A sound implementation does not ask users to refresh until it works. Each phase has an input, an output, a timeout, and a terminal state. A send-only mock makes every write instant and removes backpressure. A useful fake maintains a send buffer, drains by configured bandwidth, and fires bufferedamountlow asynchronously.
Testing combines state models, fault injection, and real browser pairs. Deterministic cases protect known contracts, randomized timing finds races, and every failing seed plus endpoint trace becomes a permanent regression.
Define the system contract first
Turn the important choices into durable contracts: validate inputs, assign state ownership, define cleanup, and specify fallback for older peers. Later optimization must not change those semantics.
- Model connecting/open/closing/closed with invalid-state throws; support message limits, delivery order, half-close, and complete frame recording.
- Bound every input by size, count, and time, returning a stable actionable error code when a budget is exceeded.
- Ship conservative defaults, server-side ceilings, and a rollout switch instead of trusting browser-provided numbers as resource budgets.
The delivery standard for A Fake DataChannel That Preserves Backpressure and Close Semantics is a usable normal path, convergent failures, bounded resources, and a state users can understand. The result is a production capability that can be explained, degraded safely, and rolled back—not a demo that works once.
What can break that contract
Failure and success must share one state model. An error toast that neither releases resources nor propagates a terminal state leaves dirty work for the next recovery attempt.
- Synchronous onmessage changes browser task ordering and hides races, while accepting send after close makes leak tests meaningless.
- Refresh and network change start two recovery paths, and duplicate side effects look like two genuine user actions.
- Ideal-size tests miss large files, long sessions, and concurrency that cross hidden limits and cause cascading failure.
How to test the contract line by line
Build golden cases from known inputs and controlled faults, then align production metrics with those results. Verification extends to production only when signals detect the same degradation early.
- Run one contract suite against the fake and two real browsers, comparing frames, backpressure pauses, cancel latency, and close errors to prevent drift.
- Disconnect, change networks, and recover mid-operation; reconcile endpoint state, persistence, and resource counts.
- Allowlist log and analytics fields, proving payloads, secrets, full IP addresses, and identifying data never leave the device.
The result must be correct, recoverable, and explainable. If any part depends on refreshing the page or an engineer guessing, the protocol loop remains incomplete.