HLD-402·Messaging & Fan-out
Expert · 2,000 ptsGlobal Real-Time Social Timeline Feed
Design a global timeline read/write architecture handling 220k+ RPS feed fetches, celebrity hot-key traffic skew, read-after-write consistency, and downstream storage brownouts.
Max p99
≤ 95 ms
Availability
≥ 99.6%
Budget
≤ $8,500
Functional Requirements
- FR-1POST /posts — Publishes a new post from author user_id.
- FR-2GET /timeline/{user_id} — Fetches the ranked recent feed for user_id.
- FR-3Read-after-write guarantee: authors and active readers must not be served stale un-invalidated caches.
- FR-4System must degrade gracefully without cascading retry storms when storage tail latency spikes 8x.
Workload Profile
Baseline Traffic85,000 Read RPS / 6,000 Write RPS
Breaking News Peak220,000 Read RPS / 15,000 Write RPS
Active Working Set64 GB hot timelines & celebrity posts
Fault ScenarioDownstream storage 8x gray latency spike in Wave 3
Consistency Model
Read-after-write consistency required (stale un-invalidated cache windows > 15s fail verification).
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