Contract
A shared agreement that defines inputs, outputs, behavior, and guarantees. In system design, contracts reduce ambiguity.
Definition
A contract is an agreement about how something behaves.
In systems, contracts show up as API behavior, event schemas, SLO targets, and what is considered in scope.
Why it matters
Contracts turn “vibes” into something you can test.
They also reduce redesign. If you agree on the contract early, you spend less time re-litigating goals later.
Common contracts in system design
- Product contract: what users can do and what “done” means.
- API contract: inputs, outputs, error behavior, timeouts.
- Data contract: schema, semantics, and ownership of fields.
- Operational contract: SLOs, on-call ownership, rollback expectations.
A useful question
When someone proposes a component, ask:
“Which part of the contract does this satisfy.”