Port mapping in Compose in Docker - Step-by-Step Execution

Process Flow - Port mapping in Compose

Start Compose file

↓

Define service

↓

Specify ports mapping

↓

Run docker-compose up

↓

Docker maps host port to container port

↓

Service accessible via host port

↓

End

This flow shows how Docker Compose reads the ports section, maps host ports to container ports, and makes the service accessible.

Execution Sample

Docker

services:
  web:
    image: nginx
    ports:
      - "8080:80"

This Compose snippet maps host port 8080 to container port 80 for the web service.

Process Table

Step	Action	Host Port	Container Port	Result
1	Read Compose file	-	-	Service 'web' found with ports mapping
2	Parse ports mapping	8080	80	Host port 8080 mapped to container port 80
3	Run docker-compose up	-	-	Container starts with port mapping
4	Access service	8080	80	Requests to localhost:8080 forwarded to container:80
5	Exit	-	-	Port mapping active while container runs

💡 Port mapping ends when container stops or Compose is shut down

Status Tracker

Variable	Start	After Step 2	After Step 3	Final
host_port	-	8080	8080	8080
container_port	-	80	80	80
service_status	not running	not running	running	running

Key Moments - 2 Insights

Why do we write "8080:80" in ports instead of just "80"?

Because "8080:80" tells Docker to map port 8080 on your computer (host) to port 80 inside the container. Just "80" would map container port 80 to a random available host port.

What happens if the host port 8080 is already in use?

Docker will fail to start the container because it cannot bind to the busy port.

Visual Quiz - 3 Questions

Look at the execution table, what is the host port mapped to container port 80?

A 443

B 80

C 8080

D 3000

💡 Hint

Check Step 2 in the execution table where ports mapping is parsed.

At which step does the container start running with port mapping?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look for the step mentioning 'Container starts with port mapping'.

If you change the ports line to "9090:80", what changes in the variable_tracker?

A host_port changes to 9090

B container_port changes to 9090

C service_status changes to stopped

D No changes

💡 Hint

Host port is the first number in the mapping, check host_port row in variable_tracker.

Concept Snapshot

Port mapping in Docker Compose:
- Use 'ports:' under service to map host:container ports
- Syntax: 'host_port:container_port'
- Allows accessing container service via host port
- Host port must be free to bind
- Mapping active while container runs

Full Transcript

This visual execution shows how Docker Compose reads the ports mapping in a service definition. The Compose file defines a service named 'web' using the nginx image. The ports section maps host port 8080 to container port 80. When 'docker-compose up' runs, Docker starts the container and sets up the port forwarding. Requests to localhost:8080 on the host machine are forwarded to port 80 inside the container, making the web service accessible. The variable tracker shows host_port as 8080 and container_port as 80, with the service status changing from not running to running. Key moments clarify why the mapping syntax uses 'host:container' and what happens if the host port is busy. The quiz tests understanding of port mapping steps and variable changes.