ref(store): Random Kafka partitioning for sessions (#1194)

Instead of using the session ID as partition key for producing sessions, assign
a random partitioning key. This ensures more even distribution of sessions on
the Kafka topic, even if clients are sending many updates for the same session.

We have noticed intermittent imbalance on the partitions. The theory is that
clients are sending repeat updates for the same session (ID) when a high number
of errors are occurring within that session. Every session update would go to
the same partition unless we use random partitioning.

CHANGELOG.md

@@ -14,7 +14,8 @@
 **Internal**:
 
 - Spread out metric aggregation over the aggregation window to avoid concentrated waves of metrics requests to the upstream every 10 seconds. Relay now applies jitter to `initial_delay` to spread out requests more evenly over time. ([#1185](https://github.com/getsentry/relay/pull/1185))
-- Add new statsd metrics for bucketing efficiency ([#1199](https://github.com/getsentry/relay/pull/1199), [#1192](https://github.com/getsentry/relay/pull/1192), [#1200](https://github.com/getsentry/relay/pull/1200))
+- Use a randomized Kafka partitioning key for sessions instead of the session ID. ([#1194](https://github.com/getsentry/relay/pull/1194))
+- Add new statsd metrics for bucketing efficiency. ([#1199](https://github.com/getsentry/relay/pull/1199), [#1192](https://github.com/getsentry/relay/pull/1192), [#1200](https://github.com/getsentry/relay/pull/1200))
 
 ## 22.2.0
 
@@ -53,6 +54,7 @@
 - Metrics extraction config, custom tags. ([#1141](https://github.com/getsentry/relay/pull/1141))
 - Update the user agent parser (uap-core Feb 2020 to Nov 2021). This allows Relay and Sentry to infer more recent browsers, operating systems, and devices in events containing a user agent header. ([#1143](https://github.com/getsentry/relay/pull/1143), [#1145](https://github.com/getsentry/relay/pull/1145))
 - Improvements to Unity OS context parsing ([#1150](https://github.com/getsentry/relay/pull/1150))
+
 **Bug Fixes**:
 
 - Support Unreal Engine 5 crash reports. ([#1132](https://github.com/getsentry/relay/pull/1132))

relay-server/src/actors/store.rs

@@ -673,8 +673,8 @@ impl KafkaMessage {
             Self::Attachment(message) => message.event_id.0,
             Self::AttachmentChunk(message) => message.event_id.0,
             Self::UserReport(message) => message.event_id.0,
-            Self::Session(message) => message.session_id,
-            Self::Metric(_message) => Uuid::nil(), // TODO(ja): Determine a partitioning key
+            Self::Session(_message) => Uuid::nil(), // Explicit random partitioning for sessions
+            Self::Metric(_message) => Uuid::nil(),  // TODO(ja): Determine a partitioning key
             Self::ProfilingTrace(_message) => Uuid::nil(), // TODO(pierre): parse session_id as uuid
             Self::ProfilingSession(_message) => Uuid::nil(), // TODO(pierre): parse session_id as uuid
         };