Migration Architecture
On-Premises Infrastructure
Self-managed Kubernetes cluster
Azure Cloud Platform
Managed, scalable, resilient
Technology Stack Evolution
Week 1-3: Failed Approach
Unpredictable lag (500ms to 8 seconds), rollback failures
Week 3-6: Pivot to Success
Version controlled schema, easy rollback
Optimized bulk transfer, handles edge cases
Sub-second sync, log-based, resilient
Progressive Traffic Shifting Strategy
Hourly Data Validation Monitoring
Real-time validation during CDC sync phase. Caught drift before it hit production.
What We Actually Learned
One Tool Can't Do Everything
SQL Server replication was supposed to be our entire migration solution. It wasn't good at any single job. The moment we split the work into schema versioning, bulk transfer, and continuous sync, everything became simpler and more reliable.
Validation Catches Everything
Hourly row count checks and checksum validation found problems we would have completely missed. Not because we're bad at testing, but because real production data is weird. Edge cases exist. Validation caught them before they mattered.
Feature Flags Are Essential
Progressive traffic shifting with feature flags meant we could ship confidently. If 1% broke, only a few requests were affected. By the time we hit 100%, we'd validated against massive scale with zero risk.
Test Failures Save Production Incidents
Week 3's failure was painful internally but invaluable. We caught it before production. We learned exactly what was wrong. We fixed it. Then we executed flawlessly because we'd already failed once in a safe place.
Observability First, Always
We deployed monitoring before we deployed apps. When something went wrong during traffic shifting, we saw it in seconds, not hours. The observability infrastructure was the thing that let us move fast safely.
Key Takeaways
Zero Downtime Migration Isn't About Perfection
It's about building escape routes before you need them. Feature flags, observability, validation, rollback procedures. If any one of those failed, we would have had an incident. Having all of them meant we could move confidently.
Failing in Testing Prevents Failing in Production
Week 3's crisis was the best thing that could have happened. We failed safely. We learned exactly what was wrong. We fixed it. Then we executed flawlessly because we'd already lived through the failure once.
Progressive Delivery is More Important Than Speed
We could have forced all 300 apps over in one big bang. We would have crashed. Instead, we moved slowly. 1%, then 10%, then 50%, then 100%. Each stage validated everything at that scale. By the time we hit 100%, we'd proven it worked 4 different ways.
The Right Tools Matter
Liquibase, DMS, and CDC weren't fancy or novel. But each one was designed for the specific job it had to do. Using them meant we weren't fighting the tools. We were using them correctly.
Observability Enables Confidence
We deployed monitoring before we deployed applications. This is backwards from how most teams work. But it's what let us move fast. We could see problems in seconds. We could make decisions with real data instead of guesses.
For Your Team
If You're Planning a Migration
Don't assume one approach will work. Test your strategy on non critical workloads first. Let it fail. Learn from the failure. Then execute on critical workloads. The time you spend failing safely will save you catastrophic failures in production.
If You're Running the Migration
Progressive delivery isn't conservative. It's confident. You're proving each stage works at real scale before moving to the next. Feature flags and observability are your safety net. Use them.
If You're Supporting the Migration
Operations needs visibility. Deploy monitoring first. Every critical path needs dashboards. Every service needs alerts. The moment something deviates from baseline, you need to know. This infrastructure is what makes fast movements safe.
Zero downtime migrations are possible. But only if you're willing to move slowly, observe constantly, and have good fallback plans. Teams that do this don't just succeed. They do it without drama.