Zero-Downtime Cloud Migration: A Practical Playbook

Migrating a production system serving hundreds of thousands of users from DigitalOcean to AWS sounds straightforward on paper. In practice, it’s a high-stakes operation where the margin for error is zero.

Here’s the playbook we developed for migrating Homewood Health’s digital mental health platform—without a single minute of downtime.

Prerequisites for Zero-Downtime

Before touching any infrastructure:

1. Complete infrastructure documentation — You can’t migrate what you don’t understand
2. Infrastructure-as-code — Terraform modules for every component
3. Comprehensive monitoring — Know your baseline metrics
4. Rollback procedures — Tested, documented, ready to execute

The Migration Strategy

Phase 1: Parallel Infrastructure

Stand up the complete AWS environment alongside the existing infrastructure:

• EC2 instances matching current compute requirements
• RDS with replication from the existing database
• S3 buckets with cross-region replication
• CloudFront distributions configured but not active
• VPC with proper network segmentation

All managed through Terraform—no manual console operations.

Phase 2: Data Synchronization

The database is always the hardest part:

# Continuous replication setup (simplified)
pg_dump source_db | pg_restore -d target_db
# Plus WAL shipping for ongoing changes

We maintained dual-write capability during the transition window. Every write hit both databases until we confirmed synchronization.

Phase 3: Traffic Migration

DNS-based cutover with aggressive TTL reduction:

1. Reduce TTL to 60 seconds, 48 hours before migration
2. Verify both environments serve identical responses
3. Update DNS to point to AWS infrastructure
4. Monitor for 24 hours with rollback ready
5. Increase TTL back to normal values

Phase 4: Cleanup

Only after confirming stable operation:

• Decommission old infrastructure
• Archive final backups
• Update documentation
• Conduct post-mortem

What We Learned

Test the rollback. We ran three mock migrations before the real one. Each revealed something we’d missed.

Over-communicate. Stakeholders got hourly updates during the migration window. No surprises.

Keep the old infrastructure running longer than you think necessary. The cost of a few extra days is nothing compared to data loss.

Results

• Zero downtime during migration
• 30% reduction in infrastructure costs
• Improved latency for Canadian users
• Complete infrastructure-as-code coverage

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Planning a cloud migration? Let’s discuss your architecture.