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48 lines
1.8 KiB
Markdown
48 lines
1.8 KiB
Markdown
# Kata 08: The Retry Policy That Respects Context
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**Target Idioms:** Retry Classification, Error Wrapping (`%w`), Timer Reuse, Context Deadlines
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**Difficulty:** 🟡 Intermediate
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## 🧠 The "Why"
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In other languages, retries are often hidden in SDKs. In Go, it’s easy to write:
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- infinite retry loops,
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- retry-on-any-error (bad),
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- retry that ignores context cancellation (worse),
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- retry implemented with repeated `time.Sleep` (hard to test, wasteful).
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This kata makes you implement a **testable**, **context-aware** retry loop.
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## 🎯 The Scenario
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You call a flaky downstream service. You should retry only on **transient** failures:
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- `net.Error` with `Timeout() == true`
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- HTTP 429 / 503 (if you model HTTP)
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- sentinel `ErrTransient`
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Everything else must fail immediately.
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## 🛠 The Challenge
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Implement:
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- `type Retryer struct { ... }`
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- `func (r *Retryer) Do(ctx context.Context, fn func(context.Context) error) error`
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### 1. Functional Requirements
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- [ ] Retries up to `MaxAttempts`.
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- [ ] Uses exponential backoff: `base * 2^attempt` with a max cap.
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- [ ] Optional jitter (deterministic in tests).
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- [ ] Stops immediately on `ctx.Done()`.
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### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
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- [ ] **Must NOT** call `time.Sleep` inside the retry loop.
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- [ ] **Must** use a `time.Timer` and `Reset` it (timer reuse).
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- [ ] **Must** wrap the final error with context (attempt count) using `%w`.
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- [ ] **Must** classify errors using `errors.Is` / `errors.As`.
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## 🧪 Self-Correction (Test Yourself)
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- **If context cancellation only stops after the sleep:** you failed.
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- **If you retry non-transient errors:** you failed classification.
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- **If you can’t test it without real time:** inject time/jitter sources.
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## 📚 Resources
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- https://go.dev/blog/go1.13-errors
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- https://pkg.go.dev/errors
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- https://pkg.go.dev/time
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