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<h1 id="custom-protocol-in-libp2p">Custom protocol in libp2p</h1>
<p>In the <a href="../tutorial_1_connect/">previous tutorial</a>, we've looked at how to create a simple ping program using the <code>nim-libp2p</code>.</p>
<p>We'll now look at how to create a custom protocol inside the libp2p</p>
<p>Let's create a <code>part2.nim</code>, and import our dependencies:
<div class="highlight"><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="kn">import</span><span class="w"> </span><span class="n">chronos</span>
<a id="__codelineno-0-2" name="__codelineno-0-2" href="#__codelineno-0-2"></a><span class="kn">import</span><span class="w"> </span><span class="n">stew</span><span class="o">/</span><span class="n">byteutils</span>
<a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></a>
<a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></a><span class="kn">import</span><span class="w"> </span><span class="n">libp2p</span>
</code></pre></div>
This is similar to the first tutorial, except we don't need to import the <code>Ping</code> protocol.</p>
<p>Next, we'll declare our custom protocol
<div class="highlight"><pre><span></span><code><a id="__codelineno-1-1" name="__codelineno-1-1" href="#__codelineno-1-1"></a><span class="k">const</span><span class="w"> </span><span class="n">TestCodec</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">&quot;/test/proto/1.0.0&quot;</span>
<a id="__codelineno-1-2" name="__codelineno-1-2" href="#__codelineno-1-2"></a>
<a id="__codelineno-1-3" name="__codelineno-1-3" href="#__codelineno-1-3"></a><span class="k">type</span><span class="w"> </span><span class="n">TestProto</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">ref</span><span class="w"> </span><span class="k">object</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">LPProtocol</span>
</code></pre></div>
We've set a <a href="https://docs.libp2p.io/concepts/protocols/#protocol-ids">protocol ID</a>, and created a custom <code>LPProtocol</code>. In a more complex protocol, we could use this structure to store interesting variables.</p>
<p>A protocol generally has two parts: a handling/server part, and a dialing/client part.
These two parts can be identical, but in our trivial protocol, the server will wait for a message from the client, and the client will send a message, so we have to handle the two cases separately.</p>
<p>Let's start with the server part:
<div class="highlight"><pre><span></span><code><a id="__codelineno-2-1" name="__codelineno-2-1" href="#__codelineno-2-1"></a><span class="k">proc</span><span class="w"> </span><span class="nf">new</span><span class="p">(</span><span class="n">T</span><span class="p">:</span><span class="w"> </span><span class="n">typedesc</span><span class="o">[</span><span class="n">TestProto</span><span class="o">]</span><span class="p">):</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="o">=</span>
<a id="__codelineno-2-2" name="__codelineno-2-2" href="#__codelineno-2-2"></a><span class="w"> </span><span class="c"># every incoming connections will in be handled in this closure</span>
<a id="__codelineno-2-3" name="__codelineno-2-3" href="#__codelineno-2-3"></a><span class="w"> </span><span class="k">proc</span><span class="w"> </span><span class="nf">handle</span><span class="p">(</span><span class="n">conn</span><span class="p">:</span><span class="w"> </span><span class="n">Connection</span><span class="p">,</span><span class="w"> </span><span class="n">proto</span><span class="p">:</span><span class="w"> </span><span class="nb">string</span><span class="p">)</span><span class="w"> </span><span class="sx">{.async.}</span><span class="w"> </span><span class="o">=</span>
<a id="__codelineno-2-4" name="__codelineno-2-4" href="#__codelineno-2-4"></a><span class="w"> </span><span class="c"># Read up to 1024 bytes from this connection, and transform them into</span>
<a id="__codelineno-2-5" name="__codelineno-2-5" href="#__codelineno-2-5"></a><span class="w"> </span><span class="c"># a string</span>
