Why did I briefly see Postgres read-only transaction errors for my Hobby/Mini/Basic instance?

Issue

I saw errors that looked like this in my logs for an app that uses a hobby/mini/basic tier Heroku Postgres instance:

PG::ReadOnlySqlTransaction:
ERROR:  cannot execute UPDATE or INSERT in a read-only transaction

Resolution

Hobby, mini, and basic tier plans are subject to unannounced maintenances and upgrades.

When these occurs the instance will temporarily be put in read-only mode while the maintenance is performed.

Please investigate upgrading to a production plan (standard and above) if this application is critical to your workflow. Production plans provide maintenance notifications, the ability to setup a maintenance window, and the ability to perform the maintenance manually at a specific time.

We have more details here:

• https://devcenter.heroku.com/articles/heroku-postgres-plans
• https://devcenter.heroku.com/articles/heroku-postgres-maintenance#limitations