On April 12th, a routine upgrade for our Analytics system was deployed at 10:29 a.m. ET. This upgrade improved the latency performance systemwide, but contained a bug which caused no data to be returned in specific queries for the Reviews system. On April 15th, this bug was discovered and reported internally at 10:52 a.m. ET. Engineers restored regular service at 12:04 p.m. ET.
We have patched the root cause and finished a comprehensive audit of our Analytics system for similar failure modes. Additionally, we have added more telemetry to allow us to prevent this class of errors in the future.