Several planned future developments in Rose and Cylc may have an impact on workflow design.

List Item Override In Site Include-Files

A few Cylc settings hold lists of task (or family) names, e.g.:

    [[special tasks]]
        clock-trigger = get-data-a, get-data-b

Currently a repeated setting completely overrides the previous value (apart from graph strings which are always additive). This means a site include-file (for example) can’t add a new site-specific clock-triggered task without writing out the complete list of all clock-triggered tasks in the workflow, which breaks the otherwise clean separation into core and site files.


In the future we plan to support add, subtract, unset, and override semantics for all items.

UM STASH in Optional App Configs

A caveat to the advice on use of option app configs in Optional App Config Files: in general you might need the ability to turn off or modify some STASH requests in the main app, not just add additional site-specific STASH. But overriding STASH in optional configs is fragile because STASH namelists names are automatically generated from a hash of the precise content of the namelist. This makes it possible to uniquely identify the same STASH requests in different apps, but if any detail of a STASH request changes in a main app its namelist name will change and any optional configs that refer to it will become divorced from their intended target.

Until this problem is solved we recommend that:

  • All STASH in main UM apps should be grouped into sensible packages that can be turned on and off in optional configs without referencing the individual STASH request namelists.

  • Or all STASH should be held in optional site configs and none in the main app. Note however that STASH is difficult to configure outside of rose edit, and the editor does not yet allow you to edit optional configs.

Modular Workflow Design

The modular workflow design concept is that we should be able to import common workflow segments at install time rather than duplicating them in each workflow. The content of a workflow module will be encapsulated in a protected namespace to avoid clashing with the importing workflow, and selected inputs and outputs exposed via a proper interface.

This should aid portable workflow design too by enabling site-specific parts of a workflow (local product generation for example) to be stored and imported on-site rather than polluting the source and revision control record of the core workflow that everyone sees.

We note that this can already be done to a limited extent by using rose suite-run to install flow.cylc fragments from an external location. However, as a literal inlining mechanism with no encapsulation or interface, the internals of the “imported” fragments would have to be compatible with the workflow definition in every respect.

See also Monolithic Or Interdependent Workflows on modular systems of workflows connected by inter-workflow triggering.