4 title: Best Practices for writing CWL
7 * Build a reusable library of components. Share tool wrappers and subworkflows between projects. Make use of and contribute to community resources such as https://github.com/common-workflow-language/workflows and http://dockstore.org
9 * When combining a parameter value with a string, such as adding a filename extension, write @$(inputs.file.basename).ext@ instead of @$(inputs.file.basename + 'ext')@. The first form is evaluated as a simple text substitution, the second form (using the @+@ operator) is evaluated as an arbitrary Javascript expression and requires that you declare @InlineJavascriptRequirement@.
11 * Avoid including @InlineJavascriptRequirement@ or @ShellCommandRequirement@ unless you specifically need them. Don't include them "just in case" because they change the default behavior and may imply extra overhead.
13 * Don't write CWL scripts that access the Arvados SDK. This is non-portable; a script that access Arvados directly won't work with @cwltool@ or crunch v2.
15 * CommandLineTools wrapping custom scripts should represent the script as an input parameter with the script file as a default value. Use @secondaryFiles@ for scripts that consist of multiple files. For example:
19 class: CommandLineTool
24 inputBinding: {position: 1}
35 inputBinding: {position: 2}
43 * You can get the designated temporary directory using @$(runtime.tmpdir)@ in your CWL file, or from the @$TMPDIR@ environment variable in your script.
45 * Use @ExpressionTool@ to efficiently rearrange input files between steps of a Workflow. For example, the following expression accepts a directory containing files paired by @_R1_@ and @_R2_@ and produces an array of Directories containing each pair.
55 InlineJavascriptRequirement: {}
59 for (var i = 0; i < inputs.inputdir.listing.length; i++) {
60 var file = inputs.inputdir.listing[i];
61 var groups = file.basename.match(/^(.+)(_R[12]_)(.+)$/);
63 if (!samples[groups[1]]) {
64 samples[groups[1]] = [];
66 samples[groups[1]].push(file);
70 for (var key in samples) {
71 dirs.push({"class": "Directory",
73 "listing": [samples[key]]});
79 * Don't specifying resource requirements in CommandLineTool. Prefer to specify them in the workflow. You can provide a default resource requirement in the top level @hints@ section, and individual steps can override it with their own resource requirement.
107 * Instead of scattering separate steps, prefer to scatter over a subworkflow.
109 With the following pattern, @step1@ has to complete for all samples to complete before @step2@ can start on any samples. This means a single long-running sample can prevent the rest of the workflow from moving on:
134 Instead, scatter over a subworkflow. In this pattern, a sample can proceed from to @step2@ as soon as @step1@ is done, independently of any other samples.
135 Example: (note, the subworkflow can also be put in a separate file)
152 outputSource: step3/out