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<h1 id="nodes">Nodes<a class="headerlink" href="#nodes" title="Permanent link">&para;</a></h1>
<p>A node wraps any callable. Its input types are inferred from the function's parameter list; its output types from the return type.</p>
<h2 id="node-types">Node types<a class="headerlink" href="#node-types" title="Permanent link">&para;</a></h2>
<table>
<thead>
<tr>
<th>Type</th>
<th>Thread model</th>
<th>Use case</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>Node&lt;Func&gt;</code></td>
<td>Dedicated thread per node</td>
<td>Default — simplest, most isolated</td>
</tr>
<tr>
<td><code>PoolNode&lt;Func&gt;</code></td>
<td>Shared <code>ThreadPool</code></td>
<td>Many nodes, resource-bounded execution</td>
</tr>
<tr>
<td><code>InterruptNode&lt;Func&gt;</code></td>
<td>Event-driven, no thread</td>
<td>Camera frame ready, timer tick, socket</td>
</tr>
<tr>
<td><code>FanoutNode&lt;T, N&gt;</code></td>
<td>Dedicated thread</td>
<td>Broadcast one item to N outputs</td>
</tr>
<tr>
<td><code>RouterNode&lt;T, N&gt;</code></td>
<td>Dedicated thread</td>
<td>Route one item to one of N outputs</td>
</tr>
<tr>
<td><code>FilterNode&lt;T&gt;</code></td>
<td>Dedicated thread</td>
<td>Pass items matching a predicate</td>
</tr>
</tbody>
</table>
<h2 id="creating-nodes">Creating nodes<a class="headerlink" href="#creating-nodes" title="Permanent link">&para;</a></h2>
<p>All node types are created via factory functions that infer types from the callable:</p>
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-0-1"><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="c1">// Free function — simplest case</span>
</span><span id="__span-0-2"><a id="__codelineno-0-2" name="__codelineno-0-2" href="#__codelineno-0-2"></a><span class="k">auto</span><span class="w"> </span><span class="n">node</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">make_node</span><span class="o">&lt;</span><span class="n">my_func</span><span class="o">&gt;</span><span class="p">();</span>
</span><span id="__span-0-3"><a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></a>
</span><span id="__span-0-4"><a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></a><span class="c1">// Stateful functor (operator() is the function)</span>
</span><span id="__span-0-5"><a id="__codelineno-0-5" name="__codelineno-0-5" href="#__codelineno-0-5"></a><span class="n">MyProcessor</span><span class="w"> </span><span class="n">proc</span><span class="p">;</span>
</span><span id="__span-0-6"><a id="__codelineno-0-6" name="__codelineno-0-6" href="#__codelineno-0-6"></a><span class="k">auto</span><span class="w"> </span><span class="n">node</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">make_node</span><span class="p">(</span><span class="n">proc</span><span class="p">);</span>
</span><span id="__span-0-7"><a id="__codelineno-0-7" name="__codelineno-0-7" href="#__codelineno-0-7"></a>
</span><span id="__span-0-8"><a id="__codelineno-0-8" name="__codelineno-0-8" href="#__codelineno-0-8"></a><span class="c1">// Pool node — shares a ThreadPool with other nodes</span>
</span><span id="__span-0-9"><a id="__codelineno-0-9" name="__codelineno-0-9" href="#__codelineno-0-9"></a><span class="k">auto</span><span class="w"> </span><span class="n">pool</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">make_shared</span><span class="o">&lt;</span><span class="n">ThreadPool</span><span class="o">&gt;</span><span class="p">(</span><span class="mi">4</span><span class="p">);</span>
</span><span id="__span-0-10"><a id="__codelineno-0-10" name="__codelineno-0-10" href="#__codelineno-0-10"></a><span class="k">auto</span><span class="w"> </span><span class="n">node</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">make_pool_node</span><span class="o">&lt;</span><span class="n">my_func</span><span class="o">&gt;</span><span class="p">(</span><span class="n">pool</span><span class="p">);</span>
</span><span id="__span-0-11"><a id="__codelineno-0-11" name="__codelineno-0-11" href="#__codelineno-0-11"></a>
</span><span id="__span-0-12"><a id="__codelineno-0-12" name="__codelineno-0-12" href="#__codelineno-0-12"></a><span class="c1">// Interrupt node — triggered externally</span>
