How to alias environment variables?

[guillaumebriday]

The documentation specifies that tags can be used to assign additional variables to particular hosts: https://kamal-deploy.org/docs/configuration/environment-variables/.

However, I need to use a specific name for my environment variable because the PostgreSQL container requires it—in my case, POSTGRES_PASSWORD.

I intend to deploy two PostgreSQL instances, each with a different password, while keeping the environment variable name consistent.

Below is a simplified version of my deployment configuration file:

accessories:
  db:
    image: postgres:16
    host: 192.168.0.1
    env:
      secret:
        - POSTGRES_USER
        - POSTGRES_DB
        - POSTGRES_PASSWORD

  db-replica:
    image: postgres:16
    host: 192.168.0.2
    env:
      secret:
        - POSTGRES_USER
        - POSTGRES_DB
        - POSTGRES_PASSWORD
        # I want to map REPLICA_POSTGRES_PASSWORD to `POSTGRES_PASSWORD` here
        # - REPLICA_POSTGRES_PASSWORD

Do you know if it possible?

[mhenrixon]

You do that with tags at the top level:

env:
  clear:
    MYSQL_USER: app
  secret:
    - MYSQL_PASSWORD
  tags:
    monitoring:
      MYSQL_USER: monitoring
    replica:
      clear:
        MYSQL_USER: readonly
      secret:
        - READONLY_PASSWORD

https://kamal-deploy.org/docs/configuration/environment-variables/#example-configuration

[frenkel]

That makes READONLY_PASSWORD available under that name, not MYSQL_PASSWORD, right? Or how does it know what name to pick for the content of READONLY_PASSWORD?

[mhenrixon]

You are right! This is how the environment variable redirection is done:

  db-replica:
    image: postgres:16
    host: 192.168.0.2
    env:
      secret:
        - POSTGRES_USER:MY_SOURCE_POSTGRES_USER
        - POSTGRES_DB:MY_SOURCE_POSTGRES_DB
        - POSTGRES_PASSWORD:MY_SOURCE_POSTGRES_PASSWORD        

The above instructs kamal to give the container secret environmnent variable of POSTGRES_USER and by reading the value from the environment variable MY_SOURCE_POSTGRES_USER

[frenkel]

Great, thanks! Were did you find this?

[mhenrixon]

Great, thanks! Were did you find this?

Experimentation and reading the source code.