Detailed explanation of the difference between docker-compose ports and expose

There are two ways to expose container ports in docker-compose: ports and expose.

ports

Ports exposes the container port to any port or specified port of the host. Usage:

ports:
 
- "80:80" # Bind the container's port 80 to the host's port 80 - "9000:8080" # Bind the container's port 8080 to the host's port 9000 - "443" # Bind the container's port 443 to any port on the host. The bound host port number is randomly assigned when the container starts

Using ports will expose the port to the host regardless of whether the host port is specified or not.

Some network applications can be run in the container. To make these applications accessible to the outside world, you can specify port mapping using the -P (uppercase) or -p (lowercase) parameters.

(1) When the -P flag is used, Docker will randomly map a port between 49000 and 49900 to the network port opened inside the container.

Using docker ps, you can see that port 49155 of the local host is mapped to port 5000 of the container. At this time, access port 49155 of the local machine to access the interface provided by the web application in the container.

$ sudo docker run -d -P training/webapp python app.py
 
$ sudo docker ps -l
 
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
 
bc533791f3f5 training/webapp:latest python app.py 5 seconds ago Up 2 seconds 0.0.0.0:49155->5000/tcp nostalgic_morse

Similarly, you can use the docker logs command to view application information.

$ sudo docker logs -f nostalgic_morse
 
* Running on http://0.0.0.0:5000/
 
10.0.2.2 - - [23/May/2014 20:16:31] "GET / HTTP/1.1" 200 -
 
10.0.2.2 - - [23/May/2014 20:16:31] "GET /favicon.ico HTTP/1.1" 404 - 

(2) -p (lowercase) can specify the IP and port to be mapped, but only one container can be bound to a specified port. The supported formats are hostPort:containerPort, ip:hostPort:containerPort, ip::containerPort.

expose

Expose the container to the container linked to the current container. Usage:

expose:
- "3000"
- "8000"

The above instructions expose ports 3000 and 8000 of the current container to the container linked to this container.

The difference from ports is that expose does not expose the port to the host.

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