From 60a6e524490b468726bd530dc31136e5e516edf6 Mon Sep 17 00:00:00 2001
From: medunes <medunes@protonmail.com>
Date: Sun, 4 Jan 2026 20:40:07 +0100
Subject: [PATCH] init commit

---
 .gitignore                                    |  1 +
 01-concurrent-aggregator/.gitignore           |  1 +
 01-concurrent-aggregator/README.md            | 48 +++++++++++++++
 01-concurrent-aggregator/go.mod               |  3 +
 .../README.md                                 | 53 +++++++++++++++++
 03-graceful-shutdown-server/README.md         | 56 ++++++++++++++++++
 04-zero-allocation-json-parser/README.md      | 58 ++++++++++++++++++
 05-context-aware-error-propagator/README.md   | 42 +++++++++++++
 06-interface-based-middleware-chain/README.md | 50 ++++++++++++++++
 07-rate-limited-fanout/README.md              | 48 +++++++++++++++
 08-retry-backoff-policy/README.md             | 47 +++++++++++++++
 09-single-flight-ttl-cache/README.md          | 42 +++++++++++++
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 11-ndjson-stream-reader/README.md             | 36 +++++++++++
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 18-embedfs-dev-prod-switch/README.md          | 54 +++++++++++++++++
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diff --git a/01-concurrent-aggregator/README.md b/01-concurrent-aggregator/README.md
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+# Kata 01: The Fail-Fast Data Aggregator
+
+**Target Idioms:** Concurrency Control (`errgroup`), Context Propagation, Functional Options
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+In other languages, you might use `Promise.all` or strict thread pools to fetch data in parallel. In Go, seasoned developers often start with `sync.WaitGroup`, but quickly realize it lacks two critical features for production: **Error Propagation** and **Context Cancellation**.
+
+If you spawn 10 goroutines and the first one fails, `WaitGroup` blindly waits for the other 9 to finish. **Idiomatic Go fails fast.**
+
+## 🎯 The Scenario
+You are building a **User Dashboard Backend**. To render the dashboard, you must fetch data from two independent, mock microservices:
+1.  **Profile Service** (Returns "Name: Alice")
+2.  **Order Service** (Returns "Orders: 5")
+
+You need to fetch these in parallel to reduce latency. However, if *either* fails, or if the global timeout is reached, the entire operation must abort immediately to save resources.
+
+## 🛠 The Challenge
+Create a `UserAggregator` struct and a method `Aggregate(id int)` that orchestrates this fetching.
+
+### 1. Functional Requirements
+* [ ] The aggregator must be configurable (timeout, logger) without a massive constructor.
+* [ ] Both services must be queried concurrently.
+* [ ] The result should combine both outputs: `"User: Alice | Orders: 5"`.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+To pass this kata, you **must** strictly adhere to these rules:
+
+* [ ] **NO `sync.WaitGroup`:** You must use `golang.org/x/sync/errgroup`.
+* [ ] **NO "Parameter Soup":** You must use the **Functional Options Pattern** for the constructor (e.g., `New(WithTimeout(2s))`).
+* [ ] **Context is King:** You must pass `context.Context` as the first argument to your methods.
+* [ ] **Cleanup:** If the Profile service fails, the Order service request must be cancelled (via Context) immediately.
+* [ ] **Modern Logging:** Use `log/slog` for structured logging.
+
+## 🧪 Self-Correction (Test Yourself)
+Run your code against these edge cases:
+
+1.  **The "Slow Poke":** * Set your aggregator timeout to `1s`.
+    * Mock one service to take `2s`.
+    * **Pass Condition:** Does your function return `context deadline exceeded` after exactly 1s?
+2.  **The "Domino Effect":**
+    * Mock the Profile Service to return an error immediately.
+    * Mock the Order Service to take 10 seconds.
+    * **Pass Condition:** Does your function return the error *immediately*? (If it waits 10s, you failed context cancellation).
+
+## 📚 Resources
+* [Go Concurrency: errgroup](https://pkg.go.dev/golang.org/x/sync/errgroup)
+* [Functional Options for Friendly APIs](https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis)
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diff --git a/01-concurrent-aggregator/go.mod b/01-concurrent-aggregator/go.mod
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+module concurrent-aggregator
+
+go 1.25.0
diff --git a/02-concurrent-map-with-sharded-locks/README.md b/02-concurrent-map-with-sharded-locks/README.md
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+# Kata 02: The Concurrent Map with Sharded Locks
+
+**Target Idioms:** Concurrency Safety, Map Sharding, `sync.RWMutex`, Avoiding `sync.Map` Pitfalls
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+Seasoned developers coming from Java might reach for `ConcurrentHashMap`-style solutions, while Pythonistas might think of GIL-protected dictionaries. In Go, you have three main options:
+1. **Naive sync.Mutex around a map** (bottlenecks under high concurrency)
+2. **sync.Map** (optimized for specific "append-only, read-heavy" cases, but opaque and often misused)
+3. **Sharded maps** (manual control, maximized throughput)
+
+The Go way is explicit control: if you know your access patterns, build a solution that fits. This kata forces you to understand *when* and *why* to choose sharding over sync.Map.
