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I'm working with a docker-compose file from an open-source repo. Notably, it's missing the version and services keys, but it still works (up until now, I have not seen a compose file without these keys).

redis:
  image: redis
  ports:
    - '6379'
   build: .
   environment:
     - LOG_LEVEL='debug'
   links:
     - redis
docker-compose up starts everything up and the app is able to talk to redis via 127.0.0.1:6379.
However, when I add the version and services keys back in, connections to redis are refused:
version: '3'
services:
  redis:
    image: redis
    ports:
      - '6379'
    build: .
    environment:
      - LOG_LEVEL='debug'
    links:
      - redis
Which results in:
[Wed Jan 03 2018 20:51:58 GMT+0000 (UTC)] ERROR { Error: Redis connection to 127.0.0.1:6379 failed - connect ECONNREFUSED 127.0.0.1:6379
    at Object.exports._errnoException (util.js:896:11)
    at exports._exceptionWithHostPort (util.js:919:20)
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1073:14)
  code: 'ECONNREFUSED',
  errno: 'ECONNREFUSED',
  syscall: 'connect',
  address: '127.0.0.1',
  port: 6379 }
Why does adding version: '3' and services: lead to failure to connect?
You don't need to specify the ports neither the links for services in the same network (compose file). You can use:
version: '3'
services:
  redis:
    image: redis
    build: .
    environment:
      - LOG_LEVEL='debug'
And then in your app code refer to redis just as 'redis:6379'. If you see the Dockerfile for the redis image you can see the port is already exposed at the end.
When you want to expose the service to a specific host port, in Docker Compose version 3 you should use this syntax:
ports:
  - '6379:6379'
Check the docs here:
  
Either specify both ports (HOST:CONTAINER), or just the container port
  (a random host port will be chosen).
                that doesn't solve the problem.  Mostly I am curious why removing  version: '3' and services: from the docker-compose file makes localhost work in the app container.  Perhaps it forces docker containers to see the network like the host machine?
– rgwozdz
                Jan 4, 2018 at 15:29
const redisHost = 'redis';
const redisPort = '6379';
let client = redis.createClient(redisPort, redisHost);
client.on('connect', () => {
  console.log(`Redis connected to ${redisHost}:${redisPort}`);
client.on('error', (err) => {
  console.log(`Redis could not connect to ${redisHost}:${redisPort}: ${err}`);
module.exports = client;
        
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