How do I handle side-effecting endpoints (e.g., POST /checkout) with shadow traffic?

Use shadow only for read paths. For writes, use synthetic payloads in a parallel test account, or implement dual-write behind a flag to a non-authoritative store, verify parity, and discard shadow writes until you are confident. Only then promote to a small canary of real writes with strong idempotency keys and compensating transactions.

What if the new service can’t match legacy latency?

Gate by user experience thresholds, not pride. If v2 p95 is >20% slower at parity load, keep optimizing under shadow. Consider caching, circuit breakers, and precompute. If latency is still high due to unavoidable dependencies, carve a thinner slice or move those dependencies first.

Do I need a service mesh to do this?

No. A mesh helps with mTLS, retries, and mirroring, but you can do mirrors in Nginx/Envoy and canaries in Argo Rollouts with ingress controllers. Just centralize your routing so you can flip and roll back in one place.

How do I prove business impact to leadership?

Report on SLO adherence during rollout, conversion or error rate deltas for the specific flow, and MTTR during incidents. Add a before/after infra cost view if the new stack is cheaper. Tie each slice to a measurable KPI and a timeline (e.g., 2-week slice, 24–48h canary).

We’ve got AI-generated code in the new service. Is that a blocker?

Not if you treat it as a draft. Run an AI code refactoring pass, add tests around edge cases, and enforce lint/type checks. We often do a vibe code cleanup to remove accidental complexity before shadowing. Don’t ship vibe code to prod without the same parity gates.

Guides · Dec 6, 2025 · 10 minute read

Ship the Strangler, Not the Rewrite: Reversible Thin Slices with Safety Nets and Shadow Traffic

A pragmatic playbook to modernize critical systems without torching your SLOs. Reversible thin slices, shadow traffic, and hard gates—no heroics required.

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Ship the Strangler, Not the Rewrite: Reversible Thin Slices with Safety Nets and Shadow Traffic

Key takeaways

Implementation checklist