Bindable environment variable not working when using Postgres connection pool on App Platform

Question

I have a Django app running on the app platform. I want to switch over to use connection pooling.

I have an existing database my_db.
I’ve set up a Postgres connection pool my_pool.
I’ve detached and reattached the postgres component to my app, configured with the new connection pool

I’ve updated my environment variable according to the the documentation here

Additionally, you can use these same variable names to refer to your database connection pool (PostgreSQL only). To do so, use the syntax ${<database component name>.<connection pool name>.VARIABLE_NAME}. For example, to create a bindable variable for a pool’s connection string, you can use ${mydb.mypool.DATABASE_URL}.

So now my (abbreviated) appspec now looks like:

databases:
- cluster_name: postgres
  db_name: my_db
  db_user: my_db
  engine: PG
  name: postgres
  production: true
  version: "13"
  
# ...

envs:
- key: DATABASE_URL
  scope: RUN_AND_BUILD_TIME
  value: ${postgres.my_pool.DATABASE_URL}

This flat out doesn’t work. The error from the build is:

07:24] django.core.exceptions.ImproperlyConfigured: Set the {postgres.my_pool.DATABASE_URL}

Which would indicate that the environment variable hasn’t been set correctly. Why doesn’t this format work?

tomahony · Answer

To follow up, if I set two environment variables:

envs:
- key: DATABASE_URL
  scope: RUN_AND_BUILD_TIME
  value: ${postgres.DATABASE_URL}
- key: DATABASE_POOL_URL
  scope: RUN_AND_BUILD_TIME
  value: ${postgres.my_pool.DATABASE_URL}

After deployment, echoing the two variables via the console confirms that ${postgres.my_pool.DATABASE_URL} format doesn’t work:

/app # echo $DATABASE_POOL_URL
${postgres.my_pool.DATABASE_URL}

The variable isn’t being replaced, it’s just being set as the literal string ${postgres.my_pool.DATABASE_URL}. In other words, variable binding isn’t working.

Bobby · Answer

Hi there,

I’ve tested this with the following app spec file and it seems to be working as expected:

alerts:
- rule: DEPLOYMENT_FAILED
- rule: DOMAIN_FAILED
databases:
- cluster_name: db-postgresql-fra1-1234
  db_name: defaultdb
  db_user: doadmin
  engine: PG
  name: db-postgresql-fra1-1234
  production: true
  version: "15"
ingress:
  rules:
  - component:
      name: sample-nodejs
    match:
      path:
        prefix: /
name: sample-nodejs
region: fra
services:
- environment_slug: node-js
  envs:
  - key: DATABASE_URL
    scope: RUN_TIME
    value: ${db-postgresql-fra1-1234.your-pool.DATABASE_URL}
  git:
    branch: main
    repo_clone_url: https://github.com/digitalocean/sample-nodejs.git
  http_port: 8080
  instance_count: 1
  instance_size_slug: basic
  name: sample-nodejs