How can we control the startup order of services in Docker compose?

To control the startup order of services in Docker Compose, you can use the depends_on option. This option allows you to specify dependencies between services, ensuring that certain services start before others. However, it is important to note that depends_on only ensures that the dependent services are started, not that they are fully ready to accept connections.

Here are the key points to consider:

1. Using depends_on:

   • The depends_on option specifies the order in which services are started. For example, if service A depends on service B, service B will start before service A.
   • Example:
     yaml Copy

     version: "3"
     services:
       web:
         build: .
         depends_on:
           - db
       db:
         image: postgres

2. Handling Service Readiness:

   • Docker Compose does not wait for a service to be fully ready (e.g., a database accepting connections) before starting the next service. It only waits until the service is running.
   • To handle this, you can use wrapper scripts or tools like wait-for-it, dockerize, or sh-compatible wait-for to poll a given host and port until it is accepting TCP connections[1][2][3][4].

3. Example with wait-for-it.sh:

   • You can wrap your service’s command with a script that waits for the dependent service to be ready.
   • Example:
     yaml Copy

     version: "3"
     services:
       web:
         build: .
         depends_on:
           - db
         command: ["./wait-for-it.sh", "db:5432", "--", "python", "app.py"]
       db:
         image: postgres

4. Custom Wrapper Script:

   • Alternatively, you can write your own wrapper script to perform a more application-specific health check.
   • Example script (wait-for-postgres.sh):
     sh Copy

     #!/bin/sh
     set -e
     host="$1"

...