<a id="__codelineno-2-6" name="__codelineno-2-6" href="#__codelineno-2-6"></a><span class="w"> </span><span class="n">echo</span><span class="w"> </span><span class="s">&quot;Got from remote - &quot;</span><span class="p">,</span><span class="w"> </span><span class="nb">string</span><span class="p">.</span><span class="n">fromBytes</span><span class="p">(</span><span class="n">await</span><span class="w"> </span><span class="n">conn</span><span class="p">.</span><span class="n">readLp</span><span class="p">(</span><span class="mi">1024</span><span class="p">))</span>
<a id="__codelineno-2-7" name="__codelineno-2-7" href="#__codelineno-2-7"></a><span class="w"> </span><span class="c"># We must close the connections ourselves when we&#39;re done with it</span>
<a id="__codelineno-2-8" name="__codelineno-2-8" href="#__codelineno-2-8"></a><span class="w"> </span><span class="n">await</span><span class="w"> </span><span class="n">conn</span><span class="p">.</span><span class="n">close</span><span class="p">()</span>
<a id="__codelineno-2-9" name="__codelineno-2-9" href="#__codelineno-2-9"></a>
<a id="__codelineno-2-10" name="__codelineno-2-10" href="#__codelineno-2-10"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">T</span><span class="p">.</span><span class="n">new</span><span class="p">(</span><span class="n">codecs</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">@[</span><span class="n">TestCodec</span><span class="o">]</span><span class="p">,</span><span class="w"> </span><span class="n">handler</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">handle</span><span class="p">)</span>
</code></pre></div>
This is a constructor for our <code>TestProto</code>, that will specify our <code>codecs</code> and a <code>handler</code>, which will be called for each incoming peer asking for this protocol.
In our handle, we simply read a message from the connection and <code>echo</code> it.</p>
<p>We can now create our client part:
<div class="highlight"><pre><span></span><code><a id="__codelineno-3-1" name="__codelineno-3-1" href="#__codelineno-3-1"></a><span class="k">proc</span><span class="w"> </span><span class="nf">hello</span><span class="p">(</span><span class="n">p</span><span class="p">:</span><span class="w"> </span><span class="n">TestProto</span><span class="p">,</span><span class="w"> </span><span class="n">conn</span><span class="p">:</span><span class="w"> </span><span class="n">Connection</span><span class="p">)</span><span class="w"> </span><span class="sx">{.async.}</span><span class="w"> </span><span class="o">=</span>
<a id="__codelineno-3-2" name="__codelineno-3-2" href="#__codelineno-3-2"></a><span class="w"> </span><span class="n">await</span><span class="w"> </span><span class="n">conn</span><span class="p">.</span><span class="n">writeLp</span><span class="p">(</span><span class="s">&quot;Hello p2p!&quot;</span><span class="p">)</span>
</code></pre></div>
Again, pretty straightforward, we just send a message on the connection.</p>
<p>We can now create our main procedure:
<div class="highlight"><pre><span></span><code><a id="__codelineno-4-1" name="__codelineno-4-1" href="#__codelineno-4-1"></a><span class="k">proc</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="sx">{.async.}</span><span class="w"> </span><span class="o">=</span>
<a id="__codelineno-4-2" name="__codelineno-4-2" href="#__codelineno-4-2"></a><span class="w"> </span><span class="k">let</span>
<a id="__codelineno-4-3" name="__codelineno-4-3" href="#__codelineno-4-3"></a><span class="w"> </span><span class="n">rng</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">newRng</span><span class="p">()</span>
<a id="__codelineno-4-4" name="__codelineno-4-4" href="#__codelineno-4-4"></a><span class="w"> </span><span class="n">testProto</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">TestProto</span><span class="p">.</span><span class="n">new</span><span class="p">()</span>
<a id="__codelineno-4-5" name="__codelineno-4-5" href="#__codelineno-4-5"></a><span class="w"> </span><span class="n">switch1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">newStandardSwitch</span><span class="p">(</span><span class="n">rng</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">rng</span><span class="p">)</span>
<a id="__codelineno-4-6" name="__codelineno-4-6" href="#__codelineno-4-6"></a><span class="w"> </span><span class="n">switch2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">newStandardSwitch</span><span class="p">(</span><span class="n">rng</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">rng</span><span class="p">)</span>
