Docker Compose is a powerful tool that allows you to define and run multi-container Docker applications. It simplifies the process of managing and orchestrating multiple containers, making it a popular choice for development and testing environments. One of the key features of Docker Compose is the depends_on option, which allows you to specify dependencies between services. In this beginner's guide, you will explore how to use depends_on effectively, ensuring your services start up in the correct order and function seamlessly. Let’s get started!! Table of contents # Docker compose depends on Difference between depends_on and links Docker compose depends on under the covers Services startup condition with service_healthy Example with depends_on and service_healthy condition Conclusion Docker compose depends on # Before we dive into depends_on, let's briefly recap Docker and Docker Compose. Docker is a containerization technology that packages and runs applications in isolated environments called containers. Docker Compose builds upon Docker, enabling you to define and manage multiple containers and their configurations in a single YAML file called docker-compose.yml. This declarative approach streamlines the process of running and orchestrating complex applications. You get free from long docker run commands using this declarative and easy-to-understand syntax that can be comprehended in a single glance. Now, where does depends_on fit into this picture? When working with multi-container applications, it's often crucial to ensure that services start up in a specific order or that the dependent service starts and is ready before the main service. For example, if your application depends on a database, the database service needs to be running before your application service starts. This is where depends_on comes in. It allows you to specify which services a particular service depends on, ensuring that the dependent services are started before the service that relies on them. Difference between depends_on and links # It's important to distinguish depends_on from links. While links create network aliases and allow containers to communicate with each other by their service names, depends_on solely focuses on startup order. It does not automatically guarantee that the dependent service is ready to accept connections or requests. You will explore how to handle readiness checks later in this guide. Let’s look at an example for an application that is a Node.js API (for Quotes) that uses Docker and has a Postgres database to store the data. The full Node.js Postgres tutorial is available if you want to learn and understand how it is built. A working docker compose file with both the Node.js API and Postgres database services using links to make it possible for the Node.js container to talk to the db can look like the below: version: '3.8'services: db: image: postgres:14.1-alpine restart: always environment: - POSTGRES_USER=postgres - POSTGRES_PASSWORD=postgres ports: - '5432:5432' volumes: - db:/var/lib/postgresql/data - ./db/init.sql:/docker-entrypoint-initdb.d/create_tables.sql api: container_name: quotes-api build: context: ./ target: production image: quotes-api restart: always links: - db ports: - 3000:3000 environment: NODE_ENV: production DB_HOST: db DB_PORT: 5432 DB_USER: postgres DB_PASSWORD: postgres DB_NAME: postgres volumes: - ./:/src - /src/node_modulesvolumes: db: driver: local You can understand more about a similar docker compose file for Docker and Postgres. The above example defines is using docker compose file version 3.8. You have defined two services in this docker-compose.yml file. The first is the db and the second one is the api. You can find see the Dockerfile used for the api too. The db service uses the official Postgres version 14.1 with the alpine flavor as it is a small image and ultimately a smaller container. It uses some environment variables and volumes, you can read about it more on Postgres with docker compose. Next, the API container is defined that uses a local Dockerfile built with the production context. It also has some needed environment variables defined and runs on port 3000. It also has volumes so that the data is persisted across container restarts. The main part here is links (highlighted in yellow in the above code snippte) says that the API links to the db and can communicate with it. If you start both services with docker compose up it will give a log like the one below: Notice that before the DB has fully started the API has started, this will be similar even if depends_on is used without a condition. In the next section, you will learn about the depends on and service_healthy condition. Docker compose depends on under the covers # Docker compose depends_on only specifies the order in which the services should start when no conditions are used. While depends_on ensures that services start in the correct order, it's important to understand that it does not guarantee that the dependent service is ready to accept connections or requests. To address this, you can use additional tools or techniques to check the readiness of the dependent service. One common approach is to use a health check script or tool that verifies the service's availability before proceeding. For example, you could use a script that checks if the PostgreSQL database is listening on port 5432 before starting your Node.js application. Wait for it is a tool you can reach but it is not the docker compose native way of waiting for the dependent service