url="#ch_zkWatches">ZooKeeper Watches</ulink>.</para> |
| </section> |
| |
| <section> |
| <title>Data Access</title> |
| |
| <para>The data stored at each znode in a namespace is read and written |
| atomically. Reads get all the data bytes associated with a znode and a |
| write replaces all the data. Each node has an Access Control List |
| (ACL) that restricts who can do what.</para> |
| |
| <para>ZooKeeper was not designed to be a general database or large |
| object store. Instead, it manages coordination data. This data can |
| come in the form of configuration, status information, rendezvous, etc. |
| A common property of the various forms of coordination data is that |
| they are relatively small: measured in kilobytes. |
| The ZooKeeper client and the server implementations have sanity checks |
| to ensure that znodes have less than 1M of data, but the data should |
| be much less than that on average. Operating on relatively large data |
| sizes will cause some operations to take much more time than others and |
| will affect the latencies of some operations because of the extra time |
| needed to move more data over the network and onto storage media. If |
| large data storage is needed, the usually pattern of dealing with such |
| data is to store it on a bulk storage system, such as NFS or HDFS, and |
| store pointers to the storage locations in ZooKeeper.</para> |
| </section> |
| |
| <section> |
| <title>Ephemeral Nodes</title> |
| |
| <para>ZooKeeper also has the notion of ephemeral nodes. These znodes |
| exists as long as the session that created the znode is active. When |
| the session ends the znode is deleted. Because of this behavior |
| ephemeral znodes are not allowed to have children.</para> |
| </section> |
| |
| <section> |
| <title>Sequence Nodes -- Unique Naming</title> |
| |
| <para>When creating a znode you can also request that |
| ZooKeeper append a monotonically increasing counter to the end |
| of path. This counter is unique to the parent znode. The |
| counter has a format of %010d -- that is 10 digits with 0 |
| (zero) padding (the counter is formatted in this way to |
| simplify sorting), i.e. "<path>0000000001". See |
| <ulink url="recipes.html#sc_recipes_Queues">Queue |
| Recipe</ulink> for an example use of this feature. Note: the |
| counter used to store the next sequence number is a signed int |
| (4bytes) maintained by the parent node, the counter will |
| overflow when incremented beyond 2147483647 (resulting in a |
| name "<path>-2147483647").</para> |
| </section> |
| |
| <section> |
| <title>Container Nodes</title> |
| |
| <para><emphasis role="bold">Added in 3.6.0</emphasis></para> |
| |
| <para>ZooKeeper has the notion of container nodes. Container nodes are |
| special purpose nodes useful for recipes such as leader, lock, etc. |
| When the last child of a container is deleted, the container becomes |
| a candidate to be deleted by the server at some point in the future.</para> |
| |
| <para>Given this property, you should be prepared to get |
| KeeperException.NoNodeException when creating children inside of |
| container nodes. i.e. when creating child nodes inside of container nodes |
| always check for KeeperException.NoNodeException and recreate the container |
| node when it occurs.</para> |
| </section> |
| </section> |
| |
| <section id="sc_timeInZk"> |
| <title>Time in ZooKeeper</title> |
| |
| <para>ZooKeeper tracks time multiple ways:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para><emphasis role="bold">Zxid</emphasis></para> |
| |
| <para>Every change to the ZooKeeper state receives a stamp in the |
| form of a <emphasis>zxid</emphasis> (ZooKeeper Transaction Id). |
| This exposes the total ordering of all changes to ZooKeeper. Each |
| change will have a unique zxid and if zxid1 is smaller than zxid2 |
| then zxid1 happened before zxid2.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">Version numbers</emphasis></para> |
| |
| <para>Every change to a node will cause an increase to one of the |
| version numbers of that node. The three version numbers are version |
| (number of changes to the data of a znode), cversion (number of |
| changes to the children of a znode), and aversion (number of changes |
| to the ACL of a znode).</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">Ticks</emphasis></para> |
| |
| <para>When using multi-server ZooKeeper, servers use ticks to define |
| timing of events such as status uploads, session timeouts, |
| connection timeouts between peers, etc. The tick time is only |
| indirectly exposed through the minimum session timeout (2 times the |
| tick time); if a client requests a session timeout less than the |
| minimum session timeout, the server will tell the client that the |
| session timeout is actually the minimum session timeout.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">Real time</emphasis></para> |
| |
| <para>ZooKeeper doesn't use real time, or clock time, at all except |
| to put timestamps into the stat structure on znode creation and |
| znode modification.</para> |
| </listitem> |
| </itemizedlist> |
| </section> |
| |
| <section id="sc_zkStatStructure"> |
| <title>ZooKeeper Stat Structure</title> |
| |
| <para>The Stat structure for each znode in ZooKeeper is made up of the |
| following fields:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para><emphasis role="bold">czxid</emphasis></para> |
| |
| <para>The zxid of the change that caused this znode to be |
| created.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">mzxid</emphasis></para> |
| |
| <para>The zxid of the change that last modified this znode.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">pzxid</emphasis></para> |
| |
| <para>The zxid of the change that last modified children of this znode.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">ctime</emphasis></para> |
| |
| <para>The time in milliseconds from epoch when this znode was |
| created.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">mtime</emphasis></para> |
| |
| <para>The time in milliseconds from epoch when this znode was last |
| modified.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">version</emphasis></para> |
| |
| <para>The number of changes to the data of this znode.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">cversion</emphasis></para> |
| |
| <para>The number of changes to the children of this znode.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">aversion</emphasis></para> |
| |
| <para>The number of changes to the ACL of this znode.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">ephemeralOwner</emphasis></para> |
| |
| <para>The session id of the owner of this znode if the znode is an |
| ephemeral node. If it is not an ephemeral node, it will be |
| zero.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">dataLength</emphasis></para> |
| |
| <para>The length of the data field of this znode.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">numChildren</emphasis></para> |
| |
| <para>The number of children of this znode.</para> |
| </listitem> |
| |
| </itemizedlist> |
| </section> |
| </section> |
| |
| <section id="ch_zkSessions"> |
| <title>ZooKeeper Sessions</title> |
| |
| <para>A ZooKeeper client establishes a session with the ZooKeeper |
| service by creating a handle to the service using a language |
| binding. Once created, the handle starts of in the CONNECTING state |
| and the client library tries to connect to one of the servers that |
| make up the ZooKeeper service at which point it switches to the |
| CONNECTED state. During normal operation will be in one of these |
| two states. If an unrecoverable error occurs, such as session |
| expiration or authentication failure, or if the application explicitly |
| closes the handle, the handle will move to the CLOSED state. |
| The following figure shows the possible state transitions of a |
| ZooKeeper client:</para> |
| |
| <mediaobject id="fg_states" > |
| <imageobject> |
| <imagedata fileref="images/state_dia.jpg"/> |
| </imageobject> |
| </mediaobject> |
| |
| <para>To create a client session the application code must provide |
| a connection string containing a comma separated list of host:port pairs, |
| each corresponding to a ZooKeeper server (e.g. "127.0.0.1:4545" or |
| "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002"). The ZooKeeper |
| client library will pick an arbitrary server and try to connect to |
| it. If this connection fails, or if the client becomes |
| disconnected from the server for any reason, the client will |
| automatically try the next server in the list, until a connection |
| is (re-)established.</para> |
| |
| <para> <emphasis role="bold">Added in 3.2.0</emphasis>: An |
| optional "chroot" suffix may also be appended to the connection |
| string. This will run the client commands while interpreting all |
