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Cage: Develop and deploy complex Docker applications

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Does your project have too many Docker services? Too many git repos? Cage makes it easy to develop complex, multi-service applications locally. It works with standard docker-compose.yml files and docker-compose, but it helps bring order to the complexity:

  • Cage provides a standardized project structure, much like Rails did for web development.
  • Cage allows you to work with multiple source repositories, and to mix pre-built Docker images with local source code.
  • Cage removes the repetitive clutter from your docker-compose.yml files.
  • Cage provides secret management, either using a single text file or Hashicorp's Vault.

For more information about Cage, see the introductory website.


First, you need to install Docker and make sure that you have at least version 1.8.1 of docker-compose:

$ docker-compose --version
docker-compose version 1.8.1, build 878cff1

We provide pre-built cage binaries for Linux and MacOS on the release page. The Linux binaries are statically linked and should work on any modern Linux distribution. To install, you can just unzip the binaries and copy them to /usr/local/bin:

unzip cage-*.zip
sudo cp cage /usr/local/bin/
rm cage-*.zip cage

If you would like to install from source, we recommend using rustup and cargo install:

curl -sSf | sh
cargo install cage

If you have trouble using cage's vault integration, try installing with cargo instead.

Note that it's possible to build cage for Windows, but it's still not yet officially supported.

Trying it out

Create a new application using cage:

$ cage new myapp
$ cd myapp

Pull the pre-built Docker images associated with this application and start up the database pod:

$ cage pull
$ cage up db

Run the rake image to initialize your database:

$ cage run rake db:create
$ cage run rake db:migrate

And bring up the rest of the app:

$ cage up

Let's take a look at the pods and services defined by this application:

$ cage status
db enabled type:placeholder
└─ db
frontend enabled type:service
└─ web ports:3000
rake enabled type:task
└─ rake

This shows us that the web service is listening on port 3000, so you should be able to access the application at http://localhost:3000. But let's make a change! First, list the available source code for the services in this app:

$ cage source ls

Try mounting the source code for rails_hello into all the containers that use it:

$ cage source mount rails_hello
$ cage up
$ cage source ls
  Cloned at src/rails_hello (mounted)

You may also notice that since myapp_rake_1 is based on the same underlying Git repository as myapp_web_1, that it also has a mount of src/rails_hello in the appropriate location. If you change the source on your host system, it will automatically show up in both containers.

Now, create an HTML file at src/rails_hello/public/index.html:

  <head><title>Sample page</title></head>
  <body><h1>Sample page</h1></body>

And reload the website in your browser. You should see the new page!

We can also run container-specific unit tests, which are specified by the container, so that you can invoke any unit test framework of your choice:

$ cage test web

And we can access individual containers using a configurable shell:

$ cage shell web

The top-level convenience commands like test and shell make it much easier to perform standard development tasks without knowing how individual containers work.

For more information, check out cage's help:

cage --help

What's a pod?

A "pod" is a tightly-linked group of containers that are always deployed together. Kubernetes defines pods as:

A pod (as in a pod of whales or pea pod) is a group of one or more containers (such as Docker containers), the shared storage for those containers, and options about how to run the containers. Pods are always co-located and co-scheduled, and run in a shared context. A pod models an application-specific “logical host” - it contains one or more application containers which are relatively tightly coupled — in a pre-container world, they would have executed on the same physical or virtual machine.

If you're using Amazon's ECS, a pod corresponds to an ECS "task" or "service". If you're using Docker Swarm, a pod corresponds to a single docker-compose.xml file full of services that you always launch as a single unit.

Pods typically talk to other pods using ordinary DNS lookups or service discovery. If a pod accepts outside network connections, it will often do so via a load balancer.

Project format

See examples/hello for a complete example.

└── pods
    ├── common.env
    ├── frontend.yml
    └── targets
        ├── development
        │   └── common.env
        ├── production
        │   ├── common.env
        │   └── frontend.yml
        └── test
            └── common.env

File types

Other commands

cage run-script

The run-script command operates similarly to npm run <script> or rake <task>. Simply define a set of named scripts in the pod's metadata:

# tasks.yml
    build: .
# tasks.metadata.yml

        - ["npm","run","populate"]

By running cage run-script populate, cage will find all services that have a populate script and run it. You can also specify a pod or service with cage run-script tasks populate.

Reporting issues

If you encounter an issue, it might help to set the following shell variables and re-run the command:

export RUST_BACKTRACE=1 RUST_LOG=cage=debug,compose_yml=debug

Development notes

Pull requests are welcome! If you're unsure about your idea, then please feel free to file an issue and ask us for feedback. We like suggestions!

Setting up tools

When working on this code, we recommend installing the following support tools:

cargo install rustfmt
cargo install cargo-watch

We also recommend installing nightly Rust, which produces better error messages and supports extra warnings using Clippy:

rustup update nightly
rustup override set nightly

If nightly produces build errors, you may need to update your compiler and libraries to the latest versions:

rustup update nightly
cargo update

If that still doesn't work, try using stable Rust instead:

rustup override set stable

If you're using nightly, run the following in a terminal as you edit:

cargo watch "test --features clippy"

If you're using stable, leave out --features clippy:

cargo watch test

Before committing your code, run:

cargo fmt

This will automatically reformat your code according to the project's conventions. We use Travis CI to verify that cargo fmt has been run and that the project builds with no warnings. If it fails, no worries—just go ahead and fix your pull request, or ask us for help.

On macOS

The openssl crate needs a compatible version of the openssl libraries. macOS ships with a version using an old API. The best solution is to install latest openssl with homebrew and link to it:

brew install openssl
export OPENSSL_INCLUDE_DIR=$(brew --prefix openssl)/include
export OPENSSL_LIB_DIR=$(brew --prefix openssl)/lib
cargo clean   # you need to do this if your macOS-versioned openssl build failed
cargo build

Official releases

To make an official release, you need to be a maintainer, and you need to have cargo publish permissions. If this is the case, first edit Cargo.toml to bump the version number, then regenerate Cargo.lock using:

cargo build

Commit the release, using a commit message of the format:



Then run:

cargo publish
git tag v$VERSION
git push; git push --tags

This will rebuild the official binaries using Travis CI, and upload a new version of the crate to