Shared maxForks=1 for a group of processes

[kwonsoobin]

Is there anyway to define a shared maxForks for multiple processes?

I have a pipeline of input | A | B | C | D. I want exactly one process instance to run at a time for B and C. For example, if C is running, I do not want B to run even if the input is different. A and D can run whenever.

I tried assigning the same label to B and C and then setting maxForks to 1 for that label in my config file as shown below. This seems to set maxForks to 1 for each process and not for both processes combined.

process {
    withLabel: one_at_a_time {
        maxForks = 1
    }
}

Any suggestions? I don't think I can define maxForks or queueSize for a workflow. My last resort would be combine B and C into one process, which I want to avoid.

[jemunro]

One solution would be to add toList and flatMap between B and C.
E.g. input | A | B | toList | flatMap | C | D
That way C won't start until B is finished.