I want to share the results of my long day of bug hunting. It all comes down to this screenshot: on the left, the visualization of my graph that does not work (note there are two merge nodes). On the right, the one that is fixed. I was composing three different graphs, and using

get_terminal_tasks()

to determine how to connect these graphs. Something has changed on the way

switch/merge

is managed and this created several terminal_tasks on one of my flows. The solution for me was to set the reference task of each flow and use

get_reference_task

instead of

get_terminal_tasks

Anyways, needed to share the small win after a frustrating bug-hunting session (because the bug would present itself randomly due to

set

not having an order)

Congratulations on the successful bug hunt!

Nice work, David!

I wonder if this is due to https://github.com/PrefectHQ/prefect/pull/2310? Calls to

switch

or

ifelse

now implicitly create and return a merged task at the end, which makes them easier to compose.

Yes I saw that PR when investigating for this issue… thanks! I think my issue may be a bit more complicated. Before, I used switch / merge to have two flows composed where only one would be executed. In the new versions of prefect, the behavior seems to be consistent, but the graph is different… it’s more difficult to understand what’s really going on… see this screenshot for example, version 0.9.4 on the right, and the latest prefect version (of today, it seems)…

Yeah, I think there's a few things we should/could do here: • We could change `ifelse`/`switch` again in some way to better handle both use cases (getting the result from two options, or just branching in the graph) • We could change how we display

merge

tasks in the visualized graph • We could change the api for expressing branches to make it easier to express branches that should execute more than one task. •

Oh, great! I will follow this issue then

Thanks, fixed in https://github.com/PrefectHQ/prefect/pull/2376