Contract Check¶
Runtime boundary contract — instantiates Invariant Guarding
Attaches preconditions, postconditions, and state assumptions to a boundary and evaluates them at runtime, so a transition is refused the moment it would break the contract.
A Contract Check attaches preconditions, postconditions, and state assumptions to a specific boundary — a function call, an API request, a module handoff — and evaluates them at runtime as the transition crosses. Its defining move is that the invariant is expressed as a contract between two parties at a named interface, checked live on the actual arguments and results of each crossing, so a caller that violates a precondition or a callee that violates a postcondition is caught at that exact boundary, in production, with the blame localized to whichever side broke its promise. It guards the live transition at the interface — not the data sitting at rest, and not a space of cases run offline.
Example¶
A payments microservice exposes charge(account, amount, currency). Its contract: a precondition that amount > 0, the currency is supported, and the account is active; a postcondition that on success the balance has decreased by exactly amount and a matching ledger entry exists. A newly deployed upstream cart service ships with a bug that, on a refund path, calls charge with a negative amount. The precondition fires at the boundary, the call is refused with a contract violation attributed to the caller, and no balance moves. Separately, a partial internal failure once left a balance decremented without a ledger entry; the postcondition assertion catches that on exit and refuses to report success. In both cases the malformed transition is stopped precisely where the two services meet, with a clear signal of which side broke the contract — rather than surfacing three hops downstream as corrupted state no one can trace.
How it works¶
- Declare the contract at the interface. Preconditions are the caller's obligation, postconditions the callee's guarantee, and invariants the state assumptions that must hold across the crossing.
- Check live on actual values. The assertions run on the real arguments, results, and state as the transition happens, not on hypothetical cases.
- Fail fast and attribute blame. A broken precondition indicts the caller, a broken postcondition the callee; the check refuses to let the crossing proceed and hands recovery back to the caller or a sibling.
- Bound to one boundary. Each contract guards a specific interface, evaluating the specific transition — not the whole store and not every path to it.
Tuning parameters¶
- Pre / post / invariant balance — how much is checked on entry vs. exit vs. across. Heavy postconditions catch internal bugs but add latency to every call.
- Enforcement mode — enforce-and-throw vs. log-and-continue vs. sampled checking. Throwing is strongest but dangerous if the contract itself is wrong; sampling trims overhead at the cost of coverage.
- Production vs. debug-only — whether contracts stay compiled in for production traffic. Keeping them on guards where real inputs arrive; stripping them saves cycles but blinds the boundary.
- Blame-attribution granularity — how precisely a breach is localized to caller vs. callee, which sets how fast an integration bug can be diagnosed.
When it helps, and when it misleads¶
Its strength is localizing responsibility at the interface: integration bugs are caught at the seam with a named culprit, instead of propagating and corrupting state several hops away where the cause is unrecoverable. This is the discipline of Design by Contract, making each boundary's obligations explicit and enforceable.[n1]
Its central failure mode is contracts disabled in production — the common practice of compiling assertions out for performance removes the guard exactly where unexpected, real-world inputs arrive, while tests that ran with contracts on lend false confidence. The classic misuse is shipping with assert disabled, so a precondition that "can never fail" silently waves a bad value through into committed state. The guarding discipline is to keep at least the cheap, high-value contracts enforced in production and to treat a contract breach as a defect to fix, not noise to silence.
How it implements the components¶
transition_scope— the contract is pinned to a named boundary (the API call or handoff), the exact crossing that must be checked.guard_condition— the pre- and post-conditions translate the invariant into a pass/refuse test evaluated as the transition crosses.validation_rule— the check evaluates the evidence (actual arguments, results, and state) that this transition preserves the invariant.
It refuses the crossing but hands recovery off; it does not restore a prior state via rollback_or_repair_policy (that's Rollback Transaction), and unlike an Invariant Test Suite it does not exercise cases offline via drift_review_cadence — it checks the one live transition in front of it.
Related¶
- Instantiates: Invariant Guarding — the Contract Check is the runtime-boundary realization of the archetype.
- Sibling mechanisms: Database Constraint · Safety Interlock · Invariant Test Suite · Policy Guardrail · Integrity Monitor · Rollback Transaction · Two-Person Rule
Editorial Notes¶
Form Classification¶
Form family: Control, Automation & Runtime
Rationale: Attaches preconditions, postconditions, and state assumptions to a boundary and evaluates them at runtime, so a transition is refused the moment it would break the contract, making its operative form a live operational control that automatically routes, enforces, adapts, or responds during execution.
Independent corroboration: The frozen evidence defines Contract Check as 'Attaches preconditions, postconditions, and state assumptions to a boundary and evaluates them at runtime, so a transition is refused the moment it would break the contract', so its operative form is Control, Automation & Runtime.
Review outcome: Independent reviewer agreement; high confidence.
Origin Attribution¶
Primary origin: Computer Science & Software Engineering
Origin pattern: Single lineage
Present-day reach: Specialized
Rationale: Design by Contract in software engineering cohered runtime checking of preconditions, postconditions, and invariants at named interfaces.
Review resolution: Both reviewers agree on computer_science. Mathematical logic is a substrate of software contracts, but the executable precondition/postcondition check is a coherent programming lineage rather than an independently originating mathematical mechanism.
Review outcome: Reconciled after independent review; high confidence.
Notes¶
Its nearest twin is the Invariant Test Suite: both encode the invariant as checkable assertions. The one-sentence separation: a Contract Check evaluates the live transition at a runtime boundary in production, while an Invariant Test Suite exercises candidate changes over cases offline before they ship.
[n1] Design by Contract — Bertrand Meyer's approach, built into the Eiffel language: callers and callees agree on preconditions, postconditions, and invariants, making each boundary's obligations explicit and checkable at runtime. ↩