| paths relative to this root (similar to the unix chroot |
| command). If used the example would look like: |
| "127.0.0.1:4545/app/a" or |
| "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002/app/a" where the |
| client would be rooted at "/app/a" and all paths would be relative |
| to this root - ie getting/setting/etc... "/foo/bar" would result |
| in operations being run on "/app/a/foo/bar" (from the server |
| perspective). This feature is particularly useful in multi-tenant |
| environments where each user of a particular ZooKeeper service |
| could be rooted differently. This makes re-use much simpler as |
| each user can code his/her application as if it were rooted at |
| "/", while actual location (say /app/a) could be determined at |
| deployment time.</para> |
| |
| <para>When a client gets a handle to the ZooKeeper service, |
| ZooKeeper creates a ZooKeeper session, represented as a 64-bit |
| number, that it assigns to the client. If the client connects to a |
| different ZooKeeper server, it will send the session id as a part |
| of the connection handshake. As a security measure, the server |
| creates a password for the session id that any ZooKeeper server |
| can validate.The password is sent to the client with the session |
| id when the client establishes the session. The client sends this |
| password with the session id whenever it reestablishes the session |
| with a new server.</para> |
| |
| <para>One of the parameters to the ZooKeeper client library call |
| to create a ZooKeeper session is the session timeout in |
| milliseconds. The client sends a requested timeout, the server |
| responds with the timeout that it can give the client. The current |
| implementation requires that the timeout be a minimum of 2 times |
| the tickTime (as set in the server configuration) and a maximum of |
| 20 times the tickTime. The ZooKeeper client API allows access to |
| the negotiated timeout.</para> |
| |
| <para>When a client (session) becomes partitioned from the ZK |
| serving cluster it will begin searching the list of servers that |
| were specified during session creation. Eventually, when |
| connectivity between the client and at least one of the servers is |
| re-established, the session will either again transition to the |
| "connected" state (if reconnected within the session timeout |
| value) or it will transition to the "expired" state (if |
| reconnected after the session timeout). It is not advisable to |
| create a new session object (a new ZooKeeper.class or zookeeper |
| handle in the c binding) for disconnection. The ZK client library |
| will handle reconnect for you. In particular we have heuristics |
| built into the client library to handle things like "herd effect", |
| etc... Only create a new session when you are notified of session |
| expiration (mandatory).</para> |
| |
| <para>Session expiration is managed by the ZooKeeper cluster |
| itself, not by the client. When the ZK client establishes a |
| session with the cluster it provides a "timeout" value detailed |
| above. This value is used by the cluster to determine when the |
| client's session expires. Expirations happens when the cluster |
| does not hear from the client within the specified session timeout |
| period (i.e. no heartbeat). At session expiration the cluster will |
| delete any/all ephemeral nodes owned by that session and |
| immediately notify any/all connected clients of the change (anyone |
| watching those znodes). At this point the client of the expired |
| session is still disconnected from the cluster, it will not be |
| notified of the session expiration until/unless it is able to |
| re-establish a connection to the cluster. The client will stay in |
| disconnected state until the TCP connection is re-established with |
| the cluster, at which point the watcher of the expired session |
| will receive the "session expired" notification.</para> |
| |
| <para>Example state transitions for an expired session as seen by |
| the expired session's watcher:</para> |
| |
| <orderedlist> |
| <listitem><para>'connected' : session is established and client |
| is communicating with cluster (client/server communication is |
| operating properly)</para></listitem> |
| <listitem><para>.... client is partitioned from the |
| cluster</para></listitem> |
| <listitem><para>'disconnected' : client has lost connectivity |
| with the cluster</para></listitem> |
| <listitem><para>.... time elapses, after 'timeout' period the |
| cluster expires the session, nothing is seen by client as it is |
| disconnected from cluster</para></listitem> |
| <listitem><para>.... time elapses, the client regains network |
| level connectivity with the cluster</para></listitem> |
| <listitem><para>'expired' : eventually the client reconnects to |
| the cluster, it is then notified of the |
| expiration</para></listitem> |
| </orderedlist> |
| |
| <para>Another parameter to the ZooKeeper session establishment |
| call is the default watcher. Watchers are notified when any state |
| change occurs in the client. For example if the client loses |
| connectivity to the server the client will be notified, or if the |
| client's session expires, etc... This watcher should consider the |
| initial state to be disconnected (i.e. before any state changes |
| events are sent to the watcher by the client lib). In the case of |
| a new connection, the first event sent to the watcher is typically |
| the session connection event.</para> |
| |
| <para>The session is kept alive by requests sent by the client. If |
| the session is idle for a period of time that would timeout the |
| session, the client will send a PING request to keep the session |
| alive. This PING request not only allows the ZooKeeper server to |
| know that the client is still active, but it also allows the |
| client to verify that its connection to the ZooKeeper server is |
| still active. The timing of the PING is conservative enough to |
| ensure reasonable time to detect a dead connection and reconnect |
| to a new server.</para> |
| |
| <para> |
| Once a connection to the server is successfully established |
| (connected) there are basically two cases where the client lib generates |
| connectionloss (the result code in c binding, exception in Java -- see |
| the API documentation for binding specific details) when either a synchronous or |
| asynchronous operation is performed and one of the following holds: |
| </para> |
| |
| <orderedlist> |
| <listitem><para>The application calls an operation on a session that is no |
| longer alive/valid</para></listitem> |
| <listitem><para>The ZooKeeper client disconnects from a server when there |
| are pending operations to that server, i.e., there is a pending asynchronous call. |
| </para></listitem> |
| </orderedlist> |
| |
| <para> <emphasis role="bold">Added in 3.2.0 -- SessionMovedException</emphasis>. There is an internal |
| exception that is generally not seen by clients called the SessionMovedException. |
| This exception occurs because a request was received on a connection for a session |
| which has been reestablished on a different server. The normal cause of this error is |
| a client that sends a request to a server, but the network packet gets delayed, so |
| the client times out and connects to a new server. When the delayed packet arrives at |
| the first server, the old server detects that the session has moved, and closes the |
| client connection. Clients normally do not see this error since they do not read |
| from those old connections. (Old connections are usually closed.) One situation in which this |
| condition can be seen is when two clients try to reestablish the same connection using |
| a saved session id and password. One of the clients will reestablish the connection |
| and the second client will be disconnected (causing the pair to attempt to re-establish |
| its connection/session indefinitely).</para> |
| |
| <para> <emphasis role="bold">Updating the list of servers</emphasis>. We allow a client to |
| update the connection string by providing a new comma separated list of host:port pairs, |
| each corresponding to a ZooKeeper server. The function invokes a probabilistic load-balancing |
| algorithm which may cause the client to disconnect from its current host with the goal |
| to achieve expected uniform number of connections per server in the new list. |
| In case the current host to which the client is connected is not in the new list |
| this call will always cause the connection to be dropped. Otherwise, the decision |
| is based on whether the number of servers has increased or decreased and by how much. |
| </para> |
| |
| <para> |
| For example, if the previous connection string contained 3 hosts and now the list contains |
| these 3 hosts and 2 more hosts, 40% of clients connected to each of the 3 hosts will |
