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43 lines
1.7 KiB
Markdown
43 lines
1.7 KiB
Markdown
# Kata 09: The Cache Stampede Shield
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**Target Idioms:** `singleflight`, TTL Cache, DoChan + Context Select, Lock Avoidance
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**Difficulty:** 🔴 Advanced
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## 🧠 The "Why"
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In many stacks, caching is “just Redis”. In Go, an in-process cache is common, but people:
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- hold locks while calling the loader (deadly),
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- refresh the same key N times concurrently (stampede),
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- can’t cancel waiters cleanly.
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This kata is about **deduplicating in-flight loads** and making waiters **context-cancellable**.
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## 🎯 The Scenario
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You have expensive per-key loads (e.g., DB or remote API). If 200 goroutines ask for the same key at once:
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- loader must run **once**
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- others must wait (or return on ctx cancel)
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- TTL must be enforced
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## 🛠 The Challenge
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Implement:
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- `type Cache[K comparable, V any] struct { ... }`
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- `Get(ctx context.Context, key K, loader func(context.Context) (V, error)) (V, error)`
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### 1. Functional Requirements
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- [ ] Return cached value if not expired.
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- [ ] If expired/missing: load once, share result to all callers.
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- [ ] Callers must be able to stop waiting via `ctx.Done()`.
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### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
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- [ ] **Must** use `golang.org/x/sync/singleflight.Group`.
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- [ ] **Must** use `DoChan` + `select` on `ctx.Done()` to cancel waiters.
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- [ ] **Must NOT** hold a mutex while calling `loader`.
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- [ ] Errors must be wrapped with key context using `%w`.
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## 🧪 Self-Correction (Test Yourself)
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- **If 200 goroutines trigger 200 loads:** you failed (no stampede protection).
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- **If a canceled context still blocks waiting:** you failed.
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- **If you lock around loader execution:** you failed (contention / deadlocks).
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## 📚 Resources
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- https://pkg.go.dev/golang.org/x/sync/singleflight
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- https://go.dev/blog/go1.13-errors
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