Understanding Heroku Redis Log Statements and Common Errors

Last updated 14 September 2016

Heroku Redis logs to the logplex which collates and publishes your application’s log-stream. You can isolate Heroku Redis events with the heroku logs command by filtering for the redis process.

$ heroku logs -p redis -t

Besides seeing system-level Redis activity, these logs are also useful for understanding your application’s use of Redis and for diagnosing common errors. This article lists common log statements, their purpose, and any action that should be taken.

SLAVE OF …

2015-08-18T17:29:39Z app[redis]: [maturing-daily-1187] * SLAVE OF would result into synchronization with the master we are already connected with. No operation performed.

This indicates that Heroku Redis is ensuring your HA standby is following your primary Redis instances.

CONFIG REWRITE

2015-08-18T17:29:39Z app[redis]: [maturing-daily-1187] # CONFIG REWRITE executed with success.

This indicates that Heroku Redis has proceed to rewrite your Redis instance configuration, this happens whenever we need to change your instance configuration or when you trigger a configuration change.

Database memory close to the limit

2016-09-12T07:27:33+00:00 app[heroku-redis]: source=REDIS_URL sample#used_memory_over_limit=10435336.0bytes message=Database memory close to the limit. Please upgrade your database plan or change your maxmemory policy.

This log is seen when a Redis instance is using more than 90% of its plan memory limit. Since you can see this log before you actually exceed the limit, you can monitor this log to make an alert.