content/failover.mdtext

Title: Failover
<a name="Failover-Overview"></a>
# Overview

OpenEJB supports stateless failover.  Specifically, the ability for an EJB
client to failover from one server to the next if a request cannot be
completed.  No application state information is communicated between the
servers, so this functionality should be used only with applications that
are inherently stateless.  A common term for this sort of setup is a server
farm.

The basic design assumption is that all servers in the same group have the
same applications deployed and are capable of doing the same job.  Servers
can be brought online and offline while clients are running.  As members
join/leave this information is sent to the client as part of normal EJB
request/response communication so active clients always have the most
current information on servers that can process their request should
communication with a particular server fail.

<a name="Failover-Failover"></a>
## Failover

On each request to the server, the client will send the version number
associated with the list of servers in the cluster it is aware of. 
Initially this version will be zero and the list will be empty.  Only when
the server sees the client has an old list will the server send the updated
list.  This is an important distinction as the list is not transmitted back
and forth on every request, only on change.  If the membership of the
cluster is stable there is essentially no clustering overhead to the
protocol -- 8 byte overhead to each request and 1 byte on each response --
so you will *not* see an exponential slowdown in response times the more
members are added to the cluster.  This new list takes affect for all
proxies that share the same connection.

When a server shuts down, more connections are refused, existing
connections not in mid-request are closed, any remaining connections are
closed immediately after completion of the request in progress and clients
can failover gracefully to the next server in the list.  If a server
crashes requests are retried on the next server in the list (or depending
on the ConnectionStrategy).  This failover pattern is followed until there
are no more servers in the list at which point the client attempts a final
multicast search (if it was created with a multicast PROVIDER_URL) before
abandoning the request and throwing an exception to the caller.

By default, the failover is ordered but random selection is supported.	The
multicast discovery aspect of the client adds a nice randomness to the
selection of the first server.

<a name="Failover-Discovery"></a>
## Discovery

Each discoverable service has a URI which is broadcast as a heartbeat to
other servers in the cluster.  This URI advertises the service's type, its
cluster group, and its location in the format of 'group:type:location'. 
Say for example "cluster1:ejb:ejbd://thehost:4201".  The URI is sent out
repeatedly in a pulse and its presence on the network indicates its
availability and its absence indicates the service is no longer available.

The sending of this pulse (the heartbeat) can be done via UDP or TCP:
multicast and "multipoint" respectively.  More on that in the following
section.  The rate at which the heartbeat is pulsed to the network can be
specified via the 'heart_rate' property.  The default is 500 milliseconds. 
This rate is also used when listening for services on the network.  If a
service goes missing for the duration of 'heart_rate' multiplied by
'max_missed_heartbeats', then the service is considered dead.

The 'group' property, cluster1 in the example, is used to dissect the
servers on the network into smaller logical clusters.  A given server will
broadcast all it's services with the group prefixed in the URI, as well it
will ignore any services it sees broadcast if they do not share the same
group name.

<a name="Failover-Multicast(UDP)"></a>
# Multicast (UDP)

Multicast is the preferred way to broadcast the heartbeat on the network. 
The simple technique of broadcasting a non-changing service URI on the
network has specific advantages to multicast.  The URI itself is
essentially stateless and there is no "i'm alive" URI or an "i'm dead" URI.

In this way the issues with UDP being unordered and unreliable melt away as
state is no longer a concern and packet sizes are always small. 
Complicated libraries that ride atop UDP and attempt to offer reliability
(retransmission) and ordering on UDP can be avoided.  As well the
advantages UDP has over TCP are retained as there are no java layers
attempting to force UDP communication to be more TCP-like.  The simple
design means UDP/Multicast is only used for discovery and from there on out
critical information is transmitted over TCP/IP which is obviously going to
do a better job at ensuring reliability and ordering.

<a name="Failover-ServerConfiguration"></a>
## Server Configuration

When you boot the server there should be a conf/multicast.properties file
containing:


    server	    = org.apache.openejb.server.discovery.MulticastDiscoveryAgent
    bind	    = 239.255.2.3
    port	    = 6142
    disabled    = true
    group	    = default


Just need to enable that by setting 'disabled=false'.  All of the above
settings except *server* can be changed.  The *port* and *bind* must
be valid for general multicast/udp network communication.

The *group* setting can be changed to further group servers that may use
the same multicast channel.  As shown below the client also has a *group*
setting which can be used to select an appropriate server from the
multicast channel.

