Data Migration Without Downtime

Moving data between systems is one of the riskiest operations in software. Here's how to do it without losing data or sleep.

Data migrations are the part of software development that keeps CTOs up at night. You're moving the most valuable thing your company has - its data - from one place to another, and you can't afford to lose a single record or have more than a few minutes of disruption. Here's how we approach it.

The dual-write pattern

Write to both the old and new system simultaneously. Read from the old system. Verify that the new system has consistent data. Switch reads to the new system. Stop writing to the old system. This pattern lets you roll back at any point by switching reads back to the old system.

Migration checklist

• Map every field from source to destination - don't skip the ones that look obvious
• Write a verification script that compares records between old and new systems
• Test the migration on a copy of production data, not a subset
• Plan for records that change during migration (new orders, updated profiles)
• Have a documented rollback procedure and test it before go-live
• Schedule during low-traffic periods, even with zero-downtime approaches

The most common failure mode

Edge cases in the data. Null values where you don't expect them, encoding issues, dates in the wrong timezone, and records that violate constraints that weren't enforced in the old system. Always run the migration against a full copy of production data before doing it for real.

Ben Arledge

CEO & CTO, CloudOwl