Getting Started

Important: Be sure to update any values in angle brackets (<...>). If you are new to Git, we recommend reading through this excellent Git Tutorial.

  1. First, check the lcr-modules repository to see if the module already exists under the modules/ directory. Then, check the lcr-modules open issues to see if the module has already been proposed. If so, reach out to the assignee if there is one, or assign yourself if it’s unassigned. Otherwise, create a new issue for the module and assign yourself.

    Important: Please make sure you’re assigned to a GitHub issue before you start developing the module to avoid duplicating efforts.

  2. Research how to best design the module. While there is no expectation that the first version is perfect, it is preferable that an honest attempt is made to collect feedback both from people with experience with the tools involved and from the literature (e.g. benchmarking studies).

  3. Install conda (Python version 3.6 or later) with Miniconda or Anaconda. Ideally, create a conda environment specific for lcr-modules development. Then, install the cookiecutter package and, if need be, Git using conda.

    # Optional: Create a conda environment for lcr-modules development
    # conda create -n lcr-modules "python>=3.6"
    # conda activate lcr-modules
    conda install cookiecutter git
  4. Clone the lcr-modules repository and the lcr-scripts repository.

    git clone
    git clone
  1. Install the custom oncopipe python package included in the lcr-modules repository, which will also install dependencies such as snakemake and pandas.

    cd lcr-modules/
    pip install -e oncopipe/
  2. Create a new branch from the master branch with the format module/<module_name>/1.0, where <module_name> refers to the core or defining software tool being used in the module. Important: Your <module_name> should only contain lowercase alphanumerical characters or underscores (i.e. no spaces).

    git checkout master  # Make sure you're on the master branch
    git pull --ff-only   # Pull the latest changes from GitHub
    git checkout -b "module/<module_name>/1.0"  # Create new branch
    git branch  # Confirm you're on the new branch (with the asterisk)
    git push -u origin "module/<module_name>/1.0"
  3. Create a new module based on the Module Template. Check out the Module Template section for details on the fields requested during module creation.

    cookiecutter "template/" --output-dir 'modules/'
    git add modules/<module_name>/1.0/
    git commit -m "Add initial draft of <module_name> version 1.0"
    git push origin "module/<module_name>/1.0"
  4. Update the basic module created from the template, which can be found under modules/<module_name>/1.0/. Parts that need to be updated are indicated by TODO comments. You can use the New Module Checklist as a guide. These changes should be regularly committed to Git and pushed to GitHub.

    git add <files>
    git commit -m "<commit message>"
    git push origin "module/<module_name>/1.0"


    Testing Your Module

    There are different methods testing your module. One approach would be to leverage the Demo Project and the associated test data. Adding to the Demo Snakefile should be self-explanatory. This method works if your module operates on the kind of samples included in the Test Data.

    Another approach to consider is testing on your own data. This might be a good way to follow up on successful tests on the Demo Project, which confirm that the syntax of the module works. Running it on a larger dataset will confirm that the commands work on a variety of samples and that the output is sensible.

  5. When you are done with your module, commit any remaining changes and merge the master branch into your module branch. You shouldn’t have any merge conflicts since any new files should be under new versions.

    git merge master
    git push origin "module/<module_name>/1.0"
  6. Submit a pull request (PR) with your module branch. After pushing to GitHub, you should be able to see a green button to create a new pull request on the lcr-modules repository page pushing the merge commit. If not, you should be able to create one for your branch on the lcr-modules active branches page.

  7. Work through the checklist that will appear when you open the PR. Once this checklist is done, you can request someone to review your PR. They can test the module if they have time and/or provide feedback on its design. Finally, once the reviewer(s) are happy, the PR can be merged. Congratulations!

Module Template

While it is technically possible to create a new module without using the module template, it’s not recommended because using the template will ensure that you are following the latest best practices for lcr-modules.

When you run the command listed in the Getting Started instructions, you will be asked for the following information:

  • module_name: This field specifies the short name for the module. The value provided here should match the value used for <module_name> in the branch name when following the Getting Started instructions.

    Important: This field should only consist of lowercase alphanumerical characters or underscores (i.e. no spaces).

  • module_author: This field specifies the full name of the person who will write the module (presumably the person entering this information).

  • original_author: This field specifies the full name of the person who originally wrote the Snakefile or script that is being used to inform the module. If the module is being written from scratch, this field can be set to N/A.

  • input_file_type and output_file_type: These fields specify the file type of the input and output files, respectively. Generally, these values will be the file extensions (e.g. bam, vcf).


    • If there is more than one input file type, just list one of them for now. The same applies for the output file type. You’ll be able to add more file types in the Snakefile based on the existing structure.
    • Each of these should only consist of lowercase alphanumerical characters or underscores (i.e. no spaces).
  • module_run_per: Possible values are tumour and sample. This field determines whether the module is intended to be run once per tumour (e.g. variant calling modules) or once per sample regardless of tissue status (e.g. BAM alignment and processing).

    Additional options will be added later, such as tumour_cohort and sample_cohort for level-3 modules (see What Are Modules? for more details).

