Step 4 - How map is synchronous

An illustrative test is one where we do not use a process for the execution, but rather just map but such that one of the inputs takes longer to process, i.e.:

// Step - 4
def waitAndReturn(it) { sleep(2000); return it }
workflow step4 {
  Channel.from( [ 1, 2, 3 ] ) \
    | map{ (it == 2) ? waitAndReturn(it) : it } \
    | map{ it + 1 } \
    | view{ it }
}

Running it:

> nextflow -q run . -entry step4
WARN: DSL 2 IS AN EXPERIMENTAL FEATURE UNDER DEVELOPMENT -- SYNTAX MAY CHANGE IN FUTURE RELEASE
2
3
4

The result may be somewhat unexpected, the order is retained even though there’s a 2 second delay between the first entry and the rest. The sleep in other words blocks all the parallel execution branches.

This is a clear indication of why it’s better to use a process to execute computations. On the other hand, as long as we stay inside the map and don’t run a process, the order is retained. This opens up possibilities that we will exploit in what follows.