Notes on the use of Staging Environments on Heroku

The setup and use of a separate staging environment is covered (almost) adequately by Heroku’s documentation on the matter.

And yet I must be missing something, as forking my app didn’t do what I expected.

In forking a running production app to create a staging clone environment what I expected to have afterwards was:

The staging app appeared to be working exactly as expected. There was a new database which contained a copy of the production database, the staging app was running the right version of the code at the appropriate URL, and it appeared to be a perfect clone of production.

Unbelievably (for me at least), the DATABASE_URL of the new staging app was set to point to the production database, including credentials. Staging was masquerading as a perfect clone of production because it was in fact pointing to the production database.

Here is the relevant portion of the Heroku documentation:

It is recommended to make sure if you have an expected Heroku Postgres setup with your target app. Please run heroku pg:info and/or heroku config command to make sure that everything has copied as you expected. If the copied database is not being the primary database (DATABASE_URL), use heroku pg:promote as described by the Heroku Postgres documentation to make it a primary database.

It does not inspire confidence that this important caveat is not highlighted more clearly, and the concepts involved not explained in more detail. In what case would we ever want to fork an app, and have it pointing at the live production database?

Another potentially important caveat, while we are forking the app, the database will be copied, i.e. the database will not be forked using the provided fork feature. This lack of clarity has likely tripped up enough people for them to warrant adding another warning to the documentation page.

I have not found these issues discussed much (or at all) online, so I can only assume that I am one of the few who has fallen foul of this. Still, if you are experimenting with staging environments on Heroku then I would recommend checking and double-checking the config of the new app (as they do indeed suggest). Heroku reduces many complex hosting-related concerns to simple commands and it’s easy to get used to that, but it seems that not all of these commands receive the same polish and attention.

Final caveat (hopefuly) is that Heroku will not necessarily use the same Postgres version: my production 9.3 database was cloned to a new instance running Postgres 9.5.

Resolving the issue

NOTE: Double and triple-check all commands before running them–the point of the article is that Heroku’s simple tooling can make it equally simple to compromise your production deployment.

The Situation

You have run Heroku’s fork command and now have a clone of your production app, but its DATABASE_URL is still pointing to that of production.

First, take a fresh backup of production before continuing:

% heroku pg:backups capture -a production-app-name

% curl `heroku pg:backups public-url -a production-app-name` -o `date +5Y5m5d`.pgbackup

With a backup safely stored, we look at the configuration of each application:

% heroku config -a production-app-name | grep URL
DATABASE_URL:               postgres://
% heroku config -a staging-app-name | grep URL
DATABASE_URL:               postgres://

Note that the staging app has a new ROSE database URL, and yet its DATABASE_URL is still pointing directly at production.

In the remaining configuration items, new credentials have been automatically set for New Relic, Papertrail and other add-ons, but for some reason not for the all-important DATABASE-URL.

The Solution

As per Heroku’s docs on the matter, we need to promote the newly-created staging database to be the primary database for the app:

% heroku pg:promote HEROKU_POSTGRESQL_ROSE_URL -a staging-app-name
Ensuring an alternate alias for existing DATABASE... done, not needed
Promoting postgresql-addon-id to DATABASE_URL on staging-app-name... done

We now confirm that the production configuration has not changed, and the staging configuration now has DATABASE_URL pointing at the new staging database, and not our production database:

% heroku config -a production-app-name | grep URL
DATABASE_URL:               postgres://
% heroku config -a staging-app-name | grep URL
DATABASE_URL:               postgres://

Updating the staging database

At some point you will want to refresh the data in staging. This can be achieved using the pg:copy command, though the first time through the output of the command will likely give you pause for thought:

% heroku pg:copy production-app-name::ROSE staging-app-name::ROSE --app staging-app-name

 !    WARNING: Destructive Action
 !    This command will remove all data from ROSE
 !    Data from ROSE will then be transferred to ROSE
 !    This command will affect the app: staging-app-name
 !    To proceed, type "staging-app-name or re-run this command with --confirm staging-app-name

> ^C !    Command cancelled.

If I could find clear guidance on how I should go about changing the colour name of my staging database I would do so, to avoid the unpleasant ambiguity in the destructive action warning.

For now, the fact that I am explicitly targetting the staging app should protect the production environment, though as we have seen this protection can easily be bypassed by the fork command as it shares the DATABASE_URL between the apps.

Allow the command to run and hopefully you should now have an up-to-date staging database.

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