Docker-compose : understanding linking environment variables - docker

Question

I'm now using docker-compose for all of my projects. Very convenient. Much more comfortable than manual linking through several docker commands.
There is something that is not clear to me yet though: the logic behind the linking environment variables.
Eg. with this docker-compose.yml:
mongodb:
image: mongo
command: "--smallfiles --logpath=/dev/null"
web:
build: .
command: npm start
volumes:
- .:/myapp
ports:
- "3001:3000"
links:
- mongodb
environment:
PORT: 3000
NODE_ENV: 'development'
In the node app, I need to retrieve the mongodb url. And if I console.log(process.env), I get so many things that it feels very random (just kept the docker-compose-related ones):
MONGODB_PORT_27017_TCP: 'tcp://172.17.0.2:27017',
MYAPP_MONGODB_1_PORT_27017_TCP_PORT: '27017',
MYAPP_MONGODB_1_PORT_27017_TCP_PROTO: 'tcp',
MONGODB_ENV_MONGO_VERSION: '3.2.6',
MONGODB_1_ENV_GOSU_VERSION: '1.7',
'MYAPP_MONGODB_1_ENV_affinity:container': '=d5c9ebd7766dc954c412accec5ae334bfbe836c0ad0f430929c28d4cda1bcc0e',
MYAPP_MONGODB_1_ENV_GPG_KEYS: 'DFFA3DCF326E302C4787673A01C4E7FAAAB2461C \t42F3E95A2C4F08279C4960ADD68FA50FEA312927',
MYAPP_MONGODB_1_PORT_27017_TCP: 'tcp://172.17.0.2:27017',
MONGODB_1_PORT: 'tcp://172.17.0.2:27017',
MYAPP_MONGODB_1_ENV_MONGO_VERSION: '3.2.6',
MONGODB_1_ENV_MONGO_MAJOR: '3.2',
MONGODB_ENV_GOSU_VERSION: '1.7',
MONGODB_1_PORT_27017_TCP_ADDR: '172.17.0.2',
MONGODB_1_NAME: '/myapp_web_1/mongodb_1',
MONGODB_1_PORT_27017_TCP_PORT: '27017',
MONGODB_1_PORT_27017_TCP_PROTO: 'tcp',
'MONGODB_1_ENV_affinity:container': '=d5c9ebd7766dc954c412accec5ae334bfbe836c0ad0f430929c28d4cda1bcc0e',
MONGODB_PORT: 'tcp://172.17.0.2:27017',
MONGODB_1_ENV_GPG_KEYS: 'DFFA3DCF326E302C4787673A01C4E7FAAAB2461C \t42F3E95A2C4F08279C4960ADD68FA50FEA312927',
MYAPP_MONGODB_1_ENV_GOSU_VERSION: '1.7',
MONGODB_ENV_MONGO_MAJOR: '3.2',
MONGODB_PORT_27017_TCP_ADDR: '172.17.0.2',
MONGODB_NAME: '/myapp_web_1/mongodb',
MONGODB_1_PORT_27017_TCP: 'tcp://172.17.0.2:27017',
MONGODB_PORT_27017_TCP_PORT: '27017',
MONGODB_1_ENV_MONGO_VERSION: '3.2.6',
MONGODB_PORT_27017_TCP_PROTO: 'tcp',
MYAPP_MONGODB_1_PORT: 'tcp://172.17.0.2:27017',
'MONGODB_ENV_affinity:container': '=d5c9ebd7766dc954c412accec5ae334bfbe836c0ad0f430929c28d4cda1bcc0e',
MYAPP_MONGODB_1_ENV_MONGO_MAJOR: '3.2',
MONGODB_ENV_GPG_KEYS: 'DFFA3DCF326E302C4787673A01C4E7FAAAB2461C \t42F3E95A2C4F08279C4960ADD68FA50FEA312927',
MYAPP_MONGODB_1_PORT_27017_TCP_ADDR: '172.17.0.2',
MYAPP_MONGODB_1_NAME: '/myapp_web_1/novatube_mongodb_1',
Don't know what to pick, and why so many entries? Is it better to use the general ones, or the MYAPP-prefixed one? Where does the MYAPP name comes from? Folder name?
Could someone clarify this?
Wouldn't it be easier to let the user define the ones he needs in the docker-compose.yml file with a custom mapping? Like:
links:
- mongodb:
- MONGOIP: IP
- MONGOPORT : PORT
What I'm saying might not have sense though. :-)

Answer 1

Environment variables are a legacy way of defining links between containers. If you are using a newer version of compose, you don't need the links declaration at all. Trying to connect to mongodb from your app container will work fine by just using the name of the service (mongodb) as a hostname, without any links defined in the compose file (instead using docker's builtin DNS resolution, check /etc/hosts, nothing in there either!)
In answer to your question about why the prefix with MYAPP, you're right. Compose prefixes the service name with the name of the folder (or 'project', in compose nomenclature). It does the same thing when creating custom networks and volumes.