Defining services in Docker - Step-by-Step Execution

Process Flow - Defining services

Write docker-compose.yml

↓

Define service name

↓

Specify image or build

↓

Set ports, volumes, environment

↓

Save and run 'docker-compose up'

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Docker creates containers for each service

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Services start and communicate

This flow shows how you write a docker-compose file to define services, then run them as containers.

Execution Sample

Docker

version: '3.8'
services:
  web:
    image: nginx:latest
    ports:
      - "8080:80"

Defines a single service named 'web' using the nginx image and maps port 8080 on the host to port 80 in the container.

Process Table

Step	Action	Details	Result
1	Read docker-compose.yml	Parse version and services	Recognizes 'web' service with nginx image
2	Create container for 'web'	Pull nginx:latest image if missing	Image downloaded or found locally
3	Map ports	Host 8080 to container 80	Port forwarding set up
4	Start container	Run nginx inside container	Container running and listening on port 80
5	Verify service	Access localhost:8080	Nginx welcome page served
6	Exit	No more steps	Setup complete, services running

💡 All services defined and started successfully

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
services.web.image	undefined	nginx:latest	nginx:latest	nginx:latest	nginx:latest
services.web.ports	undefined	undefined	["8080:80"]	["8080:80"]	["8080:80"]
container.web.status	not created	created	created	running	running

Key Moments - 3 Insights

Why do we specify ports in the service definition?

Ports map the container's internal port to the host machine so you can access the service from outside. See execution_table step 3 where port 8080 is mapped to 80.

What happens if the image is not on the local machine?

Docker pulls the image from the registry before creating the container, as shown in execution_table step 2.

How does Docker know which services to start?

Docker reads the 'services' section in the compose file and creates containers for each service listed, shown in step 1 and 4.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the container actually started?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Check the 'Action' and 'Result' columns for when the container status changes to running.

According to the variable tracker, what is the status of the container after step 3?

A not created

B running

C created

D stopped

💡 Hint

Look at 'container.web.status' value under 'After Step 3' column.

If we remove the ports section from the service, what changes in the execution?

A Container will not start

B Service will start but not accessible from host

C Image will not be pulled

D Docker-compose will throw an error

💡 Hint

Refer to the role of ports in execution_table step 3 and key moments about port mapping.

Concept Snapshot

Defining services in docker-compose:
- Use 'services:' to list containers
- Each service needs an image or build context
- Map ports to access services externally
- Run with 'docker-compose up' to start all services
- Docker pulls images if missing and creates containers
- Services communicate via network automatically

Full Transcript

Defining services in Docker means writing a docker-compose.yml file where you list each service by name. For each service, you specify an image or build instructions, ports to expose, and other settings like volumes or environment variables. When you run 'docker-compose up', Docker reads this file, pulls any missing images, creates containers for each service, sets up port forwarding, and starts the containers. This lets you run multiple connected services easily. The execution table shows each step from reading the file to starting the container. The variable tracker shows how service properties and container status change step by step. Key moments clarify why ports are needed and how images are pulled. The quiz tests understanding of when containers start, container status, and the effect of removing ports.