01Legacy adapterSERVES RESPONSE
$146.70
Discount $11.25 · Shipping $7.9502 / SYSTEM EVOLUTION
Change the engine.
Keep the contract.
Explore a safer path from legacy to modern: compare quietly, release gradually, and keep a way back.
01 / CONFIGURE
Choose your rollout.
Used in Canary mode. A stable key keeps a customer in the same bucket.
Sample request
FAILURE INJECTION
Candidate checks free shipping before discount instead of after it.
02 / OBSERVE
One request. Two paths.
REQUESTcustomer-demo-42
↓
MIGRATION ROUTERLegacy serves · candidate compares
02Modern adapterSHADOW COMPARE
$146.70
Discount $11.25 · Shipping $7.95Contract preserved.
Both implementations agree. The legacy result remains authoritative.
Deterministic routing bucket
Calculating…Legacy bucketCandidate bucketOutlined = current key
A rollback is a routing decision.
Return subsequent requests to legacy. Completed side effects are not reversed.
!
The subtle rule that a rewrite could break.
A $150 member basket gets an $11.25 discount. Its discounted subtotal is $138.75, below the $150 free-shipping threshold. The baseline charges $7.95 shipping. Turn on the pricing regression in Shadow mode to catch a candidate that silently changes this rule.
What this lab demonstrates
Compatibility before cleanup
Golden cases and a seeded 10,000-request Java comparison check business behavior before routing changes.
Evidence before promotion
Real promotion also needs representative traffic, error budgets, latency monitoring and an observation window.
An explicit rollback boundary
This read-only example has no database migration or external side effects. A flag cannot recover dropped data.