Simulation Modes

Cylc can run a workflow without running the real jobs, which may be long-running and resource-hungry.

Dummy mode replaces real jobs with background sleep jobs on the scheduler host. This avoids job runner directives that request compute resources for real workflow tasks, and it allows any workflow configuration to be run locally in dummy mode.

$ cylc play --mode=dummy <workflow-id>  # real dummy jobs


All configured task script items including init-script are ignored in dummy mode. If your init-script is required to run even local dummy jobs, the relevant environment setup should be done elsewhere.

Simulation mode does not run real jobs at all, and does not generate job log files.

$ cylc play --mode=simulation <workflow-id>  # no real jobs

Simulated Run Length

The default dummy or simulated job run length is 10 seconds. It can be changed with [runtime][<namespace>][simulation]default run length.

If [runtime][<namespace>]execution time limit and [runtime][<namespace>][simulation]speedup factor are both set, run length is computed by dividing the time limit by the speedup factor.


Dummy tasks run locally, so dummy mode does not test communication with remote job platforms. However, it is easy to write a live-mode test workflow with simple sleep 10 tasks that submit to a remote platform.

Alternate path branching is difficult to simulate effectively. You can configure certain tasks to fail via [runtime][<namespace>][simulation], but all branches based on mutually exclusive custom outputs will run because all custom outputs get artificially completed in dummy mode and in simulation mode.


Run mode is recorded in the workflow run database. Cylc will not let you restart a dummy mode workflow in live mode, or vice versa. Instead, install a new instance of the workflow and run it from scratch in the new mode.