The Strangler pattern modernizes a monolith by incrementally routing functionality to new microservices while the old system keeps running, until the monolith is gradually replaced.
Key Points: • Migration happens in small, incremental slices rather than a risky big-bang rewrite of the whole system at once. • New features can be built and shipped as microservices while the legacy system continues serving everything not yet migrated. • Each migrated slice can be tested and validated in production behind a routing layer before the old code path is retired, limiting the blast radius of any single migration step. • Teams build expertise with microservices gradually, rather than needing to master the new architecture all at once for a full cutover. • Business risk is reduced overall, since a failure in one migrated piece doesn't require rolling back the entire modernization effort.
Example: A retailer might start by extracting only the Product Search functionality into a new microservice behind a routing layer, verify it in production while the monolith still handles checkout and inventory, and then repeat the process service by service until the monolith eventually only handles a shrinking sliver of functionality.
Interview Tip: A concise interview answer is:
"The Strangler pattern lets you modernize incrementally, migrating one piece of functionality at a time behind a routing layer while the monolith keeps serving everything else. That limits risk compared to a big-bang rewrite, lets teams learn microservices gradually, and keeps the system shippable throughout the whole migration."