  • seq_type.genome, seq_type.capture, and seq_type.mrna: Possible values are omit,``unpaired``, matched_only, allow_unmatched, and no_normal, . These fields determine which sequencing data types (seq_type) are intended as input for the module and whether each seq_type is intended to be run in paired or unpaired mode, and if in paired mode, whether to allow unmatched pairs. Select omit if a seq_type is not applicable for the module or unpaired if you are running the module once per sample. For more information on the last three modes, check out the documentation for the oncopipe.generate_pairs() function. The fields correspond to whole genome, hybrid capture-based, and RNA sequencing, respectively.


    • If you selected sample for module_run_per, then you should use unpaired (or omit) here. If this is a paired analysis, you should start over (cancel with Ctrl-C) and select tumour for module_run_per.
    • If you selected tumour for module_run_per, you can select matched_only, allow_unmatched, or no_normal depending on whether the module is meant to be run on only matched tumour-normal pairs, on potentially unmatched tumour-normal pairs, or on tumours only.

Module Description

Module Structure

When you create a new module using the Getting Started instructions, you obtain the following files:

├── 1.0
│   ├── <module_name>.smk
│   ├── config
│   │   └── default.yaml
│   ├── envs
│   │   └── samtools-1.9.yaml -> ../../../../envs/samtools/samtools-1.9.yaml
│   ├── etc
│   └── schemas
│       └── base-1.0.yaml -> ../../../../schemas/base/base-1.0.yaml
  • <module_name>.smk: This Snakefile contains the rules defining the module. See Module Snakefile below for more details.
  • config/default.yaml: This configuration YAML file contains all of the user-configurable options, such as input files, conda environments, command-line options, cluster parameters, and the pairing configuration (i.e. whether/how to run samples as tumour-normal pairs).
  • envs/: This folder contains symlinks to individual conda environment YAML files from the envs/ directory, which is found in the root of the repository. These conda environment are generally tool-specific (e.g. samtools, star). Symlinks are used to keep the repository lightweight and promote reuse of conda environments between modules.
  • etc/: This folder can contain any accessory files required to run the module, such as configuration files (see manta module version 2.0 for an example). For more details, check out the Module Accessory Files and Scripts section.
  • schemas/: This folder contains symlinks to individual schema YAML files from the schemas/ directory in the root of the repository. These schemas determine the required columns in the samples table. Every module should have the base-1.0.yaml schema as a minimum requirement. For more information, check out the Required Sample Metadata section below. Symlinks are used to keep the repository lightweight and promote reuse of schemas between modules.
  • This file contains the release notes for the module. These release notes should list the changes and the rationale for each change.

Module Snakefile

This section will describe the key components of a module snakefile. It uses the star module as an example. Note that CFG refers to the module-specific configuration. In the case of the star module, this would correspond to:


Module Attribution

This section simply lists the individuals who have contributed to the module in one way or another. The Original Author refers to the person who wrote the Snakefile or script that was adapted for the module. The Module Author refers to the person who either adapted a previously written Snakefile/script or created the module from scratch. Finally, the Contributors refers to the list of individuals who have contributed to the module over time, mainly through incremental version updates.

##### ATTRIBUTION #####

# Original Author:   Nicole Thomas
# Module Author:     Bruno Grande
# Contributors:      N/A

Module Setup

There are a few standard components for the module setup and some optional components. Importing standard modules such as os (for the os.remove() function) is optional. On the other hand, importing the oncopipe module is required because it offers a suite of functions that greatly simplify the process of developing modules and facilitate configuration by the user. For brevity, the module is commonly imported with import oncopipe as op, which allows the functions to be accessible using the op prefix/namespace (e.g. op.as_one_line()).

The oncopipe.setup_module() function call is also required. This function does most of the heavy-lifting behind the scenes to streamline the process of developing modules. The arguments are self-explanatory: name is the module name, version is the module version, and subdirectories is the output subdirectories, which will be numbered automatically by oncopipe.setup_module().

The first and last subdirectories must be inputs and outputs, and they will be numbered as 00-inputs and 99-outputs, respectively. You should name the subdirectories after the tool name or the process, whatever is more evocative and specific (e.g. star over align, or mark_dups over picard).

Also, it’s worth noting that lcr-modules use a variant of semantic versioning where major versions represent changes in the number of rules in the module (or changes in the relationship between rules), whereas minor versions reprsent changes in the configuration of the module (e.g. command-line parameters).

The include statement for the utils module is optional. For more information on the include statement, you can refer to the Snakemake Includes documentation. The utils module contains rules that are generally useful (e.g. BAM file sorting, BAM file indexing). It is meant to be included into another module after it has been configured with oncopipe.setup_module(). The reason for this is that utils.smk makes use of the CFG variable to make sure it doesn’t interfere with other modules.

Finally, the localrules statement is technically optional, but it is recommended to include it in every module. For more information, you can refer to the Snakemake Local Rules documentation. Essentially, when snakemake submits jobs to a cluster, these rules are run locally instead. It is meant for quick rules (e.g. symlinking rules) that aren’t computationally intensive and could potentially get stuck in the cluster queue for much longer than they take to run.