| move to one of the new hosts in order to balance the load. The algorithm will cause the client |
| to drop its connection to the current host to which it is connected with probability 0.4 and in this |
| case cause the client to connect to one of the 2 new hosts, chosen at random. |
| </para> |
| |
| <para> |
| Another example -- suppose we have 5 hosts and now update the list to remove 2 of the hosts, |
| the clients connected to the 3 remaining hosts will stay connected, whereas all clients connected |
| to the 2 removed hosts will need to move to one of the 3 hosts, chosen at random. If the connection |
| is dropped, the client moves to a special mode where he chooses a new server to connect to using the |
| probabilistic algorithm, and not just round robin. |
| </para> |
| |
| <para> |
| In the first example, each client decides to disconnect with probability 0.4 but once the decision is |
| made, it will try to connect to a random new server and only if it cannot connect to any of the new |
| servers will it try to connect to the old ones. After finding a server, or trying all servers in the |
| new list and failing to connect, the client moves back to the normal mode of operation where it picks |
| an arbitrary server from the connectString and attempt to connect to it. If that fails, is will continue |
| trying different random servers in round robin. (see above the algorithm used to initially choose a server) |
| </para> |
| |
| </section> |
| |
| <section id="ch_zkWatches"> |
| <title>ZooKeeper Watches</title> |
| |
| <para>All of the read operations in ZooKeeper - <emphasis |
| role="bold">getData()</emphasis>, <emphasis |
| role="bold">getChildren()</emphasis>, and <emphasis |
| role="bold">exists()</emphasis> - have the option of setting a watch as a |
| side effect. Here is ZooKeeper's definition of a watch: a watch event is |
| one-time trigger, sent to the client that set the watch, which occurs when |
| the data for which the watch was set changes. There are three key points |
| to consider in this definition of a watch:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para><emphasis role="bold">One-time trigger</emphasis></para> |
| |
| <para>One watch event will be sent to the client when the data has changed. |
| For example, if a client does a getData("/znode1", true) and later the |
| data for /znode1 is changed or deleted, the client will get a watch |
| event for /znode1. If /znode1 changes again, no watch event will be |
| sent unless the client has done another read that sets a new |
| watch.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">Sent to the client</emphasis></para> |
| |
| <para>This implies that an event is on the way to the client, but may |
| not reach the client before the successful return code to the change |
| operation reaches the client that initiated the change. Watches are |
| sent asynchronously to watchers. ZooKeeper provides an ordering |
| guarantee: a client will never see a change for which it has set a |
| watch until it first sees the watch event. Network delays or other |
| factors may cause different clients to see watches and return codes |
| from updates at different times. The key point is that everything seen |
| by the different clients will have a consistent order.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">The data for which the watch was |
| set</emphasis></para> |
| |
| <para>This refers to the different ways a node can change. It |
| helps to think of ZooKeeper as maintaining two lists of |
| watches: data watches and child watches. getData() and |
| exists() set data watches. getChildren() sets child |
| watches. Alternatively, it may help to think of watches being |
| set according to the kind of data returned. getData() and |
| exists() return information about the data of the node, |
| whereas getChildren() returns a list of children. Thus, |
| setData() will trigger data watches for the znode being set |
| (assuming the set is successful). A successful create() will |
| trigger a data watch for the znode being created and a child |
| watch for the parent znode. A successful delete() will trigger |
| both a data watch and a child watch (since there can be no |
| more children) for a znode being deleted as well as a child |
| watch for the parent znode.</para> |
| </listitem> |
| </itemizedlist> |
| |
| <para>Watches are maintained locally at the ZooKeeper server to which the |
| client is connected. This allows watches to be lightweight to set, |
| maintain, and dispatch. When a client connects to a new server, the watch |
| will be triggered for any session events. Watches will not be received |
| while disconnected from a server. When a client reconnects, any previously |
| registered watches will be reregistered and triggered if needed. In |
| general this all occurs transparently. There is one case where a watch |
| may be missed: a watch for the existence of a znode not yet created will |
| be missed if the znode is created and deleted while disconnected.</para> |
| |
| <section id="sc_WatchSemantics"> |
| <title>Semantics of Watches</title> |
| |
| <para> We can set watches with the three calls that read the state of |
| ZooKeeper: exists, getData, and getChildren. The following list details |
| the events that a watch can trigger and the calls that enable them: |
| </para> |
| |
| <itemizedlist> |
| <listitem> |
| <para><emphasis role="bold">Created event:</emphasis></para> |
| <para>Enabled with a call to exists.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">Deleted event:</emphasis></para> |
| <para>Enabled with a call to exists, getData, and getChildren.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">Changed event:</emphasis></para> |
| <para>Enabled with a call to exists and getData.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">Child event:</emphasis></para> |
| <para>Enabled with a call to getChildren.</para> |
| </listitem> |
| </itemizedlist> |
| </section> |
| |
| <section id="sc_WatchRemoval"> |
| <title>Remove Watches</title> |
| <para>We can remove the watches registered on a znode with a call to |
| removeWatches. Also, a ZooKeeper client can remove watches locally even |
| if there is no server connection by setting the local flag to true. The |
| following list details the events which will be triggered after the |
| successful watch removal. |
| </para> |
| <itemizedlist> |
| <listitem> |
| <para><emphasis role="bold">Child Remove event:</emphasis></para> |
| <para>Watcher which was added with a call to getChildren.</para> |
| </listitem> |
| |
| <listitem> |
| <para><emphasis role="bold">Data Remove event:</emphasis></para> |
| <para>Watcher which was added with a call to exists or getData.</para> |
| </listitem> |
| </itemizedlist> |
| </section> |
| |
| <section id="sc_WatchGuarantees"> |
| <title>What ZooKeeper Guarantees about Watches</title> |
| |
| <para>With regard to watches, ZooKeeper maintains these |
| guarantees:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para>Watches are ordered with respect to other events, other |
| watches, and asynchronous replies. The ZooKeeper client libraries |
| ensures that everything is dispatched in order.</para> |
| </listitem> |
| </itemizedlist> |
| |
| <itemizedlist> |
| <listitem> |
| <para>A client will see a watch event for a znode it is watching |
| before seeing the new data that corresponds to that znode.</para> |
| </listitem> |
| </itemizedlist> |
| |
| <itemizedlist> |
| <listitem> |
| <para>The order of watch events from ZooKeeper corresponds to the |
| order of the updates as seen by the ZooKeeper service.</para> |
| </listitem> |
| </itemizedlist> |
| </section> |
| |
| <section id="sc_WatchRememberThese"> |
| <title>Things to Remember about Watches</title> |
| |
| <itemizedlist> |
| <listitem> |
| <para>Watches are one time triggers; if you get a watch event and |
| you want to get notified of future changes, you must set another |
| watch.</para> |
| </listitem> |
| </itemizedlist> |
| |
| <itemizedlist> |
| <listitem> |
| <para>Because watches are one time triggers and there is latency |
| between getting the event and sending a new request to get a watch |
| you cannot reliably see every change that happens to a node in |
| ZooKeeper. Be prepared to handle the case where the znode changes |
| multiple times between getting the event and setting the watch |
| again. (You may not care, but at least realize it may |
| happen.)</para> |
| </listitem> |
| </itemizedlist> |
| |
| <itemizedlist> |
| <listitem> |
| <para>A watch object, or function/context pair, will only be |
| triggered once for a given notification. For example, if the same |
| watch object is registered for an exists and a getData call for the |
| same file and that file is then deleted, the watch object would |
| only be invoked once with the deletion notification for the file. |
| </para> |
| </listitem> |
| </itemizedlist> |
| |
| <itemizedlist> |
