Tonight's goal: redraw all seven system-design architectures from memory, then check each one against its lesson diagram. No LeetCode tonight; keep doing 2 DSA problems daily.
1. Which memory sentence belongs to scaling basics?
Source invariant: “You scale up with a bigger box or out with more boxes; once data lives on more than one box, a network partition forces the CAP choice between consistency and availability.”
2. A cache answer must name which three decisions?
Source invariant: “A cache trades freshness for speed — so decide explicitly what happens on a miss, on a write (write-through vs write-back), and on eviction (LRU vs LFU).”
3. When keyed traffic uses consistent hashing, what is protected?
Source invariant: “A load balancer spreads requests across interchangeable servers; consistent hashing keeps most keys in place when a server joins or leaves the pool.”
4. Which choice matches the database choices invariant?
Source invariant: “Replication copies data to scale reads and survive failures; sharding splits data to scale writes and size; indexes buy fast reads at write cost. Choose per access pattern.”
5. URL shortener has which hot path?
Source invariant: “A tiny write path (issue an ID, base62-encode it) feeding an enormous read path — design for the reads: one lookup per redirect, cache the hot links.”
6. Which pair is correct for limiter and chat?
Source invariants: “A rate limiter answers ‘has this key spent its budget in this window?’ — token bucket and sliding window are just different bookkeeping for the same budget.” Also: “Chat is fan-out over persistent connections: WebSockets hold the pipe open, queues absorb bursts, and presence and receipts live as separate small services.”
Self-check gate
Pass tonight if
All seven designs were redrawn before looking, and each redraw includes the main boxes, arrows, bottleneck, and invariant from its source lesson.
Failure path: any diagram you cannot redraw cleanly goes on the Day 89 list. Write exactly what failed: missing box, wrong arrow, fuzzy bottleneck, or forgotten invariant.
After the redraw session, keep the daily baseline: 2 DSA problems from the plan. They are separate from tonight's system-design revision.