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Understanding a docker-compose.yml file

May 1, 2021
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Let’s take look at what the subql docker-compose.yml means. Take for example the yaml file from the starter tutorial.

version: '3'

services:
  postgres:
    image: postgres:12-alpine
    ports:
      - 5432:5432
    volumes:
      - .data/postgres:/var/lib/postgresql/data
    environment:
      POSTGRES_PASSWORD: postgres

  subquery-node:
    image: onfinality/subql-node:latest
    depends_on:
      - "postgres"
    restart: always
    environment:
      DB_USER: postgres
      DB_PASS: postgres
      DB_DATABASE: postgres
      DB_HOST: postgres
      DB_PORT: 5432
    volumes:
      - ./:/app
    command:
      - -f=/app
      - --local

  graphql-engine:
    image: onfinality/subql-query:latest
    ports:
      - 3000:3000
    depends_on:
      - "postgres"
    restart: always
    environment:
      DB_USER: postgres
      DB_PASS: postgres
      DB_DATABASE: postgres
      DB_HOST: postgres
      DB_PORT: 5432
    command:
      - --name=app
      - --playground

Service

The service tag lists all the containers which are included in the compose file and acts as there parent task. (A service definition contains configuration that is applied to each container started for that service). Here there are services defined. postgress, subquery-node and graphql-engine.

Image

The base image of a container can be defined by either using a pre-existing image on DockerHub or by building an image using a Dockerfile. Here a predefined image from DockerHub is used with the image tag. Specifically postgres:12-alpine, onfinality/subql-node:latest and onfinality/subql-query:latest.

Port

We then define which port we want to expose and the host port it should be exposed to. eg 5432:5432 for host:container. The ports here are a straight same for same mapping. (A non same mapping would be something like 8080:80).

Volumes

Volumes are Docker’s preferred way of persisting data which is generated and used by Docker containers. They are completely managed by Docker and can be used to share data between containers and the Host system.

There are three types of volumes that can be defined. Normal, path and named. Here we use path mapping such as .data/postgres:/var/lib/postgresql/data. This defines the path on the host system and mapping it to a container destination using the semi-colon (:) operator.

Restart

The restart policy here is set as always. This means to always restart the container if it stops. If it is manually stopped, it is restarted only when Docker daemon restarts or the container itself is manually restarted.

Environment

Environment variables are used to bring configuration data into your applications. This is often the case if you have some configurations that are dependent on the host operating system or some other variable things that can change.

Here we define the default credentials and database parameters. It should be a given that this is not production quality code.

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