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<title> Asynchronous Client Creation in Ditto Java Client 1.3.0 </title>
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<h1 class="post-title" itemprop="name headline">Asynchronous Client Creation in Ditto Java Client 1.3.0</h1>
<p class="post-meta">Published by <img src="https://www.gravatar.com/avatar/d923944b6a8c76e411ff838fb19acfa7?s=135" alt="A photo of Yufei Cai" style="width:50px;border-radius:50%;display:inline-block;margin-right:5px;" /><span itemprop="author" itemscope itemtype="http://schema.org/Person"><span itemprop="name"><a href="https://github.com/yufei-cai" target="_blank">Yufei Cai</a> </span></span> on <time datetime="2020-10-08T00:00:00+00:00" itemprop="datePublished">Oct 8, 2020</time> - Tags:
<a href="tag_blog.html">blog</a>
</p>
</header>
<div class="post-content" itemprop="articleBody">
<p>Before <a href="https://github.com/eclipse/ditto-clients/tree/master/java">Ditto Java Client</a> 1.3.0,
a client object connects to a configured Ditto back-end during its creation.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// create a client object and block until it connects to the Ditto back-end.</span>
<span class="kd">final</span> <span class="n">DittoClient</span> <span class="n">client</span> <span class="o">=</span> <span class="n">DittoClients</span><span class="o">.</span><span class="na">newInstance</span><span class="o">(</span><span class="n">messagingProvider</span><span class="o">);</span>
</code></pre></div></div>
<p>There are several problems with the approach.</p>
<ol>
<li>The calling thread blocks waiting for IO, namely the authentication process
and establishment of a websocket.</li>
<li>If the client is configured to reconnect, then an incorrect end-ponit configuration
makes the factory method block forever.</li>
<li>If the client is not configured to reconnect, then the factory method will throw
an exception. But it is not possible to give the client reference to the exception
handler, since the client creation did not complete. Consequently the exception handler
has no simple way to free all resources allocated for the client.</li>
</ol>
<p>1.3.0 addresses these problems by introducing an asynchronous client creation interface.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">public</span> <span class="kd">final</span> <span class="kd">class</span> <span class="nc">DittoClients</span> <span class="o">{</span>
<span class="kd">public</span> <span class="kd">static</span> <span class="n">DisconnectedDittoClient</span> <span class="nf">newDisconnectedInstance</span><span class="o">(</span><span class="n">MessagingProvider</span> <span class="n">mp</span><span class="o">);</span>
<span class="o">}</span>
<span class="kd">public</span> <span class="kd">interface</span> <span class="nc">DisconnectedDittoClient</span> <span class="o">{</span>
<span class="n">CompletionStage</span><span class="o">&lt;</span><span class="n">DittoClient</span><span class="o">&gt;</span> <span class="nf">connect</span><span class="o">();</span>
<span class="kt">void</span> <span class="nf">destroy</span><span class="o">();</span>
<span class="o">}</span>
</code></pre></div></div>
<p>The method <code class="highlighter-rouge">DittoClients.newDisconnectedInstance(MessagingProvider)</code> creates a <code class="highlighter-rouge">DisconnectedDittoClient</code>
object. The <code class="highlighter-rouge">DisconnectedDittoClient</code> has references to all resources allocated for the client and
can free them via the <code class="highlighter-rouge">destroy()</code> method. The <code class="highlighter-rouge">DisconnectedDittoClient</code> object offers no method to
interact with the Ditto API. By calling <code class="highlighter-rouge">connect()</code>, one obtains a future that yields a familiar
<code class="highlighter-rouge">DittoClient</code> object upon completion. One might use the asynchronous client creation interface thus:</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">final</span> <span class="n">DisconnectedDittoClient</span> <span class="n">disconnectedClient</span> <span class="o">=</span>
<span class="n">DittoClients</span><span class="o">.</span><span class="na">newDisconnectedInstance</span><span class="o">(</span><span class="n">messagingProvider</span><span class="o">);</span>
<span class="n">disconnectedClient</span><span class="o">.</span><span class="na">connect</span><span class="o">()</span>
<span class="o">.</span><span class="na">thenAccept</span><span class="o">(</span><span class="k">this</span><span class="o">::</span><span class="n">startUsingDittoClient</span><span class="o">)</span>
<span class="o">.</span><span class="na">exceptionally</span><span class="o">(</span><span class="n">exception</span> <span class="o">-&gt;</span> <span class="o">{</span>
<span class="k">this</span><span class="o">.</span><span class="na">handleConnectionFailure</span><span class="o">(</span><span class="n">exception</span><span class="o">);</span>
<span class="n">disconnectedClient</span><span class="o">.</span><span class="na">destroy</span><span class="o">();</span>
<span class="k">return</span> <span class="kc">null</span><span class="o">;</span>
<span class="o">});</span>
</code></pre></div></div>
<p>The asynchronous client creation interface has the following advantages.</p>
<ol>
<li>
<p>The calling thread does not block.</p>
</li>
<li>
<p>Even if configured to reconnect, the user can receive connection errors via
the connection error handler in <code class="highlighter-rouge">MessagingConfiguration</code> and shut down the client
at will.</p>
</li>
<li>
<p>When initial reconnection is disabled, the method <code class="highlighter-rouge">DisconnectedDittoClient.connect()</code>
returns a failed future on connection error. It is possible to reference the
<code class="highlighter-rouge">DittoDisconnectedClient</code> object in the future’s error handler, where the client can
be destroyed.</p>
</li>
</ol>
<p><br />
<br /></p>
<figure><img class="docimage" src="images/ditto.svg" alt="Ditto" style="max-width: 500px" /></figure>
<p><br />
The Eclipse Ditto team</p>
</div>
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