##### SETUP #####

# Import standard modules
import os

# Import package with useful functions for developing analysis modules
import oncopipe as op

# Setup module and store module-specific configuration in `CFG`
# `CFG` is a shortcut to `config["lcr-modules"]["star"]`
CFG = op.setup_module(
    name = "star",
    version = "1.0",
    subdirectories = ["inputs", "star", "sort_bam", "mark_dups", "outputs"],

# Include `utils` module
include: "../../utils/1.0/utils.smk"

# Define rules to be run locally when using a compute cluster

Module Rules

Input and Output Rules

The input and output rules serve a few purposes. First, they clearly define the entry and exit points of the module, making the module more modular and easier to tie different modules together. Second, they make it clear to anyone exploring the module output directory what the input files were and what the most useful output files (or deliverables) are. Third, by symlinking the most important files in subdirectories with the same name (i.e. 99-outputs), it makes it easier to archive those files (e.g. from scratch space to backed-up storage).

You will notice that the oncopipe.relative_symlink() function is used in the rules below rather than the standard os.symlink() function. The different between the two function is explained here: What is the difference between op.relative_symlink() and os.symlink()?.

Below is the input and output rules for the star module. Because STAR operates on paired FASTQ files, we actually need to symlink two files per sample. While this could have been achieved in two rules, it was simpler to implement as one shared rule. The output file symlinks both the BAM and BAM index (BAI) files at the same time since they need to travel together. Otherwise, I find it useful to output different file types in different subdirectories in 99-outputs; see the manta module for an example, where VCF and BEDPE files are stored separately. In this specific example, the output file rule also deletes an intermediate file. This is being done here to ensure that the downstream file exists before deleting the upstream file.

rule _star_input_fastq:
        fastq_1 = CFG["inputs"]["sample_fastq_1"],
        fastq_2 = CFG["inputs"]["sample_fastq_2"],
        fastq_1 = CFG["dirs"]["inputs"] + "fastq/{seq_type}--{genome_build}/{sample_id}.R1.fastq.gz",
        fastq_2 = CFG["dirs"]["inputs"] + "fastq/{seq_type}--{genome_build}/{sample_id}.R2.fastq.gz",
        op.relative_symlink(input.fastq_1, output.fastq_1)
        op.relative_symlink(input.fastq_2, output.fastq_2)

# The other rules, which are normally in between, were omitted

rule _star_output_bam:
      bam = CFG["dirs"]["mark_dups"] + "{seq_type}--{genome_build}/{sample_id}.sort.mdups.bam",
      bai = CFG["dirs"]["mark_dups"] + "{seq_type}--{genome_build}/{sample_id}.sort.mdups.bam.bai",
      sorted_bam = str(rules._star_symlink_sorted_bam.input.bam)
      bam = CFG["dirs"]["outputs"] + "bam/{seq_type}--{genome_build}/{sample_id}.bam"
      op.relative_symlink(input.bam, output.bam)
      op.relative_symlink(input.bai, output.bam + ".bai")
      shell("touch {input.sorted_bam}.deleted")

Target Rules

Generally, the last rule of the module snakefile is the “master target rule”. This rule is usually named _<module_name>_all (e.g. _star_all), and expands all of the output files (the files symlinked into 99-outputs) using either the samples table (CFG["samples"]) or the runs table (CFG["runs"]) depending on whether the module is run once per sample or once per tumour. The two examples below show a preview of each table and how each can be used in the target rule.

Using the Samples Table
sample_id seq_type patient_id tissue_status genome_build
TCRBOA7-T-RNA mrna TCRBOA7 tumour grch37

In the example below, since STAR is run on all RNA-seq BAM file, we are using the samples table, which has been automatically filtered for samples whose seq_type appears in the module’s pairing_config. For more information on the pairing_config, check out Pairing Configuration. Note the use of the rules variable that snakemake automatically generates for retrieving the output files from previous rules in the module.

rule _star_all:
         zip,  # Run expand() with zip(), not product()
Using the Runs Table
pair_status tumour_sample_id normal_sample_id tumour_seq_type normal_seq_type tumour_patient_id normal_patient_id tumour_tissue_status normal_tissue_status tumour_genome_build normal_genome_build
matched TCRBOA7-T-WEX TCRBOA7-N-WEX capture capture TCRBOA7 TCRBOA7 tumour normal grch37 grch37
no_normal TCRBOA7-T-RNA   mrna   TCRBOA7   tumour   grch37  

In this second example, taken from the manta module, we can see how the runs table (CFG["runs"]) is used to define the targets. Because the runs table lists tumour-normal pairs, each column from the samples table is present, but they are prefixed with tumour_ and normal_. The only column that isn’t taken from the samples table is pair_status, which described the relationship between the tumour-normal pair. Generally, this can be matched if the tumour and normal samples come from the same patient; unmatched if the two samples come from different patients; and no_normal if there is no normal paired with the tumours.

It’s worth noting that the output rule being expanded is _manta_dispatch rather than _manta_output_vcf and _manta_output_bedpe. The reason for this is technical, but briefly, it is because an input file function in the _manta_dispatch rule determines which files are converted into BEDPE format.

rule _manta_all:
            zip,  # Run expand() with zip(), not product()

Other Rules

Every other rule serve to complete the module. These other rules can vary considerably in scope. Therefore, below is a list of guiding principles to follow when designing these rules. These principles simply make it easier for users to achieve what they want. If one of these guidelines gets in the way of designing your module, feel free to employ a different approach, ideally not at the cost of flexibility for the user.

An example rule that follows most of these principles is included below (taken from the star module).

  1. Each rule should only consist of one command, unless the rule uses standard tools like gzip for additional commands. Otherwise, split into multiple rules, optionally connected using pipe() or temp() to avoid intermediate files.

