12. External Triggers


This is a new capability and its suite configuration interface may change somewhat in future releases - see Current Limitations below in Current Limitations.

External triggers allow tasks to trigger directly off of external events, which is often preferable to implementing long-running polling tasks in the workflow. The triggering mechanism described in this section replaces an older and less powerful one documented in Old-Style External Triggers (Deprecated).

If you can write a Python function to check the status of an external condition or event, the suite server program can call it at configurable intervals until it reports success, at which point dependent tasks can trigger and data returned by the function will be passed to the job environments of those tasks. Functions can be written for triggering off of almost anything, such as delivery of a new dataset, creation of a new entry in a database table, or appearance of new data availability notifications in a message broker.

External triggers are visible in suite visualizations as bare graph nodes (just the trigger names). They are plotted against all dependent tasks, not in a cycle point specific way like tasks. This is because external triggers may or may not be cycle point (or even task name) specific - it depends on the arguments passed to the corresponding trigger functions. For example, if an external trigger does not depend on task name or cycle point it will only be called once - albeit repeatedly until satisfied - for the entire suite run, after which the function result will be remembered for all dependent tasks throughout the suite run.

Several built-in external trigger functions are located in <cylc-dir>/lib/cylc/xtriggers/:

Trigger functions are normal Python functions, with certain constraints as described below in:

12.1. Built-in Clock Triggers

These are more transparent (exposed in the graph) and efficient (shared among dependent tasks) than the older clock triggers described in Clock Triggers. (However we don’t recommend wholesale conversion to the new method yet, until its interface has stabilized - see Current Limitations.)

Clock triggers, unlike other trigger functions, are executed synchronously in the main process. The clock trigger function signature looks like this:


The offset argument is a date-time duration (PT1H is 1 hour) relative to the dependent task’s cycle point (automatically passed to the function via a second argument not shown above).

In the following suite, task foo has a daily cycle point sequence, and each task instance can trigger once the wall clock time has passed its cycle point value by one hour:

    initial cycle point = 2018-01-01
        clock_1 = wall_clock(offset=PT1H):PT10S
            graph = "@clock_1 => foo"
        script = run-foo.sh

Notice that the short label clock_1 is used to represent the trigger function in the graph. The function call interval, which determines how often the suite server program checks the clock, is optional. Here it is PT10S (i.e. 10 seconds, which is also the default value).

Argument keywords can be omitted if called in the right order, so the clock_1 trigger can also be declared like this:

    clock_1 = wall_clock(PT1H)

A zero-offset clock trigger does not need to be declared under the [xtriggers] section:

    initial cycle point = 2018-01-01
            # zero-offset clock trigger:
            graph = "@wall_clock => foo"
        script = run-foo.sh

However, when xtriggers are declared the name used must contain only the letters a to z in upper or lower case and underscores.

12.2. Built-in Suite State Triggers

These can be used instead of the older suite state polling tasks described in Triggering Off Of Tasks In Other Suites for inter-suite triggering - i.e. to trigger local tasks off of remote task statuses or messages in other suites. (However we don’t recommend wholesale conversion to the new method yet, until its interface has stabilized - see Current Limitations.)

The suite state trigger function signature looks like this:

suite_state(suite, task, point, offset=None, status='succeeded',
            message=None, cylc_run_dir=None, debug=False)

The first three arguments are compulsory; they single out the target suite name (suite) task name (task) and cycle point (point). The function arguments mirror the arguments and options of the cylc suite-state command - see cylc suite-state --help for documentation.

As a simple example, consider the suites in <cylc-dir>/etc/dev-suites/xtrigger/suite_state/. The “upstream” suite (which we want to trigger off of) looks like this:

    cycle point format = %Y
    initial cycle point = 2005
    final cycle point = 2015
          graph = "foo => bar"
      script = sleep 10
      script = sleep 5; cylc message "data ready"
          x = "data ready"

It must be registered and run under the name up, as referenced in the “downstream” suite that depends on it:

  cycle point format = %Y
    initial cycle point = 2010
         upstream = suite_state(suite=up, task=foo, point=%(point)s, \
            message='data ready'):PT10S
         clock_0 = wall_clock(offset=PT0H)
           graph = """
              @clock_0 & @upstream => FAM:succeed-all => blam
        script = sleep 5
    [[foo, blam]]
        inherit = FAM

Try starting the downstream suite first, then the upstream, and watch what happens. In each cycle point the @upstream trigger in the downstream suite waits on the task foo (with the same cycle point) in the upstream suite to emit the data ready message.

