Caching

Caching in Omnibus builds are performed via Barista::Behaviors::Omnibus::CacheCallbacks

These callbacks allow the build to perform agnostic caching strategies. They can be configured to use a file system, network, or other strategies.

It works by providing executable blocks to #fetch and #update that should update and fetch artifacts to and from the cache.

Note

As Barista is not aware of the caching strategy, the callback blocks need to return a boolean to note if the block succeeded or failed

Info

Caching is not on by default.

To employ caching, pass true to Barista::Behaviors::Omnibus::Project#cache, and callbacks to each task when initializing them.

Example (Filesystem cache)

callbacks = Barista::Behaviors::Omnibus::CacheCallbacks.new

# Assuming the cache is located at `/cache`
# Copy the cached archive to a temporary directory
# and unpack it to the staging directory
callbacks.fetch do |cacher|
  dir = Dir.tempdir
  cache_path = File.join("/cache", cacher.filename)
  begin
    if File.exists?(cache_path)
      FileUtils.cp_r(cache_path, dir)
      cacher.unpack(File.join(dir, cacher.filename))
    else
      false
    end
  rescue ex
    false
  end
end

# ensure the cache directory exists
# and copy the archive to it
# `path` points to the staged aritfact archive
callbacks.update do |task, path|
  FileUtils.mkdir_p("/cache")
  FileUtils.cp(path, "/cache/#{task.tag}.tar.gz")
  true
end

Note

The artifact will always be a .tar.gz.

cacher.filename is the same as task.tag, except with the .tar.gz extension.

Barista::Behaviors::Omnibus::Cacher

Barista::Behaviors::Omnibus::Cacher is a helper object that is passed as a block parameter to the fetch calllback.

It's primary export is an unpack method, which takes a path to an archived artifact, and extracts it to the appropriate directory.

Cache busting

The cache tag (and cacher.filename) is made up of the following

• A custom prefix declared on the project (defaults to the project name)
• The name of the task
• The shasum of the task

Barista::Behaviors::Omnibus::Project#cache_tag_prefix can be set to configure the cache name according to project, platform, architecture, or a custom behavior.

Barista::Behaviors::Omnibus::Task#shasum returns a hexdigest of the task. This digest is computed from

• The version of the task (if applicable)
• The source URI of the task
• The text content of any block commands in the task (including comments)
• The license and any license files for the task
• The digest of all the commands configured on the task
• The project's shasum
• The shasum of all the task's upstream dependencies

Info

By including the shasum of a task's upstream dependencies in the cache tag, we can ensure that the task will be rebuilt if an upstream changes, without blocking any other tasks whose upstreams did not change.

Uncached tasks

It is common to not cache certain tasks. To avoid using a cache for a task, pass false to Barista::Behaviors::Omnibus::Task#cache(val)