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<section id="task-based-algorithms">
<span id="taskbasedalgorithms"></span><h1>Task Based Algorithms<a class="headerlink" href="#task-based-algorithms" title="Link to this heading">#</a></h1>
<section id="overview">
<h2>Overview<a class="headerlink" href="#overview" title="Link to this heading">#</a></h2>
<p>A <code class="docutils literal notranslate"><span class="pre">TaskBasedAlgorithm</span></code> is a C++ helper type for writing workflow-style algorithms as a sequence of
small, named tasks.</p>
<p>This is useful when the caller needs control over execution flow while reusing the same implementation.</p>
<p>The algorithm exposes a <code class="docutils literal notranslate"><span class="pre">TaskExecutionOrder</span></code> property so users can choose the task order at runtime.
The <code class="docutils literal notranslate"><span class="pre">TaskBasedAlgorithm</span></code> then stages those tasks and executes them in that order.</p>
</section>
<section id="concepts">
<h2>Concepts<a class="headerlink" href="#concepts" title="Link to this heading">#</a></h2>
<section id="taskbasedalgorithm-t">
<h3><code class="docutils literal notranslate"><span class="pre">TaskBasedAlgorithm<T></span></code><a class="headerlink" href="#taskbasedalgorithm-t" title="Link to this heading">#</a></h3>
<p>Derive your algorithm from <code class="docutils literal notranslate"><span class="pre">TaskBasedAlgorithm<YourAlgorithm></span></code>.
The base type provides task staging, dependency evaluation, output tracking and final
<code class="docutils literal notranslate"><span class="pre">OutputWorkspace</span></code> selection.</p>
</section>
<section id="algorithmtask">
<h3><code class="docutils literal notranslate"><span class="pre">AlgorithmTask</span></code><a class="headerlink" href="#algorithmtask" title="Link to this heading">#</a></h3>
<p>A task is a nested class deriving from <code class="docutils literal notranslate"><span class="pre">AlgorithmTask</span></code>.
Each task:</p>
<ul class="simple">
<li><p>has a unique task name,</p></li>
<li><p>declares expected outputs,</p></li>
<li><p>declares one or more dependency sets,</p></li>
<li><p>implements <code class="docutils literal notranslate"><span class="pre">executeImpl()</span></code>.</p></li>
</ul>
</section>
<section id="dependency-sets">
<h3>Dependency sets<a class="headerlink" href="#dependency-sets" title="Link to this heading">#</a></h3>
<p>A dependency set is a valid group of upstream task outputs.
If one set can be fulfilled, the task may run.
This allows tasks to support alternative upstream paths.</p>
<p>For example, a task can depend on:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">TaskC</span></code> output, <strong>or</strong></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">TaskB</span></code> output.</p></li>
</ul>
<p>If both are available, the active set is chosen based on proximity in <code class="docutils literal notranslate"><span class="pre">TaskExecutionOrder</span></code>.</p>
</section>
<section id="selected-output">
<h3>Selected output<a class="headerlink" href="#selected-output" title="Link to this heading">#</a></h3>
<p>A task can produce more than one output workspace.