| <listitem> |
| <para>When you disconnect from a server (for example, when the |
| server fails), you will not get any watches until the connection |
| is reestablished. For this reason session events are sent to all |
| outstanding watch handlers. Use session events to go into a safe |
| mode: you will not be receiving events while disconnected, so your |
| process should act conservatively in that mode.</para> |
| </listitem> |
| </itemizedlist> |
| </section> |
| </section> |
| |
| <section id="sc_ZooKeeperAccessControl"> |
| <title>ZooKeeper access control using ACLs</title> |
| |
| <para>ZooKeeper uses ACLs to control access to its znodes (the |
| data nodes of a ZooKeeper data tree). The ACL implementation is |
| quite similar to UNIX file access permissions: it employs |
| permission bits to allow/disallow various operations against a |
| node and the scope to which the bits apply. Unlike standard UNIX |
| permissions, a ZooKeeper node is not limited by the three standard |
| scopes for user (owner of the file), group, and world |
| (other). ZooKeeper does not have a notion of an owner of a |
| znode. Instead, an ACL specifies sets of ids and permissions that |
| are associated with those ids.</para> |
| |
| <para>Note also that an ACL pertains only to a specific znode. In |
| particular it does not apply to children. For example, if |
| <emphasis>/app</emphasis> is only readable by ip:172.16.16.1 and |
| <emphasis>/app/status</emphasis> is world readable, anyone will |
| be able to read <emphasis>/app/status</emphasis>; ACLs are not |
| recursive.</para> |
| |
| <para>ZooKeeper supports pluggable authentication schemes. Ids are |
| specified using the form <emphasis>scheme:id</emphasis>, |
| where <emphasis>scheme</emphasis> is a the authentication scheme |
| that the id corresponds to. For |
| example, <emphasis>ip:172.16.16.1</emphasis> is an id for a |
| host with the address <emphasis>172.16.16.1</emphasis>.</para> |
| |
| <para>When a client connects to ZooKeeper and authenticates |
| itself, ZooKeeper associates all the ids that correspond to a |
| client with the clients connection. These ids are checked against |
| the ACLs of znodes when a clients tries to access a node. ACLs are |
| made up of pairs of <emphasis>(scheme:expression, |
| perms)</emphasis>. The format of |
| the <emphasis>expression</emphasis> is specific to the scheme. For |
| example, the pair <emphasis>(ip:19.22.0.0/16, READ)</emphasis> |
| gives the <emphasis>READ</emphasis> permission to any clients with |
| an IP address that starts with 19.22.</para> |
| |
| <section id="sc_ACLPermissions"> |
| <title>ACL Permissions</title> |
| |
| <para>ZooKeeper supports the following permissions:</para> |
| |
| <itemizedlist> |
| <listitem><para><emphasis role="bold">CREATE</emphasis>: you can create a child node</para></listitem> |
| <listitem><para><emphasis role="bold">READ</emphasis>: you can get data from a node and list its children.</para></listitem> |
| <listitem><para><emphasis role="bold">WRITE</emphasis>: you can set data for a node</para></listitem> |
| <listitem><para><emphasis role="bold">DELETE</emphasis>: you can delete a child node</para></listitem> |
| <listitem><para><emphasis role="bold">ADMIN</emphasis>: you can set permissions</para></listitem> |
| </itemizedlist> |
| |
| <para>The <emphasis>CREATE</emphasis> |
| and <emphasis>DELETE</emphasis> permissions have been broken out |
| of the <emphasis>WRITE</emphasis> permission for finer grained |
| access controls. The cases for <emphasis>CREATE</emphasis> |
| and <emphasis>DELETE</emphasis> are the following:</para> |
| |
| <para>You want A to be able to do a set on a ZooKeeper node, but |
| not be able to <emphasis>CREATE</emphasis> |
| or <emphasis>DELETE</emphasis> children.</para> |
| |
| <para><emphasis>CREATE</emphasis> |
| without <emphasis>DELETE</emphasis>: clients create requests by |
| creating ZooKeeper nodes in a parent directory. You want all |
| clients to be able to add, but only request processor can |
| delete. (This is kind of like the APPEND permission for |
| files.)</para> |
| |
| <para>Also, the <emphasis>ADMIN</emphasis> permission is there |
| since ZooKeeper doesn’t have a notion of file owner. In some |
| sense the <emphasis>ADMIN</emphasis> permission designates the |
| entity as the owner. ZooKeeper doesn’t support the LOOKUP |
| permission (execute permission bit on directories to allow you |
| to LOOKUP even though you can't list the directory). Everyone |
| implicitly has LOOKUP permission. This allows you to stat a |
| node, but nothing more. (The problem is, if you want to call |
| zoo_exists() on a node that doesn't exist, there is no |
| permission to check.)</para> |
| |
| <section id="sc_BuiltinACLSchemes"> |
| <title>Builtin ACL Schemes</title> |
| |
| <para>ZooKeeeper has the following built in schemes:</para> |
| |
| <itemizedlist> |
| <listitem><para><emphasis role="bold">world</emphasis> has a |
| single id, <emphasis>anyone</emphasis>, that represents |
| anyone.</para></listitem> |
| |
| <listitem><para><emphasis role="bold">auth</emphasis> doesn't |
| use any id, represents any authenticated |
| user.</para></listitem> |
| |
| <listitem><para><emphasis role="bold">digest</emphasis> uses |
| a <emphasis>username:password</emphasis> string to generate |
| MD5 hash which is then used as an ACL ID |
| identity. Authentication is done by sending |
| the <emphasis>username:password</emphasis> in clear text. When |
| used in the ACL the expression will be |
| the <emphasis>username:base64</emphasis> |
| encoded <emphasis>SHA1</emphasis> |
| password <emphasis>digest</emphasis>.</para> |
| </listitem> |
| |
| <listitem><para><emphasis role="bold">ip</emphasis> uses the |
| client host IP as an ACL ID identity. The ACL expression is of |
| the form <emphasis>addr/bits</emphasis> where the most |
| significant <emphasis>bits</emphasis> |
| of <emphasis>addr</emphasis> are matched against the most |
| significant <emphasis>bits</emphasis> of the client host |
| IP.</para></listitem> |
| |
| <listitem><para><emphasis role="bold">x509</emphasis> uses the client |
| X500 Principal as an ACL ID identity. The ACL expression is the exact |
| X500 Principal name of a client. When using the secure port, clients |
| are automatically authenticated and their auth info for the x509 scheme |
| is set.</para></listitem> |
| |
| </itemizedlist> |
| </section> |
| |
| <section> |
| <title>ZooKeeper C client API</title> |
| |
| <para>The following constants are provided by the ZooKeeper C |
| library:</para> |
| |
| <itemizedlist> |
| <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_READ; //can read node’s value and list its children</para></listitem> |
| <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_WRITE;// can set the node’s value</para></listitem> |
| <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_CREATE; //can create children</para></listitem> |
| <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_DELETE;// can delete children</para></listitem> |
| <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_ADMIN; //can execute set_acl()</para></listitem> |
| <listitem><para><emphasis>const</emphasis> <emphasis>int</emphasis> ZOO_PERM_ALL;// all of the above flags OR’d together</para></listitem> |
| </itemizedlist> |
| |
| <para>The following are the standard ACL IDs:</para> |
| |
| <itemizedlist> |
| <listitem><para><emphasis>struct</emphasis> Id ZOO_ANYONE_ID_UNSAFE; //(‘world’,’anyone’)</para></listitem> |
| <listitem><para><emphasis>struct</emphasis> Id ZOO_AUTH_IDS;// (‘auth’,’’)</para></listitem> |
| </itemizedlist> |
| |
| <para>ZOO_AUTH_IDS empty identity string should be interpreted as “the identity of the creator”.</para> |
| |
| <para>ZooKeeper client comes with three standard ACLs:</para> |
| |
| <itemizedlist> |
| <listitem><para><emphasis>struct</emphasis> ACL_vector ZOO_OPEN_ACL_UNSAFE; //(ZOO_PERM_ALL,ZOO_ANYONE_ID_UNSAFE)</para></listitem> |
| <listitem><para><emphasis>struct</emphasis> ACL_vector ZOO_READ_ACL_UNSAFE;// (ZOO_PERM_READ, ZOO_ANYONE_ID_UNSAFE)</para></listitem> |
| <listitem><para><emphasis>struct</emphasis> ACL_vector ZOO_CREATOR_ALL_ACL; //(ZOO_PERM_ALL,ZOO_AUTH_IDS)</para></listitem> |
| </itemizedlist> |
| |
| <para>The ZOO_OPEN_ACL_UNSAFE is completely open free for all |
| ACL: any application can execute any operation on the node and |
| can create, list and delete its children. The |
| ZOO_READ_ACL_UNSAFE is read-only access for any |
| application. CREATE_ALL_ACL grants all permissions to the |
| creator of the node. The creator must have been authenticated by |
| the server (for example, using “<emphasis>digest</emphasis>” |
| scheme) before it can create nodes with this ACL.</para> |
| |
| <para>The following ZooKeeper operations deal with ACLs:</para> |
| |
| <itemizedlist><listitem> |
| <para><emphasis>int</emphasis> <emphasis>zoo_add_auth</emphasis> |
