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Rocket - App Container runtime

Build Status

Release early, release often: Rocket is currently a prototype and we are seeking your feedback via pull requests

Rocket is a cli for running App Containers. The goal of rocket is to be composable, secure, and fast.

Read more about Rocket in the launch announcement.

Rocket Logo

Trying out Rocket

rkt is currently supported on amd64 Linux. We recommend booting up a fresh virtual machine to test out rocket.

To install the rkt binary:

curl -L -o rocket-v0.1.0.tar.gz
tar xzvf rocket-v0.1.0.tar.gz
cd rocket-v0.1.0
./rkt help

Keep in mind while running through the examples that rkt needs to be run as root for most operations.

Rocket basics

Downloading an App Container Image (ACI)

Rocket uses content addressable storage (CAS) for storing an ACI on disk. In this example, the image is downloaded and added to the CAS.

$ rkt fetch

These files are now written to disk:

$ find /var/lib/rkt/cas/blob/

Per the App Container spec the SHA-256 is of the tarball, which is reproducible with other tools:

$ wget
$ gzip -dc etcd-v0.5.0-alpha.4-linux-amd64.aci > etcd-v0.5.0-alpha.4-linux-amd64.tar
$ sha256sum etcd-v0.5.0-alpha.4-linux-amd64.tar
f9215c18b86f406c7cec4c7b45fd8752b5bfd1a492507d647821c2ce593fbf31  etcd-v0.5.0-alpha.4-linux-amd64.tar

Launching an ACI

To run an ACI, you can either use the SHA-256 hash, or the URL which you downloaded it from:

$ rkt run
Press ^] three times to kill container.

rkt will do the appropriate ETag checking on the URL to make sure it has the most up to date version of the image.

Or, you can explicitly choose an image to run based on the SHA-256:

$ rkt run sha256-f9215c18b86f406c7cec4c7b45fd8752b5bfd1a492507d647821c2ce593fbf31
Press ^] three times to kill container.

These commands are interchangeable.

The escape character ^] is generated by Ctrl-] on a US keyboard, on other keyboard layouts that will differ (for example the Swedish keyboard layout where Ctrl-å on OS X or Ctrl-^ on Windows will generate the correct escape character).

App Container basics

App Container is a specification of an image format, runtime, and discovery protocol for running a container. We anticipate app container to be adopted by other runtimes outside of Rocket itself. Read more about it here.

To validate the rkt with the App Container validation ACIs run:

$ rkt run -volume database:/tmp \ \

Rocket internals

Rocket is designed to be modular and pluggable by default. To do this we have a concept of "stages" of execution of the container.

Execution with Rocket is divided into a number of distinct stages. The motivation for this is to separate the concerns of initial filesystem setup, execution environment, and finally the execution of the apps themselves.

Stage 0

The first step of the process, stage 0, is the actual rkt binary itself. This binary is in charge of doing a number of initial preparatory tasks:

  • Generating a Container UUID
  • Generating a Container Runtime Manifest
  • Creating a filesystem for the container
  • Setting up stage 1 and stage 2 directories in the filesystem
  • Copying the stage1 binary into the container filesystem
  • Fetching the specified ACIs
  • Unpacking the ACIs and copying each app into the stage2 directories

Given a run command such as:

rkt run --volume bind:/opt/tenant1/database \
	sha256-8a30f14877cd8065939e3912542a17d1a5fd9b4c \

a container manifest compliant with the ACE spec will be generated, and the filesystem created by stage0 should be:



  • container is the container manifest file
  • stage1 is a copy of the stage1 filesystem that is safe for read/write
  • stage1/init is the actual stage1 binary to be executed
  • stage1/opt/stage2 are copies of the unpacked ACIs

At this point the stage0 execs /stage1/init with the current working directory set to the root of the new filesystem.

Stage 1

The next stage is a binary that the user trusts to set up cgroups, execute processes, and other operations as root. This stage has the responsibility to take the execution group filesystem that was created by stage 0 and create the necessary cgroups, namespaces and mounts to launch the execution group:

  • Generate systemd unit files from the Application and Container Manifests (containing, respectively, the exec specifications of each container and the ordering given by the user)
  • Set up any external volumes (undefined at this point)
  • nspawn attaching to the bridge and launch the execution group systemd
  • Launch the root systemd
  • Have the root systemd

This process is slightly different for the qemu-kvm stage1 but a similar workflow starting at exec()'ing kvm instead of an nspawn.

Stage 2

The final stage is executing the actual application. The responsibilities of the stage2 include:

  • Launch the init process described in the Application Manifest


App Container runtime







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