How do I set up linkage between Docker containers so that restarting won't break it?

Question

I have a few Docker containers running like:

• Nginx
• Web app 1
• Web app 2
• PostgreSQL

Since Nginx needs to connect to the

Answer 1

Links are for a specific container, not based on the name of a container. So the moment you remove a container, the link is disconnected and the new container (even with the same name) will not automatically take its place.

The new networking feature allows you to connect to containers by their name, so if you create a new network, any container connected to that network can reach other containers by their name. Example:

1) Create new network

$ docker network create <network-name>       

2) Connect containers to network

$ docker run --net=<network-name> ...

or

$ docker network connect <network-name> <container-name>

3) Ping container by name

docker exec -ti <container-name-A> ping <container-name-B> 

64 bytes from c1 (172.18.0.4): icmp_seq=1 ttl=64 time=0.137 ms
64 bytes from c1 (172.18.0.4): icmp_seq=2 ttl=64 time=0.073 ms
64 bytes from c1 (172.18.0.4): icmp_seq=3 ttl=64 time=0.074 ms
64 bytes from c1 (172.18.0.4): icmp_seq=4 ttl=64 time=0.074 ms

See this section of the documentation;

Note: Unlike legacy links the new networking will not create environment variables, nor share environment variables with other containers.

This feature currently doesn't support aliases

Answer 2

This is included in the experimental build of docker 3 weeks ago, with the introduction of services: https://github.com/docker/docker/blob/master/experimental/networking.md

You should be able to get a dynamic link in place by running a docker container with the --publish-service <name> arguments. This name will be accessible via the DNS. This is persistent on container restarts (as long as you restart the container with the same service name that is of course)

Answer 3

The effect of --link is static, so it will not work for your scenario (there is currently no re-linking, although you can remove links).

We have been using two different approaches at dockerize.it to solve this, without links or ambassadors (although you could add ambassadors too).

1) Use dynamic DNS

The general idea is that you specify a single name for your database (or any other service) and update a short-lived DNS server with the actual IP as you start and stop containers.

We started with SkyDock. It works with two docker containers, the DNS server and a monitor that keeps it updated automatically. Later we moved to something more custom using Consul (also using a dockerized version: docker-consul).

An evolution of this (which we haven't tried) would be to setup etcd or similar and use its custom API to learn the IPs and ports. The software should support dynamic reconfiguration too.

2) Use the docker bridge ip

When exposing the container ports you can just bind them to the docker0 bridge, which has (or can have) a well known address.

When replacing a container with a new version, just make the new container publish the same port on the same IP.

This is simpler but also more limited. You might have port conflicts if you run similar software (for instance, two containers can not listen on the 3306 port on the docker0 bridge), etcétera… so our current favorite is option 1.

Answer 4

You can bind the connection ports of your images to fixed ports on the host and configure the services to use them instead.

This has its drawbacks as well, but it might work in your case.

Answer 5

You could also try the ambassador method of having an intermediary container just for keeping the link intact... (see https://docs.docker.com/articles/ambassador_pattern_linking/ ) for more info

Answer 6

You may use dockerlinks with names to solve this.

Most basic setup would be to first create a named database container :

$ sudo docker run -d --name db training/postgres

then create a web container connecting to db :

$ sudo docker run -d -P --name web --link db:db training/webapp python app.py

With this, you don't need to manually connect containers with their IP adresses.