<a name="Failover-MulticastClient"></a>
## Multicast Client

The multicast functionality is not just for servers to find each other in a
cluster, it can also be used for EJB clients to discover a server.  A
special "multicast://" URL can be used in the InitialContext properties to
signify that multicast should be used to seed the connection process.  Such
as:

    Properties p = new Properties();
    p.put(Context.INITIAL_CONTEXT_FACTORY,
    "org.apache.openejb.client.RemoteInitialContextFactory");
    p.put(Context.PROVIDER_URL, "multicast://239.255.2.3:6142?group=default");
    InitialContext remoteContext = new InitialContext(p);

The URL has optional query parameters such as "schemes" and "group" and
"timeout" which allow you to zero in on a particular type of service of a
particular cluster group as well as set how long you are willing to wait in
the discovery process till finally giving up.  The first matching service
that it sees "flowing" around on the UDP stream is the one it picks and
sticks to for that and subsequent requests, ensuring UDP is only used when
there are no other servers to talk to.
    
Note that EJB clients do not need to use multicast to find a server.  If
the client knows the URL of a server in the cluster, it may use it and
connect directly to that server, at which point that server will share the
full list of its peers.
    
## Multicast Servers with TCP Clients
    
Note that clients do not need to use multicast to communicate with servers.
Servers can use multicast to discover each other, but clients are still
free to connect to servers in the network using the server's TCP address.
    
    Properties p = new Properties();
    p.put(Context.INITIAL_CONTEXT_FACTORY,  "org.apache.openejb.client.RemoteInitialContextFactory");
    p.put(Context.PROVIDER_URL, "ejbd://192.168.1.30:4201");
    InitialContext remoteContext = new InitialContext(p);

When the client connects, the server will send the URLs of all the servers
in the group and failover will take place normally.

<a name="Failover-Multipoint(TCP)"></a>
# Multipoint (TCP)

As TCP has no real broadcast functionality to speak of, communication of
who is in the network is achieved by each server having a physical
connection to each other server in the network.

To join the network, the server must be configured to know the address of
at least one server in the network and connect to it.  When it does both
servers will exchange the full list of all the other servers each knows
about.	Each server will then connect to any new servers they've  just
learned about and repeat the processes with those new servers.	The end
result is that everyone has a direct connection to everyone 100% of the
time, hence the made-up term "multipoint" to describe this situation of
each server having multiple point-to-point connections which create a fully
connected graph.

On the client side things are similar.	It needs to know the address of at
least one server in the network and be able to connect to it.  When it does
it will get the full (and dynamically maintained) list of every server in
the network.  The client doesn't connect to each of those servers
immediately, but rather consults the list in the event of a failover, using
it to decide who to connect to next.

The entire process is essentially the art of using a statically maintained
list to bootstrap getting the more valuable dynamically maintained list.

{div:style=clear:both;}{div}

<a name="Failover-ServerConfiguration"></a>
## Server Configuration


In the server this list can be specified via the
`conf/multipoint.properties` file like so:

    server	    = org.apache.openejb.server.discovery.MultipointDiscoveryAgent
    bind	    = 127.0.0.1
    port	    = 4212
    disabled    = false
    initialServers = 192.168.1.20:4212, 192.168.1.30:4212, 192.168.1.40:4212


The above configuration shows the server has an port 4212 open for
connections by other servers for multipoint communication.  The
`initialServers` list should be a comma separated list of other similar
servers on the network.  Only one of the servers listed is required to be
running when this server starts up -- it is not required to list all
servers in the network.

<a name="Failover-ClientConfiguration"></a>
## Client Configuration

Configuration in the client is similar, but note that EJB clients do not
participate directly in multipoint communication and do *not* connect to
the multipoint port.  The server list is simply a list of the regular
"ejbd://" urls that a client normally uses to connect to a server.

    Properties p = new Properties();
    p.put(Context.INITIAL_CONTEXT_FACTORY, "org.apache.openejb.client.RemoteInitialContextFactory");
    p.put(Context.PROVIDER_URL, "failover:ejbd://192.168.1.20:4201,ejbd://192.168.1.30:4201");
    InitialContext remoteContext = new InitialContext(p);

    
## Considerations
    
### Network size
    
The general disadvantage of this topology is the number of connections
required.  The number of connections for the network of servers is equal to
"(n * n - n) / 2 ", where n is the number of servers.  For example, with 5
servers you need 10 connections, with 10 servers you need 45 connections,
and with 50 servers you need 1225 connections.	This is of course the
number of connections across the entire network, each individual server
only needs "n - 1" connections.
    
The handling of these sockets is all asynchronous Java NIO code which
allows the server to handle many connections (all of them) with one thread.
 From a pure threading perspective, the option is extremely efficient with
just one thread to listen and broadcast to many peers.	
    
### Double connect 
    
It is possible in this process that two servers learn of each other at the
same time and each attempts to connect to the other simultaneously,
resulting in two connections between the same two servers.  When this
happens both servers will detect the extra connection and one of the
connections will be dropped and one will be kept.  In practice this race
condition rarely happens and can be avoided almost entirely by fanning out
server startup by as little as 100 milliseconds.
    