The task’s <em>selected output</em> is the one propagated to the algorithm-level <code class="docutils literal notranslate"><span class="pre">OutputWorkspace</span></code>
when that task is last in the staged order.</p>
</section>
</section>
<section id="how-task-execution-works">
<h2>How task execution works<a class="headerlink" href="#how-task-execution-works" title="Link to this heading">#</a></h2>
<p>At a high level:</p>
<ol class="arabic simple">
<li><p><code class="docutils literal notranslate"><span class="pre">initTaskBasedAlgorithm</span></code> registers task types and declares <code class="docutils literal notranslate"><span class="pre">TaskExecutionOrder</span></code>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">configureAlgorithmTasks</span></code> validates and stages tasks in user-specified order.</p></li>
<li><p>The first staged task is seeded with <code class="docutils literal notranslate"><span class="pre">InputWorkspace</span></code>.</p></li>
<li><p>Each task evaluates whether at least one dependency set is fulfillable.</p></li>
<li><p>The task runs <code class="docutils literal notranslate"><span class="pre">executeImpl()</span></code> and publishes outputs.</p></li>
<li><p>After the final task, the selected output is written to <code class="docutils literal notranslate"><span class="pre">OutputWorkspace</span></code>.</p></li>
</ol>
</section>
<section id="writing-a-task-based-algorithm">
<h2>Writing a task based algorithm<a class="headerlink" href="#writing-a-task-based-algorithm" title="Link to this heading">#</a></h2>
<section id="derive-from-taskbasedalgorithm">
<h3>1. Derive from <code class="docutils literal notranslate"><span class="pre">TaskBasedAlgorithm</span></code><a class="headerlink" href="#derive-from-taskbasedalgorithm" title="Link to this heading">#</a></h3>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">MyWorkflow</span><span class="w"> </span><span class="k">final</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">Mantid</span><span class="o">::</span><span class="n">API</span><span class="o">::</span><span class="n">TaskBasedAlgorithm</span><span class="o"><</span><span class="n">MyWorkflow</span><span class="o">></span><span class="w"> </span><span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="w"> </span><span class="n">name</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="s">"MyWorkflow"</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="w"> </span><span class="n">summary</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="s">"Example task-based workflow"</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">version</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">init</span><span class="p">()</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">declareProperty</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">make_unique</span><span class="o"><</span><span class="n">Mantid</span><span class="o">::</span><span class="n">API</span><span class="o">::</span><span class="n">WorkspaceProperty</span><span class="o"><</span><span class="n">Mantid</span><span class="o">::</span><span class="n">API</span><span class="o">::</span><span class="n">MatrixWorkspace</span><span class="o">>></span><span class="p">(</span>
<span class="w"> </span><span class="s">"InputWorkspace"</span><span class="p">,</span><span class="w"> </span><span class="s">""</span><span class="p">,</span><span class="w"> </span><span class="n">Mantid</span><span class="o">::</span><span class="n">Kernel</span><span class="o">::</span><span class="n">Direction</span><span class="o">::</span><span class="n">Input</span><span class="p">),</span>
<span class="w"> </span><span class="s">"Input workspace"</span><span class="p">);</span>
<span class="w"> </span><span class="n">declareProperty</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">make_unique</span><span class="o"><</span><span class="n">Mantid</span><span class="o">::</span><span class="n">API</span><span class="o">::</span><span class="n">WorkspaceProperty</span><span class="o"><</span><span class="n">Mantid</span><span class="o">::</span><span class="n">API</span><span class="o">::</span><span class="n">MatrixWorkspace</span><span class="o">>></span><span class="p">(</span>
<span class="w"> </span><span class="s">"OutputWorkspace"</span><span class="p">,</span><span class="w"> </span><span class="s">""</span><span class="p">,</span><span class="w"> </span><span class="n">Mantid</span><span class="o">::</span><span class="n">Kernel</span><span class="o">::</span><span class="n">Direction</span><span class="o">::</span><span class="n">Output</span><span class="p">),</span>
<span class="w"> </span><span class="s">"Final output workspace"</span><span class="p">);</span>
<span class="w"> </span><span class="n">initTaskBasedAlgorithm</span><span class="o"><</span><span class="n">TaskA</span><span class="p">,</span><span class="w"> </span><span class="n">TaskB</span><span class="p">,</span><span class="w"> </span><span class="n">TaskC</span><span class="p">,</span><span class="w"> </span><span class="n">TaskD</span><span class="o">></span><span class="p">(</span>
<span class="w"> </span><span class="p">{</span><span class="s">"TaskA"</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskB"</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskC"</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskD"</span><span class="p">});</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">exec</span><span class="p">()</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">execTasks</span><span class="p">();</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">TaskA</span><span class="p">;</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">TaskB</span><span class="p">;</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">TaskC</span><span class="p">;</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">TaskD</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
</section>
<section id="define-tasks-and-dependencies">
<h3>2. Define tasks and dependencies<a class="headerlink" href="#define-tasks-and-dependencies" title="Link to this heading">#</a></h3>
<p>The examples below are based on <code class="docutils literal notranslate"><span class="pre">TaskBasedAlgorithmTest.h</span></code>.</p>
<section id="how-to-set-task-dependencies">
<h4>How to set task dependencies<a class="headerlink" href="#how-to-set-task-dependencies" title="Link to this heading">#</a></h4>
<p>Use <code class="docutils literal notranslate"><span class="pre">setDependantTask</span></code> to require an upstream output.