<a id="__codelineno-4-7" name="__codelineno-4-7" href="#__codelineno-4-7"></a>
<a id="__codelineno-4-8" name="__codelineno-4-8" href="#__codelineno-4-8"></a><span class="w"> </span><span class="n">switch1</span><span class="p">.</span><span class="n">mount</span><span class="p">(</span><span class="n">testProto</span><span class="p">)</span>
<a id="__codelineno-4-9" name="__codelineno-4-9" href="#__codelineno-4-9"></a>
<a id="__codelineno-4-10" name="__codelineno-4-10" href="#__codelineno-4-10"></a><span class="w"> </span><span class="n">await</span><span class="w"> </span><span class="n">switch1</span><span class="p">.</span><span class="n">start</span><span class="p">()</span>
<a id="__codelineno-4-11" name="__codelineno-4-11" href="#__codelineno-4-11"></a><span class="w"> </span><span class="n">await</span><span class="w"> </span><span class="n">switch2</span><span class="p">.</span><span class="n">start</span><span class="p">()</span>
<a id="__codelineno-4-12" name="__codelineno-4-12" href="#__codelineno-4-12"></a>
<a id="__codelineno-4-13" name="__codelineno-4-13" href="#__codelineno-4-13"></a><span class="w"> </span><span class="k">let</span><span class="w"> </span><span class="n">conn</span><span class="w"> </span><span class="o">=</span>
<a id="__codelineno-4-14" name="__codelineno-4-14" href="#__codelineno-4-14"></a><span class="w"> </span><span class="n">await</span><span class="w"> </span><span class="n">switch2</span><span class="p">.</span><span class="n">dial</span><span class="p">(</span><span class="n">switch1</span><span class="p">.</span><span class="n">peerInfo</span><span class="p">.</span><span class="n">peerId</span><span class="p">,</span><span class="w"> </span><span class="n">switch1</span><span class="p">.</span><span class="n">peerInfo</span><span class="p">.</span><span class="n">addrs</span><span class="p">,</span><span class="w"> </span><span class="n">TestCodec</span><span class="p">)</span>
<a id="__codelineno-4-15" name="__codelineno-4-15" href="#__codelineno-4-15"></a>
<a id="__codelineno-4-16" name="__codelineno-4-16" href="#__codelineno-4-16"></a><span class="w"> </span><span class="n">await</span><span class="w"> </span><span class="n">testProto</span><span class="p">.</span><span class="n">hello</span><span class="p">(</span><span class="n">conn</span><span class="p">)</span>
<a id="__codelineno-4-17" name="__codelineno-4-17" href="#__codelineno-4-17"></a>
<a id="__codelineno-4-18" name="__codelineno-4-18" href="#__codelineno-4-18"></a><span class="w"> </span><span class="c"># We must close the connection ourselves when we&#39;re done with it</span>
<a id="__codelineno-4-19" name="__codelineno-4-19" href="#__codelineno-4-19"></a><span class="w"> </span><span class="n">await</span><span class="w"> </span><span class="n">conn</span><span class="p">.</span><span class="n">close</span><span class="p">()</span>
<a id="__codelineno-4-20" name="__codelineno-4-20" href="#__codelineno-4-20"></a>
<a id="__codelineno-4-21" name="__codelineno-4-21" href="#__codelineno-4-21"></a><span class="w"> </span><span class="n">await</span><span class="w"> </span><span class="n">allFutures</span><span class="p">(</span><span class="n">switch1</span><span class="p">.</span><span class="n">stop</span><span class="p">(),</span><span class="w"> </span><span class="n">switch2</span><span class="p">.</span><span class="n">stop</span><span class="p">())</span>
<a id="__codelineno-4-22" name="__codelineno-4-22" href="#__codelineno-4-22"></a><span class="w"> </span><span class="c"># close connections and shutdown all transports</span>
</code></pre></div>
This is very similar to the first tutorial's <code>main</code>, the only noteworthy difference is that we use <code>newStandardSwitch</code>, which is similar to the <code>createSwitch</code> of the first tutorial, but is bundled directly in libp2p</p>
<p>We can now wrap our program by calling our main proc:
<div class="highlight"><pre><span></span><code><a id="__codelineno-5-1" name="__codelineno-5-1" href="#__codelineno-5-1"></a><span class="n">waitFor</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
</code></pre></div>
And that's it!
In the <a href="../tutorial_3_protobuf/">next tutorial</a>, we'll create a more complex protocol using Protobuf.</p>
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