Docker Networking - to link or not to link? - docker

Question

We have 1000s of python unit tests and to run them efficiently we parallelize them in batches. Each batch has its own Docker environment consisting of the core application and a mongo instance. It's setup something like this:
docker network create --driver bridge ut_network-1
docker network create --driver bridge ut_network-2
docker run -d --name mongo-unittest-1 --network ut_network-1 mongo:3.4.2
docker run -d --name mongo-unittest-2 --network ut_network-1 mongo:3.4.2
docker run untapt_ut --rm --network ut_network-{%} --link mongo-unittest-{%}:db python discover.py
The connection string is "mongodb://db:27017/mydb"
{%} is the number associated with the environment - so on ut_network-1, the database would be mongo-unittest-1. Note the alias to 'db'.
This works fine but I read that --link will be deprecated.
I thought the solution would be as simple as removing --link and setting the hostname:
docker run -d --hostname db --network ut_network-1 mongo:3.4.2
docker run -d --hostname db --network ut_network-2 mongo:3.4.2
docker run untapt_ut --rm --network ut_network-{%} python discover.py
However, if I do this then the application cannot find the mongo instance. Further:
I can't use --name db because Docker would attempt to create multiple containers called 'db' which it obviously cannot do (even though they are on a different network).
the default hostname of the mongo database is the first few digits of the container id. My unit tests all get the mongo database string from a secrets file which assumes the database is called 'db'.
as I said, if I use --hostname db the core app cannot find the mongo instance. But if I hard-code the container id as the server, then the core application finds the mongo instance fine.
I want to keep the alias 'db' so the unit tests can use one single source for the mongo database string that I don't need to mess with.
The documentation here implies I can use --link.
So am I doing this correctly? Or if not, how should I configure Docker networking such that I can create multiple networks and alias a 'static' hostname for 'db'?
Any advice would be much appreciated.
Thanks in advance!

Answer 1

Yes, links are being deprecated and should be avoided. For the DNS discovery, I thought the hostname would work, but I'm seeing the same results you are seeing. You could use the container name with --name db which has the unique container issue, so I recommend against that for the same reasons you've found. The best solution is to go directly to the goal of a network alias with --network-alias db:
docker run -d --network-alias db --network ut_network-1 mongo:3.4.2
docker run -d --network-alias db --network ut_network-2 mongo:3.4.2
docker run untapt_ut --rm --network ut_network-{%} python discover.py