| (zhandle_t *zh,<emphasis>const</emphasis> <emphasis>char</emphasis>* |
| scheme,<emphasis>const</emphasis> <emphasis>char</emphasis>* |
| cert, <emphasis>int</emphasis> certLen, void_completion_t |
| completion, <emphasis>const</emphasis> <emphasis>void</emphasis> |
| *data);</para> |
| </listitem></itemizedlist> |
| |
| <para>The application uses the zoo_add_auth function to |
| authenticate itself to the server. The function can be called |
| multiple times if the application wants to authenticate using |
| different schemes and/or identities.</para> |
| |
| <itemizedlist><listitem> |
| <para><emphasis>int</emphasis> <emphasis>zoo_create</emphasis> |
| (zhandle_t *zh, <emphasis>const</emphasis> <emphasis>char</emphasis> |
| *path, <emphasis>const</emphasis> <emphasis>char</emphasis> |
| *value,<emphasis>int</emphasis> |
| valuelen, <emphasis>const</emphasis> <emphasis>struct</emphasis> |
| ACL_vector *acl, <emphasis>int</emphasis> |
| flags,<emphasis>char</emphasis> |
| *realpath, <emphasis>int</emphasis> |
| max_realpath_len);</para> |
| </listitem></itemizedlist> |
| |
| <para>zoo_create(...) operation creates a new node. The acl |
| parameter is a list of ACLs associated with the node. The parent |
| node must have the CREATE permission bit set.</para> |
| |
| <itemizedlist><listitem> |
| <para><emphasis>int</emphasis> <emphasis>zoo_get_acl</emphasis> |
| (zhandle_t *zh, <emphasis>const</emphasis> <emphasis>char</emphasis> |
| *path,<emphasis>struct</emphasis> ACL_vector |
| *acl, <emphasis>struct</emphasis> Stat *stat);</para> |
| </listitem></itemizedlist> |
| |
| <para>This operation returns a node’s ACL info.</para> |
| |
| <itemizedlist><listitem> |
| <para><emphasis>int</emphasis> <emphasis>zoo_set_acl</emphasis> |
| (zhandle_t *zh, <emphasis>const</emphasis> <emphasis>char</emphasis> |
| *path, <emphasis>int</emphasis> |
| version,<emphasis>const</emphasis> <emphasis>struct</emphasis> |
| ACL_vector *acl);</para> |
| </listitem></itemizedlist> |
| |
| <para>This function replaces node’s ACL list with a new one. The |
| node must have the ADMIN permission set.</para> |
| |
| <para>Here is a sample code that makes use of the above APIs to |
| authenticate itself using the “<emphasis>foo</emphasis>” scheme |
| and create an ephemeral node “/xyz” with create-only |
| permissions.</para> |
| |
| <note><para>This is a very simple example which is intended to show |
| how to interact with ZooKeeper ACLs |
| specifically. See <filename>.../trunk/src/c/src/cli.c</filename> |
| for an example of a proper C client implementation</para> |
| </note> |
| |
| <programlisting> |
| #include <string.h> |
| #include <errno.h> |
| |
| #include "zookeeper.h" |
| |
| static zhandle_t *zh; |
| |
| /** |
| * In this example this method gets the cert for your |
| * environment -- you must provide |
| */ |
| char *foo_get_cert_once(char* id) { return 0; } |
| |
| /** Watcher function -- empty for this example, not something you should |
| * do in real code */ |
| void watcher(zhandle_t *zzh, int type, int state, const char *path, |
| void *watcherCtx) {} |
| |
| int main(int argc, char argv) { |
| char buffer[512]; |
| char p[2048]; |
| char *cert=0; |
| char appId[64]; |
| |
| strcpy(appId, "example.foo_test"); |
| cert = foo_get_cert_once(appId); |
| if(cert!=0) { |
| fprintf(stderr, |
| "Certificate for appid [%s] is [%s]\n",appId,cert); |
| strncpy(p,cert, sizeof(p)-1); |
| free(cert); |
| } else { |
| fprintf(stderr, "Certificate for appid [%s] not found\n",appId); |
| strcpy(p, "dummy"); |
| } |
| |
| zoo_set_debug_level(ZOO_LOG_LEVEL_DEBUG); |
| |
| zh = zookeeper_init("localhost:3181", watcher, 10000, 0, 0, 0); |
| if (!zh) { |
| return errno; |
| } |
| if(zoo_add_auth(zh,"foo",p,strlen(p),0,0)!=ZOK) |
| return 2; |
| |
| struct ACL CREATE_ONLY_ACL[] = {{ZOO_PERM_CREATE, ZOO_AUTH_IDS}}; |
| struct ACL_vector CREATE_ONLY = {1, CREATE_ONLY_ACL}; |
| int rc = zoo_create(zh,"/xyz","value", 5, &CREATE_ONLY, ZOO_EPHEMERAL, |
| buffer, sizeof(buffer)-1); |
| |
| /** this operation will fail with a ZNOAUTH error */ |
| int buflen= sizeof(buffer); |
| struct Stat stat; |
| rc = zoo_get(zh, "/xyz", 0, buffer, &buflen, &stat); |
| if (rc) { |
| fprintf(stderr, "Error %d for %s\n", rc, __LINE__); |
| } |
| |
| zookeeper_close(zh); |
| return 0; |
| } |
| </programlisting> |
| </section> |
| </section> |
| </section> |
| |
| <section id="sc_ZooKeeperPluggableAuthentication"> |
| <title>Pluggable ZooKeeper authentication</title> |
| |
| <para>ZooKeeper runs in a variety of different environments with |
| various different authentication schemes, so it has a completely |
| pluggable authentication framework. Even the builtin authentication |
| schemes use the pluggable authentication framework.</para> |
| |
| <para>To understand how the authentication framework works, first you must |
| understand the two main authentication operations. The framework |
| first must authenticate the client. This is usually done as soon as |
| the client connects to a server and consists of validating information |
| sent from or gathered about a client and associating it with the connection. |
| The second operation handled by the framework is finding the entries in an |
| ACL that correspond to client. ACL entries are <<emphasis>idspec, |
| permissions</emphasis>> pairs. The <emphasis>idspec</emphasis> may be |
| a simple string match against the authentication information associated |
| with the connection or it may be a expression that is evaluated against that |
| information. It is up to the implementation of the authentication plugin |
| to do the match. Here is the interface that an authentication plugin must |
| implement:</para> |
| |
| <programlisting> |
| public interface AuthenticationProvider { |
| String getScheme(); |
| KeeperException.Code handleAuthentication(ServerCnxn cnxn, byte authData[]); |
| boolean isValid(String id); |
| boolean matches(String id, String aclExpr); |
| boolean isAuthenticated(); |
| } |
| </programlisting> |
| |
| <para>The first method <emphasis>getScheme</emphasis> returns the string |
| that identifies the plugin. Because we support multiple methods of authentication, |
| an authentication credential or an <emphasis>idspec</emphasis> will always be |
| prefixed with <emphasis>scheme:</emphasis>. The ZooKeeper server uses the scheme |
| returned by the authentication plugin to determine which ids the scheme |
| applies to.</para> |
| |
| <para><emphasis>handleAuthentication</emphasis> is called when a client |
| sends authentication information to be associated with a connection. The |
| client specifies the scheme to which the information corresponds. The |
| ZooKeeper server passes the information to the authentication plugin whose |
| <emphasis>getScheme</emphasis> matches the scheme passed by the client. The |
| implementor of <emphasis>handleAuthentication</emphasis> will usually return |
| an error if it determines that the information is bad, or it will associate information |
| with the connection using <emphasis>cnxn.getAuthInfo().add(new Id(getScheme(), data))</emphasis>. |
| </para> |
| |
| <para>The authentication plugin is involved in both setting and using ACLs. When an |
| ACL is set for a znode, the ZooKeeper server will pass the id part of the entry to |
| the <emphasis>isValid(String id)</emphasis> method. It is up to the plugin to verify |
| that the id has a correct form. For example, <emphasis>ip:172.16.0.0/16</emphasis> |
| is a valid id, but <emphasis>ip:host.com</emphasis> is not. If the new ACL includes |
| an "auth" entry, <emphasis>isAuthenticated</emphasis> is used to see if the |
| authentication information for this scheme that is assocatied with the connection |
| should be added to the ACL. Some schemes |
| should not be included in auth. For example, the IP address of the client is not |
| considered as an id that should be added to the ACL if auth is specified.</para> |
| |
| <para>ZooKeeper invokes |
| <emphasis>matches(String id, String aclExpr)</emphasis> when checking an ACL. It |
| needs to match authentication information of the client against the relevant ACL |
| entries. To find the entries which apply to the client, the ZooKeeper server will |
| find the scheme of each entry and if there is authentication information |
| from that client for that scheme, <emphasis>matches(String id, String aclExpr)</emphasis> |
| will be called with <emphasis>id</emphasis> set to the authentication information |
| that was previously added to the connection by <emphasis>handleAuthentication</emphasis> and |
| <emphasis>aclExpr</emphasis> set to the id of the ACL entry. The authentication plugin |
| uses its own logic and matching scheme to determine if <emphasis>id</emphasis> is included |
| in <emphasis>aclExpr</emphasis>. |
| </para> |
| |
| <para>There are two built in authentication plugins: <emphasis>ip</emphasis> and |
