How to execute several processes sequentially, with channels in DSL2?

[satish15625]

process.txt

[mribeirodantas]

Adding maxForks 1 as a process directive will make your tasks run one by one, with no implicit parallelization. If you run the code below, you'll probably see 3 tasks concluding simultaneosly, then 6, 9 and 10. If you set maxForks 1 (now it's maxForks 3), you'll see 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 because it is run sequentially.

process FOO {
  maxForks 3
  input:
    val number
  script:
  """
  echo $number
  sleep 1
  """
}

workflow {
  Channel
    .of(1..10)
  | FOO
}

[mribeirodantas]

An update to this answer, as for the comment by Paolo below. You can make Nextflow request that tasks of a process are done one by one with maxForks 1, but there is no guarantee that they will be done one by one in the order that the channel dispatches them. The fair directive to be available soon will fix that 🥳

[pditommaso]

Sequential execution is not possible as of version 22.11.0-edge. However upcoming version of nextflow will introduce a new fair directive. When enabling it the process output is guaranteed to be produced in the same sequence as the inputs where received. For example

process sayHello {
  fair true
  input: val(x)
  output: stdout 
  """
  echo -n $x
  """
} 

workflow {
  def ch1 = channel.of('Hello','Ciao','Hola','Boujour')
  def ch2 = channel.of('world!','mondo!','mundo!','le monde!')

  ch1 | sayHello | merge(ch2) | view((x,y) -> "$x $y")
}

it will print

Hello world!
Ciao mondo!
Hola mundo!
Boujour le monde!