Some important points to note about this:

  • the function call interval, which determines how often the suite server program checks the clock, is optional. Here it is PT10S (i.e. 10 seconds, which is also the default value).
  • the suite_state trigger function, like the cylc suite-state command, must have read-access to the upstream suite’s public database.
  • the cycle point argument is supplied by a string template %(point)s. The string templates available to trigger function arguments are described in Custom Trigger Functions).

The return value of the suite_state trigger function looks like this:

results = {
    'suite': suite,
    'task': task,
    'point': point,
    'offset': offset,
    'status': status,
    'message': message,
    'cylc_run_dir': cylc_run_dir
return (satisfied, results)

The satisified variable is boolean (value True or False, depending on whether or not the trigger condition was found to be satisfied). The results dictionary contains the names and values of all of the target suite state parameters. Each item in it gets qualified with the unique trigger label (“upstream” here) and passed to the environment of dependent task jobs (the members of the FAM family in this case). To see this, take a look at the job script for one of the downstream tasks:

% cylc cat-log -f j dn f2.2011
cylc__job__inst__user_env() {
    export upstream_suite upstream_cylc_run_dir upstream_offset \
      upstream_message upstream_status upstream_point upstream_task
    upstream_message="data ready"


The task has to know the name (label) of the external trigger that it depends on - “upstream” in this case - in order to use this information. However the name could be given to the task environment in the suite configuration.

12.3. Custom Trigger Functions

Trigger functions are just normal Python functions, with a few special properties:

  • they must:
    • be defined in a module with the same name as the function;
    • be compatible with the same Python version that runs the Cylc workflow server program (see Installation for the latest version specification).
  • they can be located in:
    • <cylc-dir>/lib/cylc/xtriggers/;
    • <suite-dir>/lib/python/;
    • or anywhere in your Python library path.
  • they can take arbitrary positional and keyword arguments
  • suite and task identity, and cycle point, can be passed to trigger functions by using string templates in function arguments (see below)
  • integer, float, boolean, and string arguments will be recognized and passed to the function as such
  • if a trigger function depends on files or directories (for example) that might not exist when the function is first called, just return unsatisified until everything required does exist.


Trigger functions cannot store data Pythonically between invocations because each call is executed in an independent process in the process pool. If necessary the filesystem can be used for this purpose.

The following string templates are available for use, if the trigger function needs any of this information, in function arguments in the suite configuration:

  • %(name)s - name of the dependent task
  • %(id)s - identity of the dependent task (name.cycle-point)
  • %(point)s - cycle point of the dependent task
  • %(debug)s - suite debug mode

and less commonly needed:

  • %(user_name)s - suite owner’s user name
  • %(suite_name)s - registered suite name
  • %(suite_run_dir)s - suite run directory
  • %(suite_share_dir)s - suite share directory

If you need to pass a string template into an xtrigger function as a string literal - i.e. to be used as a template inside the function - escape it with % to avoid detection by the Cylc xtrigger parser: %%(cat)s.

Function return values should be as follows:

  • if the trigger condition is not satisfied:
    • return (False, {})
  • if the trigger condition is satisfied:
    • return (True, results)

where results is an arbitrary dictionary of information to be passed to dependent tasks, which in terms of format must:

  • be flat (non-nested);
  • contain only keys which are valid as environment variable names.

See Built-in Suite State Triggers for an example of one such results dictionary and how it gets processed by the suite.

The suite server program manages trigger functions as follows:

  • they are called asynchronously in the process pool - (except for clock triggers, which are called from the main process)
  • they are called repeatedly on a configurable interval, until satisified - the call interval defaults to PT10S (10 seconds) - repeat calls are not made until the previous call has returned
  • they are subject to the normal process pool command time out - if they take too long to return, the process will be killed
  • they are shared for efficiency: a single call will be made for all triggers that share the same function signature - i.e.the same function name and arguments
  • their return status and results are stored in the suite DB and persist across suite restarts
  • their stdout, if any, is redirected to stderr and will be visible in the suite log in debug mode (stdout is needed to communicate return values from the sub-process in which the function executes)

12.3.1. Toy Examples

A couple of toy examples in <cylc-dir>/lib/cylc/xtriggers/ may be a useful aid to understanding trigger functions and how they work. echo

The echo function is a trivial one that takes any number of positional and keyword arguments (from the suite configuration) and simply prints them to stdout, and then returns False (i.e. trigger condition not satisfied). Here it is in its entirety.

def echo(*args, **kwargs):
    print "echo: ARGS:", args
    print "echo: KWARGS:", kwargs
    return (False, {})

Here’s an example echo trigger suite:

    initial cycle point = now
        echo_1 = echo(hello, 99, qux=True, point=%(point)s, foo=10)
            graph = "@echo_1 => foo"
        script = exit 1

To see the result, run this suite in debug mode and take a look at the suite log (or run cylc run --debug --no-detach <suite> and watch your terminal). xrandom

The xrandom function sleeps for a configurable amount of time (useful for testing the effect of a long-running trigger function - which should be avoided) and has a configurable random chance of success. The function signature is:

xrandom(percent, secs=0, _=None, debug=False)

The percent argument sets the odds of success in any given call; secs is the number of seconds to sleep before returning; and the _ argument (underscore is a conventional name for a variable that is not used, in Python) is provided to allow specialization of the trigger to (for example) task name, task ID, or cycle point (just use the appropriate string templates in the suite configuration for this).