Use <code class="docutils literal notranslate"><span class="pre">addDependantTaskSet</span></code> to support alternative dependency sets.</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">TaskB</span><span class="w"> </span><span class="k">final</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">AlgorithmTask</span><span class="w"> </span><span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="k">explicit</span><span class="w"> </span><span class="n">TaskB</span><span class="p">(</span><span class="n">MyWorkflow</span><span class="w"> </span><span class="o">*</span><span class="n">parent</span><span class="p">)</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">AlgorithmTask</span><span class="p">(</span><span class="n">parent</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskB"</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">setExpectedOutputs</span><span class="p">({</span><span class="s">"TaskBOutput1"</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskBOutput2"</span><span class="p">});</span>
<span class="w"> </span><span class="c1">// Set 0: depend on TaskA output</span>
<span class="w"> </span><span class="n">setDependantTask</span><span class="p">(</span><span class="s">"TaskA"</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskAOutput"</span><span class="p">,</span><span class="w"> </span><span class="s">"InputWorkspace"</span><span class="p">);</span>
<span class="w"> </span><span class="c1">// Set 1: alternative dependency on TaskC</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">altSet</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">addDependantTaskSet</span><span class="p">();</span>
<span class="w"> </span><span class="n">setDependantTask</span><span class="p">(</span><span class="s">"TaskC"</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskCOutput"</span><span class="p">,</span><span class="w"> </span><span class="s">"InputWorkspace"</span><span class="p">,</span><span class="w"> </span><span class="n">altSet</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">executeImpl</span><span class="p">()</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">inputWS</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">getDependantWorkspace</span><span class="p">(</span><span class="s">"InputWorkspace"</span><span class="p">);</span>
<span class="w"> </span><span class="c1">// ... do work</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
</pre></div>
</div>
<p>If no output name is supplied (<code class="docutils literal notranslate"><span class="pre">setDependantTask("TaskD")</span></code>), all expected outputs from
that upstream task are made available using their output names as aliases.</p>
</section>
<section id="how-to-set-task-outputs">
<h4>How to set task outputs<a class="headerlink" href="#how-to-set-task-outputs" title="Link to this heading">#</a></h4>
<p>Declare expected outputs with <code class="docutils literal notranslate"><span class="pre">setExpectedOutputs</span></code> and publish each output via <code class="docutils literal notranslate"><span class="pre">outputWorkspace</span></code>.</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">TaskD</span><span class="w"> </span><span class="k">final</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">AlgorithmTask</span><span class="w"> </span><span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="k">explicit</span><span class="w"> </span><span class="n">TaskD</span><span class="p">(</span><span class="n">MyWorkflow</span><span class="w"> </span><span class="o">*</span><span class="n">parent</span><span class="p">)</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">AlgorithmTask</span><span class="p">(</span><span class="n">parent</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskD"</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">setExpectedOutputs</span><span class="p">({</span><span class="s">"TaskDOutput1"</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskDOutput2"</span><span class="p">});</span>
<span class="w"> </span><span class="n">setSelectedOutput</span><span class="p">(</span><span class="s">"TaskDOutput2"</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">executeImpl</span><span class="p">()</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">ws1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="cm">/* create output */</span><span class="p">;</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">ws2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="cm">/* create output */</span><span class="p">;</span>
<span class="w"> </span><span class="n">outputWorkspace</span><span class="p">(</span><span class="n">ws1</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskDOutput1"</span><span class="p">);</span>
<span class="w"> </span><span class="n">outputWorkspace</span><span class="p">(</span><span class="n">ws2</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskDOutput2"</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
</pre></div>
</div>
</section>
<section id="how-to-specify-a-default-taskexecutionorder">
<h4>How to specify a default <code class="docutils literal notranslate"><span class="pre">TaskExecutionOrder</span></code><a class="headerlink" href="#how-to-specify-a-default-taskexecutionorder" title="Link to this heading">#</a></h4>
<p>Pass a default vector to <code class="docutils literal notranslate"><span class="pre">initTaskBasedAlgorithm</span></code> in <code class="docutils literal notranslate"><span class="pre">init()</span></code>.
