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| |
| <h1>EJB Client/Server Failover</h1> |
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| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <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> |
| </div> |
| <div class="paragraph"> |
| <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> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_client_behavior">Client Behavior</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <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> |
| </div> |
| <div class="paragraph"> |
| <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> |
| </div> |
| <div class="paragraph"> |
| <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> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_discovery">Discovery</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <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:jbake-date: 2018-12-05 |
| :jbake-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> |
| </div> |
| <div class="paragraph"> |
| <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> |
| </div> |
| <div class="paragraph"> |
| <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> |
| </div> |
| </div> |
| </div> |
| <h1 id="_details" class="sect0">Details</h1> |
| <div class="paragraph"> |
| <p>Multicast</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><a href="multicast-discovery.html">Multicast UDP Discovery</a></p> |
| </li> |
| <li> |
| <p><a href="multipulse-discovery.html">Multipulse UDP Discovery</a></p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Multipoint</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><a href="multipoint-discovery.html">Multipoint TCP Discovery</a></p> |
| </li> |
| <li> |
| <p><a href="multipoint-considerations.html">Considerations</a></p> |
| </li> |
| <li> |
| <p><a href="multipoint-recommendations.html">Recommendations</a></p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Logging</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><a href="failover-logging.html">Failover Logging Events</a></p> |
| </li> |
| </ul> |
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