Cache Buildkite Plugin

A Buildkite plugin to store ephemeral cache files between builds.

Often builds involve fetching and processing large amounts of data that don't change much between builds, for instance downloading npm/gem/pip/cocoapod packages from central registries, or shared compile cache for things like ccache, or large virtual machine images that can be re-used.

Buildkite recommends using Artifacts for build artifacts that are the result of a build and useful for humans, where as we see cache as being an optional byproduct of builds that doesn't need to be content addressable.

For example, caching the node_modules folder as long as the package-lock.json file does not change can be done as follows:

steps:
  - label: ':nodejs: Install dependencies'
    command: npm ci
    plugins:
      - cache#v0.4.0:
          manifest: package-lock.json
          path: node_modules
          restore: file
          save: file

Mandatory parameters

path (string)

The file or folder to cache.

At least one of the following

restore (string, specific values)

The maximum caching level to restore, if available. See the available caching levels

save (string, specific values)

The level to use for saving the cache. See the available caching levels

Options

backend (string)

Defines how the cache is stored and restored. Can be any string (see Customizable Backends), but the plugin natively supports the following:

• fs (default)
• s3

fs

Very basic local filesystem backend.

The BUILDKITE_PLUGIN_FS_CACHE_FOLDER environment variable defines where the copies are (default: /var/cache/buildkite). If you don't change it, you will need to make sure that the folder exists and buildkite-agent has the propper permissions, otherwise the plugin will fail.

IMPORTANT: the fs backend just copies files to a different location in the current agent, as it is not a shared or external resource, its caching possibilities are quite limited.

s3

Store things in an S3 bucket. You need to make sure that the aws command is available and appropriately configured.

You also need the agent to have access to the following defined environment variables:

• BUILDKITE_PLUGIN_S3_CACHE_BUCKET: the bucket to use (backend will fail if not defined)
• BUILDKITE_PLUGIN_S3_CACHE_PREFIX: optional prefix to use for the cache within the bucket

Setting the BUILDKITE_PLUGIN_S3_CACHE_ONLY_SHOW_ERRORS environment variable will reduce logging of file operations towards S3.

manifest (string, required if using file caching level)

A path to a file or folder that will be hashed to create file-level caches.

It will cause an unrecoverable error if either save or restore are set to file and this option is not specified.

Caching levels

This plugin uses the following hierarchical structure for caches to be valid (meaning usable), from the most specific to the more general:

• file: only as long as a manifest file does not change (see the manifest option)
• step: valid only for the current step
• branch: when the pipeline executes in the context of the current branch
• pipeline: all builds and steps of the pipeline
• all: all the time

When restoring from cache, all levels, in the described order, up to the one specified will be checked. The first one available will be restored and no further levels or checks will be made.

Customizable backends

One of the greatest flexibilities of this plugin is its flexible backend architecture. You can provide whatever value you want for the backend option of this plugin (X for example) as long as there is an executable script accessible to the agent named cache_X that respects the following execution protocol:

• cache_X exists $KEY

Should exit successfully (0 return code) if any previous call to this very same plugin was made with cache_x save $KEY. Any other exit code will mean that there is no valid cache and will be ignored.

• cache_X get $KEY $FILENAME

Will restore whatever was previously saved on $KEY (using the save call described next) to the file or folder $FILENAME. A non-0 exit code will cause the whole execution to halth and the current step to fail.

You can assume that all calls like this will be preceded by an exists call to ensure that there is something to get.

• cache_X save $KEY $FILENAME

Will save whatever is in the $FILENAME path (which can be a file or folder) in a way that can be identified by the string $KEY. A non-0 return code will cause the whole execution to halt and the current step to fail.

• should fail with error 255 on any instance, preferably without output

Examples

You can always have more complicated logic by using the plugin multiple times with different levels and on different steps. In the following example the node_modules folder will be saved and restored with the following logic:

• first step:
  • if the package-lock.json file has not changed, node_modules will be restored as is, run the npm install (that should do nothing because no dependencies changed), and skip saving the cache because it already exists
  • if the package-lock.json file has changed, it will restore step-level, branch-level and pipeline-level caches of the node_modules folder (the first one that exists), run npm install (that should be quick, just installing the differences), and then save the resulting node_modules folder as a file-level cache
• second step:
  • will restore the file-level cache of the node_modules folder saved by the first step and run npm test
• third step (that will only run on the master branch):
  • will restore the file-level cache saved by the first step, run npm run deploy and finally save the contents of the node_modules folder as a pipeline-level cache for usage as a basis even when the lockfile changes (in the first step)

steps:
  - label: ':nodejs: Install dependencies'
    command: npm ci 
    plugins:
      - cache#v0.4.0:
          manifest: package-lock.json
          path: node_modules
          restore: pipeline
          save: file
  - wait: ~
  - label: ':test_tube: Run tests'
    command: npm test # does not save cache, not necessary
    plugins:
      - cache#v0.4.0:
          manifest: package-lock.json
          path: node_modules
          restore: file
  - wait: ~  # don't run deploy until tests pass
  - label: ':goal_net: Save stable cache after deployment'
    if: build.branch == "master"
    command: npm run deploy
    plugins:
      - cache#v0.4.0:
          manifest: package-lock.json
          path: node_modules
          restore: file
          save: pipeline

License

MIT (see LICENSE)