Pre-Arm
The rolling window of trace data captured before an alert fires — and why it changes incident response.
Pre-Arm
Pre-arm is the window of trace data captured before an alert threshold is crossed.
Why it matters
In most incident scenarios, the cause precedes the symptom. A slow upstream database call degrades a service. The degraded service starts returning 500s. The 500s trigger an alert. The alert pages the oncall engineer.
By the time the engineer is paged, the cause is already in the past. Without pre-arm, the trace data explaining the cause may have been flushed or never captured at all.
Pre-arm ensures that the causal trace — the chain of events that led to the alert — is retained and available the moment the alert fires. The oncall engineer does not need to reconstruct history. They read it.
How it works
The SDK captures causal events continuously. Events are held in a rolling buffer. The pre-arm window defines how far back that buffer reaches.
When an alert fires (detected by the Incidentary API based on the event stream, or triggered manually via the ingest endpoint), events within the pre-arm window are retained permanently as part of the incident record. Events outside the window are discarded per your retention policy.
Timeline
─────────────────────────────────────────────────────────►
│ │
│◄─── pre-arm window ─────────►│
│ (events retained) alert fires
│
5 min before alertConfiguration
Configure the pre-arm window in the SDK init options or via environment variable:
| Option | Environment Variable | Default | Description |
|---|---|---|---|
preArmWindowMs | INCIDENTARY_PRE_ARM_WINDOW_MS | 300000 (5 min) | How far back the pre-arm buffer reaches |
const client = new IncidentaryClient({
apiKey: process.env.INCIDENTARY_API_KEY,
serviceName: 'my-service',
workspaceId: process.env.INCIDENTARY_WORKSPACE_ID,
preArmWindowMs: 600_000, // 10 minutes
});Longer pre-arm windows retain more history but consume more memory in the SDK process buffer. The default 5-minute window covers the vast majority of incident causality chains.