An example xrandom trigger suite is <cylc-dir>/etc/dev-suites/xtriggers/xrandom/.

12.4. Current Limitations

The following issues may be addressed in future Cylc releases:

  • trigger labels cannot currently be used in conditional (OR) expressions in the graph; attempts to do so will fail validation.
  • aside from the predefined zero-offset wall_clock trigger, all unique trigger function calls must be declared with all of their arguments under the [scheduling][xtriggers] section, and referred to by label alone in the graph. It would be convenient (and less verbose, although no more functional) if we could just declare a label against the common arguments, and give remaining arguments (such as different wall clock offsets in clock triggers) as needed in the graph.
  • we may move away from the string templating method for providing suite and task attributes to trigger function arguments.

12.5. Filesystem Events?

Cylc does not have built-in support for triggering off of filesystem events such as inotify on Linux. There is no cross-platform standard for this, and in any case filesystem events are not very useful in HPC cluster environments where events can only be detected at the specific node on which they were generated.

12.6. Continuous Event Watchers?

For some applications a persistent process that continually monitors the external world is better than discrete periodic checking. This would be more difficult to support as a plugin mechanism in Cylc, but we may decide to do it in the future. In the meantime, consider implementing a small daemon process as the watcher (e.g. to watch continuously for filesystem events) and have your Cylc trigger functions interact with it.

12.7. Old-Style External Triggers (Deprecated)


This mechanism is now technically deprecated by the newer external trigger functions (External Triggers). (However we don’t recommend wholesale conversion to the new method yet, until its interface has stabilized - see Current Limitations.)

These old-style external triggers are hidden task prerequisites that must be satisfied by using the cylc ext-trigger client command to send an associated pre-defined event message to the suite along with an ID string that distinguishes one instance of the event from another (the name of the target task and its current cycle point are not required). The event ID is just an arbitrary string to Cylc, but it can be used to identify something associated with the event to the suite - such as the filename of a new externally-generated dataset. When the suite server program receives the event notification it will trigger the next instance of any task waiting on that trigger (whatever its cycle point) and then broadcast (see Cylc Broadcast) the event ID to the cycle point of the triggered task as $CYLC_EXT_TRIGGER_ID. Downstream tasks with the same cycle point therefore know the new event ID too and can use it, if they need to, to identify the same new dataset. In this way a whole workflow can be associated with each new dataset, and multiple datasets can be processed in parallel if they happen to arrive in quick succession.

An externally-triggered task must register the event it waits on in the suite scheduling section:

# suite "sat-proc"
    cycling mode = integer
    initial cycle point = 1
    [[special tasks]]
        external-trigger = get-data("new sat X data avail")
            graph = get-data => conv-data => products

Then, each time a new dataset arrives the external detection system should notify the suite like this:

$ cylc ext-trigger sat-proc "new sat X data avail" passX12334a

where “sat-proc” is the suite name and “passX12334a” is the ID string for the new event. The suite passphrase must be installed on triggering account.


Only one task in a suite can trigger off a particular external message. Other tasks can trigger off the externally triggered task as required, of course.

<cylc-dir>/etc/examples/satellite/ext-triggers/suite.rc is a working example of a simulated satellite processing suite.

External triggers are not normally needed in date-time cycling suites driven by real time data that comes in at regular intervals. In these cases a data retrieval task can be clock-triggered (and have appropriate retry intervals) to submit at the expected data arrival time, so little time is wasted in polling. However, if the arrival time of the cycle-point-specific data is highly variable, external triggering may be used with the cycle point embedded in the message:

# suite "data-proc"
    initial cycle point = 20150125T00
    final cycle point   = 20150126T00
    [[special tasks]]
        external-trigger = get-data("data arrived for $CYLC_TASK_CYCLE_POINT")
            graph = init-process => get-data => post-process

Once the variable-length waiting is finished, an external detection system should notify the suite like this:

$ cylc ext-trigger data-proc "data arrived for 20150126T00" passX12334a

where “data-proc” is the suite name, the cycle point has replaced the variable in the trigger string, and “passX12334a” is the ID string for the new event. The suite passphrase must be installed on the triggering account. In this case, the event will trigger for the second cycle point but not the first because of the cycle-point matching.