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In the previous exercise, we saw that the two tasks were running simultaneously.
We would like now to make sure that t2 only runs once t1 is complete.
For this, we have to define a trigger

Triggers are used to declare dependencies between two tasks.

For instance, the second task might need data created by the first task.

When ecFlow tries to start a task, it evaluates the trigger expression.

If the condition is correct, the task is started, otherwise, the task stays queued.

Triggers can be between tasks, or between families, or a mixture.

Remember the two rules:

A family is complete when all its tasks are complete
A task will be started if its triggers and the triggers of all its parent families evaluate to true

A node can only have one trigger expression, but very complex expressions can be built (and keep in mind that the triggers of the parent nodes are also implicit triggers).

Sometimes triggers are also used to prevent too many jobs from running at the same time. In this case, the use of a limit may be a better solution (we will cover limits later on, in the Limits section).

Nodes can be addressed in trigger expressions using full names: /test/f1/t1 refers to the task t1, and /test/f1 refers to the family f1.

In some contexts, ecFlow will accept relative names, such as ../t1.

trigger /test/f1/t1 == complete

Triggers can be very complex, and ecFlow supports all kinds of conditions
(not, and, or, ...), in addition, they can also reference Node attributes like
event, meter, variable, repeat, limits and generated variables.

Text

# Definition of the suite test.
suite test
   edit ECF_INCLUDE "$HOME/course"   # replace '$HOME' with the path to your home directory
   edit ECF_HOME    "$HOME/course"
   family f1
     edit SLEEP 20
     task t1
     task t2
         trigger t1 eq complete
   endfamily
endsuite

Python

The trigger expression can be checked, this is especially important when dealing with very large suites and complex triggers.

import os
from ecflow import Defs,Suite,Family,Task,Edit,Trigger

def create_family_f1():
    return Family("f1",
                Edit(SLEEP=20),
                Task("t1"),
                Task("t2",Trigger("t1 == complete")))
             
print("Creating suite definition")
home = os.path.join(os.getenv("HOME"), "course")
defs = Defs( 
        Suite("test",
            Edit(ECF_INCLUDE=home,ECF_HOME=home),
            create_family_f1()))
print(defs) 

print("check trigger expressions")
check = defs.check()
assert len(check) == 0, check 
 
print("Checking job creation: .ecf -> .job0")  
print(defs.check_job_creation())

print("Saving definition to file 'test.def'")
defs.save_as_defs("test.def")

What to do

Edit the suite definition file to add the trigger.
Replace the suite
python: python3 test.py ; python3 client.py
text: ecflow_client --suspend=/test ; ecflow_client --replace=/test test.def
Observe the tasks in ecflow_ui .
See the triggers by selecting t1 or t2.
See the trigger relation by clicking on the trigger tab.
Search any reference to t1 by using the search menu.
Introduce an error in the trigger expression and ensure that this error is trapped. i.e. change the trigger to.
Trigger("t == complete") # there is no node with name t, this should be reported as an error