python - Docker [Errno 111] Connect call failed ('127.0.0.1', 6379)

Question

I am trying to follow the tutorial here https://channels.readthedocs.io/en/latest/tutorial/part_2.html and check if channel layer can communicate with Redis. The only different thing I'm doing is that I'm using docker-compose and running the entire thing on a docker container, and that seems to be messing up with everything. This is the error message I'm getting when I try to

run async_to_sync(channel_layer.send)('test_channel', {'type': 'hello'})

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/lib/python3.7/site-packages/asgiref/sync.py", line 116, in __call__
    return call_result.result()
  File "/usr/local/lib/python3.7/concurrent/futures/_base.py", line 428, in result
    return self.__get_result()
  File "/usr/local/lib/python3.7/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/usr/local/lib/python3.7/site-packages/asgiref/sync.py", line 156, in main_wrap
    result = await self.awaitable(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/channels_redis/core.py", line 293, in send
    async with self.connection(index) as connection:
  File "/usr/local/lib/python3.7/site-packages/channels_redis/core.py", line 820, in __aenter__
    self.conn = await self.pool.pop()
  File "/usr/local/lib/python3.7/site-packages/channels_redis/core.py", line 70, in pop
    conns.append(await aioredis.create_redis(**self.host, loop=loop))
  File "/usr/local/lib/python3.7/site-packages/aioredis/commands/__init__.py", line 175, in create_redis
    loop=loop)
  File "/usr/local/lib/python3.7/site-packages/aioredis/connection.py", line 113, in create_connection
    timeout)
  File "/usr/local/lib/python3.7/asyncio/tasks.py", line 414, in wait_for
    return await fut
  File "/usr/local/lib/python3.7/site-packages/aioredis/stream.py", line 24, in open_connection
    lambda: protocol, host, port, **kwds)
  File "/usr/local/lib/python3.7/asyncio/base_events.py", line 958, in create_connection
    raise exceptions[0]
  File "/usr/local/lib/python3.7/asyncio/base_events.py", line 945, in create_connection
    await self.sock_connect(sock, address)
  File "/usr/local/lib/python3.7/asyncio/selector_events.py", line 473, in sock_connect
    return await fut
  File "/usr/local/lib/python3.7/asyncio/selector_events.py", line 503, in _sock_connect_cb
    raise OSError(err, f'Connect call failed {address}')
ConnectionRefusedError: [Errno 111] Connect call failed ('127.0.0.1', 6379)

I've checked a few post and saw that many suggested this is because Redis isn't running. I know that Redis exist since docker ps shows that

CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
2ccab2cfc570        test_web            "python manage.py ru…"   7 minutes ago       Up 7 minutes        0.0.0.0:8000->8000/tcp   test_web_1
6da398f093fc        redis:2.8           "docker-entrypoint.s…"   10 minutes ago      Up 10 minutes       0.0.0.0:6379->6379/tcp   relaxed_aryabhata

Any idea what I can do right now? I'm really new to these

Answer 1

Try changing 127.0.0.1:6379 to redis:6379.

Although Redis is running, your python container isn't able to communicate with it; this is because it's trying to connect to 127.0.0.1:6379, but from the container's perspective, there's nothing running there. This can be a bit frustrating to debug, but it's a bit easier if you keep in mind that containers get their own network namespace. As a result, python's localhost != redis's localhost != your host machine's localhost.

Luckily, it's easy to connect containers that are sharing the same bridge, and by default, docker-compose creates a single bridge network and connects all your containers to them, providing the necessary DNS to allow them to discover one another. As a result, container-to-container communication works simply by using the service name.

As a note, it's possible to run containers in the same namespace, and to run in them in the namespace of the host, via the --net=container:<container-id> or --net=host flag. This is especially useful for running debugging tools in a container and attaching them to the network namespace of either another container or the host, e.g. using netshoot to see what ports are listening within the container (exposed or not), docker run --rm -it --net container:test_web_1 nicolaka/netshoot netstat -tulpn.