| <emphasis>digest</emphasis>. Additional plugins can adding using system properties. At |
| startup the ZooKeeper server will look for system properties that start with |
| "zookeeper.authProvider." and interpret the value of those properties as the class name |
| of an authentication plugin. These properties can be set using the |
| <emphasis>-Dzookeeeper.authProvider.X=com.f.MyAuth</emphasis> or adding entries such as |
| the following in the server configuration file:</para> |
| |
| <programlisting> |
| authProvider.1=com.f.MyAuth |
| authProvider.2=com.f.MyAuth2 |
| </programlisting> |
| |
| <para>Care should be taking to ensure that the suffix on the property is unique. If there are |
| duplicates such as <emphasis>-Dzookeeeper.authProvider.X=com.f.MyAuth -Dzookeeper.authProvider.X=com.f.MyAuth2</emphasis>, |
| only one will be used. Also all servers must have the same plugins defined, otherwise clients using |
| the authentication schemes provided by the plugins will have problems connecting to some servers. |
| </para> |
| </section> |
| |
| <section id="ch_zkGuarantees"> |
| <title>Consistency Guarantees</title> |
| |
| <para>ZooKeeper is a high performance, scalable service. Both reads and |
| write operations are designed to be fast, though reads are faster than |
| writes. The reason for this is that in the case of reads, ZooKeeper can |
| serve older data, which in turn is due to ZooKeeper's consistency |
| guarantees:</para> |
| |
| <variablelist> |
| <varlistentry> |
| <term>Sequential Consistency</term> |
| |
| <listitem> |
| <para>Updates from a client will be applied in the order that they |
| were sent.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term>Atomicity</term> |
| |
| <listitem> |
| <para>Updates either succeed or fail -- there are no partial |
| results.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term>Single System Image</term> |
| |
| <listitem> |
| <para>A client will see the same view of the service regardless of |
| the server that it connects to.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term>Reliability</term> |
| |
| <listitem> |
| <para>Once an update has been applied, it will persist from that |
| time forward until a client overwrites the update. This guarantee |
| has two corollaries:</para> |
| |
| <orderedlist> |
| <listitem> |
| <para>If a client gets a successful return code, the update will |
| have been applied. On some failures (communication errors, |
| timeouts, etc) the client will not know if the update has |
| applied or not. We take steps to minimize the failures, but the |
| guarantee is only present with successful return codes. |
| (This is called the <emphasis>monotonicity condition</emphasis> in Paxos.)</para> |
| </listitem> |
| |
| <listitem> |
| <para>Any updates that are seen by the client, through a read |
| request or successful update, will never be rolled back when |
| recovering from server failures.</para> |
| </listitem> |
| </orderedlist> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term>Timeliness</term> |
| |
| <listitem> |
| <para>The clients view of the system is guaranteed to be up-to-date |
| within a certain time bound (on the order of tens of seconds). |
| Either system changes will be seen by a client within this bound, or |
| the client will detect a service outage.</para> |
| </listitem> |
| </varlistentry> |
| </variablelist> |
| |
| <para>Using these consistency guarantees it is easy to build higher level |
| functions such as leader election, barriers, queues, and read/write |
| revocable locks solely at the ZooKeeper client (no additions needed to |
| ZooKeeper). See <ulink url="recipes.html">Recipes and Solutions</ulink> |
| for more details.</para> |
| |
| <note> |
| <para>Sometimes developers mistakenly assume one other guarantee that |
| ZooKeeper does <emphasis>not</emphasis> in fact make. This is:</para> |
| |
| <variablelist> |
| <varlistentry> |
| <term>Simultaneously Consistent Cross-Client Views</term> |
| |
| <listitem> |
| <para>ZooKeeper does not guarantee that at every instance in |
| time, two different clients will have identical views of |
| ZooKeeper data. Due to factors like network delays, one client |
| may perform an update before another client gets notified of the |
| change. Consider the scenario of two clients, A and B. If client |
| A sets the value of a znode /a from 0 to 1, then tells client B |
| to read /a, client B may read the old value of 0, depending on |
| which server it is connected to. If it |
| is important that Client A and Client B read the same value, |
| Client B should should call the <emphasis |
| role="bold">sync()</emphasis> method from the ZooKeeper API |
| method before it performs its read.</para> |
| |
| <para>So, ZooKeeper by itself doesn't guarantee that changes occur |
| synchronously across all servers, but ZooKeeper |
| primitives can be used to construct higher level functions that |
| provide useful client synchronization. (For more information, |
| see the <ulink |
| url="recipes.html">ZooKeeper Recipes</ulink>. |
| <emphasis>[tbd:..]</emphasis>).</para> |
| </listitem> |
| </varlistentry> |
| </variablelist> |
| </note> |
| </section> |
| |
| <section id="ch_bindings"> |
| <title>Bindings</title> |
| |
| <para>The ZooKeeper client libraries come in two languages: Java and C. |
| The following sections describe these.</para> |
| |
| <section> |
| <title>Java Binding</title> |
| |
| <para>There are two packages that make up the ZooKeeper Java binding: |
| <emphasis role="bold">org.apache.zookeeper</emphasis> and <emphasis |
| role="bold">org.apache.zookeeper.data</emphasis>. The rest of the |
| packages that make up ZooKeeper are used internally or are part of the |
| server implementation. The <emphasis |
| role="bold">org.apache.zookeeper.data</emphasis> package is made up of |
| generated classes that are used simply as containers.</para> |
| |
| <para>The main class used by a ZooKeeper Java client is the <emphasis |
| role="bold">ZooKeeper</emphasis> class. Its two constructors differ only |
| by an optional session id and password. ZooKeeper supports session |
| recovery accross instances of a process. A Java program may save its |
| session id and password to stable storage, restart, and recover the |
| session that was used by the earlier instance of the program.</para> |
| |
| <para>When a ZooKeeper object is created, two threads are created as |
| well: an IO thread and an event thread. All IO happens on the IO thread |
| (using Java NIO). All event callbacks happen on the event thread. |
| Session maintenance such as reconnecting to ZooKeeper servers and |
| maintaining heartbeat is done on the IO thread. Responses for |
| synchronous methods are also processed in the IO thread. All responses |
| to asynchronous methods and watch events are processed on the event |
| thread. There are a few things to notice that result from this |
| design:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para>All completions for asynchronous calls and watcher callbacks |
| will be made in order, one at a time. The caller can do any |
| processing they wish, but no other callbacks will be processed |
| during that time.</para> |
| </listitem> |
| |
| <listitem> |
| <para>Callbacks do not block the processing of the IO thread or the |
| processing of the synchronous calls.</para> |
| </listitem> |
| |
| <listitem> |
| <para>Synchronous calls may not return in the correct order. For |
| example, assume a client does the following processing: issues an |
| asynchronous read of node <emphasis role="bold">/a</emphasis> with |
| <emphasis>watch</emphasis> set to true, and then in the completion |
| callback of the read it does a synchronous read of <emphasis |
| role="bold">/a</emphasis>. (Maybe not good practice, but not illegal |
| either, and it makes for a simple example.)</para> |
| |
| <para>Note that if there is a change to <emphasis |
| role="bold">/a</emphasis> between the asynchronous read and the |
| synchronous read, the client library will receive the watch event |
| saying <emphasis role="bold">/a</emphasis> changed before the |
| response for the synchronous read, but because the completion |
| callback is blocking the event queue, the synchronous read will |
| return with the new value of <emphasis role="bold">/a</emphasis> |
| before the watch event is processed.</para> |
| </listitem> |
| </itemizedlist> |
| |
| <para>Finally, the rules associated with shutdown are straightforward: |
| once a ZooKeeper object is closed or receives a fatal event |
| (SESSION_EXPIRED and AUTH_FAILED), the ZooKeeper object becomes invalid. |
| On a close, the two threads shut down and any further access on zookeeper |
| handle is undefined behavior and should be avoided. </para> |
| <section id="sc_java_client_configuration"> |
| <title><emphasis role="bold">Client Configuration Parameters</emphasis></title> |