Users can still override this property.</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">initTaskBasedAlgorithm</span><span class="o"><</span><span class="n">TaskA</span><span class="p">,</span><span class="w"> </span><span class="n">TaskB</span><span class="p">,</span><span class="w"> </span><span class="n">TaskC</span><span class="p">,</span><span class="w"> </span><span class="n">TaskD</span><span class="o">></span><span class="p">(</span>
<span class="w"> </span><span class="p">{</span><span class="s">"TaskA"</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskB"</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskC"</span><span class="p">,</span><span class="w"> </span><span class="s">"TaskD"</span><span class="p">});</span>
</pre></div>
</div>
<p>Optionally override <code class="docutils literal notranslate"><span class="pre">constructTaskExecutionOrder()</span></code> if the default order must be assembled dynamically.</p>
</section>
</section>
</section>
<section id="validation-and-behaviour-notes">
<h2>Validation and behaviour notes<a class="headerlink" href="#validation-and-behaviour-notes" title="Link to this heading">#</a></h2>
<ul class="simple">
<li><p>Invalid task names in <code class="docutils literal notranslate"><span class="pre">TaskExecutionOrder</span></code> are rejected.</p></li>
<li><p>Duplicate task names in <code class="docutils literal notranslate"><span class="pre">TaskExecutionOrder</span></code> are rejected.</p></li>
<li><p>By default, the input workspace is cloned before task execution.
Use <code class="docutils literal notranslate"><span class="pre">setMutableInput(true)</span></code> if in-place mutation is intended.</p></li>
</ul>
</section>
<section id="future-work">
<h2>Future work<a class="headerlink" href="#future-work" title="Link to this heading">#</a></h2>
<p>The following improvements are planned/possible extensions:</p>
<ul class="simple">
<li><p>Allow repeated task names in <code class="docutils literal notranslate"><span class="pre">TaskExecutionOrder</span></code>, if certain tasks can be executed more than once.</p></li>
<li><p>Conduct more validation of the dependency structure at initialization as opposed to run-time.</p></li>
<li><p>Consider formally building in conditional skipping of specified tasks.</p></li>
<li><p>Explore parallel execution of independent task branches.</p></li>
<li><p>Ultimately, it would be great to have a flowchart-like GUI allowing users to specify the flow of tasks.</p></li>
</ul>
</section>
<section id="see-also">
<h2>See also<a class="headerlink" href="#see-also" title="Link to this heading">#</a></h2>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">Framework/API/inc/MantidAPI/TaskBasedAlgorithm.h</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">Framework/API/test/TaskBasedAlgorithmTest.h</span></code></p></li>
</ul>
</section>
</section>
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<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#overview">Overview</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#concepts">Concepts</a><ul class="nav section-nav flex-column">
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#taskbasedalgorithm-t"><code class="docutils literal notranslate"><span class="pre">TaskBasedAlgorithm<T></span></code></a></li>
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<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#derive-from-taskbasedalgorithm">1. Derive from <code class="docutils literal notranslate"><span class="pre">TaskBasedAlgorithm</span></code></a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#define-tasks-and-dependencies">2. Define tasks and dependencies</a><ul class="nav section-nav flex-column">
<li class="toc-h4 nav-item toc-entry"><a class="reference internal nav-link" href="#how-to-set-task-dependencies">How to set task dependencies</a></li>
<li class="toc-h4 nav-item toc-entry"><a class="reference internal nav-link" href="#how-to-set-task-outputs">How to set task outputs</a></li>
<li class="toc-h4 nav-item toc-entry"><a class="reference internal nav-link" href="#how-to-specify-a-default-taskexecutionorder">How to specify a default <code class="docutils literal notranslate"><span class="pre">TaskExecutionOrder</span></code></a></li>
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