How Reddit Migrated Comments Functionality from Python to Go

Reddit migrated their comments functionality from a legacy Python monolith to a Go microservice, handling their largest dataset and highest write throughput. The migration used "tap compare" for read operations and "sister datastores" for writes, allowing validation with real traffic while maintaining zero risk. Key challenges included cross-language serialization issues, database access pattern differences, and race conditions in verification. The migration succeeded with zero user disruption and delivered an unexpected bonus: p99 latency was cut in half, dropping from occasional 15-second spikes to consistently under 100 milliseconds.