---
name: system-design-case-catalog
description: "Answer classic system design problems as constraint-to-solution sketches and coach interview practice: URL shortener, rate limiter, news feed, chat, notification, autocomplete, crawler, unique id. Use for interview practice or naming the closest known shape for a new problem."
metadata:
  triggers:
    keywords:
      - system design interview
      - design twitter
      - design url shortener
      - news feed
      - design chat system
      - web crawler
      - unique id generator
      - video streaming
      - video publishing
      - ride hailing
      - ride dispatch
      - payment ledger
      - payment timeout
      - flash sale
      - multi-tenant SaaS
      - cache stampede
      - notification outage
      - chat reconnect
      - database migration
---

# Case Catalog

## **Priority: P2 (MEDIUM)**

Each classic problem has one defining constraint. Name it first; the rest of the design follows.

## Defining Constraints

| Problem | Defining constraint | Decisions that follow |
| --- | --- | --- |
| URL shortener | Read-heavy by ~100:1, key must be short and unique | Base62 of a distributed counter, cache-first read path, 301 vs 302 choice |
| Rate limiter | Decision must be cheap, shared, and correct under concurrency | Token bucket in a shared counter, fail-open or fail-closed rule, `429` plus `Retry-After` |
| News feed | Fan-out cost versus read latency, with celebrity skew | Push for normal accounts, pull for celebrities, hybrid merge at read |
| Chat | Delivery guarantees and presence at persistent-connection scale | WebSocket gateways, per-conversation ordering, offline queue, read receipts |
| Notification | Multi-channel delivery with retries and dedupe | Queue per channel, idempotency key, user preference and quiet hours |
| Autocomplete | Sub-100ms prefix lookup over a huge term space | Trie or prefix index in memory, precomputed top-k per prefix, async rebuild |
| Web crawler | Politeness and dedupe at scale, not raw fetching | Frontier queue per host, robots cache, URL fingerprint dedupe, freshness policy |
| Unique id | Ordered, unique, generated without a central lock | Snowflake-style timestamp plus node plus sequence; clock-skew handling |
| Video streaming | Bitrate ladder and CDN economics, not the upload | Transcode pipeline per rendition, adaptive manifests (HLS/DASH), edge cache hit ratio as the cost lever |
| Ride hailing | Geo matching under moving supply and demand | Geohash or S2 cells, driver location stream with TTL, matching window and surge as a pricing signal |
| Payment ledger | Exactly-once effect under retries and partial failure | Idempotency key per attempt, double-entry ledger, reconciliation job against the processor |

## Coaching Mode

- Mock rounds, the clock, the rubric, and the debrief live in `system-design-interview-coaching`; this catalog is its problem bank.
- Give the model answer only after the candidate commits to an approach; the defining constraint above is the follow-up question when they stall.
- For production practice, lazy-load [production case packs](references/production-case-packs.md). They are synthetic fixtures unless a named primary source says otherwise; never present their numbers or timelines as deployed facts.

## Production Case Routing

- Choose one pack only after stating the profile, workload, SLO, team, budget, and invariant. Recalculate all numbers for the user's system.
- Carry the pack's selected views through the HLD-to-LLD trace; use only the view that answers the current question and do not create a second diagramming lane.


## Reuse Rules

- Map a new problem to the nearest catalog shape, then re-derive the numbers. The shape transfers, the sizing never does.
- State where the analogy breaks before borrowing the design.
- A catalog answer is a starting hypothesis, not a substitute for intake and estimation.

## Anti-Patterns

- **No pattern-matching without numbers**: a known shape still needs this system's QPS and data volume.
- **No interview answer as a build plan**: production adds migration, cost, compliance, and team constraints.
- **No full solution dump in coaching mode**: the value is in the questions asked, not the answer given.

## References

- [Common Designs](references/common-designs.md) - per-problem sketch with constraints, components, and trade-offs