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Keyed timers — detect the silence

Intermediate ⏱ 15 min streaming · process · timers

You will build: a watchdog that fires when a device goes quiet — per-key timeout timers that turn absence of events into an event.

heartbeats → streaming:watchdog (process + timer) → (silence alerts out)

Prerequisites​

  • Comfortable with Windows & aggregations. Windows aggregate what arrives — this tutorial is about what doesn't.

1 · Scaffold​

pulse new fleetwatch --source webhook --stage streaming:watchdog

2 · Declare the keyed timer​

stages:
  - name: watchdog
    engine: streaming
    operators:
      - type: process
        key: device_id
        timeout: "10m"
        timer: processing      # processing · event
        onTimeout: emit        # emit · drop
        passThrough: true

Each device_id gets its own timer. Every event from that device resets it; if timeout elapses with no event, the operator emits a timeout event for that key. passThrough: true keeps normal heartbeats flowing downstream too.

3 · Deploy and heartbeat two devices​

pulse secret set PULSE_WEBHOOK_SECRET dev-secret   # the scaffolded webhook source requires it
pulse deploy .
pulse events publish --topic fleetwatch.in --value '{"device_id":"d-1","battery":88}'
pulse events publish --topic fleetwatch.in --value '{"device_id":"d-2","battery":91}'

4 · Watch one go silent​

pulse events tail --topic fleetwatch.watchdog.out

Keep d-1 heartbeating; stop sending for d-2. When d-2's timer expires, a timeout event for that key only appears — d-1's timer, still being reset, never fires.

tip

For a quick demo, drop the timeout to "30s" — the semantics are identical.

5 · Chain the alert​

Add a rule-based stage after the watchdog to route timeout events to your on-call flow — or an mcp stage to open the ticket directly (see Acting with MCP tools).

What just happened​

You used the process operator — a keyed process function with per-key state and timeout timers. Windows answer "what happened per key in this period"; process answers "what should have happened and didn't". Together they cover both halves of monitoring: activity and silence.

Troubleshooting​

The timeout never fires

With timer: processing the clock is wall-time from the last event for that key — confirm you actually stopped sending for it. timeout must be positive, and onTimeout: emit (not drop) is what makes the silence visible downstream.

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