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| <h1>EJB Client/Server Failover |
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| <p>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.</p> |
| |
| <p>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.</p> |
| |
| <h1>Client Behavior</h1> |
| |
| <p>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 <em>not</em> 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.</p> |
| |
| <p>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 <code>ConnectionStrategy</code>). 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 <code>PROVIDER_URL</code> starting with |
| <code>multicast://</code>) before abandoning the request and throwing an exception to |
| the caller.</p> |
| |
| <p>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.</p> |
| |
| <h1>Discovery</h1> |
| |
| <p>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.</p> |
| |
| <p>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.</p> |
| |
| <p>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.</p> |
| |
| <h1>Details</h1> |
| |
| <p>Multicast</p> |
| |
| <ul> |
| <li><a href="multicast-discovery.html">Multicast UDP Discovery</a></li> |
| <li><a href="multipulse-discovery.html">Multipulse UDP Discovery</a></li> |
| </ul> |
| |
| <p>Multipoint</p> |
| |
| <ul> |
| <li><a href="multipoint-discovery.html">Multipoint TCP Discovery</a></li> |
| <li><a href="multipoint-considerations.html">Considerations</a></li> |
| <li><a href="multipoint-recommendations.html">Recommendations</a></li> |
| </ul> |
| |
| <p>Logging</p> |
| |
| <ul> |
| <li><a href="failover-logging.html">Failover Logging Events</a></li> |
| </ul> |
| |
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