Update release document

releases/0.5.2.md

@@ -2,7 +2,14 @@
 
 ## Overview
 
-A quick release to improve postgres support.
+A quick release to avoid a bug leading to incredibly large cleanup queue sizes and resulting in high
+CPU use. Upgrading to 0.5.2 is recommended for any deployment that recently upgraded to 0.5.0 or
+0.5.1.
+
+### Bugfixes
+
+- [At-Most Once Cleanup Jobs](#at-most-once-cleanup-jobs)
+
 
 ### Changes
 
@@ -18,6 +25,27 @@ of pict-rs.
 
 ## Descriptions
 
+### At-Most Once Cleanup Jobs
+
+pict-rs 0.5.2 now is smarter about queueing certain cleanup jobs. Previously, it might queue an
+arbitrary number of the same cleanup job while under significant load, resulting in many redundant
+jobs consuming CPU. The reason for this is pict-rs' strategy for handling variants and proxied media
+that hadn't been accessed within the configured timeframe was to queue a job to check every 30
+seconds. This is fine in normal conditions, but can be a problem after upgrading to 0.5 from 0.4.
+
+During the upgrade, pict-rs marks all variants as having been accessed at the time of the upgrade,
+since this information was not tracked in 0.4. This results in many variants being queued for
+cleanup exactly 1 week (or otherwise configured cache duration) after upgrading. When the cleanup
+queue fills with these cleanup jobs, it takes longer to process them all than the 30 second window
+between queues of the general cleanup jobs, resulting in redundant cleanup jobs that spawn the same
+redundant removal jobs, increasing the queue size by the number of outdated variants each time.
+
+By adding an option to the queueing system to mark certain jobs as unique, pict-rs can now ensure
+that only one copy of these general cleanup jobs can be queued or running at any given time,
+ensuring that all individual variant cleanup jobs will be queued after a general cleanup job runs
+before the next general cleanup job is queued.
+
+
 ### Support TLS in Postgres Connections
 
 The postgres repo now supports some new options to enable connecting via TLS. If the postgres
@@ -56,6 +84,7 @@ PICTRS__REPO__USE_TLS=true
 PICTRS__REPO__CERTIFICATE_FILE=/path/to/ca/certificate.crt
 ```
 
+
 ### Compiler Version Requirements
 
 pict-rs 0.5.2 now takes advantage of rust's native support for AFIT (Async Fn In Trait) for the