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<div class="section" id="add-trigger">
<span id="index-0"></span><span id="id1"></span>
<div class="line-block">
<div class="line">In the previous exercise we saw that the two tasks were running simultaneously.</div>
<div class="line">We would like now to make sure that <strong>t2</strong> only runs once <strong>t1</strong> is complete.</div>
<div class="line">For this we have to define a <a class="reference internal" href="../../../glossary/#term-trigger"><em class="xref std std-term">trigger</em></a></div>
</div>
<div class="highlight-python"><pre># Definition of the suite test.
suite test
   edit ECF_INCLUDE "$HOME/course"
   edit ECF_HOME    "$HOME/course"
   family f1
     edit SLEEP 20
     task t1
     task t2
         trigger t1 eq complete
   endfamily
endsuite</pre>
</div>
<p>In python this would be:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c">##!/usr/bin/env python2.5</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">ecflow</span> 

<span class="k">def</span> <span class="nf">create_family_f1</span><span class="p">():</span>
    <span class="n">f1</span> <span class="o">=</span> <span class="n">ecflow</span><span class="o">.</span><span class="n">Family</span><span class="p">(</span><span class="s">&quot;f1&quot;</span><span class="p">)</span>
    <span class="n">f1</span><span class="o">.</span><span class="n">add_variable</span><span class="p">(</span><span class="s">&quot;SLEEP&quot;</span><span class="p">,</span><span class="mi">20</span><span class="p">)</span>
    <span class="n">f1</span><span class="o">.</span><span class="n">add_task</span><span class="p">(</span><span class="s">&quot;t1&quot;</span><span class="p">)</span>
    <span class="n">t2</span> <span class="o">=</span> <span class="n">f1</span><span class="o">.</span><span class="n">add_task</span><span class="p">(</span><span class="s">&quot;t2&quot;</span><span class="p">)</span>  
    <span class="n">t2</span><span class="o">.</span><span class="n">add_trigger</span><span class="p">(</span><span class="s">&quot;t1 eq complete&quot;</span><span class="p">)</span> 
    <span class="k">return</span> <span class="n">f1</span>
    
<span class="n">defs</span> <span class="o">=</span> <span class="n">ecflow</span><span class="o">.</span><span class="n">Defs</span><span class="p">(</span><span class="s">&quot;test.def&quot;</span><span class="p">)</span>
<span class="n">suite</span> <span class="o">=</span> <span class="n">defs</span><span class="o">.</span><span class="n">add_suite</span><span class="p">(</span><span class="s">&quot;test&quot;</span><span class="p">)</span>
<span class="n">suite</span><span class="o">.</span><span class="n">add_variable</span><span class="p">(</span><span class="s">&quot;ECF_INCLUDE&quot;</span><span class="p">,</span><span class="n">os</span><span class="o">.</span><span class="n">getenv</span><span class="p">(</span><span class="s">&quot;HOME&quot;</span><span class="p">)</span> <span class="o">+</span> <span class="s">&quot;/course&quot;</span><span class="p">)</span>
<span class="n">suite</span><span class="o">.</span><span class="n">add_variable</span><span class="p">(</span><span class="s">&quot;ECF_HOME   &quot;</span><span class="p">,</span><span class="n">os</span><span class="o">.</span><span class="n">getenv</span><span class="p">(</span><span class="s">&quot;HOME&quot;</span><span class="p">)</span> <span class="o">+</span> <span class="s">&quot;/course&quot;</span><span class="p">)</span>

<span class="n">suite</span><span class="o">.</span><span class="n">add_family</span><span class="p">(</span> <span class="n">create_family_f1</span><span class="p">()</span> <span class="p">)</span>
</pre></div>
</div>
<div class="line-block">
<div class="line">Triggers are used to declare <a class="reference internal" href="../../../glossary/#term-dependencies"><em class="xref std std-term">dependencies</em></a> between two tasks.</div>
<div class="line">For instance, the second task might need data created by the first task.</div>
</div>
<div class="line-block">
<div class="line">When ecFlow tries to start a task, it evaluates the <a class="reference internal" href="../../../glossary/#term-trigger"><em class="xref std std-term">trigger</em></a> expression.</div>
<div class="line">If the condition is correct, the task is started, otherwise the task</div>
<div class="line">stays <a class="reference internal" href="../../../glossary/#term-queued"><em class="xref std std-term">queued</em></a>.</div>
</div>
<div class="line-block">
<div class="line">Triggers can be between tasks, or between families, or a mixture.</div>
<div class="line">Remember the two rules:</div>
</div>
<ul class="simple">
<li>A family is <a class="reference internal" href="../../../glossary/#term-complete"><em class="xref std std-term">complete</em></a> when all its tasks are <a class="reference internal" href="../../../glossary/#term-complete"><em class="xref std std-term">complete</em></a>.</li>
<li>A task will be started if its triggers and the triggers of all is
parent families evaluate to true.</li>
</ul>
<div class="line-block">
<div class="line">A <a class="reference internal" href="../../../glossary/#term-node"><em class="xref std std-term">node</em></a> can only have one trigger expression, but very complex</div>
<div class="line">expressions can be built (and keep in mind that the triggers of the</div>
<div class="line">parent nodes are also implicit triggers).</div>
</div>
<div class="line-block">
<div class="line">Sometimes triggers are also used to prevent too many jobs from running</div>
<div class="line">at the same time. In this case the use of a <a class="reference internal" href="../../../glossary/#term-limit"><em class="xref std std-term">limit</em></a> may be a better</div>
<div class="line">solution (we will cover limits later on in the <a class="reference internal" href="/wiki/display/ECFLOW/Limits#limits"><em>Limits</em></a> section).</div>
</div>
<div class="line-block">
<div class="line">Triggers can be very complex, and ecFlow supports all kinds of conditions</div>
<div class="line">(not, and, or, ...)</div>
</div>
<p>What to do:</p>
<ol class="arabic simple">
<li>Edit the <a class="reference internal" href="../../../glossary/#term-suite-definition"><em class="xref std std-term">suite definition</em></a> file to add the <a class="reference internal" href="../../../glossary/#term-trigger"><em class="xref std std-term">trigger</em></a>.</li>
<li>Play the definition again.</li>
<li>Observe the tasks in <a class="reference internal" href="../../../glossary/#term-ecflowview"><em class="xref std std-term">ecFlowview</em></a>.</li>
<li>See the triggers by selecting <strong>t1</strong> or <strong>t2</strong> and clicking on the <img alt="triggers" src="/wiki/download/attachments/7373012/triggers.jpg" /> icon.</li>
<li>See the trigger relation by clicking on the arrow.</li>
<li>See the triggers in the tree, using the Show menu.</li>
<li>Search any reference to <strong>t1</strong> by using the <img alt="search" src="/wiki/download/attachments/7373012/search.jpg" /> icon</li>
</ol>
</div>