| <para> |
| The following list contains configuration properties for the Java client. You can set any |
| of these properties using Java system properties. For server properties, please check the |
| following reference |
| <ulink url="zookeeperAdmin.html#sc_configuration">Server configuration section.</ulink> |
| </para> |
| <variablelist> |
| <varlistentry> |
| <term>zookeeper.sasl.client</term> |
| <listitem> |
| <para>Set the value to <emphasis role="bold">false</emphasis> to disable |
| SASL authentication. Default is <emphasis role="bold">true</emphasis>.</para> |
| </listitem> |
| </varlistentry> |
| <varlistentry> |
| <term>zookeeper.sasl.clientconfig</term> |
| <listitem> |
| <para>Specifies the context key in the JAAS login file. Default is "Client".</para> |
| </listitem> |
| </varlistentry> |
| <varlistentry> |
| <term>zookeeper.sasl.client.username</term> |
| <listitem> |
| <para>Traditionally, a principal is divided into three parts: the primary, the instance, and the realm. |
| The format of a typical Kerberos V5 principal is primary/instance@REALM. |
| zookeeper.sasl.client.username specifies the primary part of the server principal. Default |
| is "zookeeper". Instance part is derived from the server IP. Finally server's principal is |
| username/IP@realm, where username is the value of zookeeper.sasl.client.username, IP is |
| the server IP, and realm is the value of zookeeper.server.realm.</para> |
| </listitem> |
| </varlistentry> |
| <varlistentry> |
| <term>zookeeper.server.realm</term> |
| <listitem> |
| <para>Realm part of the server principal. By default it is the client principal realm.</para> |
| </listitem> |
| </varlistentry> |
| <varlistentry> |
| <term>zookeeper.disableAutoWatchReset</term> |
| <listitem> |
| <para>This switch controls whether automatic watch resetting is enabled. Clients automatically |
| reset watches during session reconnect by default, this option allows the client to turn off |
| this behavior by setting zookeeper.disableAutoWatchReset to <emphasis role="bold">true</emphasis>. |
| </para> |
| </listitem> |
| </varlistentry> |
| <varlistentry> |
| <term>zookeeper.client.secure</term> |
| <listitem> |
| <para> |
| If you want to connect to the server secure client port, you need to set this property to |
| <emphasis role="bold">true</emphasis> |
| on the client. This will connect to server using SSL with specified credentials. Note that |
| it requires the Netty client. |
| </para> |
| </listitem> |
| </varlistentry> |
| <varlistentry> |
| <term>zookeeper.clientCnxnSocket</term> |
| <listitem> |
| <para> |
| Specifies which ClientCnxnSocket to be used. Possible values are |
| <emphasis role="bold">org.apache.zookeeper.ClientCnxnSocketNIO</emphasis> |
| and |
| <emphasis role="bold">org.apache.zookeeper.ClientCnxnSocketNetty</emphasis> |
| . Default is |
| <emphasis role="bold">org.apache.zookeeper.ClientCnxnSocketNIO</emphasis> |
| . If you want to connect to server's secure client port, you need to set this property to |
| <emphasis role="bold">org.apache.zookeeper.ClientCnxnSocketNetty</emphasis> |
| on client. |
| </para> |
| </listitem> |
| </varlistentry> |
| <varlistentry> |
| <term>zookeeper.ssl.keyStore.location and zookeeper.ssl.keyStore.password</term> |
| <listitem> |
| <para>Specifies the file path to a JKS containing the local credentials to be used for SSL connections, |
| and the password to unlock the file. |
| </para> |
| </listitem> |
| </varlistentry> |
| <varlistentry> |
| <term>zookeeper.ssl.trustStore.location and zookeeper.ssl.trustStore.password</term> |
| <listitem> |
| <para>Specifies the file path to a JKS containing the remote credentials to be used for SSL connections, |
| and the password to unlock the file. |
| </para> |
| </listitem> |
| </varlistentry> |
| <varlistentry> |
| <term>jute.maxbuffer</term> |
| <listitem> |
| <para>It specifies the maximum size of the incoming data from the server. The default value is 4194304 |
| Bytes , or just 4 MB. This is really a sanity check. The ZooKeeper server is designed to store and send |
| data on the order of kilobytes. If incoming data length is more than this value, an IOException |
| is raised.</para> |
| </listitem> |
| </varlistentry> |
| <varlistentry> |
| <term>zookeeper.kinit</term> |
| <listitem> |
| <para>Specifies path to kinit binary. Default is "/usr/bin/kinit".</para> |
| </listitem> |
| </varlistentry> |
| </variablelist> |
| </section> |
| </section> |
| |
| <section> |
| <title>C Binding</title> |
| |
| <para>The C binding has a single-threaded and multi-threaded library. |
| The multi-threaded library is easiest to use and is most similar to the |
| Java API. This library will create an IO thread and an event dispatch |
| thread for handling connection maintenance and callbacks. The |
| single-threaded library allows ZooKeeper to be used in event driven |
| applications by exposing the event loop used in the multi-threaded |
| library.</para> |
| |
| <para>The package includes two shared libraries: zookeeper_st and |
| zookeeper_mt. The former only provides the asynchronous APIs and |
| callbacks for integrating into the application's event loop. The only |
| reason this library exists is to support the platforms were a |
| <emphasis>pthread</emphasis> library is not available or is unstable |
| (i.e. FreeBSD 4.x). In all other cases, application developers should |
| link with zookeeper_mt, as it includes support for both Sync and Async |
| API.</para> |
| |
| <section> |
| <title>Installation</title> |
| |
| <para>If you're building the client from a check-out from the Apache |
| repository, follow the steps outlined below. If you're building from a |
| project source package downloaded from apache, skip to step <emphasis |
| role="bold">3</emphasis>.</para> |
| |
| <orderedlist> |
| <listitem> |
| <para>Run <command>ant compile_jute</command> from the ZooKeeper |
| top level directory (<filename>.../trunk</filename>). |
| This will create a directory named "generated" under |
| <filename>.../trunk/src/c</filename>.</para> |
| </listitem> |
| |
| <listitem> |
| <para>Change directory to the<filename>.../trunk/src/c</filename> |
| and run <command>autoreconf -if</command> to bootstrap <emphasis |
| role="bold">autoconf</emphasis>, <emphasis |
| role="bold">automake</emphasis> and <emphasis |
| role="bold">libtool</emphasis>. Make sure you have <emphasis |
| role="bold">autoconf version 2.59</emphasis> or greater installed. |
| Skip to step<emphasis role="bold"> 4</emphasis>.</para> |
| </listitem> |
| |
| <listitem> |
| <para>If you are building from a project source package, |
| unzip/untar the source tarball and cd to the<filename> |
| zookeeper-x.x.x/src/c</filename> directory.</para> |
| </listitem> |
| |
| <listitem> |
| <para>Run <command>./configure <your-options></command> to |
| generate the makefile. Here are some of options the <emphasis |
| role="bold">configure</emphasis> utility supports that can be |
| useful in this step:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para><command>--enable-debug</command></para> |
| |
| <para>Enables optimization and enables debug info compiler |
| options. (Disabled by default.)</para> |
| </listitem> |
| |
| <listitem> |
| <para><command>--without-syncapi </command></para> |
| |
| <para>Disables Sync API support; zookeeper_mt library won't be |
| built. (Enabled by default.)</para> |
| </listitem> |
| |
| <listitem> |
| <para><command>--disable-static </command></para> |
| |
| <para>Do not build static libraries. (Enabled by |
| default.)</para> |
| </listitem> |
| |
| <listitem> |
| <para><command>--disable-shared</command></para> |
| |
| <para>Do not build shared libraries. (Enabled by |
| default.)</para> |
| </listitem> |
| </itemizedlist> |
| |
| <note> |
| <para>See INSTALL for general information about running |
| <emphasis role="bold">configure</emphasis>.</para> |
| </note> |
| </listitem> |
| |
| <listitem> |
| <para>Run <command>make</command> or <command>make |
| install</command> to build the libraries and install them.</para> |
| </listitem> |
| |
| <listitem> |
| <para>To generate doxygen documentation for the ZooKeeper API, run |
| <command>make doxygen-doc</command>. All documentation will be |
| placed in a new subfolder named docs. By default, this command |
| only generates HTML. For information on other document formats, |
| run <command>./configure --help</command></para> |
| </listitem> |
| </orderedlist> |
| </section> |
| |
| <section> |
| <title>Using the C Client</title> |
| |
| <para>You can test your client by running a ZooKeeper server (see |
| instructions on the project wiki page on how to run it) and connecting |
| to it using one of the cli applications that were built as part of the |
| installation procedure. cli_mt (multithreaded, built against |
| zookeeper_mt library) is shown in this example, but you could also use |
