Ordenando a execução dos containers

Baixar o Docker Compose para eliminar os container criados para o Host Driver:

~/compose$ docker compose -f host-compose.yaml down
[+] Running 3/3
 ✔ Container compose-nginx-1  Removed                                         0.0s
 ✔ Container compose-curl-1   Removed                                         0.0s
 ✔ Network compose_default    Removed                                         0.7s

Remover o arquivo host-compose.yaml:

~/compose$ rm host-compose.yaml

Simulação de cenário

Ao executar o comando docker compose up -d os containers são criados numa ordem aleatória, pois as especificações do arquivos compose.yaml são apenas declarativas.

~/compose$ docker compose up -d
[+] Running 2/2
 ✔ Container compose-kubenews-1  Starte...                                0.6s
 ✔ Container compose-postgre-1   Started                                  0.6s

Porém, muitas vezes, existem dependências entre os containers. Neste exemplo, o container kubenews depende do container postgre. Ou seja, a aplicação depende do banco de dados para se conectar a ele.

Faz sentido criar o container postgre antes do kubenews, por isso é necessário especificar as dependências dos containers no arquivo compose.yaml.

Ordenando a execução

Edição do arquivo compose.yaml para inserir no container kubenews a sua dependência do postgre:

~/compose$ vim compose.yaml

services:
  postgre:
    image: postgres:13.16
    ports:
      - 5432:5432
    environment:
      POSTGRES_PASSWORD: kubenews
      POSTGRES_USER: kubenews
      POSTGRES_DB: kubenews
    volumes:
      - postgre-docker-vol:/var/lib/postgresql/data
    networks:
      - kubenews-net
  kubenews:
    image: marcelodpbarbieri/kube-news:v2
    ports:
      - 8080:8080
    dependes_on:
      - postgre      
    environment:
      DB_DATABASE: kubenews
      DB_USERNAME: kubenews
      DB_PASSWORD: kubenews
      DB_HOST: postgre
    networks:
      - kubenews-net
      
volumes:
  postgre-docker-vol:

networks:
  kubenews-net:
    name: network-externa
    external: true
    driver: bridge

Destaque para:

services:
  postgre:
    ...
  kubenews:
    ...
    dependes_on:
      - postgre
    ...

Com isso a ordem de execução especificada é aplicada:

~/compose$ docker compose down
[+] Running 2/2
 ✔ Container compose-kubenews-1  Remove...                               10.6s
 ✔ Container compose-postgre-1   Removed                                  0.6s
 
~/compose$ docker compose up -d
[+] Running 2/2
 ✔ Container compose-postgre-1   Started                                  0.8s
 ✔ Container compose-kubenews-1  Starte...                                1.1s

O depends_on é uma lista. Com isso é possível especificar que um container depende de um ou mais containers.

Por exemplo, criar mais um container nginx:

services:
  ...
  nginx:
    image: nginx
    depends_on: 
      - kubenews
  ...

~/compose$ docker compose down
[+] Running 4/4
 ✔ Container compose-kubenews-1  Remove...                               10.6s
 ✔ Container compose-postgre-1   Removed                                  0.5s
 ✔ Network compose_default       Removed                                  0.6s
 
~/compose$ docker compose up -d
[+] Running 4/4
 ✔ Network compose_default       Created                                  0.1s
 ✔ Container compose-postgre-1   Started                                  0.5s
 ✔ Container compose-kubenews-1  Starte...                                0.7s
 ✔ Container compose-nginx-1     Started                                  1.2s

Poderia especificar que o container postgre depende do container nginx, e que o container nginx deixa de depende do kubenews:

services:
  postgre:
    image: postgres:13.16
    ports:
      - 5432:5432
    depends_on:
      - nginx
    ...
  nginx:
    image: nginx

O container nginx foi criado antes do container postgre:

~/compose$ docker compose down
[+] Running 4/4
 ✔ Container compose-kubenews-1  Remove...                               10.5s
 ✔ Container compose-postgre-1   Removed                                  0.5s
 ✔ Container compose-nginx-1     Removed                                  0.5s
 ✔ Network compose_default       Removed                                  0.8s

~/compose$ docker compose up -d
[+] Running 4/4
 ✔ Network compose_default       Created                                  0.2s
 ✔ Container compose-nginx-1     Started                                  0.5s
 ✔ Container compose-postgre-1   Started                                  0.8s
 ✔ Container compose-kubenews-1  Starte...                                1.1s
marcelo@Latitude5440:~/compose$

Dependência Circular

Cuidado para não criar uma dependência circular. No exemplo acima se o container nginx continuasse dependente do container kubenews.

No arquivo compose.yaml é possível verificar as especificações de que os containers são dependentes entre si:

• nginx depende do kubenews

• kubenews depende do postgre

• postgre depende do nginx

services:
  postgre:
    image: postgres:13.16
    ports:
      - 5432:5432
    depends_on:
      - nginx
    environment:
      POSTGRES_PASSWORD: kubenews
      POSTGRES_USER: kubenews
      POSTGRES_DB: kubenews
    volumes:
      - postgre-docker-vol:/var/lib/postgresql/data
    networks:
      - kubenews-net
  kubenews:
    image: marcelodpbarbieri/kube-news:v2
    ports:
      - 8080:8080
    depends_on:
      - postgre
    environment:
      DB_DATABASE: kubenews
      DB_USERNAME: kubenews
      DB_PASSWORD: kubenews
      DB_HOST: postgre
    networks:
      - kubenews-net
  nginx:
    image: nginx
    depends_on:
      - kubenews

volumes:
  postgre-docker-vol:

networks:
  kubenews-net:
    name: network-externa
    external: true
    driver: bridge

~/compose$ docker compose down
dependency cycle detected: kubenews -> postgre -> nginx -> kubenews

Retirando a dependência inserida para o container postgre a dependência cíclica é resolvida:

services:
  postgre:
    image: postgres:13.16
    ports:
      - 5432:5432
    environment:
      POSTGRES_PASSWORD: kubenews
      POSTGRES_USER: kubenews
      POSTGRES_DB: kubenews
    volumes:
      - postgre-docker-vol:/var/lib/postgresql/data
    networks:
      - kubenews-net
      ...

~/compose$ docker compose down
[+] Running 4/4
 ✔ Container compose-nginx-1     Removed                                  0.5s
 ✔ Container compose-kubenews-1  Remove...                               10.5s
 ✔ Container compose-postgre-1   Removed                                  0.4s
 ✔ Network compose_default       Removed                                  0.8s

~/compose$ docker compose up -d
[+] Running 4/4
 ✔ Network compose_default       Created                                  0.2s
 ✔ Container compose-postgre-1   Started                                  0.5s
 ✔ Container compose-kubenews-1  Starte...                                0.8s
 ✔ Container compose-nginx-1     Started                                  1.2s