To connect to a source container you use the --link <container-name|id>:<alias> option when launching a new container. The container name refers to the source container we defined in the previous step while the alias defines the friendly name of the host.

By setting an alias we separate how our application is configured to how the infrastructure is called. This means the application configuration doesn't need to change as it's connected to other environments.

How Links Work

In this example, we bring up a Alpine container which is linked to our redis-server. We've defined the alias as redis. When a link is created, Docker will do two things.

First, Docker will set some environment variables based on the linked to the container. These environment variables give you a way to reference information such as Ports and IP addresses via known names.

You can output all the environment variables with the env command. For example:

docker run --link redis-server:redis alpine env

Secondly, Docker will update the HOSTS file of the container with an entry for our source container with three names, the original, the alias and the hash-id. You can output the containers host entry using cat /etc/hosts

docker run --link redis-server:redis alpine cat /etc/hosts

Example

With a link created you can ping the source container in the same way as if it were a server running in your network.

docker run --link redis-server:redis alpine ping -c 1 redis

With a link created, applications can connect and communicate with the source container in the usual way, independent of the fact both services are running in containers.

Example Application

Here is a simple node.js application which connects to redis using the hostname redis.

docker run -d -p 3000:3000 --link redis-server:redis katacoda/redis-node-docker-example

Test Connection

Sending an HTTP request to the application will store the request in Redis and return a count. If you issue multiple requests, you'll see the counter increment as items are persisted.

curl docker:3000

In the same way, you can connect source containers to applications, you can also connect them to their own CLI tools.

Launching Redis CLI

The command below will launch an instance of the Redis-cli tool and connect to the redis server via it's alias.

docker run -it --link redis-server:redis redis redis-cli -h redis

Issuing Commands

The command KEYS * will output the contents stored currently in the source redis container.

Type QUIT to exit the CLI.