    This guideline ensures that rules are modular and can easily be rearranged by the user. It also enables tool-specific conda environments (e.g. samtools, star) to be used, which is not possible is more than one tool is used in a rule.

  2. For input files, use rules references to previous output (or input) files wherever possible. You should wrap any references to rules with str().

    These rules references minimizes the risk that two files get out of sync, e.g. if you update an upstream output file and forget to update every downstream occurrence of that file.

    The str() function ensures that the rules reference isn’t considered as an explicit dependency on whatever rule is specified. Otherwise, users won’t be able to provide an alternative rule to generate the input in question.

  3. Reference data should be provided as input files and ideally have rules in the reference_files workflow so they can be generated automatically. If a reference file has parameters, these can be exposed to the user under the reference_params section in the module configuration.

    Having reference data as input files ensures that rules are re-run if the reference data is updated. For more information on the reference_files workflow, check out the Reference Files Workflow section.

  4. The output (and input) files should use values in the CFG["dirs"], which correspond to the subdirectory names provided to setup_module().

    This allows the user to easily adjust the output directory for the entire module.

  5. Avoid using non-standard wildcards. The standard wildcards for sample-based modules are: seq_type, genome_build, and sample_id. The standard wildcards for tumour-based modules are: seq_type, genome_build, sample_id, tumour_id, and normal_id.

    Adding new wildcards makes it hard to connect different modules together. For example, if module A adds an ffpe_status wildcard and module B depends on module A, module B will have to include ffpe_status as a wildcard, even though it’s not relevant to module B. You can thus see how this would result in the steady accumulation of wildcards. To change the behaviour of a module/rule based on sample metadata, see the Conditional Module Behaviour section below.

  6. For log files, use the corresponding subdirectory names in CFG["logs"].

    The directories in CFG["logs"] are automatically timestamped, which allows the log files from each run to be stored separately for posterity.

  7. Store stdout and stderr in separate log files, unless the tool outputs to stdout, in which case only stderr needs to be stored.

    Storing stdout and stderr in separate files makes it easier to know what output came from where, and it prevent potential issues with truncated log files.

  8. Create an opts entry under param for all command-line options that are not linked to a {...} value, which are configured in the default.yaml file.

    As you can see in the example below, every option under shell is associated with a value taken from the rule (e.g. --genomeDir {input.index}), whereas it completely lacks “standalone options” (e.g. --runMode alignReads). This guideline is to allow the user to have absolute control over the parameterization of the command-line tool.

  9. Re-use (or provide) tool-specific conda environments for each rule needing one, which are configured in the default.yaml file. This can be skipped if the rule only uses standard UNIX tools (e.g. gzip, awk) or if it uses the run directive (instead of the shell directive).

    Conda environments simplify software installation for a module and ensure reproducibility by specifying tool versions. Even if a rule only uses standard UNIX tools, it might still be worth using the coreutils conda environment to avoid OS variations (e.g. GNU vs BSD for sed).

  10. Add the threads and resources (mem_mb) directives for all non-local rules, which are configured in the default.yaml file.

    These directives are essential for running the module on a compute cluster. The values should be as low as possible while ensuring that most jobs are run within a reasonable amount of time (to minimize time spent in the queue).

  11. Use the shell directive for rules with the conda directive. Use the run directive instead if more complicated logic is required.

    The as_one_line() function is meant to be used with the triple-quoted (""") strings for long commands. The benefits of using this function are: (1) spaces are automatically added at the end of each line; (2) double-quotes do not need to be escaped; and (3) cleaner commands that are easier to organize using indentation. For example, any pipes (|) or double-ampersands (&&) can be indented to indicate the separation between two commands.

rule _star_run:
      fastq_1 = str(rules._star_input_fastq.output.fastq_1),
      fastq_2 = str(rules._star_input_fastq.output.fastq_2),
      index = reference_files("genomes/{{genome_build}}/star_index/star-2.7.3a/gencode-{}/overhang-{}".format(
         CFG["reference_params"]["gencode_release"], CFG["reference_params"]["star_overhang"]
      gtf = reference_files("genomes/{{genome_build}}/annotations/gencode_annotation-{}.gtf".format(
      bam = CFG["dirs"]["star"] + "{seq_type}--{genome_build}/{sample_id}/Aligned.out.bam"
      stdout = CFG["logs"]["star"] + "{seq_type}--{genome_build}/{sample_id}/star.stdout.log",
      stderr = CFG["logs"]["star"] + "{seq_type}--{genome_build}/{sample_id}/star.stderr.log"
      opts = CFG["options"]["star"],
      prefix = CFG["dirs"]["star"] + "{seq_type}--{genome_build}/{sample_id}/",
      star_overhang = CFG["reference_params"]["star_overhang"]
      mem_mb = CFG["mem_mb"]["star"]
      STAR {params.opts} --readFilesIn {input.fastq_1} {input.fastq_2} --genomeDir {input.index}
      --outFileNamePrefix {params.prefix} --runThreadN {threads} --sjdbGTFfile {input.gtf}
      --sjdbOverhang {params.star_overhang} > {log.stdout} 2> {log.stderr}
      rmdir {params.prefix}/_STARtmp

Module Cleanup

Every module ends with a clean-up step. At the moment, this mainly consists of outputting the module configuration, including the samples and runs, to disk for future reference. These files are output in a timestampted directory in the logs/ subdirectory. Additionally, this function will delete the CFG variable from the environment to ensure it does not interfere with other modules.

