Creating and Managing Heroku Postgres Follower Databases
Last updated 20 July 2015
Table of Contents
You can also use
heroku pg:psql with followers to safely run ad-hoc queries against your production data.
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.
Followers are only supported on Standard, Premium, and Enterprise tier database plans. Follow these steps to upgrade from a Hobby tier (dev or basic) plan to a production plan.
Followers like all Heroku Postgres databases are charged on a pro-rated basis based on the plan of the follower.
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.
The follower must be able to accommodate the current data volume of the master database. If an attempt is made to create a follower that cannot accommodate the data volume, the follower will not be created and an error message will indicate the minimum plan needed.
When a database is ready to support followers, that information will be shown in
$ heroku pg:info === HEROKU_POSTGRESQL_PURPLE_URL (DATABASE_URL) ... Fork/Follow: Available ...
The lag between a master and follower databases varies greatly depending on the amount and frequency of data updates. It is possibile for long running queries on the follower to increase your lag time, though once those queries are done your follower should catch up. Under normal usage it is common for your follower to be within a few seconds or less of your leader.
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 pg:info === HEROKU_POSTGRESQL_CHARCOAL_URL (DATABASE_URL) Plan: Standard 0 Status: available ...
If more than one database is listed, the one currently serving as the master will most often be the one assigned to the
DATABASE_URL (listed after the database name).
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.
Followers do not have to be the same database plan as their master. Production tier database plans can be followed by, and can follow, all other production tier plans. If you are on an older 32-bit machine then the follower may only be followed by the same plan, you can identify this by running heroku pg:info on your database.
$ heroku addons:create 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
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
Unfollowing a database is not the same as de-provisioning it. You will still be charged for the database. To completely de-provision a database use the
heroku addons:destroy HEROKU_POSTGRESQL_WHITE command.
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.
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
Heroku Postgres premium plans have the HA feature with automated failover. Read more about how it works.
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.
Follower databases are guaranteed to be provisioned on geographically separate infrastructure than the primary database, providing purely additive uptime over a single database.
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.