use case low latency sensor stream

Spikeforge — UC-4: Ultra-Low-Latency Sensor-Stream Inference

A fully scoped production use case. Extends the reference slice in use_case_streaming_timeseries.md; consumes the shared enablers from production_toolkit_plan.md; listed in the umbrella production_use_cases.md.

One-line summary. Per-sample decisions on radar/LiDAR/RF/vibration streams with hard latency bounds, on CPU with no neuromorphic chip.

What it demonstrates. The SNN's deterministic per-step latency: decisions are made incrementally as samples arrive, without buffering a full window, and the value is measured in p50/p99 scheduling — not power.

Design/spec only. Every claim about current code is grounded in a file/line reference so a Code-mode agent can execute this file-by-file. No implementation starts in this document.

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1. Problem and users

What. Given a high-rate sensor stream (radar/LiDAR/RF/vibration samples arriving per step), emit a decision or score every K samples with a hard bound on p99 end-to-end latency and jitter, running on CPU (GPU optional) with no neuromorphic hardware.

Users. Real-time signal engineers (radar/RF/SDR, tactile/vibration sensing); robotics teams that need a bounded control-loop decision; researchers comparing temporal SNN latency against framed baselines.

Why SNN here. A recurrent spiking model carries state across steps, so a decision needs only the current sample plus carried membrane state — no fixed frame buffer — and per-step work is bounded and deterministic, which is what real-time budgets require.

Success criteria (SLAs).

• Latency: p99 end-to-end per-decision latency ≤ the configured budget on CPU (target: ≤ 5 ms per step for a small topology); jitter bounded (max − p50 ≤ budget/2).
• Quality: task-specific metric (per-sample accuracy / AUROC) above a dataset-specific floor defined at kickoff; the floor is asserted in CI on the synthetic fixture.
• Determinism: repeated runs reproduce the same decision sequence for the same seed and input (determinism.py).
• Zero hardware: laptop or small CPU container.

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2. Reference architecture

flowchart LR
    A[Sensor stream] --> B[Per-sample normalize]
    B --> C[Encoding contract]
    C --> D[Stateful InferenceSession step]
    D --> E[Per-step head and decision rule]
    D --> F[DeploymentBundle]
    F --> D
    E --> G[spikeforge-serve predict and stream]
    G --> H[spikeforge-clients SDK]
    G --> I[Latency metrics]
    J[spikeforge-io high-rate replay] --> G
    K[Offline training] --> F

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Legend: the pipeline is deliberately one sample (or one micro-batch) at a time; the latency harness is the new machinery.

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3. Offline: data, encoding, model, training

3.1 Data

• Sources: a public radar/RF or vibration dataset plus a synthetic high-rate generator for CI (paralleling sequence_source.py).
• Contract: per-sample normalization stats and channel order are frozen in preprocessing.json in the bundle; the stream is [N, D] at sensor rate, streamed sample-by-sample with no lookahead.
• Repo fit: high-rate replay belongs to spikeforge-io (adapters.py, replay.py).

3.2 Encoding

• Primary coding: delta (per-sample temporal change is the signal) with latency as the alternative for spike-time-sensitive tasks and rate as the baseline. A frozen EncodeSpec pins the choice in the bundle.
• Input shape: [T, B, D] with T=1 per call in the streaming path; batch only for offline training, so per-step cost is bounded.

3.3 Model

• Topology: fc_small (per-step MLP) as the minimal-latency baseline; recurrent_net/sequence_mlp for temporal accuracy when budget allows; a latency-tuned TopologySpec (spec.py).
• Head: per-step classifier or score with a decision rule (threshold / hysteresis) applied to the carried state.
• Reuse: TrainingEngine, surrogate gradients, checkpointing (checkpoint_mixin.py).

3.4 Evaluation

• Task metric (accuracy / AUROC) above the floor; latency distribution (p50/p99, jitter, throughput) is a first-class result, not an afterthought.
• Validation: NIR drift and determinism (determinism.py); manifest per run (manifest.py).