# Perform some clean-up tasks, including storing the module-specific
# configuration on disk and deleting the `CFG` variable

Module Configuration

One of the core principles of lcr-modules is configurability, and this is primarily achieved by storing anything that can be adjusted in a configuration file separate from the Snakefile. For most modules, there will be a single configuration file called default.yaml. On the other hand, some modules might have multiple configuration files to account for different scenarios. For this reason, there is a config/ subdirectory for each module where all of these configuration files live.

In theory, configuration YAML files can take on any structure. However, it helps both module users and developers to start with a standard structure. This also facilitates feature development. Below is a description of each section of a typical default.yaml file using the star module as an example.

Configuration Features

Requiring User Intervention

Make sure that anything that needs to be updated by the user contains __UPDATE__ in the configuration file. You can see examples in the excerpts below taken from the star default configuration. If the string __UPDATE__ is detected anywhere in the module configuration, an error will inform the user that they need to update a configuration field.

Directory Placeholders

Since the module developer won’t know where the lcr-modules (and lcr-scripts, if applicable) repository will be located, one of the features of the setup_module() function in oncopipe is to replace the following directory placeholders with their actual values. This way, you can specify file paths relative to these directories. See the README for the list of Directory Placeholders.

Configuring Header

Each module configuration should fall under the lcr-modules and <module_name> (e.g. star) keys. The lcr-modules top-level configuration key is considered reserved for use by modules in this project and the oncopipe package. This ensures that the module configuration is properly siloed and avoids clashes with other configuration set by the user.


Configuring Input and Reference Files

Virtually all modules will have input files, and many will also require reference files. These are defined using the inputs and reference_params keys, respectively.

The input files will generally be set to __UPDATE__ since they need to be specified by the user. This can be done in the configuration file or in the Snakefile (see the Demo Snakefile for an example). Either way, the available wildcards are usually listed in a comment. If not, you can always look at the wildcards in the output files of the rule using the inputs configuration section. In general, these are {seq_type}, {genome_build}, and {sample_id}.

One advantage of specifying the input files in the Snakefile (as opposed to in the configuration file) is that the user can provide Input File Functions rather than a string.

While conceptually similar to input files, reference files are handled differently in lcr-modules. They are generally genome build–specific rather than sample-specific. Accordingly, they need to be generated separately. In the past, this was often done in a time-consuming ad hoc way where the commands used to generate the reference files were often not tracked. A reference_files workflow was developed as part of lcr-modules to streamline this process and promote reproducibility. Most reference files depend only on the genome build and thus required no intervention from the user since the genome_build is a standard wildcard. However, some reference files require additional parameterization (e.g. the amount of splice-junction overhang when building a STAR index). These parameters are exposed to the user under the reference_params section. Some parameters are so important that they will be commented out with #! to require user intervention, such as the star_overhang parameter in the example below.

For more information on the approach taken in reference_files and its benefits and limitations, check out the section on the Reference Files Workflow.

   # The inputs can be configured here or in the Snakefile
   # Available wildcards: {seq_type} {genome_build} {sample_id}
   sample_fastq_1: "__UPDATE__"
   sample_fastq_2: "__UPDATE__"

   # Ideally, `star_overhang` = max(read_length) - 1
   # STAR indices were precomputed for "74" and "99"
   star_overhang: "__UPDATE__"
   # The Gencode release to use for the transcript annotation
   gencode_release: "33"

Configuring Scratch Subdirectories

The scratch_subdirectories section provides the user with the ability of storing intermediate files in a scratch directory. Essentially, the listed subdirectories, which must match the names provided to the subdirectories argument in oncopipe.setup_module(), will be made into symlinks to corresponding directories in a scratch space. This scratch space is also specified by the user, generally with the scratch_directory key under _shared.

Note that if you’ve already run your Snakefile, the subdirectories will already exist as actual directories and not symlinks. Accordingly, you will have to delete them before adding another entry to scratch_subdirectories. Otherwise, you will run into an error.

scratch_subdirectories: ["star", "sort_bam"]

Configuring Options

The options section specifies the command-line options for each tool used in the module (where such options exist). Generally, any command-line option not linked to a placeholder (e.g. {input}, {output}, {params}) should be listed under the tool’s corresponding entry in options. This provides the user with ultimate control over how the tool is run without having to deal with the Snakefile.

Even if a tool has no command-line options beyond those already used in the Snakefile, it is useful to include an entry under options with an empty string in case options appear in future versions of the tool. For example, if the user wants to use a command-line option available in a later version of a tool, they can update the conda environment (see Configuring Conda Environments) and replace the empty string under options with the new option, thus avoiding any editing of the underlying Snakefile.

In the example below, you can see that any command-line options associated with a snakemake parameter (e.g. --sjdbOverhang, --runThreadN) or an input/output file (e.g. --readFilesIn, --outFileNamePrefix) are not included here. Instead, they reside in the star snakefile.

