hardDesignConceptMidSeniorStaffLead
How do you decide where one service ends and the next begins, and when is a modular monolith the more honest answer?
7 min readUpdated 2026-08-15
Answer
Short answer
Draw boundaries on data ownership so exactly one service writes each table, then accept that a workflow crossing them needs a saga of compensating steps and an outbox to make the write and the publish atomic.
Microservices are useful only when the service boundaries match ownership and change patterns. Good microservices design asks which data changes together, which team owns the decisions, and where transactions need to end. Weak service boundaries split code by nouns and create distributed transactions. Strong service boundaries may lead to microservices, a modular monolith, or a hybrid, depending on where coordination is cheapest.
Microservices should reduce coordination, not multiply it across a transaction.
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Likely follow-ups
- Two services keep needing the same customer record. What are your options besides sharing the table?
- A saga's compensating step fails. What now?
- How would you extract one service out of an existing monolith without a big-bang cutover?
- Your consumers receive the same event twice. Whose problem is that, and where is it solved?
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