Step 5 - Introduce an ID

If we can not guarantee the order of the different parallel branches, we should introduce a branch ID. This may be a label, a sample ID, a batch ID, etc. It’s the unit of parallelization.

// Step - 5
process addTuple {
  input:
    tuple val(id), val(input)
  output:
    tuple val("${id}"), val(output)
  exec:
    output = input + 1
}
workflow step5 {
  Channel.from( [ 1, 2, 3 ] ) \
    | map{ el -> [ el.toString(), el ]} \
    | addTuple \
    | view{ it }
}

We can run this code sample in the same way as the previous examples:

> nextflow -q run . -entry step5
WARN: DSL 2 IS AN EXPERIMENTAL FEATURE UNDER DEVELOPMENT -- SYNTAX MAY CHANGE IN FUTURE RELEASE
[3, 4]
[1, 2]
[2, 3]

Please note that the function to add 1 remains exactly the same, we only added the id as the first element of the tuple in both input and output. As such we keep a handle on which sample is which, by means of the key in the tuple.

Note: Later, we will extend this tuple and add configuration parameters to it… but this was supposed to go step by step.