Custom Bootstrap Actions - Amazon ParallelCluster
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Custom Bootstrap Actions

Amazon ParallelCluster can run arbitrary code either before (pre-install) or after (post-install) the main bootstrap action when the cluster is created. In most cases, this code is stored in Amazon Simple Storage Service (Amazon S3) and accessed through an HTTPS connection. The code is run as root and can be in any script language that's supported by the cluster OS. Often the code is in Bash or Python.

Pre-install actions are called before any cluster deployment bootstrap action is started, such as configuring NAT, Amazon Elastic Block Store (Amazon EBS) or the scheduler. Some pre-install actions include modifying storage, adding extra users, and adding packages.

Post-install actions are called after the cluster bootstrap processes are complete. Post-install actions serve the last actions to occur before an instance is considered fully configured and complete. Some post-install actions include changing scheduler settings, modifying storage, and modifying packages.

You can pass argument to scripts by specifying them during configuration. For this, you pass them double-quoted to the pre-install or post-install actions.

If a pre-install or post-install action fails, the instance bootstrap also fails. Success is signaled with an exit code of zero (0). Any other exit code indicates the instance bootstrap failed.

You can differentiate between running head and compute nodes. Source the /etc/parallelcluster/cfnconfig file and evaluate the cfn_node_type environment variable that have a value of "MasterServer" and "ComputeFleet" for the head and compute nodes, respectively.

#!/bin/bash . "/etc/parallelcluster/cfnconfig" case "${cfn_node_type}" in MasterServer) echo "I am the head node" >> /tmp/head.txt ;; ComputeFleet) echo "I am a compute node" >> /tmp/compute.txt ;; *) ;; esac


The following configuration settings are used to define pre-install and post-install actions and arguments.

# URL to a preinstall script. This is run before any of the boot_as_* scripts are run # (no default) pre_install = https://<bucket-name> # Arguments to be passed to preinstall script # (no default) pre_install_args = argument-1 argument-2 # URL to a postinstall script. This is run after any of the boot_as_* scripts are run # (no default) post_install = https://<bucket-name> # Arguments to be passed to postinstall script # (no default) post_install_args = argument-3 argument-4


The first two arguments — $0 and $1 — are reserved for the script name and url.

$0 => the script name $1 => s3 url $n => args set by pre/post_install_args


The following steps create a simple post-install script that installs the R packages in a cluster.

  1. Create a script.

    #!/bin/bash echo "post-install script has $# arguments" for arg in "$@" do echo "arg: ${arg}" done yum -y install "${@:2}"
  2. Upload the script with the correct permissions to Amazon S3. If public read permissions aren't appropriate for you, use either s3_read_resource or s3_read_write_resource parameters to grant access. For more information, see Working with Amazon S3.

    $ aws s3 cp --acl public-read /path/to/ s3://<bucket-name>/

    If the script was edited on Windows, line endings must be changed from CRLF to LF before the script is uploaded to Amazon S3.

  3. Update the Amazon ParallelCluster configuration to include the new post-install action.

    [cluster default] ... post_install = https://<bucket-name> post_install_args = 'R curl wget'

    If the bucket doesn't have public-read permission, use s3 as the URL protocol.

    [cluster default] ... post_install = s3://<bucket-name>/ post_install_args = 'R curl wget'
  4. Launch the cluster.

    $ pcluster create mycluster
  5. Verify the output.

    $ less /var/log/cfn-init.log 2019-04-11 10:43:54,588 [DEBUG] Command runpostinstall output: post-install script has 4 arguments arg: s3://<bucket-name>/ arg: R arg: curl arg: wget Loaded plugins: dkms-build-requires, priorities, update-motd, upgrade-helper Package R-3.4.1-1.52.amzn1.x86_64 already installed and latest version Package curl-7.61.1-7.91.amzn1.x86_64 already installed and latest version Package wget-1.18-4.29.amzn1.x86_64 already installed and latest version Nothing to do