Creating and Managing Heroku Postgres Follower Databases

Last Updated: 06 February 2015

follow postgres replica upgrade

Database replication serves many purposes including increasing read throughput with a master-slave configuration, additional availability with a hot standby, serving as a reporting database, and seamless migrations and upgrades. Though these strategies all serve different purposes they are all based on the ability to create and manage copies of a master (or lead) database. On Heroku Postgres this functionality is exposed as the follow feature.

A database follower is a read-only copy of the master database that stays up-to-date with the master database data. As writes and other data modifications are committed in the master database the changes are streamed, in real-time, to the follower databases.

Create a follower

A follower can be created for any Standard, Premium, or Enterprise tier database that is itself not a follower (that is, followers cannot be chained). Followers cannot be created for a period on newly forked databases (this applies to both explicit forks and forks created through unfollow). The exact timeframe varies depending on the size of the database to be followed, and is typically between a few minutes and a few hours. When a database is ready to support followers, that information will be shown in heroku pg:info:

$ heroku pg:info
=== HEROKU_POSTGRESQL_PURPLE_URL (DATABASE_URL)
...
Fork/Follow: Available
...

To create a follower database you must first know the add-on name of the master database. Use heroku pg:info to find its HEROKU_POSTGRESQL_*COLOR*_URL name.

$ heroku pg:info
=== HEROKU_POSTGRESQL_CHARCOAL_URL (DATABASE_URL)
Plan:        Standard 0
Status:      available
...

Create a follower database by provisioning a new heroku-postgresql Standard, Production or Enterprise tier add-on database and specify the master database to follow with the --follow flag. The flag can take either the config var name of the database on the same app, an argument of the form appname::HEROKU_POSTGRES_COLOR_URL, or the full url of any Heroku Postgres database.

$ heroku addons:add heroku-postgresql:standard-2 --follow HEROKU_POSTGRESQL_CHARCOAL_URL
Adding heroku-postgresql:standard-2 to sushi... done, v71 ($200/mo)
Attached as HEROKU_POSTGRESQL_WHITE
Follower will become available for read-only queries when up-to-date
Use `heroku pg:wait` to track status

Preparing a follower can take anywhere from several minutes to several hours, depending on the size of your dataset. The heroku pg:wait command outputs the provisioning status of any new databases.

$ heroku pg:wait
Waiting for database HEROKU_POSTGRESQL_WHITE_URL... available

Unfollow

The heroku pg:unfollow command stops the follower from receiving updates from its master database and transforms it into a full read/write database containing all of the data received up to that point. This creates a database fork.

$ heroku pg:unfollow HEROKU_POSTGRESQL_WHITE_URL
!    HEROKU_POSTGRESQL_WHITE_URL will become writable and no longer
!    follow HEROKU_POSTGRESQL_CHARCOAL. This cannot be undone.

!    WARNING: Potentially Destructive Action
!    This command will affect the app: sushi
!    To proceed, type "sushi" or re-run this command with --confirm sushi

> sushi
Unfollowing... done

Database upgrades and migrations with changeovers

In addition to providing data redundancy, followers can also be used to change database plans with minimal downtime. To upgrade or migrate your database with a follower, see this guide on upgrading Heroku Postgres.

Monitoring followers

Because followers are asynchronously updated they may be behind their leader by some number of commits. You can view the number of commits your follower is behind by running heroku pg:info. If your follower is increasing in the number of commits it is behind it may be due to long running transactions on your database. Using the pg-extras plugin you can run heroku pg:ps to get currently running transactions, then if any have been running for longer than expected you may cancel those with:

$ heroku pg:kill PROCESSID

High availability with followers

Having a follower provisioned, even if not being actively used as a read slave, ensures that you always have a hot standby available for immediate promotion in situations where the primary database becomes corrupted or unavailable. As a general practice, applications desiring high-availability should provision a hot standby follower.

Manual failover

Heroku does not automatically promote a follower database when the primary database is corrupt or inaccessible. If this functionality is required, use a premium or enterprise tier plan that does offer HA. Performing a database failover is the same manual process as a database migration starting with the prevent data updates step.