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| |
| <h1>Multicast (UDP) Discovery</h1> |
| </div> |
| </div> |
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| |
| <div class="col-md-12"> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_server_configuration">Server Configuration</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>When you boot the server there should be a <code>conf/multicast.properties</code> |
| file containing:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-properties" data-lang="properties">server = org.apache.openejb.server.discovery.MulticastDiscoveryAgent |
| bind = 239.255.2.3 |
| port = 6142 |
| disabled = true |
| group = default</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Just need to enable that by setting <code>disabled=false</code>. All of the above |
| settings except <code>server</code> can be changed. The <code>port</code> and <code>bind</code> must be |
| valid for general multicast/udp network communication.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The <code>group</code> setting can be changed to further group servers that may use |
| the same multicast channel. As shown below the client also has a <code>group</code> |
| setting which can be used to select an appropriate server from the |
| multicast channel.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_multicast_client">Multicast Client</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>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 <code>multicast://</code> URL can be used in the <code>InitialContext</code> |
| properties to signify that multicast should be used to seed the |
| connection process. Such as:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-java" data-lang="java">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);</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The URL has optional query parameters such as <code>schemes</code> and <code>group</code> and |
| <code>timeout</code> 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.</p> |
| </div> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_multicast_servers_with_tcp_clients">Multicast Servers with TCP Clients</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-java" data-lang="java">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);</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>When the client connects, the server will send the URLs of all the |
| servers in the group and failover will take place normally.</p> |
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