Initial commit: Go SRE interview exercises

Includes solved 02-worker-pool and 09-safe-counter (mutex, atomic, sharded). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01GbnJj9ZqEx9nUvzkjdwfMq
2026-08-28 22:18:09 +02:00
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+# 04 — Log Crunching: Top-N
+
+Looks like a warm-up. It's actually a trap-density championship.
+
+## Trap 1: `bufio.Scanner`'s 64KB guillotine
+
+`Scanner` refuses tokens above 64KB by default and stops with
+`bufio.ErrTooLong` — and if you forget to check `sc.Err()`, it stops
+*silently*, and your report just... misses the tail of the file. Real-world
+lines that exceed 64KB: JSON-formatted logs, stack traces, requests with fat
+query strings. The fix:
+
+```go
+sc.Buffer(make([]byte, 0, 1024*1024), 1024*1024)
+```
+
+An enormous number of production log processors have shipped with this bug.
+Checking `sc.Err()` after the loop is the difference between a tool and a liar.
+
+## Trap 2: maps have no order — by design, with a great backstory
+
+Go **deliberately randomizes** map iteration order (a random seed per map, and
+the runtime even randomizes the starting bucket) so nobody can accidentally
+depend on it. To rank by count you must extract into a slice and sort:
+
+```go
+sort.Slice(out, func(i, j int) bool { return out[i].Count > out[j].Count })
+```
+
+Since 1.24 maps use a Swiss-table implementation (flat, SIMD-probed, cache
+friendlier) — fun fact to drop if asked "what's inside a Go map."
+
+## Trap 3: `strings.Fields` vs `strings.Split`
+
+`Split(s, " ")` produces empty strings for repeated spaces; `Fields` splits on
+any run of whitespace (incl. tabs) and never yields empties. For log parsing,
+`Fields` is almost always what you mean.
+
+## The senior follow-up: "the file is 500 GB"
+
+Have a ladder ready:
+
+1. **This solution already streams** — memory is O(distinct endpoints), not
+   O(file). Say that first; many candidates don't realize their answer scales.
+2. **Cardinality explosion** (endpoints contain IDs): normalize paths
+   (`/users/123` → `/users/:id`) or cap the map with count-min sketch /
+   space-saving algorithm — approximate top-K in bounded memory. Naming
+   "count-min sketch" in an SRE interview is a mic-drop; it's what real
+   metrics pipelines do to survive high-cardinality labels.
+3. **Parallelize**: shard by hash(endpoint) across workers, merge partials —
+   map-reduce in one sentence.
+4. **Only need top-N, not full sort?** A min-heap of size N gives
+   O(distinct · log N) instead of O(distinct · log distinct). `container/heap`
+   exists; nobody remembers its interface; everyone respects you for trying.