| cli_st (singlethreaded, built against zookeeper_st library):</para> |
| |
| <para><command>$ cli_mt zookeeper_host:9876</command></para> |
| |
| <para>This is a client application that gives you a shell for |
| executing simple ZooKeeper commands. Once successfully started |
| and connected to the server it displays a shell prompt. You |
| can now enter ZooKeeper commands. For example, to create a |
| node:</para> |
| |
| <para><command>> create /my_new_node</command></para> |
| |
| <para>To verify that the node's been created:</para> |
| |
| <para><command>> ls /</command></para> |
| |
| <para>You should see a list of node who are children of the root node |
| "/".</para> |
| |
| <para>In order to be able to use the ZooKeeper API in your application |
| you have to remember to</para> |
| |
| <orderedlist> |
| <listitem> |
| <para>Include ZooKeeper header: #include |
| <zookeeper/zookeeper.h></para> |
| </listitem> |
| |
| <listitem> |
| <para>If you are building a multithreaded client, compile with |
| -DTHREADED compiler flag to enable the multi-threaded version of |
| the library, and then link against against the |
| <emphasis>zookeeper_mt</emphasis> library. If you are building a |
| single-threaded client, do not compile with -DTHREADED, and be |
| sure to link against the<emphasis> zookeeper_st |
| </emphasis>library.</para> |
| </listitem> |
| </orderedlist> |
| |
| <para>Refer to <xref linkend="ch_programStructureWithExample"/> |
| for examples of usage in Java and C. |
| <emphasis>[tbd]</emphasis> |
| </para> |
| </section> |
| </section> |
| </section> |
| |
| <section id="ch_guideToZkOperations"> |
| <title>Building Blocks: A Guide to ZooKeeper Operations</title> |
| |
| <para>This section surveys all the operations a developer can perform |
| against a ZooKeeper server. It is lower level information than the earlier |
| concepts chapters in this manual, but higher level than the ZooKeeper API |
| Reference. It covers these topics:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para><xref linkend="sc_connectingToZk" /></para> |
| </listitem> |
| </itemizedlist> |
| |
| <section id="sc_errorsZk"> |
| <title>Handling Errors</title> |
| |
| <para>Both the Java and C client bindings may report errors. The Java client binding does so by throwing KeeperException, calling code() on the exception will return the specific error code. The C client binding returns an error code as defined in the enum ZOO_ERRORS. API callbacks indicate result code for both language bindings. See the API documentation (javadoc for Java, doxygen for C) for full details on the possible errors and their meaning.</para> |
| </section> |
| |
| <section id="sc_connectingToZk"> |
| <title>Connecting to ZooKeeper</title> |
| |
| <para></para> |
| </section> |
| |
| <section id="sc_readOps"> |
| <title>Read Operations</title> |
| |
| <para></para> |
| </section> |
| |
| <section id="sc_writeOps"> |
| <title>Write Operations</title> |
| |
| <para></para> |
| </section> |
| |
| <section id="sc_handlingWatches"> |
| <title>Handling Watches</title> |
| |
| <para></para> |
| </section> |
| |
| <section id="sc_miscOps"> |
| <title>Miscelleaneous ZooKeeper Operations</title> |
| <para></para> |
| </section> |
| |
| |
| </section> |
| |
| <section id="ch_programStructureWithExample"> |
| <title>Program Structure, with Simple Example</title> |
| |
| <para><emphasis>[tbd]</emphasis></para> |
| </section> |
| |
| <section id="ch_gotchas"> |
| <title>Gotchas: Common Problems and Troubleshooting</title> |
| |
| <para>So now you know ZooKeeper. It's fast, simple, your application |
| works, but wait ... something's wrong. Here are some pitfalls that |
| ZooKeeper users fall into:</para> |
| |
| <orderedlist> |
| <listitem> |
| <para>If you are using watches, you must look for the connected watch |
| event. When a ZooKeeper client disconnects from a server, you will |
| not receive notification of changes until reconnected. If you are |
| watching for a znode to come into existance, you will miss the event |
| if the znode is created and deleted while you are disconnected.</para> |
| </listitem> |
| |
| <listitem> |
| <para>You must test ZooKeeper server failures. The ZooKeeper service |
| can survive failures as long as a majority of servers are active. The |
| question to ask is: can your application handle it? In the real world |
| a client's connection to ZooKeeper can break. (ZooKeeper server |
| failures and network partitions are common reasons for connection |
| loss.) The ZooKeeper client library takes care of recovering your |
| connection and letting you know what happened, but you must make sure |
| that you recover your state and any outstanding requests that failed. |
| Find out if you got it right in the test lab, not in production - test |
| with a ZooKeeper service made up of a several of servers and subject |
| them to reboots.</para> |
| </listitem> |
| |
| <listitem> |
| <para>The list of ZooKeeper servers used by the client must match the |
| list of ZooKeeper servers that each ZooKeeper server has. Things can |
| work, although not optimally, if the client list is a subset of the |
| real list of ZooKeeper servers, but not if the client lists ZooKeeper |
| servers not in the ZooKeeper cluster.</para> |
| </listitem> |
| |
| <listitem> |
| <para>Be careful where you put that transaction log. The most |
| performance-critical part of ZooKeeper is the transaction log. |
| ZooKeeper must sync transactions to media before it returns a |
| response. A dedicated transaction log device is key to consistent good |
| performance. Putting the log on a busy device will adversely effect |
| performance. If you only have one storage device, put trace files on |
| NFS and increase the snapshotCount; it doesn't eliminate the problem, |
| but it can mitigate it.</para> |
| </listitem> |
| |
| <listitem> |
| <para>Set your Java max heap size correctly. It is very important to |
| <emphasis>avoid swapping.</emphasis> Going to disk unnecessarily will |
| almost certainly degrade your performance unacceptably. Remember, in |
| ZooKeeper, everything is ordered, so if one request hits the disk, all |
| other queued requests hit the disk.</para> |
| |
| <para>To avoid swapping, try to set the heapsize to the amount of |
| physical memory you have, minus the amount needed by the OS and cache. |
| The best way to determine an optimal heap size for your configurations |
| is to <emphasis>run load tests</emphasis>. If for some reason you |
| can't, be conservative in your estimates and choose a number well |
| below the limit that would cause your machine to swap. For example, on |
| a 4G machine, a 3G heap is a conservative estimate to start |
| with.</para> |
| </listitem> |
| </orderedlist> |
| </section> |
| |
| <appendix id="apx_linksToOtherInfo"> |
| <title>Links to Other Information</title> |
| |
| <para>Outside the formal documentation, there're several other sources of |
| information for ZooKeeper developers.</para> |
| |
| <variablelist> |
| <varlistentry> |
| <term>ZooKeeper Whitepaper <emphasis>[tbd: find url]</emphasis></term> |
| |
| <listitem> |
| <para>The definitive discussion of ZooKeeper design and performance, |
| by Yahoo! Research</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term>API Reference <emphasis>[tbd: find url]</emphasis></term> |
| |
| <listitem> |
| <para>The complete reference to the ZooKeeper API</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><ulink |
| url="http://us.dl1.yimg.com/download.yahoo.com/dl/ydn/zookeeper.m4v">ZooKeeper |
| Talk at the Hadoup Summit 2008</ulink></term> |
| |
| <listitem> |
| <para>A video introduction to ZooKeeper, by Benjamin Reed of Yahoo! |
| Research</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><ulink |
| url="https://cwiki.apache.org/confluence/display/ZOOKEEPER/Tutorial">Barrier and |
| Queue Tutorial</ulink></term> |
| |
| <listitem> |
| <para>The excellent Java tutorial by Flavio Junqueira, implementing |
| simple barriers and producer-consumer queues using ZooKeeper.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><ulink |
| url="https://cwiki.apache.org/confluence/display/ZOOKEEPER/ZooKeeperArticles">ZooKeeper |
| - A Reliable, Scalable Distributed Coordination System</ulink></term> |
| |
| <listitem> |
| <para>An article by Todd Hoff (07/15/2008)</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><ulink url="recipes.html">ZooKeeper Recipes</ulink></term> |
| |
| <listitem> |
| <para>Pseudo-level discussion of the implementation of various |
| synchronization solutions with ZooKeeper: Event Handles, Queues, |
| Locks, and Two-phase Commits.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry> |
| <term><emphasis>[tbd]</emphasis></term> |
| |
| <listitem> |
| <para>Any other good sources anyone can think of...</para> |
| </listitem> |
| </varlistentry> |
| </variablelist> |
| </appendix> |
| </article> |