How do links and scaling work together in docker compose?

Question

Here's my understanding of docker compose:

• You can "scale" a service to run it in multiple containers at once.
• If you link service A to service B, service A has access to a container running service B.

Is my understanding correct, and if so, where does a link connect if there are multiple containers running the service?

Answer 1

First of all I would clarify that, by default, with or without linking a container with other, all container has visibility to other containers running in the same host (using the container IP). You can change this behavior using the icc=true flag in docker daemon.

In respect of the links with docker-compose, these are generated when the container with the links are created. Let's see it with an example. Using this docker-compose.yml

web:   build: .   command: python app.py   ports:    - "5000:5000"   volumes:    - .:/code   links:    - redis redis:   image: redis 

After running docker-compose up -d the web_1 container is linked with the container with redis_1:

        (...)         "Links": [         "/compose_redis_1:/compose_web_1/compose_redis_1",         "/compose_redis_1:/compose_web_1/redis",         "/compose_redis_1:/compose_web_1/redis_1"     ], (...) 

Now we want to scale the redis service using docker-compose scale redis=2. After running it (and create a new container redis_2), the links in web_1 keeps unchanged.

        (...)         "Links": [         "/compose_redis_1:/compose_web_1/compose_redis_1",         "/compose_redis_1:/compose_web_1/redis",         "/compose_redis_1:/compose_web_1/redis_1"     ], (...) 

It is needed to stop, remove and run web_1 to see these links created:

docker-compose stop web docker-compose rm web docker-compose run -d web docker inspect compose_web_run_2 (...)        "Links": [         "/compose_redis_1:/compose_web_run_2/compose_redis_1",         "/compose_redis_2:/compose_web_run_2/compose_redis_2",         "/compose_redis_1:/compose_web_run_2/redis",         "/compose_redis_1:/compose_web_run_2/redis_1",         "/compose_redis_2:/compose_web_run_2/redis_2"     ],(...) 

And the /etc/hosts of web_1 container:

172.17.0.24 7be2dabea910 127.0.0.1   localhost ::1 localhost ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters 172.17.0.21 compose_redis_1 8a1297a5b3e4 172.17.0.23 compose_redis_2 069dd46836aa 172.17.0.21 redis 8a1297a5b3e4 compose_redis_1 172.17.0.21 redis_1 8a1297a5b3e4 compose_redis_1 172.17.0.23 redis_2 069dd46836aa compose_redis_2 

So to generate the new links, you is is needed to stop, remove, and run again the container.