Zero-Downtime Database Migrations at Scale
Migrating 500TB of data from MongoDB to PostgreSQL while serving production traffic. The strategies, tools, and lessons that made it possible.
Migrating 500 terabytes of production data from one database to another while maintaining 99.99% uptime sounds impossible. Our team proved it wasn't. Here's how we did it.
The challenge was daunting. Five years of accumulated data across dozens of MongoDB collections needed to move to PostgreSQL. The business requirement was clear: zero downtime. No maintenance windows. No impact to users.
Our strategy relied on three key principles: dual-write, incremental migration, and careful validation. We built a abstraction layer that wrote to both databases simultaneously while reading from MongoDB. This gave us time to migrate historical data without pressure.
The migration itself used custom ETL pipelines written in Go for performance. We batched records, transformed schemas on the fly, and handled failures gracefully with automatic retries. Progress was tracked in a separate coordination database, allowing us to resume from any point.
Validation was continuous and multi-layered. Checksum comparisons ensured data integrity. Automated tests ran queries against both databases, comparing results. We even built a shadow traffic system that replayed production reads against PostgreSQL, logging discrepancies.
The cutover was anticlimactic—exactly as planned. We gradually shifted read traffic to PostgreSQL over 48 hours, monitoring error rates and performance metrics. When 100% of reads hit PostgreSQL without issues, we stopped dual-writes and decommissioned MongoDB.
Total migration time: 6 weeks. User-facing incidents: zero. Data loss: zero. The key was thorough planning, automated testing, and the patience to move incrementally rather than attempting a risky big-bang migration.