get a python docker container to interact with a redis docker container

Question

I'm new to docker, redis and any kind of networking, (I know python at least!). Firstly I have figured out how to get a redis docker image and run it in a docker container:

docker run --name some-redis -d redis

As I understand this redis instance has port 6379 available to connect to other containers.

docker network inspect bridge

   "Containers": {
        "2ecceba2756abf20d5396078fd9b2ecf0d60ab04ca6b8df5e1b631b6fb5e9a85": {
            "Name": "some-redis",
            "EndpointID": "09f0069dae3632a2456cb4d82ad5e7c9782a2b58cb7a4ee655f57b5c410c3e87",
            "MacAddress": "02:42:ac:11:00:02",
            "IPv4Address": "172.17.0.2/16",
            "IPv6Address": ""
        }

If I run the following command I can interact with the redis instance and generate key:value pairs:

docker run -it --link some-redis:redis --rm redis redis-cli -h redis -p 6379
set 'a' 'abc'
>OK
get 'a'
>"abc"
quit

I have figured out how to make and run a docker container with the redis library installed that will run a python script as follows:

Here is my Dockerfile:

FROM python:3
ADD redis_test_script.py /
RUN pip install redis 
CMD [ "python", "./redis_test_script.py" ]

Here is redis_test_script.py:

import redis
print("hello redis-py")

Build the docker image:

docker build -t python-redis-py .

If I run the following command the script runs in its container:

docker run -it --rm --name pyRed python-redis-py

and returns the expected:

>hello redis-py

It seems like both containers are working ok, the problem is connecting them both together, I would like to ultimately use python to perform operation on the redis container. If I modify the script as follows and rebuild the image for the python container it fails:

import redis
print("hello redis-py")
r = redis.Redis(host="localhost", port=6379, db=0)
r.set('z', 'xyz')
r.get('z')

I get several errors:

...
OSError: [Errno 99] Cannot assign requested address
...
redis.exceptions.ConnectionError: Error 99 connecting to localhost:6379. Cannot assign requested address.
.....

It looks like they're not connecting, I tried again using the bridge IP in the python script:

r = redis.Redis(host="172.17.0.0/16", port=6379, db=0)

and get this error:

redis.exceptions.ConnectionError: Error -2 connecting to 172.17.0.0/16:6379. Name or service not known.

and I tried the redis sub IP:

r = redis.Redis(host="172.17.0.2/16", port=6379, db=0)

and I get this error:

redis.exceptions.ConnectionError: Error -2 connecting to 172.17.0.2/16:6379. Name or service not known.

It feels like I'm fundamentally misunderstanding something about how to get the containers to talk to each other. I've read quite a lot of documentation and tutorials but as I say have no networking experience and have not previously used docker so any helpful explanations and/or solutions would be really great.

Many thanks

Answer 1

That's all about Docker networking. Fast solution - use host network mode for both containers. Drawback is low isolation, but you will get it working fast:

docker run -d --network=host redis ...
docker run --network=host python-redis-py ...

Then to connect from python to redis just use localhost as a hostname.

Better solution is to use docker user-defined bridge network

# create network
docker network create foo
docker run -d --network=foo --name my-db redis ...
docker run    --network=foo python-redis-py ...

Note that in this case you cannot use localhost but instead use my-db as a hostname. That's why I've used --name my-db parameter when starting first container. In user-defined bridge networks containers reach each other by theirs names.