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| <div class='page-header'> |
| |
| <h1>Multipoint Recommendations</h1> |
| </div> |
| </div> |
| </div> |
| <div class="row"> |
| |
| <div class="col-md-12"> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>As mentioned the <code>initialServers</code> 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.</p> |
| </div> |
| <div class="paragraph"> |
| <p>However to ensure that the bootstrapping process can occur successfully, |
| the <code>initialServers</code> property of the <code>conf/multipoint.properties</code> file |
| must be set carefully and with a specific server start order in mind. |
| Each server consults its <code>initialServers</code> list exactly once in the |
| bootstrapping phase at startup, after that time connections are made |
| dynamically.</p> |
| </div> |
| <div class="paragraph"> |
| <p>This means that at least one of the servers listed in <code>initialServers</code> |
| 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.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_failed_scenario_background">Failed scenario background</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>As an example of a failed scenario, imagine there are three servers; |
| <code>server1</code>, <code>server2</code>, <code>server3</code>. They are setup only to point to the |
| server in front of them making a chain:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>server1; <code>initialServers = server2</code></p> |
| </li> |
| <li> |
| <p>server2; <code>initialServers = server3</code></p> |
| </li> |
| <li> |
| <p>server3; <code>initialServers = <strong><blank></strong></code></p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Which is essentially <code>server1</code> → <code>server2</code> → <code>server3</code>. This scenario |
| could work, but they servers would have to be started in exactly the |
| opposite order:</p> |
| </div> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p><code>server3</code> starts</p> |
| </li> |
| <li> |
| <p><code>server2</code> starts</p> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p><em>static:</em> connect to <code>server3</code></p> |
| </li> |
| </ol> |
| </div> |
| </li> |
| <li> |
| <p><code>server1</code> starts</p> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p><em>static:</em> connect to <code>server2</code></p> |
| </li> |
| <li> |
| <p><em>dynamic:</em> connect to <code>server3</code></p> |
| </li> |
| </ol> |
| </div> |
| </li> |
| </ol> |
| </div> |
| <div class="paragraph"> |
| <p>At this point all servers would be fully connected. But the above setup |
| is flawed and could easily fail. The first flaw is <code>server3</code> lists |
| nothing in its <code>initialServers</code> list, so if it were restarted it would |
| leave the multipoint network and not know how to get back in.</p> |
| </div> |
| <div class="paragraph"> |
| <p>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:</p> |
| </div> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p><code>server1</code> starts</p> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p><em>static:</em> connect to <code>server2</code> - failed, <code>server2</code> not started.</p> |
| </li> |
| </ol> |
| </div> |
| </li> |
| <li> |
| <p><code>server2</code> starts</p> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p><em>static:</em> connect to <code>server3</code> - failed, <code>server3</code> not started.</p> |
| </li> |
| </ol> |
| </div> |
| </li> |
| <li> |
| <p><code>server3</code> starts</p> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p>no connection attempts, initialServers list is empty.</p> |
| </li> |
| </ol> |
| </div> |
| </li> |
| </ol> |
| </div> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The above setup is easily fixable via better configuration. If <code>server3</code> |
| listed both <code>server1</code> and <code>server2</code> 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.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_bootstrapping_three_servers_or_less">Bootstrapping Three Servers or Less</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>In a three sever scenario, we recommend simply having all three servers |
| list all three servers.</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>server1/conf/multipoint.properties</p> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>initialServers = server1, server2, server3</code></p> |
| </li> |
| </ul> |
| </div> |
| </li> |
| <li> |
| <p>server2/conf/multipoint.properties</p> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>initialServers = server1, server2, server3</code></p> |
| </li> |
| </ul> |
| </div> |
| </li> |
| <li> |
| <p>server3/conf/multipoint.properties</p> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>initialServers = server1, server2, server3</code></p> |
| </li> |
| </ul> |
| </div> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_bootstrapping_four_servers_or_more">Bootstrapping Four Servers or More</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>server1/conf/multipoint.properties</p> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>initialServers = server2</code></p> |
| </li> |
| </ul> |
| </div> |
| </li> |
| <li> |
| <p>server2/conf/multipoint.properties</p> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>initialServers = server1</code></p> |
| </li> |
| </ul> |
| </div> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Root <code>server1</code> would list root <code>server2</code> so they would always be linked |
| to each other regardless of start order or if one of them went down. |
| <code>Server1</code> could be shutdown and reconnect on startup to the full |
| multipoint network through <code>server2</code>, and vice versa.</p> |
| </div> |
| <div class="paragraph"> |
| <p>All other servers would simply list the root servers (<code>server1</code>, |
| <code>server2</code>) in their initialServers list.</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>server3/conf/multipoint.properties</p> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>initialServers = server1, server2</code></p> |
| </li> |
| </ul> |
| </div> |
| </li> |
| <li> |
| <p>server4/conf/multipoint.properties</p> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>initialServers = server1, server2</code></p> |
| </li> |
| </ul> |
| </div> |
| </li> |
| <li> |
| <p>serverN/conf/multipoint.properties</p> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>initialServers = server1, server2</code></p> |
| </li> |
| </ul> |
| </div> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>As long as at least one root server (<code>server1</code> or <code>server2</code>) was |
| running, you can bring other servers on and offline at will and always |
| have a fully connected graph.</p> |
| </div> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_setting_initialservers_overrides">Setting initialServers overrides</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Always remember that any property in a |
| <code>conf/<server-service>.properties</code> file can be overridden on the command |
| line or via system properties. So it is possible easily set the |
| <code>initialServers</code> list in startup scripts.</p> |
| </div> |
| <div class="paragraph"> |
| <p>A bash example might look something like:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-java" data-lang="java">!/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</code></pre> |
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