Zero-Downtime Cloud Migration
Migrated 100TB to the cloud with zero customer-visible downtime
An enterprise SaaS client
An enterprise SaaS with brutal uptime SLAs needed to leave a colo facility that was being decommissioned. They had 100TB of operational data, customers in three regulated industries, and contracts that paid penalties for any minute of downtime. The path of least resistance - a maintenance-window cutover - was off the table. I led the migration as a parallel-run: every byte was replicated continuously, traffic shifted gradually behind a routing layer, and any phase could be reversed in minutes. We finished the migration with literally zero customer-visible downtime.
This is a representative architecture study based on real project patterns. Specific metrics and client details have been generalized to protect confidentiality.
Results
What changed, in numbers
The metrics the engagement is measured by.
0 minutes
Downtime
during the entire migration
100TB+
Data Migrated
with zero data loss
40%
Cost Reduction
infrastructure cost savings
+35%
Performance
improvement in p95 response times
Challenge
What was broken
An on-premise footprint that was rotting, contractual SLAs that didn't tolerate downtime, and a regulator that needed to approve the new architecture before any customer data crossed the boundary. The application also had a 15-year-old core with quirks the original engineers were no longer at the company to explain.
Solution
The shape of the fix
A parallel-run migration with continuous replication, per-tenant traffic shifting behind a routing edge, instant rollback at every phase, and weekly chaos exercises to prove the rollback worked. Boring on the day of cutover - which is exactly the goal.
Approach
How I tackled it
The concrete moves that took the project from broken to shipped.
Stood up the target AWS environment as a full parallel deployment, not a phased one
Built continuous replication for both transactional and blob data with end-to-end checksum validation
Added a routing edge that could shift traffic per-tenant, per-region, per-feature with instant rollback
Rehearsed cutover and failback weekly in a chaos-engineering style, including pulled-cable tests
Coordinated with the regulator early so the architecture was pre-approved before live data moved
Decommissioned the old environment only after 30 days of zero-issue parallel run
Outcomes
What shipped, and what it changed
Measured results from the engagement, told as a story rather than a scoreboard.
Zero minutes of customer-visible downtime across the entire 6-month migration
100TB+ of data migrated with zero data-loss incidents
Cut steady-state infrastructure spend by 40% versus the colo footprint
Improved p95 application response time by 35% on the new platform
Cleared regulator review on the new architecture without findings
Stack
Technologies used
Linked entries open the technology page with related studies, playbooks, and notes.
Services
How I helped
The specific services involved in this engagement. Each links to a deeper breakdown.
Lessons
What I would tell the next team
The takeaways I carry into every similar engagement.
Parallel-run is the answer to almost every 'how do we migrate without downtime' question
Rollback is not a hope. If you have not rehearsed it under load this week, you do not have it
Regulators say yes faster when you bring them in early. They hate surprises more than they hate change
Related
Other studies you might recognize
Engagements with overlapping problem shapes, industries, or stacks.
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