September 15, 2013

Running Redis 2.6 on Docker


With the end of my free tier eligibility looming on AWS, I took advantage of the Rackspace developer discount and set up a new account and personal server this weekend. One of the technologies I’ve been most interested in recently is Docker, a container engine for Linux which aims to be the intermodal shipping container for applications and their dependencies. A brand-new box seemed like the perfect time to dig in to it.

To get a sense of what Docker is and how it works, I’d recommend going through the getting started tutorial as well as some of the examples in the documentation. However, here’s a very brief rundown:

  • Docker uses LXC and a union file system (AUFS) to run isolated containers (not VMs).
  • Software and its dependencies are packaged into an image.
  • Images are immutable and stateless.
  • Images can be committed in layers to form more complex images.
  • An image running a process is called a container, which is stateful.
  • A container exists as running or stopped, and can be run interactively or in the background.
  • Images are lightweight, perhaps 100Mb for a Redis server running on Ubuntu.
  • Containers are not virtual machines, so they are lightening-fast to boot and lightweight on resources.

One of the documentation examples describes setting up a Redis service. Following the example was straightforward, but I felt it was missing two things when I was finished. First, it uses Redis 2.4, which is already quite out of date (as of this writing, Redis 2.8 is nearing release). Plus, it felt awkward having to specify a lengthy command and config file each time the container started.

Installing Redis 2.6

The first thing to do is start a container from a base image, in this case the ubuntu image pulled during setup:

sudo docker run -i -t ubuntu /bin/bash

This results in a root shell on a new, running Ubuntu container. A few things will be needed in order to download, build and test Redis:

apt-get install wget build-essential tcl8.5

To install Redis 2.6, it’s necessary to build it from source:

wget get
tar xvf redis-2.6.16.tar.gz
cd redis-2.6.16

Build it, run some tests and install once completed:

make test
make install

Lastly, move the config file to a more standard location:

mkdir /etc/redis
mv redis.conf /etc/redis/redis.conf

Verify the server is working by running the following:

redis-server /etc/redis/redis.conf

Run commands and images

In the example, the resulting container is committed to an image, and then run:

sudo docker commit <container_id> crsmithdev/redis
sudo docker run -d -p 6379 crsmithdev/redis2.6 /usr/bin/redis-server /etc/redis/redis.conf

This is still a bit clunky — why do redis-server and the config file have to be specified each time it’s run? Forunately, they can be built into the image itself:

sudo docker commit -run='{"Cmd":["/usr/local/bin/redis-server", "/etc/redis/redis.conf"]}' \
    <container_id> crsmithdev/redis2.6

That way, you can run the container like so:

sudo docker run -d -p 6379:6379 crsmithdev/redis2.6

Specifying -p 6379:6379 ensures that the server’s port 6379 is mapped to the container’s port 6379. Otherwise, Docker will assign a random local server port in the 49000s, which is probably unwanted in most non-development environments.

Note that it is still possible to override the image-specified run command. The following will open a shell using the image, instead of launching Redis:

sudo docker run -i -t crsmithdev/redis2.6 /bin/bash

Handling data

One important point: what about the data and log files that result from the Redis process? Every time I run Redis from that image, I get a new container with fresh data. That’s ideal for some situations, but less so for for others: in a production environment, it’s entirely possible I’d want to be able to start a new Redis container, but be able to load a dumpfile from a previous one.

Fortunately, you can share one or more volumes with the host server easily. Modify redis.conf on the container to specify a dumpfile location of your choice:

dir /data/redis

Then, run the image specifying a mount point on the server:

sudo docker run -d -p 6379:6379 -v /mnt/redis:/data/redis:rw crsmithdev/redis2.6

Connecting via redis-cli and executing SAVE should result in a dump.rdb file in /mnt/redis. Redis will be logging to stdout unless specified otherwise, so the logs are viewable using a Docker command:

sudo docker logs <container_d>

If you specify a different logfile location in redis.conf, it’s possible to add a second volume to the run command.


And that’s it. This image can then be downloaded and started, producing a running, fully-functional Redis server in literally a few seconds.

You can grab my image here.

UPDATE 9-15 - updated container run arguments, and added a bit about volumes.