graph traversal utility

[hjoliver]

See discussion at #6221 - we need a way for users to easily see what happens downstream of arbitrary points in the graph, particularly prior to staging a runtime intervention.

static

Plot all paths downstream of chosen task(s) in the graph, e.g.:

$ cylc graph [--from-task=4/model] [--max-edges=10] [--stop-cycle=5]

live (GUI)

Plot all paths downstream of chosen task(s) in the graph, as above, but get the result into the datastore somehow.

• could be done with simulation mode, with a special flow number to avoid impacting a subsequent real flow?

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notes

• to see if a cut branch is just a self-terminating side-branch: follow until it dead-ends (probably a side-branch) or hits the next cycle point (probably not a side branch)
• highlight any off-flow prerequisites

[oliver-sanders]

I think this is potentially a good idea, it might be a useful tool in workflow development and testing amongst other things.

I think some SoD interventions are going to push us towards developing some graph traversal functionality anyway. E.G. the proposed group trigger functionality will need to be able to identify start tasks within the group. So that might play into the development of a cylc graph command.

From the perspective of helping users to perform interventions, this would help, bit I think it is more important for us to develop simpler interventions that do not require graph inspection in the first place. That's not to say this isn't a potentially useful tool, but users shouldn't have to go charting a course through the graph to perform targetted interventions. The group-trigger proposal will hopefully avoid the need for users to identify the upstream tasks (to set/trigger) and the downstream tasks (to hold/remove) when re-running subgraphs of tasks (and may also avoid some bugbears around parentless tasks) so is a preferable solution IMO.