Advanced Features

WDL also provides features for implementing more complex workflows. For example, hello_task introduced in the previous example can be called in parallel across many different input files using the well-known scatter-gather pattern:

Example: hello_parallel.wdl

version 1.3

import "hello.wdl"

workflow hello_parallel {
  input {
    Array[File] files
    String pattern
  }

  scatter (path in files) {
    call hello.hello_task {
      infile = path,
      pattern = pattern
    }
  }

  output {
    # WDL implicitly implements the 'gather' step, so the output of
    # a scatter is always an array with the elements in the same
    # order as the input array. Since hello_task.matches is an array,
    # all the results will be gathered into an array-of-arrays.
    Array[Array[String]] all_matches = hello_task.matches
  }
}

{
  "hello_parallel.pattern": "^[a-z_]+$",
  "hello_parallel.files": ["data/greetings.txt", "data/hello.txt"]
}

{
  "hello_parallel.all_matches": [["hi_world"], ["hello"]]
}