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55 lines
2.0 KiB
Markdown
55 lines
2.0 KiB
Markdown
# Kata 18: embed.FS Dev/Prod Switch Without Handler Forks
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**Target Idioms:** `embed`, `io/fs`, Build Tags, `fs.Sub`, Same Handler Code Path
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**Difficulty:** 🟡 Intermediate
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## 🧠 The "Why"
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Embedding assets is great for production (single binary), but terrible for frontend iteration if every CSS tweak needs a rebuild.
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Idiomatic Go solves this with:
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- compile-time selection via build tags
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- a shared `fs.FS` abstraction so handler code doesn’t branch on “dev/prod”.
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## 🎯 The Scenario
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You run a small internal dashboard:
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- Prod: ship a single binary (assets embedded).
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- Dev: designers update `static/` and `templates/` live without recompiling.
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## 🛠 The Challenge
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Create a server that serves:
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- templates from `templates/`
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- static assets from `static/`
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### 1. Functional Requirements
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- [ ] `GET /` renders an HTML template.
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- [ ] `GET /static/...` serves static files.
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- [ ] Dev mode serves from disk; prod mode serves embedded.
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- [ ] Handler code is identical in both modes.
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### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
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- [ ] **Build tags:** two files:
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- `assets_dev.go` with `//go:build dev`
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- `assets_prod.go` with `//go:build !dev`
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- [ ] **Return `fs.FS`:** `func Assets() (templates fs.FS, static fs.FS, err error)`
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- [ ] **Use `fs.Sub`:** exported FS must have *clean roots* (no `static/static/...` path bugs).
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- [ ] **No runtime env checks in handlers:** mode selection must be compile-time.
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- [ ] **Single `http.FileServer` setup:** no duplicated handler logic for dev vs prod.
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## 🧪 Self-Correction (Test Yourself)
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1. **Live Reload**
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- Build with `-tags dev`.
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- Modify a CSS file and refresh.
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- **Pass:** change shows without rebuild.
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2. **Binary Portability**
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- Build without tags.
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- Delete `static/` and `templates/` from disk.
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- **Pass:** server still serves assets/templates.
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3. **Prefix Correctness**
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- Request `/static/app.css`.
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- **Pass:** works in both modes (no 404 due to prefix mismatch).
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## 📚 Resources
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- https://pkg.go.dev/embed
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- https://pkg.go.dev/io/fs
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- https://pkg.go.dev/io/fs#Sub
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