How long should a shadow phase run?

Long enough to cover traffic shape changes and edge cases—usually 48–72 hours, including a peak period. If your workload has weekly seasonality, run it a full week.

When do I turn on dual-writes?

After shadow equivalence on reads is stable and you’ve added idempotency keys. Start with low-risk entities, monitor reconciliation drift, and only switch reads after a week of clean dual-writes.

What if the canary fails but only for one tenant or region?

Scope the flag and traffic routing by tenant/region label. Contain blast radius by rolling back only the affected segment; keep others advancing. This is where per-tenant flags and Istio subset routing earn their keep.

Guides · Nov 24, 2025 · 9 minute read

Modernization Without the Meltdown: Reversible Thin Slices with Safety Nets and Shadow Traffic

How to move a legacy system—without lighting it on fire—using thin slices, shadowing, and kill switches that actually work.

Back to all posts

Modernization Without the Meltdown: Reversible Thin Slices with Safety Nets and Shadow Traffic

Key takeaways

Implementation checklist