Practice Strangler Fig pattern vocabulary: strangle the legacy system, coexistence phase, feature parity, gradually migrating traffic, and the dark system concept.
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Where does the name 'Strangler Fig pattern' come from?
The strangler fig is a tree that wraps around a host, eventually replacing it. Martin Fowler used this metaphor for the pattern: new code wraps around the legacy system, gradually taking over functionality until the old system can be removed.
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In the Strangler Fig pattern, what does 'new code handles new requests' mean?
The Strangler Fig strategy routes new functionality to the new system while letting the legacy system continue handling existing features. Over time, more and more requests are 'strangled' — migrated — to the new system.
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What is the 'coexistence phase' in a Strangler Fig migration?
The coexistence phase is when both systems run in parallel. A routing layer (proxy, feature flag, or API gateway) directs traffic. This phase ends when the new system reaches full feature parity and all traffic is migrated.
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What does 'feature parity' mean in the context of a legacy system migration?
Feature parity means the new system can do everything the old system could do. You can't retire the legacy system until you've achieved feature parity — otherwise you'd break functionality that users depend on.
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What does it mean when engineers say 'the old system is now dark'?
'The old system is dark' means it's been decommissioned — no traffic, no requests, shut down. Getting the legacy system to 'go dark' is the final milestone of a successful Strangler Fig migration.