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PATTERN Cited by 1 source

Evidence fan-out then synthesize

Pattern

When investigating an operational event, gather evidence from multiple independent sources in parallel (fan-out), then join all evidence into a single synthesis step (reduce). Wall-clock time = slowest collector, not sum of all collectors.

Evidence categories (Blueberry example)

Alert context
    ├─ Historical: postmortems, runbooks, prior incidents
    ├─ Live operational: metrics, traces, error patterns
    ├─ Changes: recent deploys, feature flags, config changes
    └─ Team-contextual: team-specific sources
         ↓
    [Join] → Grounded synthesis

Why parallel matters for on-call

An on-call triage needs context from 4-6 independent data sources. Sequential collection would take 12-18 minutes; parallel collection achieves ~3 minutes (bounded by slowest source). The difference is operationally significant: 3 minutes lands an explanation before the incident room fully forms; 18 minutes arrives after the team has already context-switched to manual investigation.

Design principle

"Historical priors should inform the investigation, but live evidence should decide it." — the synthesis step compares historical context against live signals and narrows based on where evidence actually converges. (Source: sources/2026-07-14-instacart-blueberry-on-call-reasoning-harness)

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