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A REST HTTP gateway for ZooKeeper
=================================

Specification Version: 2

ZooKeeper is meant to enable distributed coordination and also store
system configuration and other relatively small amounts of information
that must be stored in a persistent and consistent manner. The
information stored in ZooKeeper is meant to be highly available to a
large number of nodes in a distributed-computing cluster.

ZooKeeper offers a client-side library that supports rich semantics
that include strict ordering guarantees on operations, the creation of
ephemeral znodes, and the ability to watch for changes to state.
However, where clients need simple "CRUD" (create, read, update,
delete) operations, the ZooKeeper libraries can be cumbersome, both to
the programmers who have to use them (who are increasingly used to
REST-style APIs), and to the operators who have to deploy and update
them (for whom deploying and updating client libraries can be very
painful).

It turns out that most languages comes with client libraries for HTTP
that are easy and familiar to program against, and deployed as part of
the language runtime. Thus, for simple CRUD clients, an HTTP gateway
would be a less cumbersome interface than the ZooKeeper library.

This document describes a gatway for using HTTP to interact with a
ZooKeeper repository.

Binding ZooKeeper to HTTP
-------------------------

Encoding
--------

UTF-8 unless otherwise noted

Paths
-----

A ZooKeeper paths are mapped to IRIs and URIs as follows. ZK paths
are converted to IRIs by simply percent-encoding any characters in the
ZK path that are not allowed in IRI paths. ZK paths are converted to
URIs by mapping them first to IRIs, then converting to URIs in the
standard way.

Going from URIs and IRIs is the reverse of the above but for one
difference: any "." and ".." segments in an IRI or URI must be folded
before conversion. (Fortunately, ZK does not allow "." and ".."
segments in its paths.)

ZK and IRIs recommend the same practices when it comes to Unicode
normalization: ultimately, normalization is left to application
designers, but both recommend that application designers use NFC as a
best practice.

Root
----

The following examples assume that the ZooKeeper znode heirarchy is
bound to the root of the HTTP servers namespace. This may not be the
case in practice however, the gateway may bind to some prefix, for
example the URL for accessing /a/b/c may be:

  http://localhost/zookeeper/znodes/v1/a/b/c

This is perfectly valid. Users of the REST service should be aware of
this fact and code their clients to support any root (in this case
"/zookeeper" on the server localhost).


Basics: GET, PUT, HEAD, and DELETE
----------------------------------

HTTP's GET, PUT, HEAD, and DELETE operations map naturally to
ZooKeeper's "get," "set," "exists," and "delete" operations.

ZooKeeper znodes have a version number that changes each time the
znode's value is updated. This number is returned by "get," "set," and
"exists" operations. The "set" and "delete" operations optionally take
a version number. If one is supplied, then "set" or "delete" will fail
if the current version of the znode doesn't match the version-number
supplied in the call. This mechanism supports atomic read-modify-write
cycles. Set/delete requests may include an optional parameter
"version" which defaults to no version check.


Getting ZooKeeper children
--------------------------

We overload the GET method to return the children of a ZooKeeper. In
particular, the GET method takes an optional parameter "view" which
could be set to one of type values, either "data" or "children". The
default is "data". Thus, to get the children of a znode named
"/a/b/c", then the GET request should start:

  GET /znodes/v1/a/b/c?view=children HTTP/1.1

If the requested view is "data", then the data of a znode is returned
as described in the previous section. If the requested view is
"children", then a list of children is returned in either an XML
document, or in a JSON object. (The default is JSON, but this can be
controlled changed by setting the Accept header.)


Creating a ZooKeeper session
----------------------------

In order to be able to create ephemeral nodes you first need to start
a new session.

  POST /sessions/v1?op=create&expire=<SECONDS> HTTP/1.1

If the session creation is successful, then a 201 code will be returned.

A session is just an UUID that you can pass around as a parameter and
the REST server will foward your request on the attached persistent 
connection.

Keeping a session alive
-----------------------

To keep a session alive you must send hearbeat requests:

  PUT /sessions/v1/<SESSION-UUID> HTTP/1.1

Closing a ZooKeeper session
---------------------------

You can close a connection by sending a DELETE request.

  DELETE /sessions/v1/<SESSION-UUID> HTTP/1.1

If you don't close a session it will automatically expire after
the amount of time you specified on creation. 

Creating a ZooKeeper znode
--------------------------

We use the POST method to create a ZooKeeper znode. For example, to
create a znode named "c" under a parent named "/a/b", then the POST
request should start:

  POST /znodes/v1/a/b?op=create&name=c HTTP/1.1

If the creation is successful, then a 201 code will be returned. If
it fails, then a number of different codes might be returned
(documented in a later subsection).

ZooKeeper's create operation has a flag that tells the server to
append a sequence-number to the client-supplied znode-name in order to
make the znode-name unique. If you set this flag and ask to create a
znode named "/a/b/c", and a znode named "/a/b" already exists, then
"create" will create a znode named "/a/b/c-#" instead, where "#" is and
integer required to generate a unique name in for format %10d.

To obtain this behavior, an additional "sequence=true" parameter
should be added to the parameters of the POST. (Note that "sequence"
is an optional parameter, that defaults to "false"; this default may
be provided explicitly if desired.)

On success the actual path of the created znode will be returned.

