worldmonitor/todos/173-pending-p2-sequential-per-region-persist-1600ms.md

status, priority, issue_id, tags, dependencies

status	priority	issue_id	tags	dependencies
pending	p2	173	code-review phase-0 regional-intelligence performance redis

Sequential per-region persist pipelines waste ~1600ms wall-clock

Problem Statement

scripts/seed-regional-snapshots.mjs:181-203 awaits persistSnapshot(snapshot) per region in a sequential for loop. Each call issues 2 round-trips (dedup SETNX + data pipeline). 8 regions × 2 round-trips = 16 serial round-trips = ~1600ms wall-clock. Regions are fully independent (dedup keys are region-scoped) so this is safe to parallelize.

Findings

• scripts/seed-regional-snapshots.mjs:181-203 — sequential for-loop over regions with await persistSnapshot inside.

Proposed Solutions

Option 1: Parallelize with Promise.all

Replace the sequential for-loop with Promise.all(regions.map(async region => ...)). Saves ~1400ms wall-clock.

Pros: Trivial change; region-independent writes make this safe; failures are isolated per region via Promise.allSettled pattern. Cons: Partial failure handling needs explicit allSettled to not block other regions. Effort: Small. Risk: Low.

Option 2: Batch pipeline all 8 regions into one redis.pipeline() call

Collect all writes, emit a single multi-region pipeline. Saves ~1500ms.

Pros: Single round-trip for writes. Cons: Dedup SETNX results need to be checked before emitting the data writes, so you still need two phases; more invasive refactor. Effort: Medium. Risk: Medium.

Recommended Action

Option 1 with Promise.allSettled. Combined with #172, drops total runtime from ~3.4s to <800ms.

Technical Details

persistSnapshot currently does:

1. SETNX dedup:{region}:{hash} 1 (1 round-trip)
2. pipeline: SET by-id, SET latest, EXPIRE... (1 round-trip)

Each region's dedup key is namespaced by region, so there is no cross-region write ordering constraint. Switching from for (const r of regions) await persist(r) to Promise.allSettled(regions.map(persist)) preserves per-region dedup guarantees while removing the inter-region serialization.

Acceptance Criteria

• All 8 regions persisted in parallel.
• Failed regions don't block other regions (use Promise.allSettled).
• Existing tests still pass.

Work Log

Resources

• PR #2940
• Spec: docs/internal/pro-regional-intelligence-upgrade.md