dummy mode: consider removing

[oliver-sanders]

Cylc 7 had two "dummy" modes:

dummy: Basically comments out all script* and environment so that you are running "blank" tasks, but still submitting them to the relevant platforms with the appropriate directives. This was apparently used by some as a validation test.

dummy-local: Also comments out the host and directives to make the "blank" tasks run locally.

Cylc 8 only has one mode called "dummy" which matches the behaviour of Cylc 7's "dummy-local".

Considering that "simulation" mode does everything that "dummy" mode does, without the unnecessary side effect of local submissions (which have the potential to flood the system!), simulation mode is a better tool for learning/testing. It also has options for simulating job failure and will play properly with the optional outputs system in due course.

So it's unclear what purpose the "dummy" mode (aka "dummy-local") has in Cylc 8. I raised this Discourse post to see if the community had any use cases and separately polled MO users. No responses so far.

Given changes currently underway to implement "skip" mode, maintaining the "dummy" mode code is about to become a bit burdensome, so we should consider removing it if unused.

[hjoliver]

I have no problem with ditching dummy mode in favour of simulation mode, so long as we can make simulation work fully correctly.

For historical context, dummy mode predates simulation mode because it was easy to implement without breaking anything - all you have to do is ignore the script and environment content in tasks and everything else works exactly the same.

But simulation mode is fundamentally quite different, and more difficult (at least a bit more difficult) to get right.

Anyhow, that said, let's ditch dummy mode.

[hjoliver]

Also see #5975 - that seems a more natural fit for dummy mode than sim mode. In principle we could run some real jobs in sim mode, but I'm not sure how easy that would be to implement.

[hjoliver]

Tentatively assigning @wxtim as it seems like we will agree to do this, and Tim's "cup runneth over" with run-mode knowledge ATM.

[wxtim]

Deleting dummy mode is easy - it's just deleting stuff. It removes quite a lot of logic.

I have a nebulous idea that we might find a need to reinstate the "modify the scripts and return to live mode" code pathway later, though I'm not quite sure what for (disable err-script? provide a standardized err-script?). Not a good enough idea to keep code which could be easily re-instated.

Also see #5975 - that seems a more natural fit for dummy mode than sim mode. In principle we could run some real jobs in sim mode, but I'm not sure how easy that would be to implement.

At one point the skip mode PR offered this, but we decided to get it in rather than messing about. It's probably not super tricky though. Will look into it whilst it's all fresh in my mind.

[hjoliver]

At one point the skip mode PR offered this ...

I guess that explains why that Issue was closed by merging the skip mode PR. I don't think it should have been - if you agree, can you reopen it? (we may close it for other reasons anyway, but I don't think it's quite resolved yet).