setup-ucacher

This action installs Earthly ucacher.

Universal caching with ucacher

ucacher is a CLI tool that tracks which files are accessed by commands, skips unnecessary executions, and restores cached outputs when possible, resulting in significantly reduced execution times.

Quick start

Install and configure ucacher

Add ucacher to your GitHub Actions workflow:

steps:
- uses: earthly/setup-ucacher@main
  ...

Cache your commands

To cache a command, simply wrap it with ucacher:

steps:
  ...
  - run: ucacher <your_command>

• First Run: The command runs as usual, with a slight overhead to upload output files.
• Subsequent Runs: ucacher detects if the command’s output is unaffected by files changed. If so, it skips execution and restores the cached output files instantly.

How ucacher works

ucacher utilizes ptrace to monitor files read (inputs) and written (outputs) by a command. When the command completes successfully, it uploads output files to persistent storage (e.g., GitHub Actions cache) along with some build metadata, in particular the hashes of all input files at the time they were read.

On subsequent runs, ucacher checks for matching initial conditions (input files content, arguments, environment variables, system architecture, etc.). If they match, it skips execution and restores the cached output files instead.

Example

Suppose the following steps on a node-js project, that run the unit tests of the client and server application components:

steps:
  ...
- uses: earthly/setup-ucacher@main
  ...
  - run: ucacher yarn test --client
  - run: ucacher yarn test --server

and the workflow already run for that branch. Now, if a change in server-impl.js is pushed, then:

• ucacher yarn test --client would return the output from a compatible past execution right away, since that file is not used in the client tests.
• ucacher yarn test --server would execute or return a cached result, depending on whether a compatible past execution is found (command already run for that file contents).

The reason why ucacher determines that this file isn’t used is indirect, not by explicitly checking it, but by finding a previous execution whose input file contents that match the current filesystem state. This indicates that this file (or potentially other files) is irrelevant to this specific command instance.

Customizing caching with ignored files and environment variables

To improve caching performance, certain files and environment variables can be ignored so that changes to them won’t prevent cache hits.

.ucacherignore

Place a .ucacherignore file in the root directory to define file patterns to ignore. Each line is a glob pattern that ucacher will use to filter out files.

.git/**/*
node_modules/**/*
/tmp/**/*

.ucacherignore.env

Define environment variable patterns to ignore in .ucacherignore.env:

GITHUB_TOKEN
ACTIONS_RUNTIME_TOKEN
GITHUB_PATH
GITHUB_OUTPUT
GITHUB_ENV

Complete example here: .ucacherignore.env

This prevents environment changes that don’t impact command output from invalidating the cache.

Comparison with actions/cache

actions/cache is the official GitHub Action designed to cache dependencies and build outputs in GitHub Actions workflows. Here it is how they compare to each other:

Feature	actions/cache	ucacher
API	File-based	Command-based
Skipping	Manual via cache-hit	Automatic
Output restore	Not supported	Automatic
Persistence	GitHub cache	GitHub cache, S3, Minio (upcoming)

• actions/cache: Primarily for dependency caching, requiring explicit file lists. Often used to avoid downloading dependencies multiple times, saving network and compute resources.
• ucacher: Like actions/cache, it can cache dependencies (for example: ucacher npm install), but it can also be used to cache the execution of any other command and restore its outputs, so successive commands depending on these files are not affected.

Best practices

1. Split commands: Divide large commands (e.g., yarn test) into smaller shards to maximize cache hit chances when only a subset of source files change.
2. Disable daemon modes: If tools support daemon modes, consider disabling them for ucacher to track inputs and outputs accurately.
3. Use ignore files: For maximum cache efficiency, use .ucacherignore and .ucacherignore.env to exclude irrelevant files and variables from tracking.

Limitations

• OS support: Only Linux runners supported.
• Architecture support: Currently only supports amd64.
• Inter-Process communication: External processes not part of the spawned command tree (like daemon or client-server setups) won’t have their file interactions tracked.

Licensing and privacy

• License: ucacher is free to use; it may become open-source in the future.
• Data collection: ucacher collects limited runtime metrics for maintenance, such as repository and organization names, commit author, and ucacher-specific errors. Source code or environment data is never collected.

Who is using ucacher?

(in chronological order)

1. First spot available!
2. ...

Using ucacher? Send a PR adding your repo at the end of the previous list

Feedback

Please give us feedback to keep improving it by opening an issue in this repo. In particular, we'd like to know:

• Is it useful for you? Share some metrics with us
• Is performance not as good as you’d like?
• Is ucacher missing some file accesses?
• Which feature would you like us to work next?
• ...

Thanks!