If you want to create an ephemeral node you need to specify an
additional "ephemeral=true" parameter. (Note that "ephemeral" is an optional
parameter, that defaults to "false")

(Note: ZooKeeper also allows the client to set ACLs for the
newly-created znode. This feature is not currently supported by the
HTTP gateway to ZooKeeper.)


Content types and negotiation
-----------------------------

ZooKeeper REST gateway implementations may support three content-types
for request and response messages:

* application/octet-stream

  HEAD   - returns nothing (note below: status = 204)
  GET    - returns the znode data as an octet-stream
  PUT    - send binary data, returns nothing
  POST   - send binary data, returns the name of the znode
  DELETE - returns nothing

  For PUT and HEAD some other content-type (i.e. JSON or XML) must be
  used to access the Stat information of a znode.

* application/json, application/javascript & application/xml

  HEAD   - returns nothing
  GET    - returns a STAT or CHILD structure
  PUT    - send binary data, returns a STAT structure (sans data field)
  POST   - send binary data, returns a PATH structure
  DELETE - returns nothing

  (structures defined below)

  Results returning DATA may include an optional "dataformat"
  parameter which has two possible values; base64 (default) or
  utf8. This allows the caller to control the format of returned data
  and may simplify usage -- for example cat'ing results to the command
  line with something like curl, or accessing a url through a browser.
  Care should be exercised however, if utf8 is used on non character
  data errors may result.

  "application/javascript" requests may include an optional "callback"
  parameter. The response is wrapped in a callback method of your
  choice. e.g. appending &callback=foo to your request will result in
  a response body of: foo(...). Callbacks may only contain
  alphanumeric characters and underscores.

PATH
  path : string
  uri: string

  path is the full path to the znode as seen by ZooKeeper

  uri is the full URI of the znode as seen by the REST server, does not
  include any query parameters (i.e. it's the path to the REST resource)

SESSION
  id : string UUID
  uri : string

CHILD
  PATH
  child_uri_template: string
  children : [ string* ]

  The children list of strings contains only the name of the child
  znodes, not the full path.

  child_uri_template is a template for URI of child znodes as seen by the
  REST server. e.g. "http://localhost:9998/znodes/v1/foo/{child}", where
  foo is the parent node, and {child} can be substituted with the name
  of each child in the children array in order to access that resource.
  This template is provided to simplify child access.
    
STAT
  PATH
  encoding : value of "base64" or "utf8"
  data     : base64 or utf8 encoded string
  stat :
    czxid          : number
    mzxid          : number
    ctime          : number
    mtime          : number
    version        : number
    cversion       : number
    aversion       : number
    ephemeralOwner : number
    datalength     : number
    numChildren    : number
    pzxid          : number


Error Codes
-----------

The ZooKeeper gateway uses HTTP response codes as follows:

   * 200 (Success) - ZOK for "get" "set" "delete", "yes" case of "exists" (json/xml)
   * 201 (Created) - ZOK for "create"
   * 204 (No Content) - ZOK for "yes" case of "exists" (octet)
   * 400 (Bad Request) - ZINVALIDACL, ZBADARGUMENTS, version param not a number
   * 401 (Unauthorized) - ZAUTHFAILED
   * 404 (Not Found) - ZOK for "no" case of "exists;" ZNONODE for "get," "set," and "delete"
   * 409 (Conflict) - ZNODEEXISTS, ZNONODE for "create," ZNOTEMPTY, 
   * 412 (Precondition Failed) - ZBADVERSION
   * 415 (Unsupported Media Type) - if content-type of PUT or POST is not "application/octet-stream"
   * 500 (Internal Server Error) - Failure in gateway code
   * 501 (Not Implemented) - HTTP method other than GET, PUT, HEAD, DELETE
   * 502 (Bad Gateway) - All other ZooKeeper error codes
   * 503 (Service Unavailable) - ZSESSIONEXPIRED, ZCONNECTIONLOSS, (gateway will try to reestablish the connection, but will not hold the request waiting...)
   * 504 (Gateway Timeout) - ZOPERATIONTIMEOUT, or ZooKeeper does not return in a timely manner

Note that these are the codes used by the HTTP-to-Gateway software
itself. Depending on how this software is configured into a Web
server, the resulting Web Server might behave differently, e.g., it
might do redirection, check other headers, etc.

Error Messages
--------------

Error messages are returned to the caller, format is dependent on the
format requested in the call. 

* application/octet-stream

  A string containing the error message. It should include the request
  and information detailing the reason for the error.

* application/json

  { "request":"GET /a/b/c", "message":"Node doesn't exist" }

* application/xml

<?xml version="1.0" encoding="UTF-8"?>
<error>
  <request>GET /a/b/c</request>
  <message>Node doesn't exist</message>
</error>


Binding ZooKeeper to an HTTP server
-----------------------------------

It might be sage to assume that everyone is happy to run an Apache
server, and thus write a "mod_zookeeper" for Apache that works only
for the Apache Web Server. However, different operational
environments prefer different Web Servers, and it would be nice to
support more than one Web server.

Issues:

   * Configuration.

   * Defining a root: Need to provide a URL alias and associate it
     with a server. Need to be able to map different aliases to
     different servers (implemented via multiple ZK connections).

   * Sharing connection across multiple processes.

   * Asynchronous.

   * Adaptors.

   * Code re-use.


Authentication -- TBD, not currently supported

...the config file should contain authentication material for the gateway

...the config file should contain an ACL list to be passed along to "create"

...would we ever want to authenticate each request to ZooKeeper?...