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Caper (Cromwell Assisted Pipeline ExecutoR) is a wrapper Python package for Cromwell.


Caper wraps Cromwell to run pipelines on multiple platforms like GCP (Google Cloud Platform), AWS (Amazon Web Service) and HPCs like SLURM, SGE, PBS/Torque and LSF. It provides easier way of running Cromwell server/run modes by automatically composing necessary input files for Cromwell. Caper can run each task on a specified environment (Docker, Singularity or Conda). Also, Caper automatically localizes all files (keeping their directory structure) defined in your input JSON and command line according to the specified backend. For example, if your chosen backend is GCP and files in your input JSON are on S3 buckets (or even URLs) then Caper automatically transfers s3:// and http(s):// files to a specified gs:// bucket directory. Supported URIs are s3://, gs://, http(s):// and local absolute paths. You can use such URIs either in CLI and input JSON. Private URIs are also accessible if you authenticate using cloud platform CLIs like gcloud auth, aws configure and using ~/.netrc for URLs.

Installation for Google Cloud Platform and AWS

See this for details.

Installation for AWS

See this for details.


  1. Make sure that you have Java (>= 11) and Python>=3.6 installed on your system and pip to install Caper.

    $ pip install caper
  2. If you see an error message like caper: command not found then add the following line to the bottom of ~/.bashrc and re-login.

    export PATH=$PATH:~/.local/bin
  3. Choose a backend from the following table and initialize Caper. This will create a default Caper configuration file ~/.caper/default.conf, which have only required parameters for each backend. caper init will also install Cromwell/Womtool JARs on ~/.caper/. Downloading those files can take up to 10 minutes. Once they are installed, Caper can completely work offline with local data files.

    Backend Description
    local local computer without cluster engine.
    slurm SLURM cluster.
    sge Sun GridEngine cluster.
    pbs PBS cluster.
    lsf LSF cluster.
    sherlock Stanford Sherlock (based on slurm backend).
    scg Stanford SCG (based on slurm backend).
    $ caper init [BACKEND]
  4. Edit ~/.caper/default.conf and follow instructions in there. DO NOT LEAVE ANY PARAMETERS UNDEFINED OR CAPER WILL NOT WORK CORRECTLY

Docker, Singularity and Conda

For local backends (local, slurm, sge, pbs and lsf), you can use --docker, --singularity or --conda to run WDL tasks in a pipeline within one of these environment. For example, caper run ... --singularity docker://ubuntu:latest will run each task within a Singularity image built from a docker image ubuntu:latest. These parameters can also be used as flags. If used as a flag, Caper will try to find a default docker/singularity/conda in WDL. e.g. All ENCODE pipelines have default docker/singularity images defined within WDL’s meta section (under key caper_docker or default_docker).

IMPORTANT: Docker/singularity/conda defined in Caper’s configuration file or in CLI (--docker, --singularity and --conda) will be overriden by those defined in WDL task’s runtime. We provide these parameters to define default/base environment for a pipeline, not to override on WDL task’s runtime.

For Conda users, make sure that you have installed pipeline’s Conda environments before running pipelines. Caper only knows Conda environment’s name. You don’t need to activate any Conda environment before running a pipeline since Caper will internally run conda run -n ENV_NAME COMMANDS for each task.

Take a look at the following examples:

$ caper run test.wdl --docker # can be used as a flag too, Caper will find docker image from WDL if defined
$ caper run test.wdl --singularity docker://ubuntu:latest
$ caper submit test.wdl --conda your_conda_env_name # running caper server is required

An environemnt defined here will be overriden by those defined in WDL task’s runtime. Therefore, think of this as a base/default environment for your pipeline. You can define per-task environment in each WDL task’s runtime.

For cloud backends (gcp and aws), you always need to use --docker (can be skipped). Caper will automatically try to find a base docker image defined in your WDL. For other pipelines, define a base docker image in Caper’s CLI or directly in each WDL task’s runtime.

Singularity and Docker Hub pull limit

If you provide a Singularity image based on docker docker:// then Caper will locally build a temporary Singularity image (*.sif) under SINGULARITY_CACHEDIR (defaulting to ~/.singularity/cache if not defined). However, Singularity will blindly pull from DockerHub to quickly reach a daily pull limit. It’s recommended to use Singularity images from shub:// (Singularity Hub) or library:// (Sylabs Cloud).

Important notes for Conda users

Since Caper>=2.0 you don’t have to activate Conda environment before running pipelines. Caper will internally run conda run -n ENV_NAME /bin/bash Just make sure that you correctly installed given pipeline’s Conda environment(s).