</span><span id="__span-0-13"><a id="__codelineno-0-13" name="__codelineno-0-13" href="#__codelineno-0-13"></a><span class="k">auto</span><span class="w"> </span><span class="n">sched</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">make_shared</span><span class="o">&lt;</span><span class="n">ThreadPool</span><span class="o">&gt;</span><span class="p">(</span><span class="mi">2</span><span class="p">);</span>
</span><span id="__span-0-14"><a id="__codelineno-0-14" name="__codelineno-0-14" href="#__codelineno-0-14"></a><span class="k">auto</span><span class="w"> </span><span class="n">node</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">make_interrupt_node</span><span class="o">&lt;</span><span class="n">produce_frame</span><span class="o">&gt;</span><span class="p">(</span><span class="n">sched</span><span class="p">,</span><span class="w"> </span><span class="n">out</span><span class="o">&lt;</span><span class="s">&quot;frame&quot;</span><span class="o">&gt;</span><span class="p">{});</span>
</span><span id="__span-0-15"><a id="__codelineno-0-15" name="__codelineno-0-15" href="#__codelineno-0-15"></a><span class="n">camera_sdk</span><span class="p">.</span><span class="n">on_frame_ready</span><span class="p">(</span><span class="n">node</span><span class="p">.</span><span class="n">get_trigger</span><span class="p">());</span>
</span></code></pre></div>
<h2 id="channel-capacity">Channel capacity<a class="headerlink" href="#channel-capacity" title="Permanent link">&para;</a></h2>
<p>Each node's input FIFO has a configurable capacity (default 5):</p>
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-1-1"><a id="__codelineno-1-1" name="__codelineno-1-1" href="#__codelineno-1-1"></a><span class="k">auto</span><span class="w"> </span><span class="n">node</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">make_node</span><span class="o">&lt;</span><span class="n">my_func</span><span class="o">&gt;</span><span class="p">(</span><span class="cm">/*capacity=*/</span><span class="mi">20</span><span class="p">);</span>
</span><span id="__span-1-2"><a id="__codelineno-1-2" name="__codelineno-1-2" href="#__codelineno-1-2"></a><span class="k">auto</span><span class="w"> </span><span class="n">node</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">make_pool_node</span><span class="o">&lt;</span><span class="n">my_func</span><span class="o">&gt;</span><span class="p">(</span><span class="n">pool</span><span class="p">,</span><span class="w"> </span><span class="cm">/*capacity=*/</span><span class="mi">20</span><span class="p">);</span>
</span></code></pre></div>
<p>When an upstream push would exceed capacity, <code>ChannelOverflowError</code> is thrown and the item is dropped. See <a href="../error-handling/">Error Handling &amp; Events</a> to observe and react to this.</p>
<h2 id="source-nodes">Source nodes<a class="headerlink" href="#source-nodes" title="Permanent link">&para;</a></h2>
<p>A node with no inputs is a source. It self-submits immediately on <code>start()</code> and re-submits after each execution:</p>
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-2-1"><a id="__codelineno-2-1" name="__codelineno-2-1" href="#__codelineno-2-1"></a><span class="k">static</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="nf">produce</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
</span><span id="__span-2-2"><a id="__codelineno-2-2" name="__codelineno-2-2" href="#__codelineno-2-2"></a><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">this_thread</span><span class="o">::</span><span class="n">sleep_for</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">chrono</span><span class="o">::</span><span class="n">milliseconds</span><span class="p">(</span><span class="mi">10</span><span class="p">));</span>
</span><span id="__span-2-3"><a id="__codelineno-2-3" name="__codelineno-2-3" href="#__codelineno-2-3"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="o">++</span><span class="n">counter</span><span class="p">;</span>
</span><span id="__span-2-4"><a id="__codelineno-2-4" name="__codelineno-2-4" href="#__codelineno-2-4"></a><span class="p">}</span>
</span><span id="__span-2-5"><a id="__codelineno-2-5" name="__codelineno-2-5" href="#__codelineno-2-5"></a><span class="k">auto</span><span class="w"> </span><span class="n">src</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">make_node</span><span class="o">&lt;</span><span class="n">produce</span><span class="o">&gt;</span><span class="p">();</span>
</span></code></pre></div>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>Source nodes must sleep or yield to avoid overflowing their output channel. The channel capacity provides the only bound.</p>