      --runMode alignReads
      --twopassMode Basic
      --genomeLoad NoSharedMemory
      --readFilesCommand zcat
      --outSAMtype BAM Unsorted
      --outSAMattrIHstart 0
      --chimOutType WithinBAM SoftClip
      --chimSegmentMin 20
   utils_bam_sort: ""
   utils_bam_markdups: ""
   utils_bam_index: "-b"

You will also notice the various utils_bam_* fields. These correspond to rules in the utils module. For example, the following utils rule can index a BAM file, and you can see how it has an opts parameter that looks up the utils_bam_index field under options. If it doesn’t find a value, it defaults to "-b". In this case, the module developer exposed these fields to the user by including them in the star default configuration. In this case, the same default values as in the utils module were used, but that might not always be the case depending on the module.

# _utils_bam_index: Index a BAM file
      bam = CFG["dirs"]["_parent"] + "{prefix}/{suffix}.bam"
      bam = CFG["dirs"]["_parent"] + "{prefix}/{suffix}.bam.bai"
   # ...
      opts = CFG["options"].get("utils_bam_index", "-b"),
      prefix = CFG["dirs"]["_parent"] + "{prefix}/{suffix}"
   # ...
      samtools index {params.opts} -@ {threads}
      {input.bam} > {log.stdout} 2> {log.stderr}

Configuring Conda Environments

The conda environments that power each module are listed under conda_envs. These allow for specific versions of tools to be automatically installed, which facilitates reproducibility. Each module will specify a set of default versions of each tool. The user can update this conda environments (e.g. to use a more recent version), but this might break the module if there are backwards-incompatible changes to the tool’s command-line interface.

Each conda environment should ideally be tool-specific because that promotes re-use of environments between modules. Otherwise, commonly used tools such as samtools would be included in multiple module-specific environments. This also allows for easier tracking of the tool versions in the file names. This can only be achieved if each module rule is indeed only using one tool, which should be the case.

Note that Snakemake expects the paths to be relative to the Snakefile. This is automatically handled by the oncopipe.setup_module() function, so the paths provided under conda_envs in the module configuration are expected to be relative to the working directory (usually where you run the snakemake command). In the example below, you can see the {MODSDIR} directory placeholder being used such that the paths are portably regardless of where the user stores the lcr-modules repository (as long as repository is specified under _shared). For more information, check out Directory Placeholders.

   star: "{MODSDIR}/envs/star-2.7.3a.yaml"
   samtools: "{MODSDIR}/envs/samtools-1.9.yaml"
   sambamba: "{MODSDIR}/envs/sambamba-0.7.1.yaml"

Creating Conda Environments

This is a suggested workflow for creating conda environments for your modules. The commands are based on the example of creating a conda environment for the STAR aligner.

  1. Create a conda environment for the tool in question. For example, in the case of STAR, the following command would create a conda environment called test-star and install the latest version of the star package from the bioconda Anaconda channel along with its dependencies

    conda create -c bioconda -n test-<star> <star>
  2. Optionally, activate this conda environment before testing any STAR commands. This testing can occur in bash scripts or in a snakefile as long as the rule(s) don’t activate their own conda environment, thus relying on the global shell environment.

    conda activate test-<star>
    bash run_<star> ...
  3. Create a subdirectory in envs/ named after the tool once you are ready to add the new conda environments to the lcr-modules repository. Again, for the STAR environment, this would look like:

    mkdir envs/<star>/
  4. Determine the version of the tool that was installed by conda. You can usually achieve this by grepping the tool from the output of conda env export.

    conda env export -n test-<star> --no-builds | grep <star>
  5. Output the conda environment specification into a file named after the tool and its version. Be sure to use the --no-builds command-line option to omit build ID, which tend to cause problems recreating environments later.

    conda env export -n test-<star> --no-builds > envs/<star>/<star>-<2.7.3a>.yaml
  6. Symlink this new conda environment YAML file into your module’s envs/ directory. The symlinks prevent the repository from becoming bloated with duplicate files. This approach also promotes the re-use of conda environments across modules.

    cd modules/<star>/<1.0>/envs/
    ln -s ../../../../envs/<star>/<star>-<2.7.3a>.yaml ./

Configuring Compute Resources

Many users will be launching the modules on a high-performance computing cluster. Hence, all non-local rules should have sensible default values for resources such as CPU (threads) and memory (mem_mb). These settings should strike a balance between the time spent waiting in the queue (with higher resource values) and the time spent running (with lower resource values).

  • ``threads``: The number of logical cores to allocate. This number is typically passed to a command-line argument such as --threads or --cores. Make sure to check the tool’s actual CPU usage. If it’s consistently lower or higher than the specified amount, consider adjusting the value.
  • ``mem_mb``: The amount of memory to allocate in megabytes (MB). This number is usually best determined empirically based on actual tool runs. This can be done in a number of ways, including monitoring top/htop or inspecting “Maximum resident set size” when the command is prepended with /usr/bin/time -v.
   star: 12
   utils_bam_sort: 12
   utils_bam_markdups: 12
   utils_bam_index: 6

   star: 40000
   utils_bam_sort: 12000
   utils_bam_markdups: 8000
   utils_bam_index: 4000

Pairing Configuration

The pairing_config section is where the module is configured to run for each sequencing data type (seq_type). Two examples are included below to illustrate how the pairing_config is used. Check out the Pairing Configuration Options section for more details on each field (e.g. run_paired_tumours).

