Prevent delayed telemetry from moving a van backwards
A device buffers updates through a tunnel, then sends them out of order. The marker jumps to an older point and the arrival-age label resets incorrectly.
- Focused work estimate
- 2h 30m + prerequisites
- Priority in the scenario
- High
- Engineering practice
- Event ordering · Device identity · Stream processing
Estimated field mix
- Real-time systems70%
- Data engineering30%
Field percentages are editorial estimates of the ticket's engineering focus. They total 100%; they are not measured time, proficiency scores, or ownership evidence.
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Project context
The fictional Cedar delivery depot dispatches vans through a browser map and accessible list. Synthetic telemetry arrives through a gateway with device sequence numbers. This is operational fleet software practice; no real driver tracking, employee scoring, or routing safety guarantee is part of the project.
Setup prerequisites
- Synthetic vehicle telemetry with device sessions and sequence numbers
- Dispatch assignment and authorized region API contracts
Preceding work
Complete these dependencies, or supply their agreed outputs before taking this ticket.
Acceptance criteria
- Per-device session and sequence identify whether a fix advances accepted position.
- Duplicates and earlier fixes do not replace current position or refresh its age.
- A new device session follows an explicit reset/authority policy rather than comparing unrelated sequence counters.
Implementation constraints
- Arrival time alone cannot establish device order; keep bounded diagnostics for rejected categories.
Verification to include
- Deliver sequence 40, 42, 41, and 42 again; accepted position remains at 42.
- Restart the device session with sequence 1 and replay an old-session fix; the documented session policy chooses one authority.
Deliverables
- Device-session telemetry reducer and reordered-trace tests
Rollout and recovery
Replay fixed traces before live pilot; freeze a device at its last confirmed fix when session authority is ambiguous.
Value of the work
For the engineer: Practice geospatial boundaries, stream ordering, snapshot recovery, and concurrent operational commands.
For the team: Inspect whether a developer keeps dispatch decisions grounded in current authorized data and exposes uncertainty during outages.
Evidence boundaries
Outcome Evidence: Tests, patches, and runbooks are requested deliverables. They become Outcome Evidence only through a qualified Mission and immutable Evidence IDs.
Ownership Evidence: Independent adaptation must be observed under a declared verification policy and cite immutable Evidence IDs. Completing a planning ticket establishes no Ownership Evidence.