SYSTEM Cited by 1 source
Zalando Returns Monolith¶
Definition¶
The Zalando Returns Monolith is the legacy monolithic application from which the Zalando Returns team extracted returns logic into the Returns Microservice. Zalando refers to it in the source post only as "a soon-to-be legacy monolithic application" — no official name is disclosed.
Role during the parallel run¶
- Remains the source of truth for all returns requests during the migration.
- Responds to client requests on the hot path — the monolith's response is what the client sees.
- Fires the async
/consistency-checksPOST to the Returns microservice after responding to the client. - Carries the full load until each endpoint reaches its consistency threshold and cuts over to the microservice via Skipper proxy rule.
Known limitations (from the source)¶
- Parts of the returns logic in the monolith had no tests, so behaviour under edge cases was not verified.
- "Dark corners" in the code base whose behaviour was unknown or forgotten.
- Criticality prevented downtime during cutover, which excluded big-bang replacement.
These constraints motivated the parallel-run choice over test-driven migration alone.
(Source: sources/2021-11-03-zalando-parallel-run-pattern-a-migration-technique-in-microservices)