Reddit Migrates Comment Backend from Python to Go Microservice to Halve Latency

"Reddit has completed amajor rebuild of its comment backend, migrating from a legacy Python system to a domain-specific Go microservice to improve performance and reliability. The change addresses long-standing challenges with latency and scalability in one of Reddit's highest-write systems, while laying the groundwork for modernizing other core models. The migration followed a multi-phase strategy designed to preserve correctness and minimize user disruption."

"Reddit engineers first implemented all comment read endpoints in Go and validated them using a tap-compare testing approach. In this method, a portion of live traffic is sent to the new service, and its responses are compared against the legacy Python system. In contrast, only the original responses are returned to users. This allowed engineers to detect discrepancies safely in production before fully switching traffic."

"Writes were more complex because comment creation touches multiple datastores: PostgreSQL for persistence, Memcached for caching, and Redis for change data capture (CDC) events. To prevent conflicts with production data, Reddit deployed sister data stores for the Go service during tap-compare tests. These stores mirrored the production schema but handled only test writes, enabling the team to validate behavior without risking live data corruption. In total, Reddit tested three write endpoints across three datastores, creating 18 separate validation paths."

Reddit migrated the comment backend from a legacy Python system to a domain-specific Go microservice to improve performance and reliability and address latency and scalability challenges in a high-write system. Engineers implemented read endpoints in Go first and validated them via a tap-compare testing approach that routed a portion of live traffic to the new service while returning only original Python responses to users. Writes involved PostgreSQL, Memcached, and Redis, so Reddit deployed sister datastores that mirrored production schema for safe test writes. Three write endpoints across three datastores produced 18 validation paths. The migration revealed edge cases requiring CDC consumer validation and mitigation of increased database load from direct Go writes.

Read at InfoQ