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In Docker 1.8 the Docker Engine gained support for volume drivers, and networking drivers followed in Docker 1.9. This is now the preferred way to extend Docker's functionality with third-party plugins.

Powerstrip is now deprecated, and no longer under development by ClusterHQ, however we will continue to review and merge patches submitted by the community.

The goals of Powerstrip have been achieved, and we would like to thank everyone at Docker and the wider ecosystem who contributed to the Docker extensions projects.

Powerstrip: A tool for prototyping Docker extensions


At ClusterHQ we are participating in the ongoing effort in the Docker community to add an extensions API to Docker. You can join this effort at #docker-extensions on Freenode.

While this work is ongoing there is interest from the community to start prototyping extensions today. Enter Powerstrip.

What is Powerstrip?

Powerstrip is implemented as a configurable, pluggable HTTP proxy for the Docker API which lets you plug multiple Docker extension prototypes into the same Docker daemon.

For example, you can have a storage adapter (e.g. Flocker) running alongside a networking adapter (e.g. Weave), all playing nice with your choice of orchestration framework.

Crucially for the community, this immediately enables composition of prototypes of Docker extensions.

This is intended to allow quick prototyping, in order to figure out which integration points are needed in order to turn such prototypical adapters into real Docker extensions.

How it works

Powerstrip works by implementing chained blocking webhooks to arbitrary Docker API calls.

This is inspired by moby/moby#6982.

A note on nomenclature: we are calling the things that plug into the powerstrip "adapters" because it works with the metaphor, and may help disambiguate Powerstrip adapters from the Docker extensions they are prototyping.

Target audience

The target audience of this project is folks who to want to write Docker extensions, not end users. See the Powerstrip adapters section below for a list of adapters that you can use with Docker.

Goal of project

It should eventually be possible to run, for example, a Powerstrip-enabled Docker Swarm with Flocker and Weave both loaded as extensions.

version: 1
  "POST /*/containers/create":
    pre: [flocker, weave]
  "POST /*/containers/*/start":
    post: [weave]
  weave: http://weave/extension
  flocker: http://flocker/flocker-adapter

This example might allow an orchestration framework to move (reschedule) stateful containers while their Weave IP and Flocker volumes move around with them.

The Powerstrip configuration file can match any of the Docker API endpoints.

This enables you to modify any of the Docker behaviour and means Powerstrip will adapt easily to future changes in the Docker HTTP API.

Try it out

Powerstrip ships as a Docker image, and adapters can be any HTTP endpoint, including other linked Docker containers.

Slowreq is a trivial Powerstrip adapter (container) which adds a 1 second delay to all create commands.

Try it out like this (assuming logged into an Ubuntu Docker host).

If you are using boot2docker, drop the sudo and also unset DOCKER_TLS_VERIFY.

$ cd ~/
$ mkdir powerstrip-demo
$ cat > powerstrip-demo/adapters.yml <<EOF
version: 1
  "POST /*/containers/create":
    pre: [slowreq]
  slowreq: http://slowreq/slowreq-adapter

$ sudo docker run -d --name powerstrip-slowreq \
           --expose 80 \
$ sudo docker run -d --name powerstrip \
           -v /var/run/docker.sock:/var/run/docker.sock \
           -v $PWD/powerstrip-demo/adapters.yml:/etc/powerstrip/adapters.yml \
           --link powerstrip-slowreq:slowreq \
           -p 2375:2375 \

# Note how the second command takes a second longer than the first.
$ time sudo docker run ubuntu echo hello
$ time DOCKER_HOST=localhost:2375 docker run ubuntu echo hello

Warning: Powerstrip exposes the Docker API unprotected on port 2375. Only use it in private, secure development environments.

Issues: If you are using SELinux and having some issues, disable it or run the following commands:

$ sudo grep docker /var/log/audit/audit.log | audit2allow -M mypol
$ sudo semodule -i mypol.pp

Powerstrip adapters

This is a list of current and upcoming Powerstrip adaptors. Submit a pull request to add yours:

Read on for detailed info on writing your own adapter.

Writing an adapter

A adapter is just a single HTTP POST API endpoint. Use your favourite framework and language to write it.

Pre-hook adapter endpoints receive POSTs like this

Pre-hooks get called when the client has sent a request to the proxy, but before that request is passed through to the Docker daemon. This gives the adapter the opportunity to modify or delay the request.

POST /adapter HTTP/1.1
Content-type: application/json
Content-length: ...

    PowerstripProtocolVersion: 1,
    Type: "pre-hook",
    ClientRequest: {
        Method: "POST",
        Request: "/v1.16/container/create",
        Body: "{ ... }" or null

And they respond with:

Content-type: application/json

    PowerstripProtocolVersion: 1,
    ModifiedClientRequest: {
        Method: "POST",
        Request: "/v1.16/container/create",
        Body: "{ ... }" or null

So that, for example, they can rewrite a GET request string, or modify the JSON in a POST body.

Alternatively, pre-hooks can respond with an HTTP error code, in which case the call is never passed through to the Docker daemon, and instead the error is returned straight back to the client.

Pre-hooks must not change the scope of which endpoint is being matched - rewriting the Request should only be used for modifying GET arguments (e.g. after a '?' in the URL).

