Networks

Docker networks are used to allow containers to communicate with one another. By default, instead of using an actual IP - adress, docker handles containers by their names and automatically map them to a network.

Docker compose

When running services with docker compose, a dedicated network will be created where each service can access the other - even when no ports are exposed - using their hostname

• Assume there’s two containers: mongodb & node-app
• node-app can then use an address such as mongodb://user:password@mongodb/collection to securely connect to a mongo database.
• An alternative is to figure the IP - address used by mongodb using sudo docker inspect database. This is, however, not recommended

Alternatively, if one has to access the local machine from inside a container, it can be done by using host.docker.internal instead of localhost

Basic Commands

# Inspect a container to figure the IP - adress
sudo docker inspect <database-container>
 
# Create a network
sudo docker network create favorites-net
 
# Run a container and assign it to a network
sudo docker run --name database --rm -d --network favorites-net mongo

Once the network is created, connecting to it from inside a node.js app can be done as follows:

mongoose.connect(
  'mongodb://database:27017/swfavorites',
  { useNewUrlParser: true },
  (err) => {
    if (err) {
      console.log(err);
    } else {
      app.listen(3000);
    }
  }
);

If you’re using Docker compose, it will create a single network for all services