Incremental extraction replaces a bounded capability while retaining working paths. The hard part is data and behavior ownership, not merely proxy routing.
Before you start
You should understand HTTP, database transactions and the difference between one process and independently failing services. Draw the participants and message direction before choosing a pattern. Include timeout, duplicate delivery and recovery in the model instead of considering only successful requests.
The practical goal is to reason through this situation: Move one read capability behind a new interface before extracting its writes. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Select a bounded capability
Choose manageable coupling rather than extracting a random table.
Step 2: Preserve authoritative ownership
Specify which component owns reads and writes at each stage.
Step 3: Migrate incrementally
Introduce an interface and route controlled traffic before removing the old path.
Worked scenario
Move one read capability behind a new interface before extracting its writes.
A read capability moves behind a new interface while writes remain authoritative in the monolith. A replicated view can support the new read path under a freshness contract. Introducing uncoordinated dual writes creates new divergence risk; routing alone does not solve migration of data ownership.
Common mistake
Dual writes create consistency failure modes unless deliberately coordinated.
Verify the behavior
Compare old and new outputs and interrupt migration to verify a safe fallback path.
Interview exercise
Choose the first extraction.
Answer and reasoning
Select a well-understood capability with manageable coupling and define authoritative data ownership throughout the transition.
Continue learning
Compare the scenario with the Microservices interview questions and test your understanding with the Microservices MCQs. For terminology and implementation details, consult the reference material.