Sharing a database between Rails apps on Heroku

Heroku has a convenient new way to share add-ons between apps. Since their postgres service is one such add-on, it becomes extremely easy to get two apps to talk to the same database.

The easiest and most natural way to create the postgres add-on initially is simply to deploy the first Rails application to Heroku. Other than adding the pg and rails_12factor gems to the production group of the Gemfile as usual, no other steps need to be taken here.

The second application can be created and deployed in a similar fashion, and Heroku will automatically add the postgresql add-on and configure it to point to a different database. The first step, then, is to immediately remove this add-on from the second app with heroku addons:destroy heroku-postgresql.

Next, you need to find out the name of the first app’s postgresql add-on. Run heroku addons from the first app’s local folder. The output will contain something along the lines of heroku-postgresql (<db name>). We are interested in the <db name> part, and it usually comes in the form ancient-savannah-12345 as per Heroku’s default naming scheme.

Now the second app can be linked to the first app’s postgresql add-on by running heroku addons:attach <db name> -a <second-app-name> --as <meaningful-name>. The --as <meaningful-name> part is optional, but it’s not a bad idea to give this shared add-on a name that reflects its role, the fact that it’s shared, or what have you.

Finally, the second app needs to be configured to point to the correct Heroku database. Near the bottom of config/database.yml there are commented-out instructions for doing just that. Start by removing the default configuration of the production db, and then uncomment

  production:
    url: <%= ENV['DATABASE_URL'] %>

Since the second app’s original postgres add-on was destroyed in the first step, the 'DATABASE_URL' environment variable is not set. In the first app’s local folder, run heroku config:get DATABASE_URL. The output will look like this:

$ heroku config:get DATABASE_URL
postgres://XXXXX:YYYYY@#####.compute-1.amazonaws.com:####/#####

This full string is the address for the first app’s postgres server together with the app’s authentication credentials.

Now, in the second app’s local folder run heroku config:set DATABASE_URL=<postgres server>. With that done, commit and deploy to Heroku. To test that the setup is functional, run heroku pg:psql from the second app’s folder. You should be able to connect to the shared database and run queries.