Docker image wait for postgres image to start

I have a standard Python docker image that needs to start after postgers is properly started in its standard image.

I understand that I can add this Bash command in the docker-compose file:

command: bash -c 'while !</dev/tcp/db/5432; do sleep 1; done; npm start'
depends_on:
  - mypostgres

But I don't have bash installed in the standard python docker image, and I'm trying to keep the installation minimal.

Is there a way to wait for postgres without having bash installed in my image?

CodePudding user response：

I have a standard Python docker image that needs to start after postgres is properly started in its standard image.

You mentioned "Python docker image", but you appear to be calling npm start, which is a node.js application, not a Python application.

The standard Python images do have bash installed (as do the official Node images):

$ docker run -it --rm python:3.10 bash
root@c9bdac2e23f9:/#

However, just checking for the port to be available may be insufficient in any case, so really what you want is to execute a query against the database and only continue once the query is successful.

A common solution is to install the postgres cli and run psql in a loop, like this:

until psql -h $HOST -U $USER -d $DB_NAME -c 'select 1' >/dev/null 2>&1; do
  echo 'Waiting for database...'
  sleep 1
done

You can use environment variables or a .pgpass file to provide the appropriate password.

If you are building a custom image, it may be better to place this logic in your ENTRYPOINT script rather than embedding it in the command field of your docker-compose.yaml.

If you don't want to psql, you can write the same logic in Python or Node utilizing whatever Postgres bindings are available (e.g., something like psycopg2 for Python).

A better solution is to make your application robust in the face of database failures, because this allows your application to continue running if the database is briefly unavailable during a restart.