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.
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
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
PROVIDER_URL starting with
multicast://) before abandoning the request and throwing an exception to
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.
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.
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