How to connect to a Redis container using Docker Compose?

Question

This docker-compose.yml seems to work OK:

version: '3'
services:
  web:
    image: web-app
    command: bundle exec rackup
    ports:
     - "9292:9292"
    links:
      - redis
  redis:
    image: redis

Command:

docker build -t web-app .; docker-compose up

The Web App is expecting a REDIS_URL config. Web App Dockerfile:

ENV REDIS_URL redis:6379

It seems that the Web App cannot connect to the Redis:

Redis::CannotConnectError - Error connecting to Redis on 127.0.0.1:6379 (Errno::ECONNREFUSED):

So, what is the Redis URL for the Web App?

Do I need to expose the Redis port (6379)?

EDIT:

Thanks to @Andy Shinn, I see the redis url should be redis://6379.
Now I get Errno::EINVAL - Invalid argument - connect(2) for 0.0.24.235:6379. This may have to do with Redis v 3.2.8. I'll try another version and see if it works.

EDIT #2:

3.2.8 works fine with the app on my local machine, so that is not the issue.

Solution

The issue is that redis is "variable" that will be interpolated in the Dockerfile.

ENV REDIS_URL redis:6379

But the Redis URL should start with the literal redis.

So, the fix is updating the docker compose:

redis_db:
  image: redis:3.2.8

The desired config is:

REDIS_URL=redis://redis_db:6379

Thanks to BMitch and Andy Shinn for helping me figure this out.