DSL 2

Nextflow provides a syntax extension that allows the definition of module libraries and simplifies the writing of complex data analysis pipelines.

To enable this feature you need to define the following directive at the beginning of your workflow script:

nextflow.enable.dsl=2

Tip

As of version 22.03.0-edge Nextflow defaults to DSL 2 if no version version is specified explicitly. You can restore the previous behavior setting in into your environment the following variable:

export NXF_DEFAULT_DSL=1

Note

As of version 22.03.0-edge the DSL version specification (either 1 or 2) can also be specified in the Nextflow configuration file using the same notation shown above.

Function

Nextflow allows the definition of custom functions in the workflow script using the following syntax:

def <function name> ( arg1, arg, .. ) {
    <function body>
}

For example:

def foo() {
    'Hello world'
}

def bar(alpha, omega) {
    alpha + omega
}

The above snippet defines two simple functions, that can be invoked in the workflow script as foo() which returns the Hello world string and bar(10,20) which returns the sum of two parameters (30 in this case).

Note

Functions implicitly return the result of the last evaluated statement.

The keyword return can be used to explicitly exit from a function and return the specified value. For example:

def fib( x ) {
    if( x <= 1 )
        return x
    else
        fib(x-1) + fib(x-2)
}

Process

Process definition

The new DSL separates the definition of a process from its invocation. The process definition follows the usual syntax as described in the process documentation. The only difference is that the from and into channel declarations have to be omitted.

Then a process can be invoked as a function in the workflow scope, passing the expected input channels as parameters as if it were a custom function. For example:

nextflow.enable.dsl=2

process foo {
    output:
      path 'foo.txt'

    script:
      """
      your_command > foo.txt
      """
}

process bar {
    input:
      path x

    output:
      path 'bar.txt'

    script:
      """
      another_command $x > bar.txt
      """
}

workflow {
    data = channel.fromPath('/some/path/*.txt')
    foo()
    bar(data)
}

Warning

A process component can be invoked only once in the same workflow context.

Process composition

Processes having matching input-output declaration can be composed so that the output of the first process is passed as input to the next process. Taking in consideration the previous example, it’s possible to write the following:

workflow {
    bar(foo())
}

Process output

A process output can also be accessed using the out attribute on the corresponding process object. For example:

workflow {
    foo()
    bar(foo.out)
    bar.out.view()
}

When a process defines two or more output channels, each of them can be accessed using the array element operator e.g. out[0], out[1], etc. or using named outputs (see below).

Process named output

The emit option can be added to the process output definition to assign a name identifier. This name can be used to reference the channel within the caller scope. For example:

process foo {
  output:
    path '*.bam', emit: samples_bam

  '''
  your_command --here
  '''
}

workflow {
    foo()
    foo.out.samples_bam.view()
}

Process named stdout

The emit option can be used also to name the stdout:

process sayHello {
    input:
        val cheers

    output:
        stdout emit: verbiage

    script:
    """
    echo -n $cheers
    """
}

workflow {
    things = channel.of('Hello world!', 'Yo, dude!', 'Duck!')
    sayHello(things)
    sayHello.out.verbiage.view()
}

Workflow

Workflow definition

The workflow keyword allows the definition of sub-workflow components that enclose the invocation of one or more processes and operators:

workflow my_pipeline {
    foo()
    bar( foo.out.collect() )
}

For example, the above snippet defines a workflow component, named my_pipeline, that can be invoked from another workflow component definition as any other function or process with my_pipeline().

Workflow parameters

A workflow component can access any variable and parameter defined in the outer scope:

params.data = '/some/data/file'

workflow my_pipeline {
    if( params.data )
        bar(params.data)
    else
        bar(foo())
}

Workflow input

A workflow component can declare one or more input channels using the take keyword. For example:

workflow my_pipeline {
    take: data
    main:
        foo(data)
        bar(foo.out)
}

Warning

When the take keyword is used, the beginning of the workflow body must be identified with the main keyword.

Then, the input can be specified as an argument in the workflow invocation statement:

workflow {
    my_pipeline( channel.from('/some/data') )
}

Note

Workflow inputs are always channels by definition. If a basic data type is provided instead, such as a number, string, list, etc, it is implicitly converted to a value channel.

Workflow output

A workflow component can declare one or more output channels using the emit keyword. For example:

workflow my_pipeline {
    main:
      foo(data)
      bar(foo.out)
    emit:
      bar.out
}

Then, the result of the my_pipeline execution can be accessed using the out property, i.e. my_pipeline.out. When multiple output channels are declared, use the array bracket notation to access each output channel as described for the Process output definition.

Workflow named output

If the output channel is assigned to an identifier in the emit declaration, such identifier can be used to reference the channel within the caller scope. For example:

workflow my_pipeline {
   main:
     foo(data)
     bar(foo.out)
   emit:
     my_data = bar.out
}

Then, the result of the above snippet can accessed using my_pipeline.out.my_data.