</div>
<h2 id="sink-nodes">Sink nodes<a class="headerlink" href="#sink-nodes" title="Permanent link">&para;</a></h2>
<p>A node with a <code>void</code> return is a sink — it consumes items without producing output:</p>
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-3-1"><a id="__codelineno-3-1" name="__codelineno-3-1" href="#__codelineno-3-1"></a><span class="k">static</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">print_it</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">x</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="sc">&#39;\n&#39;</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
</span><span id="__span-3-2"><a id="__codelineno-3-2" name="__codelineno-3-2" href="#__codelineno-3-2"></a><span class="k">auto</span><span class="w"> </span><span class="n">snk</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">make_node</span><span class="o">&lt;</span><span class="n">print_it</span><span class="o">&gt;</span><span class="p">();</span>
</span></code></pre></div>
<h2 id="error-handler">Error handler<a class="headerlink" href="#error-handler" title="Permanent link">&para;</a></h2>
<p>When a node's function throws an unhandled exception, the default behaviour is to stop the node (disabling its channels so the shutdown cascades downstream). Install a handler to override:</p>
<div class="language-cpp highlight"><pre><span></span><code><span id="__span-4-1"><a id="__codelineno-4-1" name="__codelineno-4-1" href="#__codelineno-4-1"></a><span class="w"> </span><span class="c1">// Return true → skip this invocation, keep the node running.</span>
</span><span id="__span-4-2"><a id="__codelineno-4-2" name="__codelineno-4-2" href="#__codelineno-4-2"></a><span class="w"> </span><span class="c1">// Return false → stop the node (downstream drains then also stops).</span>
</span><span id="__span-4-3"><a id="__codelineno-4-3" name="__codelineno-4-3" href="#__codelineno-4-3"></a><span class="w"> </span><span class="n">proc</span><span class="p">.</span><span class="n">set_error_handler</span><span class="p">([](</span><span class="n">std</span><span class="o">::</span><span class="n">string_view</span><span class="w"> </span><span class="n">name</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">exception_ptr</span><span class="w"> </span><span class="n">ep</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
</span><span id="__span-4-4"><a id="__codelineno-4-4" name="__codelineno-4-4" href="#__codelineno-4-4"></a><span class="w"> </span><span class="k">try</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">rethrow_exception</span><span class="p">(</span><span class="n">ep</span><span class="p">);</span><span class="w"> </span><span class="p">}</span>
</span><span id="__span-4-5"><a id="__codelineno-4-5" name="__codelineno-4-5" href="#__codelineno-4-5"></a><span class="w"> </span><span class="k">catch</span><span class="w"> </span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">exception</span><span class="o">&amp;</span><span class="w"> </span><span class="n">e</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
</span><span id="__span-4-6"><a id="__codelineno-4-6" name="__codelineno-4-6" href="#__codelineno-4-6"></a><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cerr</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;[&quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">name</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;] skipping item — &quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">e</span><span class="p">.</span><span class="n">what</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="sc">&#39;\n&#39;</span><span class="p">;</span>
</span><span id="__span-4-7"><a id="__codelineno-4-7" name="__codelineno-4-7" href="#__codelineno-4-7"></a><span class="w"> </span><span class="p">}</span>
</span><span id="__span-4-8"><a id="__codelineno-4-8" name="__codelineno-4-8" href="#__codelineno-4-8"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nb">true</span><span class="p">;</span>
</span><span id="__span-4-9"><a id="__codelineno-4-9" name="__codelineno-4-9" href="#__codelineno-4-9"></a><span class="w"> </span><span class="p">});</span>
</span></code></pre></div>
<p>See <a href="../error-handling/">Error Handling &amp; Events</a> for the full picture.</p>
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