to be ready. So what is the native docker compose depends_on way to do it, you will know about it next. Services startup condition with service_healthy # Docker Compose added a new condition called service_started and service_healthy conditions. The service_healthy condition allows you to specify that a service should only start after another service has successfully started with a health check. This provides a more fine-grained level of control in conjunction with depends_on. With service_healthy, you can specify a condition that checks the health of the dependent service. For example, you can specify that a service should only start after the dependent service has started and is listening on a specific port. This ensures that the dependent service is ready to accept connections before the service that depends on it starts. You can use just depends_on with docker compose but it will only make sure that the dependent services start before the one that depends on it. To make sure that the dependent service has started and is ready/healthy to start accepting requests/connections use depends_on with a health check on the dependent service. You can have a look at an example in the next section. Example with depends_on and service_healthy condition # Let's consider a practical example to illustrate how to use depends_on with the service_healthy condition. You will use the same Node.js API that connects to a PostgreSQL database to fetch quotes. You want to ensure that the PostgreSQL database starts and can accept connections before your Node.js application starts. Here's how you can achieve this with a docker-compose.yml file: version: '3.8'services: db: image: postgres:14.1-alpine restart: always healthcheck: test: ["CMD-SHELL", "pg_isready -U postgres"] interval: 5s timeout: 5s retries: 5 environment: - POSTGRES_USER=postgres - POSTGRES_PASSWORD=postgres ports: - '5432:5432' volumes: - db:/var/lib/postgresql/data - ./db/init.sql:/docker-entrypoint-initdb.d/create_tables.sql api: container_name: quotes-api build: context: ./ target: production image: quotes-api restart: always depends_on: db: condition: service_healthy ports: - 3000:3000 environment: NODE_ENV: production DB_HOST: db DB_PORT: 5432 DB_USER: postgres DB_PASSWORD: postgres DB_NAME: postgres volumes: - ./:/src - /src/node_modulesvolumes: db: driver: local In this example as the above one, you have two services: db and api. The db service uses the official PostgreSQL 14.1 Alpine image and exposes port 5432. It also defines environment variables for the database user, password, and database name. The api service represents your Node.js application, which depends on the db service. This dependency is specified using the depends_on option. The Node.js app specifies that it depends on the db service. The additional condition on the depends on you have specified is the dependent service has to be healthy with service_healthy. It has been highlighted in yellow in the above code. This makes sure that the api services starts only after the db service is healthy (able to receive connections). On the db service a healthcheck is defined that uses the pg_isready tool. There are some options for the health check, first one is the test for the health check. To check if the DB is healthy you are using the pg_isready tool that checks the connection status of a PostgreSQL server, the exit status for this command determines the status. There are other options for health check like interval, timeout, and retires. For this example, you are specifying a retry of a maximum of 5 times every 5-second interval with a timeout of 5 seconds. All this has been highlighted in yellow in the above code snippet. There is also the service_completed_successfully option. This option specifies that a dependency is expected to run to successful completion before starting a dependent service. It is another option you can use if you deem it is the right one. For this example, you will use the serivice_healthy option as specified in the above docker-compose file. When you run docker compose up, Docker Compose will start the db service first, followed by the api service. This ensures that the PostgreSQL database runs before your Node.js application attempts to connect to it. As seen above, the Node.js API this time only started after the Postgres database had started and was deemed healthy by the pg_isready tool that tried to connect using the postgres user. So once you see this log, you can go to http://localhost/quotes and you are sure to see the output on the database. Which takes us to the end of this beginner's guide. Conclusion # The depends_on option in Docker Compose is a valuable tool for ensuring that services start-up in the correct order with dependiences in a healthy state. By specifying dependencies between services, you can avoid errors and ensure your application functions seamlessly. However, it's important to remember that depends_on only controls the startup order and does not guarantee service readiness by default. Use additional tools or techniques, such as health checks, to verify the availability of dependent services before starting services that rely on them. By understanding and effectively using depends_on, you can streamline the development and deployment of your multi-container applications, ensuring they run smoothly and reliably. I hope you learned the proper way to use docker compose depends on, keep containerizing and using docker compose more effectively!

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