Workflow entrypoint

A workflow definition which does not declare any name (also known as implicit workflow) is the entry point of execution for the workflow application.

Note

Implicit workflow definition is ignored when a script is included as a module. This allows the writing of a workflow script that can be used either as a library module or as an application script.

Tip

A different workflow entrypoint can be specified using the -entry command line option.

Workflow composition

Workflows defined in your script or imported with Module inclusion can be invoked and composed as any other process in your application.

workflow flow1 {
    take: data
    main:
        foo(data)
        bar(foo.out)
    emit:
        bar.out
}

workflow flow2 {
    take: data
    main:
        foo(data)
        baz(foo.out)
    emit:
        baz.out
}

workflow {
    take: data
    main:
        flow1(data)
        flow2(flow1.out)
}

Note

Nested workflow execution determines an implicit scope. Therefore the same process can be invoked in two different workflow scopes, like for example foo in the above snippet that is used both in flow1 and flow2. The workflow execution path, along with the process names, determines the fully qualified process name that is used to distinguish the two different process invocations, i.e. flow1:foo and flow2:foo in the above example.

Tip

The fully qualified process name can be used as a valid process selector in the nextflow.config file and it has priority over the simple process name.

Modules

The new DSL allows the definition of module scripts that can be included and shared across workflow applications.

A module script (or simply, module) can contain the definition of functions, processes and workflows as described in the previous sections.

Note

Functions, processes and workflows are globally referred to as components.

Module inclusion

A component defined in a module script can be imported into another Nextflow script using the include keyword.

For example:

include { foo } from './some/module'

workflow {
    data = channel.fromPath('/some/data/*.txt')
    foo(data)
}

The above snippet includes a process with name foo defined in the module script in the main execution context. This way, foo` can be invoked in the workflow scope.

Nextflow implicitly looks for the script file ./some/module.nf resolving the path against the including script location.

Note

Relative paths must begin with the ./ prefix. Also, the include statement must be defined outside of the workflow definition.

Multiple inclusions

A Nextflow script allows the inclusion of an arbitrary number of modules and components. When multiple components need to be included from the same module script, the component names can be specified in the same inclusion using the curly brackets notation as shown below:

include { foo; bar } from './some/module'

workflow {
    data = channel.fromPath('/some/data/*.txt')
    foo(data)
    bar(data)
}

Module aliases

When including a module component, it’s possible to specify an alias with the as keyword. This allows the inclusion and the invocation of components with the same name in your script using different names. For example:

include { foo } from './some/module'
include { foo as bar } from './other/module'

workflow {
    foo(some_data)
    bar(other_data)
}

The same is possible when including the same component multiple times from the same module script as shown below:

include { foo; foo as bar } from './some/module'

workflow {
    foo(some_data)
    bar(other_data)
}

Module parameters

A module script can define one or more parameters using the same syntax of a Nextflow workflow script:

params.foo = 'Hello'
params.bar = 'world!'

def sayHello() {
    println "$params.foo $params.bar"
}

Then, parameters are inherited from the including context. For example:

params.foo = 'Hola'
params.bar = 'Mundo'

include {sayHello} from './some/module'

workflow {
    sayHello()
}

The above snippet prints:

Hola Mundo

Note

The module inherits the parameters defined before the include statement, therefore any further parameter set later is ignored.

Tip

Define all pipeline parameters at the beginning of the script before any include declaration.

The option addParams can be used to extend the module parameters without affecting the external scope. For example:

include {sayHello} from './some/module' addParams(foo: 'Ciao')

workflow {
    sayHello()
}

The above snippet prints:

Ciao world!

Finally, the include option params allows the specification of one or more parameters without inheriting any value from the external environment.

Module templates

The module script can be defined in an external template file. With DSL2 the template file can be placed under the templates directory where the module script is located.

For example, let’s suppose to have a project L with a module script defining 2 processes (P1 and P2) and both use templates. The template files can be made available under the local templates directory:

Project L
    |-myModules.nf
    |-templates
        |-P1-template.sh
        |-P2-template.sh

Then, we have a second project A with a workflow that includes P1 and P2:

Pipeline A
    |-main.nf

Finally, we have a third project B with a workflow that includes again P1 and P2:

Pipeline B
    |-main.nf

With the possibility to keep the template files inside the project L, A and B can use the modules defined in L without any changes. A future prject C would do the same, just cloning L (if not available on the system) and including its module script.

Beside promoting sharing modules across pipelines, there are several advantages in keeping the module template under the script path:

1. module components are self-contained,

2. module components can be tested independently from the pipeline(s) importing them,

3. it is possible to create libraries of module components.

