Detailed Component Design

1. ID Generation Service

How it works: A global counter encoded in base62 (62⁷ ≈ 3.5 trillion codes). To avoid a single hot counter, use range allocation — a coordinator hands each app server a block of IDs (e.g. 1M at a time); servers mint codes locally with no per-request coordination.

Concurrency: Because each server owns a distinct range, two concurrent creates can never produce the same code — no collision possible, no collision-retry read on the hot path.
Unpredictability: Sequential codes are enumerable (scrape /aaaaaab, /aaaaaac…). Mitigate by padding/obfuscating the counter (e.g. bit-shuffle or XOR with a secret), or switch to a pre-generated random pool (KGS) where codes are random by construction.
Outage: If the coordinator dies, servers keep draining their local buffered range so creation continues; on partition, ranges are disjoint by design so a split-brain can't mint duplicates. Buffer size trades memory for outage tolerance.

2. Caching (the hot redirect path)

Viral link: A single code hammered by millions gets served from Redis (hot cache) and/or a CDN at the edge, so neither the redirect service nor the DB is overwhelmed. Cache hit → 302 in a few ms.
Miss / expiry: On a cache miss, the redirect service reads the mapping from the DB, serves the 302, and populates the cache for the next request.
Invalidation: When a link is disabled/edited, purge it from both CDN and Redis so stale mappings don't keep redirecting.
Eviction: LRU so hot codes stay resident and cold codes fall out; memory stays bounded.

3. Partitioning

Single-shard lookup: Partition key = hash(short_code) → every redirect routes to exactly one shard, no scatter-gather, O(1).
Even spread + hotspots: Random-ish codes hash uniformly, so traffic spreads evenly. A super-hot code can't overload one shard because requests are absorbed by Redis/CDN and read replicas before reaching the partition.

4. Rate Limiting (brief)

Per-IP / per-token throttling at the edge so one abusive client can't mint millions of links; return 429 with Retry-After so clients back off.

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