War Story: Ditching MongoDB 6 for PostgreSQL 17 Saved Our 50M User App from Data Inconsistencies Our social commerce platform hit 50 million monthly active users in Q3 2024, powering 1.2 million daily transactions across 30 countries. For three years, we’d run on MongoDB 6, lured by its flexible document model and horizontal scaling. But by mid-2024, data inconsistencies were threatening our core business: duplicate orders, missing user preferences, mismatched inventory counts, and support tickets piling up at a rate of 2,000 per day. The Slow Creep of Inconsistency MongoDB’s eventual consistency model worked fine when we had 5 million users. But as we scaled, sharded clusters and cross-region replication introduced race conditions we couldn’t ignore. A user updating their shipping address might see the change reflected in the app but not in the order processing pipeline. Flash sales triggered duplicate order creation because write conflicts weren’t handled atomically.…