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| <div class="sect1"> |
| <h2 id="connection_pool">Connection Pool</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>There are situations where you don’t want to carry around connections in your code.</p> |
| </div> |
| <div class="paragraph"> |
| <p>For example if you have a microservice application, you might have multiple services accessing PLC data.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Keeping an open connection for each of these is rather problematic as in contrast to modern computers, PLCs usually are only able to manage a small number of concurrent connections.</p> |
| </div> |
| <div class="paragraph"> |
| <p>I think a Siemens S7-1200 is limited to 7 concurrent connections and a Siemens LOGO can’t do more than 3.</p> |
| </div> |
| <div class="paragraph"> |
| <p>One alternative would be to open and close the connections all the time. |
| This however puts an unnecessary stress on the PLCs as for protocols like the S7 or the ADS protocol a handshake of multiple requests and responses has to be executed before being able to communicate.</p> |
| </div> |
| <div class="paragraph"> |
| <p>As usually these services only require ad-hoc access to a PLC, it would be ideal to share connections between services.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Another problem occurs when you open a connection and keep it open for a prolonged time period. |
| Here it can happen quite often that a connection is terminated because the PLC has been turned off.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Per default the PLC connection has no means of automatically re-connecting.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The <code>PooledPlcDriverManager</code> can help you with both of these scenarios.</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="the_pooledplcdrivermanager">The PooledPlcDriverManager</h3> |
| <div class="paragraph"> |
| <p>The <code>PooledPlcDriverManager</code> is a wrapper around the normal <code>PlcDriverManager</code>.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The main difference is that as soon as a connection is requested, it will look if one already exists and return that.</p> |
| </div> |
| <div class="paragraph"> |
| <p>If no connection exists it will create a new one and keep it in the pool.</p> |
| </div> |
| <div class="paragraph"> |
| <p>There are no API differences from using the normal <code>PlcDriverManager</code> so you call <code>createConnection</code> and <code>close</code> just like with the normal PLC4X API.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The structure of the code is now more or less like if you would create a new connection for every request and close it afterwards.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Another benefit of the <code>PooledPlcDriverManager</code> is that it will check a connection for any problems before returning it to the client.</p> |
| </div> |
| <div class="paragraph"> |
| <p>So if a connection was terminated, it will detect this and create a new connection.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="example">Example</h3> |
| <div class="paragraph"> |
| <p>Here comes a little example program utilizing the <code>PooledPlcDriverManager</code>:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="prettyprint highlight"><code data-lang="java"> PlcDriverManager driverManager = new PooledPlcDriverManager(); |
| |
| // This just simulates a scenario where a lot of connections would be created and immediately destroyed again. |
| for(int i = 0; i < 100; i++) { |
| try(PlcConnection connection = driverManager.getConnection("...")) { |
| |
| ... do something ... (please refer to the PLC4J getting started for details) |
| |
| } |
| }</code></pre> |
| </div> |
| </div> |
| <div class="admonitionblock note"> |
| <table> |
| <tr> |
| <td class="icon"> |
| <i class="fa icon-note" title="Note"></i> |
| </td> |
| <td class="content"> |
| When sharing connections between multiple services, be sure to share the instance of the <code>PooledPldDriverManager</code> as otherwise you will result in having multiple pools. |
| </td> |
| </tr> |
| </table> |
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