Important notes for Stanford HPC (Sherlock and SCG) users

DO NOT INSTALL CAPER, CONDA AND PIPELINE’S WDL ON $SCRATCH OR $OAK STORAGES. You will see Segmentation Fault errors. Install these executables (Caper, Conda, WDL, …) on $HOME OR $PI_HOME. You can still use $OAK for input data (e.g. FASTQs defined in your input JSON file) but not for outputs, which means that you should not run Caper on $OAK. $SCRATCH and $PI_SCRATCH are okay for both input and output data so run Caper on them. Running Croo to organize outputs into $OAK is okay.

Running pipelines on HPCs

Use --singularity or --conda in CLI to run a pipeline inside Singularity image or Conda environment. Most HPCs do not allow docker. For example, submit caper run ... --singularity as a leader job (with long walltime and not-very-big resources like 2 cpus and 5GB of RAM). Then Caper’s leader job itself will submit its child jobs to the job engine so that both leader and child jobs can be found with squeue or qstat.

Here are some example command lines to submit Caper as a leader job. Make sure that you correctly configured Caper with caper init and filled all parameters in the conf file ~/.caper/default.conf.

There are extra parameters --db file --file-db [METADATA_DB_PATH_FOR_CALL_CACHING] to use call-caching (restarting workflows by re-using previous outputs). If you want to restart a failed workflow then use the same metadata DB path then pipeline will start from where it left off. It will actually start over but will reuse (soft-link) previous outputs.

# make a separate directory for each workflow.

# Example for Stanford Sherlock
$ sbatch -p [SLURM_PARTITON] -J [WORKFLOW_NAME] --export=ALL --mem 5G -t 4-0 --wrap "caper run [WDL] -i [INPUT_JSON] --singularity --db file --file-db [METADATA_DB_PATH_FOR_CALL_CACHING]"

# Example for Stanford SCG
$ sbatch -A [SLURM_ACCOUNT] -J [WORKFLOW_NAME] --export=ALL --mem 5G -t 4-0 --wrap "caper run [WDL] -i [INPUT_JSON] --singularity --db file --file-db [METADATA_DB_PATH_FOR_CALL_CACHING]"

# Example for General SLURM cluster
$ sbatch -A [SLURM_ACCOUNT_IF_NEEDED] -p [SLURM_PARTITON_IF_NEEDED] -J [WORKFLOW_NAME] --export=ALL --mem 5G -t 4-0 --wrap "caper run [WDL] -i [INPUT_JSON] --singularity --db file --file-db [METADATA_DB_PATH_FOR_CALL_CACHING]"

# Example for SGE
$ echo "caper run [WDL] -i [INPUT_JSON] --conda --db file --file-db [METADATA_DB_PATH_FOR_CALL_CACHING]" | qsub -V -N [JOB_NAME] -l h_rt=144:00:00 -l h_vmem=3G

# Check status of leader job
$ squeue -u $USER | grep -v [WORKFLOW_NAME]

# Kill the leader job then Caper will gracefully shutdown to kill its children.
$ scancel [LEADER_JOB_ID]

How to customize resource parameters for HPCs

Each HPC backend (slurm, sge, pbs and lsf) has its own resource parameter. e.g. slurm-resource-param. Find it in Caper’s configuration file (~/.caper/default.conf) and edit it. For example, the default resource parameter for SLURM looks like the following:

slurm-resource-param=-n 1 --ntasks-per-node=1 --cpus-per-task=${cpu} ${if defined(memory_mb) then "--mem=" else ""}${memory_mb}${if defined(memory_mb) then "M" else ""} ${if defined(time) then "--time=" else ""}${time*60} ${if defined(gpu) then "--gres=gpu:" else ""}${gpu}

This should be a one-liner with WDL syntax allowed in ${} notation. i.e. Cromwell’s built-in resource variables like cpu(number of cores for a task), memory_mb(total amount of memory for a task in MB), time(walltime for a task in hour) and gpu(name of gpu unit or number of gpus) in ${}. See for WDL syntax. This line will be formatted with actual resource values by Cromwell and then passed to the submission command such as sbatch and qsub.

Note that Cromwell’s implicit type conversion (WomLong to String) seems to be buggy for WomLong type memory variables such as memory_mb and memory_gb. So be careful about using the + operator between WomLong and other types (String, even Int). For example, ${"--mem=" + memory_mb} will not work since memory_mb is WomLong type. Use ${"if defined(memory_mb) then "--mem=" else ""}{memory_mb}${"if defined(memory_mb) then "mb " else " "} instead. See for details.


See details.