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4. Online: bundle, runtime, service

4.1 Deployment bundle

• model.spkf with manifest.json (incl. the latency budget), weights.pt, encode_config.json, preprocessing.json, graph.nir.json, checksums/signature.
• Anchors: bundle.py, bundle_manifest.py.

4.2 Stateful runtime

• InferenceSession.load(bundle), .reset(), .step(frame) -> Prediction, .run_stream(frames); per-step state via StateTree. The shared per-step body is the artifact under latency test (step.py).

4.3 Service

• spikeforge-serve: POST /v1/predict, POST /v1/reset, GET|WS /v1/stream, GET /health, GET /metrics, GET /v1/bundle (app.py, service.py).
• Streaming is the primary path; bounded queue depth and backpressure reject rather than delay; request timeouts honour the latency budget.

4.4 Clients and I/O

• spikeforge-clients SDKs drive the stream (client.py); spikeforge-io replays a recorded high-rate stream at real time into /v1/stream (replay.py).

4.5 Observability

• Prometheus over the registry (registry.py): per-request latency histogram (p50/p99/p999), jitter, queue depth, drop count, throughput, spike rate per step.
• Serving benchmark: p50/p99 latency, throughput at concurrency N, cold start, peak memory — wired into the regression gate (compare.py); the tail-latency gate is the whole point of this use case.

4.6 Compression option

• Optional pruning (pruning.py) to shrink per-step cost, with weight-only quantization (quantize.py) for edge CPU. Drift is recorded; the latency budget is re-checked after compression.

4.7 Test-deploy matrix row

• Test-deploy the per-step topology on reference and norse (and lava_loihi2 if the chain lowers) with a parity report (test_deploy.py); estimate: true, available: false with a reason when an SDK is absent.

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5. Phased delivery

Phase	Deliverable	Depends on	Acceptance
P0	High-rate source adapter + synthetic generator + frozen per-sample spec	—	stream reproducible; normalization frozen
P1	Low-latency topology trained; quality + latency report	P0	quality above floor; per-step cost measured
P2	InferenceSession per-step path + tests	W1	step latency bounded; state persists
P3	DeploymentBundle export/import + tamper check	W1	fresh-process rebuild is exact
P4	spikeforge-serve /stream + /predict + /reset	W2, W3	in-process parity; backpressure rejects, never delays
P5	/metrics, serving benchmark, p99 tail-latency CI gate	W6	p99 ≤ budget in CI; jitter bounded
P6	Container, promotion/rollback, drift monitor, compressed artifact	W7	promote/rollback demo; drift alarm fires

MVP = P0–P4. That is the smallest end-to-end slice that demonstrates the chip-less latency story.

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6. Dependencies and out of scope

Depends on: PT-W1/W2 (released spikeforge/serving/ + frozen EncodeSpec); PT-W3 spikeforge-serve; PT-W6 observability + serving benchmarks (the tail-latency gate); PT-W7 I/O adapters. Tracked by umbrella issue #12; reference implementation UC-1 (released in spikeforge 0.3.0).

Out of scope: measured power (estimate: true); hard real-time OS/kernel tuning and CPU pinning beyond best-effort reporting; custom hardware timing; safety certification.

7. Risks

Risk	Mitigation
Python/service overhead dominates per-step cost	benchmark in-process step vs served latency; document the split honestly
Tail latency noisy in CI	assert p99 with a generous margin plus a jitter bound; record the environment
Encoding lookahead sneaks in	encode is strictly one sample; windowing is forbidden in this task
Compression regresses latency	re-run the latency gate after pruning/quantization

8. GitHub issue payload

• Title: [UC-4] Ultra-low-latency sensor-stream inference
• Labels: enhancement, architecture
• Body: see plans/use_case_low_latency_sensor_stream.md — goal, reference architecture (per-sample encode → InferenceSession.step → per-step head → spikeforge-serve → clients/io → /metrics), delta primary coding, fc_small/recurrent_net, acceptance (p99 ≤ budget, jitter ≤ budget/2, quality above floor), MVP phases P0–P4, dependencies PT-W3/W6/W7.