Post-hook adapter endpoints receive POSTs like this

Post-hooks get called after the response from Docker is complete but before it has been sent back to the client. Both the initial request and the Docker response are included in the POST body.

Plugins thus get a chance to modify or delay the response from Docker to the client.

POST /adapter HTTP/1.1

    PowerstripProtocolVersion: 1,
    Type: "post-hook",
    ClientRequest: {
        Method: "POST",
        Request: "/v1.16/containers/create",
        Body: "{ ... }"
    ServerResponse: {
        ContentType: "text/plain",
        Body: "{ ... }" response string
                        or null (if it was a GET request),
        Code: 404

The adapter responds with:

    PowerstripProtocolVersion: 1,
    ModifiedServerResponse: {
        ContentType: "application/json",
        Body: "{ ... }",
        Code: 200

This gives the post-hook a chance to convert a Docker error into a success if it thinks it can.


Both pre- and post-hooks can be chained: the response from the N'th hook is passed in as the request to the N+1'th in list order according to the YAML configuration.

If any hook returns an HTTP error response, the rest of the chain is cancelled, and the error returned to the client. You can think of this like Twisted Deferred chains where hooks are like callbacks. The exception to this is when the Docker API returns an error: the post-hooks are still run in that case, because we thought adapter authors would like to know about Docker error messages.

Defining Endpoints

Endpoints are defined using UNIX shell-like globbing. The request POST /v1.16/container/create would be matched by all of the following endpoint definitions:

  • POST /v1.16/containers/create
  • POST /v1*/containers/create
  • POST /*/containers/create
  • POST /*/*/create
  • * /*/containers/create
  • POST /v[12]/containers/create

Note: Query arguments are stripped for matching purposes.

Any of the Docker endpoints can be matched - so for example the following routes are perfectly valid:

  • POST /*/containers/create
  • POST /*/containers/*/start
  • POST /*/containers/*/stop
  • POST /*/containers/*/kill

A useful resource when defining your endpoints is the Docker remote API documentation


Powerstrip does not support, and will silently skip over certain types of hooks in the following cases:

  • pre-hooks for request bodies with content-types other than application/json, such as build contexts POSTed in the build API call.
  • post-hooks for responses with content-type application/vnd.docker.raw-stream, such as "hijacked" responses in the attach API call.

For responses that are streamed back from the Docker daemon without proper framing (such as build and pull API call responses):

  • if post-hooks are not added:
    • responses will be streamed to the client as they come in from the Docker daemon.
  • otherwise, if post-hooks are added, then:
    • responses will be buffered and then delivered to the post-hook chain as a single body.

Recommended deployment

For now, Powerstrip does not support TLS, but given that it should only be used for prototyping in local development environments, that's OK.

It's recommended that adapters run in containers that are linked (with Docker links) to the proxy container. Plugins should listen on port 80.

Then you can just specify the URL using e.g. http://adapter/, assuming "adapter" is the link alias. (See example under "Try it out").


We'd love your help with Powerstrip. If you have any questions or need help, besides filing a GitHub issue with feature requests or bug reports you can also join us on the #clusterhq or #docker-extensions channel on the IRC network.

We plan to do CI with from for unit tests. Or maybe Travis-CI. Integration tests will exist but only get run manually for now.

Possible fates for a request

There are a few different paths that an HTTP request can take.

Here are some of them:

  • Client req => Plugin pre-hook returns OK => Docker => Plugin post-hook => Client response
  • Client req => Plugin pre-hook returns error code => error response to client (don't pass through request to Docker)
  • Client req => Plugin pre-hook => Docker => Error response from Docker to adapter post-hook => Pass through error response to client
  • Client req => Plugin pre-hook => Docker => Plugin post-hook => error response to client

Possible improvements

  • A Continue response argument could be added to allow chain cancellation with a non-error response.
  • Verbose logging (to stdout) as an optional argument/yaml configuration flag, to help adapter authors debugging adapters.
    • Define the logging/traceability story (adapters and powerstrip log to stdout?).
  • A public list of all known Powerstrip hooks (GitHub links + Docker Hub names).
  • Version the webhooks and the configuration.
  • Publish standard testing framework for adapters.
  • Expose headers as well as (instead of) just content-type. For both pre and post-hooks.
  • Run all the hooks in case of an error condition, do give them a chance to unwind things.
  • Have an explicit "unwinder" hook-type for pre-hooks, to differentiate error-handling post-hooks from regular post-hooks.


Fixed in master:

  • Add integration tests against real Docker for run, build and pull, fix various bugs exposed therein.
  • In particular, fix docker attach, streaming responses when there are no post-hooks, GET requests, skip pre-hooks with application/tar handling, stdin handling for attach.


  • Initial release

Additional Adapter Ideas

  • A post hook for containers => start that will block until the container is fully connected to the weave bridge
  • A pre hook for containers => create that will inject ENV variables loaded from consul or etcd
  • A post hook for containers => {start,stop} that will update consul or etcd with the containers exposed endpoints


Copyright 2015 ClusterHQ, Inc

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


Powerstrip: A tool for prototyping Docker extensions







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