What is Compose Service? | DevOps Dictionary

Compose Service — One Component of Your Application Stack

What Is a Compose Service in Simple Terms?

In Docker Compose, a service is one named component of your application. Your application might have three services: api, postgres, and redis. Each service definition tells Docker exactly how to run that component — which image to use, which ports to expose, which environment variables to set, which volumes to mount, and which other services it depends on.

YAML

version: "3.8"
 
services:
 
  payment-api:          # <- service name (also DNS hostname)
    image: payment-api:v3.1.0
    ports:
      * "8080:8080"
    environment:
      DB_HOST: postgres  # reaches postgres service by name
    depends_on:
      postgres:
        condition: service_healthy
    restart: unless-stopped
    deploy:
      resources:
        limits:
          cpus: "1.5"
          memory: 512M
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:8080/health"]
      interval: 30s
      timeout: 5s
      retries: 3
 
  postgres:
    image: postgres:15-alpine
    environment:
      POSTGRES_PASSWORD: secret
    volumes:
      * postgres-data:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 10s
      retries: 5
 
volumes:
  postgres-data:

All Service Configuration Keys

YAML

services:
  myservice:
    image: myimage:tag          # image to use
    build: .                    # or build from Dockerfile
    container_name: myservice   # fixed container name
    ports:
      * "8080:80"
    environment:
      KEY: value
    env_file:
      * .env
    volumes:
      * data:/app/data
      * ./config:/app/config:ro
    networks:
      * mynetwork
    depends_on:
      other:
        condition: service_healthy
    restart: unless-stopped
    command: ["node", "server.js"]   # override CMD
    entrypoint: ["/entrypoint.sh"]   # override ENTRYPOINT
    working_dir: /app
    user: "1000:1000"
    read_only: true
    tmpfs:
      * /tmp
    logging:
      driver: json-file
      options:
        max-size: "100m"
    deploy:
      resources:
        limits:
          cpus: "1"
          memory: 512M
    profiles:
      * debug              # only starts when debug profile active

Service Scaling

Bash

# Scale a service to 3 instances
docker compose up -d --scale payment-api=3
 
# IMPORTANT: remove container_name before scaling
# A fixed container_name conflicts when scaling to multiple instances
 
# Check which instances are running
docker compose ps
# NAME              STATUS
# project-api-1     Up
# project-api-2     Up
# project-api-3     Up

PLACEMENT PRO TIP
**Tip:** The service name in Docker Compose is also the DNS hostname other services use to reach it. Name your services after what they do (`api`, `postgres`, `redis`) not after the image they use. This makes your application config readable and easy to understand.

COMMON MISTAKE / WARNING
**Common Mistake:** Setting `container_name` on services you want to scale. Docker Compose cannot create two containers with the same name — scaling a service with a fixed `container_name` fails immediately.