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App Container and Docker

A core principle of the App Container (appc) specification is that it is open: multiple implementations of the spec should exist and be developed independently. Even though the spec is young and pre-1.0, it has already seen a number of implementations.

With this in mind, over the last few weeks we have been working on ways to make appc interoperable with the Docker v1 Image format. As we discovered, the two formats are sufficiently compatible that Docker v1 Images can easily be run alongside appc images (ACIs). Today we want to describe two different demonstrations of this interoperability, and start a conversation about closer integration between the Docker and appc communities.

rkt Running Docker Images

rkt is an App Container implementation that fully implements the current state of the spec. This means it can download, verify and run App Container Images (ACIs). And now along with ACI support the latest release of rkt, v0.3.2, can download and run container images directly from the Docker Hub or any other Docker Registry:

$ rkt --insecure-skip-verify run docker://redis docker://tenstartups/redis-commander
rkt: fetching image from docker://redis
rkt: warning: signature verification has been disabled
Downloading layer: 511136ea3c5a64f264b78b5433614aec563103b4d4702f3ba7d4d2698e22c158
      _.-``    `.  `_.  ''-._           Redis 2.8.19 (00000000/0) 64 bit
  .-`` .-```.  ```\/    _.,_ ''-._
 (    '      ,       .-`  | `,    )     Running in stand alone mode
 |`-._`-...-` __...-.``-._|'` _.-'|     Port: 6379
 |    `-._   `._    /     _.-'    |     PID: 3
[3] 12 Feb 09:09:19.071 # Server started, Redis version 2.8.19
# redis will be  running on and redis-commander on

Docker Running App Container Images

At the same time as adding Docker support to rkt, we have also opened a pull-request that enables Docker to run appc images (ACIs). This is a simple functional PR that includes many of the essential features of the image spec. Docker API operations such as image list, run image by appc image ID and more work as expected and integrate with the native Docker experience. As a simple example, downloading and running an etcd ACI works seamlessly with the addition of this patchset:

$ docker pull --format aci
$ docker run --format aci
2015/02/12 11:21:05 no data-dir provided, using default data-dir ./default.etcd
2015/02/12 11:21:05 etcd: listening for peers on http://localhost:2380
2015/02/12 11:21:05 etcd: listening for peers on http://localhost:7001

For more details, check out the PR itself.

Docker and App Container: Looking forward

We think App Container represents the next logical iteration in what a container image format, runtime engine, and discovery protocol should look like. App Container is young but we want to continue to get wider community feedback and see the spec evolve into something that can work for a number of runtimes.

Before appc spec reaches 1.0 (stable) status, we would like feedback from the Docker community on what might need to be modified in the spec in order for it to be supported natively in Docker. To gather feedback and start the discussion, we have put up a proposal to add appc support to Docker.

We are looking forward to getting additional feedback from the Docker community on this proposal. Working together, we can create a better appc spec for everyone to use, and over time, work towards a shared standard.

Join us on a mission to create a secure, composable, and standards-based container runtime. If you are interested in hacking on rkt or App Container we encourage you to get involved:


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App Container

Help Wanted

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If you want more background on the appc spec, we encourage you to read our first blog post about the App Container spec and Rocket. Also read more in a recent Q&A with