In this first example, we continue with the star module. Here, the pairing configuration only lists mrna (i.e. RNA-seq data) as a supported seq_type. In the future, additional sequencing data types could be added, such as mirna for miRNA sequencing data. For mrna, the star module is configured to run on all samples in unpaired mode. This is achieved by first disabling paired mode (run_paired_tumours as False) and then ensuring that any paired tumours are forced to run in unpaired mode (run_paired_tumours_as_unpaired as True). Setting run_unpaired_tumours_with to "no_normal" is meant to clarify that the unpaired tumours should be included; otherwise, they would be omitted since the default for run_unpaired_tumours_with is None.

      run_paired_tumours: False
      run_unpaired_tumours_with: "no_normal"
      run_paired_tumours_as_unpaired: True

This second example was taken from the manta module. As you can see, the module can handle genome, capture, and mrna data. It treats genome and capture data the same way, namely by allowing unpaired tumours to be analyzed using unmatched normals (as opposed to a truly unpaired analysis without a normal sample). Also, paired tumours are not unnecessarily run as unpaired. In contrast, mrna data is run specifically in an unpaired fashion without a normal sample because tumour RNA-seq alignments generally do not have matched normal RNA-seq data.

      run_paired_tumours: True
      run_unpaired_tumours_with: "unmatched_normal"
      run_paired_tumours_as_unpaired: False
      run_paired_tumours: True
      run_unpaired_tumours_with: "unmatched_normal"
      run_paired_tumours_as_unpaired: False
      run_paired_tumours: False
      run_unpaired_tumours_with: "no_normal"
      run_paired_tumours_as_unpaired: True

Pairing Configuration Options

Here’s a brief description of each of the options that go into a pairing_config. Here, the term “unpaired tumour” refers to tumours that lack a matched normal sample with the same seq_type.

  • run_paired_tumours: Possible values are True or False. This option determines whether to run paired tumours. Setting this to False is useful for naturally unpaired or tumour-only analyses (e.g. for RNA-seq), which is normally done while setting run_paired_tumours_as_unpaired to True in case there are any paired tumours.
  • run_unpaired_tumours_with: Possible values are None, "unmatched_normal", or "no_normal". This option determines what to pair with unpaired tumours. Specifying None means that unpaired tumours will be skipped for the given module. This option cannot be set to None if run_paired_tumours_as_unpaired is True. Specifying "unmatched_normal" means that unpaired tumours will be run by being paired with the unmatched normal sample given by unmatched_normal_id (see below). Specifying "no_normal" means that unpaired tumours will be run without a normal sample. Note that modules need to be specifically configured to be run in paired and/or unpaired mode, since the commands of the underlying tools probably need to be tailored accordingly.
  • run_paired_tumours_as_unpaired: Possible values are True or False. This option determines whether paired tumours should be run as unpaired (i.e. separate from their matched normal sample). This is useful for benchmarking purposes or preventing unwanted paired analyses (e.g. in RNA-seq analyses intended to be tumour-only).

Module Accessory Files and Scripts

When you create a new module from the cookiecutter template, you will notice an empty etc/ subdirectory in your module directory. This folder is meant to contain any additional file required to run your module. For example, the manta module requires configuration files, which are stored in etc/. Scripts can also be stored in this directory. That said, if a script is generally useful, you might want to submit a pull request to the lcr-scripts repository. The purpose of this separate repository is to avoid storing useful scripts in a nested directory within the lcr-modules repository. Just as with lcr-modules, lcr-scripts are versioned and come with conda environment YAML files. For your convenience, there is a SCRIPTSDIR directory placeholder you can use in your default configuration file.

You can look at how the script from lcr-scripts is used in the manta module version 2.0.

Advanced Module Features

Required Sample Metadata

Every module requires the samples table, which contains metadata on the samples being analyzed. The minimum set of columns expected by lcr-modules are the sample_id, patient_id, seq_type, and tissue_status columns (see Required Columns for more info). These requirements are spelled out using schemas in YAML format. The base requirements can be found in schemas/base/base-1.0.yaml.

Some modules will need additional metadata (e.g. the strandedness of RNA-seq libraries). These extra requirements should also be described in schema files. To promote modularity, each required column should have its own file to promote modularity. An exception can be made for a set of columns should always be present together. The new schemas should be stored in the shared schemas/ directory and then symlinked into individual modules. Symlinks are used to keep the repository lightweight and promote reuse of schemas between modules.

An example single-column schema file can be found in schemas/ffpe_status/ffpe_status-1.0.yaml, where as a multi-column schema file should look like the base schema, i.e. schemas/base/base-1.0.yaml.

Important: Read the section below on Conditional Module Behaviour for an explanation on why you should avoid adding new wildcards beyond the standard ones described in Other Rules.

Conditional Module Behaviour

One size doesn’t always fit all, so modules sometimes have to tailor their behaviour based on sample attributes. Snakemake offers more than one avenue to implement these conditional behaviours. The simplest approach is to create parallel rules, which will handle samples differently based on the file names, potentially using wildcard constraints. However, this approach has two major issues.

First, the resulting parallel rules are mostly identical except for a few, often minor differences (e.g. a single command-line argument). This redundancy violates the DRY Principle, making the module harder to maintain and more vulnerable to bugs. This pitfall can be avoided by merging the two rules and using the Switch on Wildcard Value function from oncopipe described below.

