Making reproducible workflows with

27-Nov-2024

Nextflow features

• Generalisable
• Portable
• Scalable
• Platform-agnostic
• Based on Groovy and Java
• Large active community in e.g. nf-core

Concepts and nomenclature

• Channels contain data, e.g. input files
• Processes run some kind of code, e.g. a script or a program
• Tasks are instances of a process, one per process input
• Each task is run in its own, isolated sub-directory

Anatomy of a process

process GET_SRA_BY_ACCESSION {

    input:
    val(sample)

    output:
    path("${sample}.fastq.gz")

    script:
    """
    fastq-dump ${sample} > ${sample}.fastq.gz
    """
}

Anatomy of a process

process GET_SRA_BY_ACCESSION {

    input:
    val(sample)

    output:
    tuple val(sample), path("${sample}.fastq.gz")

    script:
    """
    fastq-dump ${sample} > ${sample}.fastq.gz
    """
}

Anatomy of a process

process GET_SRA_BY_ACCESSION {

    cpus 2
    memory '8 GB'

    input:
    val(sample)

    output:
    tuple val(sample), path("${sample}.fastq.gz")

    script:
    """
    fastq-dump ${sample} > ${sample}.fastq.gz
    """
}

Anatomy of a process

process GET_SRA_BY_ACCESSION {

    cpus 2
    memory '8 GB'

    conda 'sra-tools=2.11.0'
    container 'ncbi/sra-tools:2.11.0'

    input:
    val(sample)

    output:
    tuple val(sample), path("${sample}.fastq.gz")

    script:
    """
    fastq-dump ${sample} > ${sample}.fastq.gz
    """
}

Anatomy of a workflow

workflow {

    // Define SRA input data channel
    ch_sra_ids = Channel.fromList ( ["SRR935090", "SRR935091", "SRR935092"] )

    // Define the workflow
    GET_SRA_BY_ACCESSION (
        ch_sra_ids
    )
}

Anatomy of a workflow

workflow {

    // Define SRA input data channel
    ch_sra_ids = Channel.fromList ( ["SRR935090", "SRR935091", "SRR935092"] )

    // Define the workflow
    GET_SRA_BY_ACCESSION (
        ch_sra_ids
    )
    RUN_FASTQC (
        GET_SRA_BY_ACCESSION.out
    )
}

Anatomy of a workflow

workflow {

    // Define SRA input data channel
    ch_sra_ids = Channel.fromList ( ["SRR935090", "SRR935091", "SRR935092"] )

    // Define the workflow
    GET_SRA_BY_ACCESSION (
        ch_sra_ids
    )
    RUN_FASTQC (
        GET_SRA_BY_ACCESSION.out
    )
    RUN_MULTIQC (
        RUN_FASTQC.out.collect()
    )
}

Executing Nextflow

Execute a workflow

$ nextflow run main.nf

Re-run using cached results

$ nextflow run main.nf -resume

Execute with a specific configuration file

$ nextflow run main.nf -c nextflow.config

Supply a custom parameter

$ nextflow run main.nf --param "value"

Use a pre-defined configuration profile

$ nextflow run main.nf -profile uppmax

Use Docker or Apptainer

$ nextflow run main.nf -with-docker
$ nextflow run main.nf -with-apptainer

Snakemake and Nextflow differences

	Snakemake	Nextflow
Language	Python	Groovy
Data	Everything is a file	Can use both files and values
Execution	Working directory	Each job in its own directory
Philosophy	“Pull”	“Push”
Dry-runs	Yes	No

• Question: But, which one is the best?
• Answer: Both - it’s mostly up to personal preference!

Questions?