Ultimately, having multiple template locations allows a more structured organization within the same project. If a project has several module components, and all them use templates, the project could group module scripts and their templates as needed. For example:

baseDir
    |-main.nf
    |-Phase0-Modules
        |-mymodules1.nf
        |-mymodules2.nf
        |-templates
            |-P1-template.sh
            |-P2-template.sh
    |-Phase1-Modules
        |-mymodules3.nf
        |-mymodules4.nf
        |-templates
            |-P3-template.sh
            |-P4-template.sh
    |-Phase2-Modules
        |-mymodules5.nf
        |-mymodules6.nf
        |-templates
            |-P5-template.sh
            |-P6-template.sh
            |-P7-template.sh

Channel forking

Using the new DSL, Nextflow channels are automatically forked when connecting two or more consumers.

For example:

channel
    .from('Hello','Hola','Ciao')
    .set{ cheers }

cheers
    .map{ it.toUpperCase() }
    .view()

cheers
    .map{ it.reverse() }
    .view()

The same is valid for the result (channel) of a process execution. Therefore a process output can be consumed by two or more processes without the need to fork it using the into operator, making the writing of workflow scripts more fluent and readable.

Pipes

The pipe operator

Nextflow processes and operators can be composed using the | pipe operator. For example:

process foo {
    input:
    val data

    output:
    val result

    exec:
    result = "$data world"
}

workflow {
   channel.from('Hello','Hola','Ciao') | foo | map { it.toUpperCase() } | view
}

The above snippet defines a process named foo and invokes it passing the content of the data channel. The result is then piped to the map operator which converts each string to uppercase and finally, the last view operator prints it.

The and operator

The & and operator allows feeding of two or more processes with the content of the same channel(s). For example:

process foo {
    input:
    val data

    output:
    val result

    exec:
    result = "$data world"
}

process bar {
    input:
    val data

    output:
    val result

    exec:
    result = data.toUpperCase()
}

workflow {
    channel.from('Hello') | map { it.reverse() } | (foo & bar) | mix | view
}

In the above snippet the channel emitting the Hello string is piped with the map which reverses the string value. Then, the result is passed to both foo and bar processes which are executed in parallel. Each process outputs a channel, and the two channels are merged into a single channel using the mix operator. Finally the result is printed using the view operator.

Tip

The break-line operator \ can be used to split long statements over multiple lines. The above snippet can also be written as:

workflow {
    channel.from('Hello') \
      | map { it.reverse() } \
      | (foo & bar) \
      | mix \
      | view
}

DSL2 migration notes

• DSL2 final version is activated using the declaration nextflow.enable.dsl=2 in place of nextflow.preview.dsl=2.

• Process inputs of type set have to be replaced with tuple.

• Process outputs of type set have to be replaced with tuple.

• Process output option mode flatten is no longer available. Replace it using the flatten operator on the corresponding output channel.

• Anonymous and unwrapped includes are not supported anymore. Replace them with an explicit module inclusion. For example:

  include './some/library'
  include bar from './other/library'
  
  workflow {
    foo()
    bar()
  }
  

  Should be replaced with:

  include { foo } from './some/library'
  include { bar } from './other/library'
  
  workflow {
    foo()
    bar()
  }
  

• The use of unqualified value and file elements into input tuples is not allowed anymore. Replace them with a corresponding val or path qualifier:

  process foo {
  input:
    tuple X, 'some-file.bam'
  
  script:
    '''
    your_command --in $X some-file.bam
    '''
  }
  

  Use:

  process foo {
  input:
    tuple val(X), path('some-file.bam')
  
  script:
    '''
    your_command --in $X some-file.bam
    '''
  }
  

• The use of unqualified value and file elements into output tuples is not allowed anymore. Replace them with a corresponding val or path qualifier:

  process foo {
  output:
    tuple X, 'some-file.bam'
  
  script:
    X = 'some value'
    '''
    your_command > some-file.bam
    '''
  }
  

  Use:

  process foo {
  output:
    tuple val(X), path('some-file.bam')
  
  script:
    X = 'some value'
    '''
    your_command > some-file.bam
    '''
  }
  

• Operator bind has been deprecated by DSL2 syntax

• Operator operator << has been deprecated by DSL2 syntax.

• Operator choice has been deprecated by DSL2 syntax. Use branch instead.

• Operator close has been deprecated by DSL2 syntax.

• Operator create has been deprecated by DSL2 syntax.

• Operator countBy has been deprecated by DSL2 syntax.

• Operator into has been deprecated by DSL2 syntax since it’s not needed anymore.

• Operator fork has been renamed to multiMap.

• Operator groupBy has been deprecated by DSL2 syntax. Replace it with groupTuple

• Operator print and println have been deprecated by DSL2 syntax. Use view instead.

• Operator separate has been deprecated by DSL2 syntax.

• Operator spread has been deprecated with DSL2 syntax. Replace it with combine.

• Operator route has been deprecated by DSL2 syntax.