Blocker

Blocker is a ridiculously simple stateless volume plugin for Docker. It makes using Amazon Elastic Block Store volumes trivial, a pretty useful thing for microservices like databases.

Blocker was designed to work with Docker Machine and Swarm and does as little as possible. It mounts and unmounts volumes, and that's about it.

To make an EBS volume accessible to a container, pass --volume-driver blocker and a -v volume mount specification to the docker run command:

docker run \
    --volume-driver blocker \
    -v <ebs-volume-id>:<container-path> \
    ...

In this example, <ebs-volume-id> is the EBS volume identifier, typically in the form vol-00000000, and <container-path> is the path within the container at which the volume will be mounted.

For example, to run a MongoDB container with a persistent volume vol-933e6c67, run this:

docker run \
    --volume-driver blocker \
    -v vol-933e6c67:/data/db \
    mongo

EBS volumes must be properly initialized before using them. Per Blocker's stance on simplicity, it doesn't attempt to do anything fancy here. This likely entails creating a filesystem, for example, since EBS creates blank volumes by default. See this handy guide for more details on how to do this.

The target volume must be in the same AWS region and availability zone as the machine running Docker. Blocker will print these out when it starts up. The daemon will automatically attach and detach volumes as necessary.

Installation

To install Blocker, just run this on the host running Docker:

wget -qO- \
    https://raw.githubusercontent.com/joeduffy/blocker/master/res/get.sh | sh

If you're running Docker Swarm, you'll want to run this on the master and agents.

The install script installs an Upstart service named blocker whose output is logged to the /var/log/upstart/blocker.log file. If all has gone well you'll see information something like this:

2015/10/25 18:07:11 blocker:starting up...
2015/10/25 18:07:11 Auto-detected EC2 information:
2015/10/25 18:07:11     InstanceId        : i-5bdf67b9
2015/10/25 18:07:11     Region            : us-west-2
2015/10/25 18:07:11     Availability Zone : us-west-2a
2015/10/25 18:07:11 Ready to go; listening on socket /var/run/blocker.sock...

Additional information for all mounting and unmounting activities is logged.

Note, AWS authentication information must be available before starting Blocker. See this guide for details on how this is done. In short, the easiest is to generate an ~/.aws/credentials file containing an aws_access_key_id and aws_secret_access_key. This is subtly different than what you get from running aws configure, though it's very close. An alternative is to export AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables, but this is a bit tricky because the Upstart process needs access to them.

Other Platforms

At present, only Linux x64 is supported as a host platform. I am open to supporting other hosts if you want to submit a pull request.

Note that supporting Mac OS X and Windows isn't necessary, since they run Docker within a Docker Machine. To use Blocker in these cases, merely provision a machine -- let's say default -- and then SSH into it and run the installation as specified above. I.e.:

$ docker machine create default --driver virtualbox
$ docker machine ssh default
<in the machine>
$ wget -qO- ... installation shown earlier ...

Other Cloud Storage Providers

I am not opposed to supporting cloud storage providers other than Amazon. In fact, the code is setup to do this fairly easily (the VolumeDriver interface simply needs multiple implementations). If you want to contribute this, I'm happy to accept pull requests, so long as it doesn't complicate the original intent of keeping this driver as simple as possible.