moby/docs/sources/examples/linking_into_redis.rst

title

Linking to an Redis container

description

Running redis linked into your web app

keywords

docker, example, networking, redis, link

Linking Redis

Building a redis container to link as a child of our web application.

Building the redis container

Lets build a redis image with the following Dockerfile.

git clone https://github.com/antirez/redis.git
cd redis
git checkout 2.6

# Save this Dockerfile to the root of the redis repository.  

# Build redis from source
# Make sure you have the redis source code checked out in
# the same directory as this Dockerfile
FROM ubuntu

RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
RUN apt-get update
RUN apt-get upgrade -y

RUN apt-get install -y gcc make g++ build-essential libc6-dev tcl

ADD . /redis

RUN (cd /redis && make)

RUN mkdir -p /redis-data
VOLUME ["/redis-data"]
EXPOSE 6379

ENTRYPOINT ["/redis/src/redis-server"]
CMD ["--dir", "/redis-data"]

# docker build our new redis image from source
docker build -t redis-2.6 .

We need to EXPOSE the default port of 6379 so that our link knows what ports to connect to our redis container on. If you do not expose any ports for the image then docker will not be able to establish the link between containers.

Run the redis container

docker run -d -e PASSWORD=docker -name redis redis-2.6 --requirepass docker

This will run our redis container wit the password docker to secure our service. By specifying the -name flag on run we will assign the name redis to this container. If we do not specify a name for our container via the -name flag docker will automatically generate a name for us. We can issue all the commands that you would expect; start, stop, attach, using the name for our container. The name also allows us to link other containers into this one.

Linking redis as a child

Next we can start a new web application that has a dependency on redis and apply a link to connect both containers. If you noticed when running our redis server we did not use the -p flag to publish the redis port to the host system. Redis exposed port 6379 via the Dockerfile and this is all we need to establish a link.

Now lets start our web application with a link into redis.

docker run -t -i -link redis:db -name webapp ubuntu bash

root@4c01db0b339c:/# env

HOSTNAME=4c01db0b339c
DB_NAME=/webapp/db
TERM=xterm
DB_PORT=tcp://172.17.0.8:6379
DB_PORT_6379_TCP=tcp://172.17.0.8:6379
DB_PORT_6379_TCP_PROTO=tcp
DB_PORT_6379_TCP_ADDR=172.17.0.8
DB_PORT_6379_TCP_PORT=6379
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/
DB_ENV_PASSWORD=docker
SHLVL=1
HOME=/
container=lxc
_=/usr/bin/env
root@4c01db0b339c:/#

When we inspect the environment of the linked container we can see a few extra environment variables have been added. When you specified -link redis:db you are telling docker to link the container named redis into this new container with the alias db. Environment variables are prefixed with the alias so that the parent container can access network and environment information from the containers that are linked into it.

# The name of the child container
DB_NAME=/webapp/db

# The default protocol, ip, and port of the service running in the container
DB_PORT=tcp://172.17.0.8:6379

# A specific protocol, ip, and port of various services
DB_PORT_6379_TCP=tcp://172.17.0.8:6379
DB_PORT_6379_TCP_PROTO=tcp
DB_PORT_6379_TCP_ADDR=172.17.0.8
DB_PORT_6379_TCP_PORT=6379

# Get environment variables of the container 
DB_ENV_PASSWORD=docker

Accessing the network information along with the environment of the child container allows us to easily connect to the redis service on the specific ip and port and use the password specified in the environment.