# Multipoint Configuration Recommendations
    
As mentioned above the {{initialServers}} is only used for bootstrapping
the multipoint network.  Once running, all servers will dynamically
establish direct connections with each other and there is no single point
of failure.
    
However to ensure that the bootstrapping process can occur successfully,
the {{initialServers}} property of the {{conf/multipoint.properties}} file
must be set carefully and with a specific server start order in mind.  Each
server consults its {{initialServers}} list exactly once in the
bootstrapping phase at startup, after that time connections are made
dynamically.
    
This means that at least one of the servers listed in {{initialServers}}
must already be running when the server starts or the server might never
become introduced and connected to all the other servers in the network.
    
## Failed scenario (background)
    
As an example of a failed scenario, imagine there are three servers;
server1, server2, server3.  They are setup only to point to the server in
front of them making a chain:
    
  * server1; initialServers = server2
  * server2; initialServers = server3
  * server3; initialServers = <blank>
    
Which is essentially server1 -> server2 -> server3.  This scenario could
work, but they servers would have to be started in exactly the opposite
order:
    
 # server3 starts
 # server2 starts
 #* static: connect to server3
 # server1 starts
 #* static: connect to server2
 #* dynamic: connect to server3
    
At this point all servers would be fully connected.  But the above setup is
flawed and could easily fail.  The first flaw is server3 lists nothing in
its {{initialServers}} list, so if it were restarted it would leave the
multipoint network and not know how to get back in.

The second flaw is if you started them in any other order, you would also
not get a fully connected multipoint network.  Say the servers were started
in "front" order:
    
 # server1 starts
 #* static: connect to server2 - failed, server2 not started.
 # server2 starts
 #* static: connect to server3 - failed, server3 not started.
 # server3 starts
 #* no connection attempts, initialServers list is empty.
    
After startup completes, all servers will be completely isolated and
failover will not work.  The described setup is weaker than it needs to be.
Listing just one server means the listed server is a potential point of
weakness.  As a matter of trivia, it is interesting to point out that you
could bring a fourth server online temporarily that lists all three
servers.  Once it makes the introductions and all servers learn of each
other, you could shut it down again.
    
The above setup is easily fixable via better configuration.  If server3
listed both server1 and server2 in its initialServers list, rather than
listing nothing at all, then all servers would fully discover each other
regardless of startup order; assuming all three servers did eventually
start.
    
## Bootstrapping Three Servers or Less
    
In a three sever scenario, we recommend simply having all three servers
list all three servers.  

 * server1/conf/multipoint.properties
 **    initialServers = server1, server2, server3
 * server2/conf/multipoint.properties
 **	initialServers = server1, server2, server3
 * server3/conf/multipoint.properties
 **	initialServers = server1, server2, server3

There's no harm to a server listing itself.  It gives you one clean list to
maintain and it will work even if you decide not to start one of the three
servers.
    
## Bootstrapping Four Servers or More

In a scenario of four or more, we recommend picking at least to servers and
focus on always keeping at least one of them running.  Lets refer to them
as "root" servers for simplicity sake.
    
 * server1/conf/multipoint.properties
 **    initialServers = server2
 * server2/conf/multipoint.properties
 **    initialServers = server1
    
Root server1 would list root server2 so they would always be linked to each
other regardless of start order or if one of them went down.  Server1 could
be shutdown and reconnect on startup to the full multipoint network through
server2, and vice versa.

All other servers would simply list the root servers (server1, server2) in
their initialServers list.

 * server3/conf/multipoint.properties
 **    initialServers = server1, server2
 * server4/conf/multipoint.properties
 **    initialServers = server1, server2
 * serverN/conf/multipoint.properties
 **    initialServers = server1, server2

As long as at least one root server (server1 or server2) was running, you
can bring other servers on and offline at will and always have a fully
connected graph.

Of course all servers once running and connected will have a full list of
all other servers in the network, so if at any time the "root" servers
weren't around to make initial introductions to new servers it would be no
trouble.  It's possible to reconfigure new servers to point at any other
server in the network as all servers will have the full list.  So these
"root" servers are no real point of failure in function, but only of
convenience.
    
## Command line overrides
    
Always remember that any property in a conf/<server-service>.properties
file can be overridden on the command line or via system properties.  So it
is possible easily set the initialServers list in startup scripts.

A bash example might look something like:

    !/bin/bash

    OPENEJB_HOME=/opt/openejb-3.1.3
    INITIAL_LIST=$(cat /some/shared/directory/our_initial_servers.txt)

    $OPENEJB_HOME/bin/openejb start -Dmultipoint.initialServers=$INITIAL_LIST