<div  id="add-trigger">
<span id="index-0"></span><span id="id1"></span>
<div >
<div >In the previous exercise we saw that the two tasks were running simultaneously.</div>
<div >We would like now to make sure that <strong>t2</strong> only runs once <strong>t1</strong> is complete.</div>
<div >For this we have to define a <a  href="/wiki/display/ECFLOW/Glossary#term-trigger"><em >trigger</em></a></div>
</div>
<div ><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 ><div ><pre><span >##!/usr/bin/env python2.5</span>
<span >import</span> <span >os</span>
<span >import</span> <span >ecflow</span> 

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

<span >suite</span><span >.</span><span >add_family</span><span >(</span> <span >create_family_f1</span><span >()</span> <span >)</span>
</pre></div>
</div>
<div >
<div >Triggers are used to declare <a  href="/wiki/display/ECFLOW/Glossary#term-dependencies"><em >dependencies</em></a> between two tasks.</div>
<div >For instance, the second task might need data created by the first task.</div>
</div>
<div >
<div >When ecFlow tries to start a task, it evaluates the <a  href="/wiki/display/ECFLOW/Glossary#term-trigger"><em >trigger</em></a> expression.</div>
<div >If the condition is correct, the task is started, otherwise the task</div>
<div >stays <a  href="/wiki/display/ECFLOW/Glossary#term-queued"><em >queued</em></a>.</div>
</div>
<div >
<div >Triggers can be between tasks, or between families, or a mixture.</div>
<div >Remember the two rules:</div>
</div>
<ul >
<li>A family is <a  href="/wiki/display/ECFLOW/Glossary#term-complete"><em >complete</em></a> when all its tasks are <a  href="/wiki/display/ECFLOW/Glossary#term-complete"><em >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 >
<div >A <a  href="/wiki/display/ECFLOW/Glossary#term-node"><em >node</em></a> can only have one trigger expression, but very complex</div>
<div >expressions can be built (and keep in mind that the triggers of the</div>
<div >parent nodes are also implicit triggers).</div>
</div>
<div >
<div >Sometimes triggers are also used to prevent too many jobs from running</div>
<div >at the same time. In this case the use of a <a  href="/wiki/display/ECFLOW/Glossary#term-limit"><em >limit</em></a> may be a better</div>
<div >solution (we will cover limits later on in the <a  href="/wiki/display/ECFLOW/Limits#limits"><em>Limits</em></a> section).</div>
</div>
<div >
<div >Triggers can be very complex, and ecFlow supports all kinds of conditions</div>
<div >(not, and, or, ...)</div>
</div>
<p>What to do:</p>
<ol >
<li>Edit the <a  href="/wiki/display/ECFLOW/Glossary#term-suite-definition"><em >suite definition</em></a> file to add the <a  href="/wiki/display/ECFLOW/Glossary#term-trigger"><em >trigger</em></a>.</li>
<li>Play the definition again.</li>
<li>Observe the tasks in <a  href="/wiki/display/ECFLOW/Glossary#term-ecflowview"><em >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>