Broadcast Tutorial

This tutorial walks you through using cylc broadcast which can be used to change task runtime configuration in a running workflow, on-the-fly.

Purpose

cylc broadcast can be used to change any [runtime] setting whilst the workflow is running.

The standard use of cylc broadcast is to update the workflow to an unexpected change in configuration, for example modifying the host a task runs on.

Standalone Example

Create a new workflow in the cylc-src directory called tutorial-broadcast:

mkdir ~/cylc-src/tutorial-broadcast
cd ~/cylc-src/tutorial-broadcast

Copy the following configuration into a flow.cylc file:

[scheduling]
    initial cycle point = 1012
    [[graph]]
        R1 = wipe_log => announce
        PT1H = announce[-PT1H] => announce

[runtime]
    [[wipe_log]]
        # Delete any files in the workflow's "share" directory.
        script = rm "${CYLC_WORKFLOW_SHARE_DIR}/knights" || true

    [[announce]]
        script = echo "${CYLC_TASK_CYCLE_POINT} - ${MESSAGE}" >> "${FILE}"
        [[[environment]]]
            WORD = ni
            MESSAGE = We are the knights who say \"${WORD}\"!
            FILE = "${CYLC_WORKFLOW_SHARE_DIR}/knights"

We now have a workflow with an announce task which runs every hour, writing a message to a log file (share/knights) when it does so. For the first cycle the log entry will look like this:

10120101T0000Z - We are the knights who say "ni"!

The cylc broadcast command enables us to change runtime configuration whilst the workflow is running. For instance we could change the value of the WORD environment variable using the command:

cylc broadcast tutorial-broadcast -n announce -s "[environment]WORD=it"

• tutorial-broadcast is the name of the workflow.

• -n announce tells Cylc we want to change the runtime configuration of the announce task.

• -s "[environment]WORD=it" changes the value of the WORD environment variable to it.

Run the workflow then try using the cylc broadcast command to change the message:

cylc validate .
cylc install
cylc play tutorial-broadcast
cylc broadcast tutorial-broadcast -n announce -s "[environment]WORD=it"

Inspect the share/knights file, you should see the message change at certain points.

Stop the workflow:

cylc stop tutorial-broadcast

In-Situ Example

We can call cylc broadcast from within a task’s script. This effectively provides the ability for tasks to communicate between themselves.

It is almost always better for tasks to communicate using files but there are some niche situations where communicating via cylc broadcast is justified. This tutorial walks you through using cylc broadcast to communicate between tasks.

Add the following recurrence to the graph section:

PT3H = announce[-PT1H] => change_word => announce

The change_word task runs the cylc broadcast command to randomly change the WORD environment variable used by the announce task.

Add the following runtime configuration to the runtime section:

[[change_word]]
    script = """
        # Select random word.
        IFS=',' read -r -a WORDS <<< $WORDS
        WORD=${WORDS[$(date +%s) % ${#WORDS[@]}]}

        # Broadcast random word to the announce task.
        cylc broadcast $CYLC_WORKFLOW_ID -n announce -s "[environment]WORD=${WORD}"
    """
    [[[environment]]]
        WORDS = ni, it, ekke ekke ptang zoo boing

Re-install and run the workflow, and inspect the log. You should see the message change randomly after every third entry (because the change_word task runs every 3 hours) e.g:

10120101T0000Z - We are the knights who say "ni"!
10120101T0100Z - We are the knights who say "ni"!
10120101T0200Z - We are the knights who say "ni"!
10120101T0300Z - We are the knights who say "ekke ekke ptang zoo boing!"

Stop the workflow:

cylc stop tutorial-broadcast