Second, it requires the module developer to encode the sample attributes in the file names. While this is not a severe limitation on its own, it complicates the task of connecting modules together because the file names in downstream modules will need to include every wildcard from upstream modules. This would not only lead to unsustainably long file names, but the file names of a module shouldn’t depend on which modules are upstream to ensure modularity. The accumulation of module-specific wildcards can be avoided using the Switch on Sample Metadata function from oncopipe described below.

To give a specific example, let’s say the salmon module requires the strandedness of the RNA-seq samples, so this information is encoded in the file name, e.g. {sample_id}.{strandedness}.quant. Once we have quantified gene expression in all RNA-seq samples, we wish to perform cohort-wide correction for library size. Unfortunately, we need to pull the information about strandedness from the sample metadata in order to find the salmon output files because it’s part of the file names, even though that information isn’t relevant to our library size correction module.

Important: The oncopipe.switch_on_wildcard() and oncopipe.switch_on_column() functions do not currently support Directory Placeholders. This issue will track the implementation.

Switch on Wildcard Value

You can use the oncopipe.switch_on_wildcard() function to dynamically set the value of an input file or parameter for a snakemake rule based on the value of a wildcard. The first argument (wildcard) is the name of the wildcard, and the second argument (options) is a dictionary mapping possible values for the wildcard to the corresponding values that should be returned.

This dictionary can make use of special keys. The most important one to note is the "_default" special key, whose associated value is selected if the wildcard value isn’t among the other keys. You should check out oncopipe.switch_on_wildcard() to find out about the other special keys. (What does the underscore prefix mean?)

By default, the oncopipe.switch_on_wildcard() will replace any placeholders (using the same format as the shell directive; e.g. {wildcards.seq_type}) with the actual values. This beheviour can be tweaked with the format (default = True) and strict (default = False) optional arguments. See the function docstring for more information on these optional arguments.

An example taken from the manta module is included below (only relevant parts are shown). Here, the _manta_configure rule needs to use a different configuration file based on the sequencing data type (seq_type). Specifically, we wish to provide the high-sensitivity configuration if the seq_type is RNA-seq (mrna) or capture-based sequencing (capture), or the default configuration otherwise. Accordingly, the first argument is "seq_type".

rule _manta_configure:
        config = op.switch_on_wildcard("seq_type", CFG["switches"]["manta_config"])

The second argument is a reference to the module configuration (CFG), specifically the switches section. Since YAML files are parsed as nested dictionaries, it is straightforward to store the mapping between wildcard values and desired return values in the default.yaml configuration file. The relevant part from the YAML file is included below.

        _default: "{MODSDIR}/etc/manta_config.default.ini"
        mrna: "{MODSDIR}/etc/manta_config.high_sensitivity.ini"
        capture: "{MODSDIR}/etc/manta_config.high_sensitivity.ini"

CFG["switches"]["manta_config"] contains the dictionary representation of the manta_config section from the YAML file shown above. You can see how the "_default" special key is being used here (see Switch on Wildcard Value for more info) as well as the {MODSDIR} placeholder for the module subdirectory (see Directory Placeholders for more info).

# This is the dictionary stored in `CFG["switches"]["manta_config"]`
    '_default': '{MODSDIR}/etc/manta_config.default.ini',
    'mrna': '{MODSDIR}/etc/manta_config.high_sensitivity.ini',
    'capture': '{MODSDIR}/etc/manta_config.high_sensitivity.ini'

Switch on Sample Metadata

As I mentioned in Conditional Module Behaviour, adding wildcards for conditional behaviour in a Snakefile is unsustainable and goes against the core principle of modularity. One workaround is to query the metadata for each sample (or each tumour-normal pair) and to update the tool command accordingly. The approach is similar to a Switch on Wildcard Value, but with a few notable differences.

The function to use is oncopipe.switch_on_column(() where the first argument (column) is the column name, the second argument (samples) is the samples data frame (typically CFG["samples"]), and the third argument (options) is a dictionary mapping possible values in the column to the corresponding values that should be returned. This dictionary follows the same structure as the Switch on Wildcard Value. An additional albeit optional argument is called match_on, which needs to be set to either "tumour" (default) or "normal" to determine whether the function uses the wildcards.tumour_id or wildcards.normal_id to look up a sample ID. The function will automatically use wildcards.seq_type to also filter on sequencing data type.

At the moment, this function only works for tumour-based modules (e.g. paired variant calling). It should soon be generalized to also work with sample-based modules (e.g. STAR alignment). This issue is tracked here.

The code block below shows how we could achieve the same outcome using oncopipe.switch_on_column() for the example given in Switch on Wildcard Value. The only difference other than the function name is the addition of the samples argument before providing the same options dictionary. By default, the function will use wildcards.tumour_id (and wildcards.seq_type) to look up the sample in CFG["samples"]. In practice, you would simply use oncopipe.switch_on_wildcard() since seq_type is available as a wildcard.

rule _manta_configure:
        config = op.switch_on_column("seq_type", CFG["samples"], CFG["switches"]["manta_config"])

Switch on File Contents

The behaviour of some module depends on the contents (or existence) of input or intermediate files. The best way to address this is using Snakemake Checkpoints. They are a bit complicated to implement, but you can look at the manta module (version 1.0) for an example. Do note that checkpoints can be slow because the function using the checkpoint is run sequentially for each sample.