+
+## 🎯 The Scenario
+You're building a real-time **API Rate Limiter** that tracks request counts per user ID. The system handles 50k+ RPS with 95% reads (checking limits) and 5% writes (incrementing counters). A single mutex would serialize all operations-unacceptable. `sync.Map` might work but obscures memory usage and lacks type safety.
+
+## 🛠 The Challenge
+Implement `ShardedMap[K comparable, V any]` with configurable shard count that provides safe concurrent access.
+
+### 1. Functional Requirements
+* [ ] Type-safe generic implementation (Go 1.18+)
+* [ ] `Get(key K) (V, bool)` - returns value and existence flag
+* [ ] `Set(key K, value V)` - inserts or updates
+* [ ] `Delete(key K)` - removes key
+* [ ] `Keys() []K` - returns all keys (order doesn't matter)
+* [ ] Configurable number of shards at construction
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+* [ ] **NO `sync.Map`**: Implement sharding manually with `[]map[K]V` and `[]sync.RWMutex`
+* [ ] **Smart Sharding**: Use `fnv64` hashing for key distribution (don't rely on Go's random map iteration)
+* [ ] **Read Optimization**: Use `RLock()` for `Get()` operations when safe
+* [ ] **Zero Allocation Hot-Path**: `Get()` and `Set()` must not allocate memory in the critical section (no string conversion, no boxing)
+* [ ] **Clean `Keys()`**: Implement without data races, even while concurrent writes occur
+
+## 🧪 Self-Correction (Test Yourself)
+1. **The Contention Test**:
+    - Run 8 goroutines doing only `Set()` operations with sequential keys
+    - With 1 shard: Should see heavy contention (use `go test -bench=. -cpuprofile` to verify)
+    - With 64 shards: Should see near-linear scaling
+
+2. **The Memory Test**:
+    - Store 1 million `int` keys with `interface{}` values
+    - **Fail Condition**: If your solution uses more than 50MB extra memory vs baseline map
+    - **Hint**: Avoid `string(key)` conversions; use type-safe hashing
+
+3. **The Race Test**:
+    - Run `go test -race` with concurrent read/write/delete operations
+    - Any race condition = automatic failure
+
+## 📚 Resources
+* [Go Maps Don't Appear to be O(1)](https://dave.cheney.net/2018/05/29/how-the-go-runtime-implements-maps-efficiently-without-generics)
+* [When to use sync.Map](https://dave.cheney.net/2017/07/30/should-i-use-sync-map)
+* [Practical Sharded Maps](https://github.com/orcaman/concurrent-map)
diff --git a/03-graceful-shutdown-server/README.md b/03-graceful-shutdown-server/README.md
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+# Kata 03: The Graceful Shutdown Server
+**Target Idioms:** Context Propagation, Signal Handling, Channel Coordination, Resource Cleanup
+**Difficulty:** 🔴 Advanced
+
+## 🧠 The "Why"
+In other ecosystems, graceful shutdown is often framework magic (Spring's `@PreDestroy`, Django's `close()`). Go forces explicit lifecycle management. The mismatch: developers used to automatic cleanup often leak goroutines, drop in-flight requests, or corrupt data during shutdown.
+
+The Go way: **Own your lifecycle**. Every goroutine you spawn must have a controlled shutdown path.
+
+## 🎯 The Scenario
+Build an **HTTP Server with Background Worker** that must:
+1. Accept HTTP requests (handled by a pool of worker goroutines)
+2. Run a background cache warmer every 30 seconds
+3. Maintain persistent database connections
+4. Shutdown within 10 seconds when receiving SIGTERM, completing in-flight requests but rejecting new ones
+
+## 🛠 The Challenge
+Implement `Server` struct with `Start() error` and `Stop(ctx context.Context) error` methods.
+
+### 1. Functional Requirements
+* [ ] HTTP server on configurable port with request timeout
+* [ ] Worker pool (configurable size) processes requests via channel
+* [ ] Background cache warmer ticks every 30s (use `time.Ticker`)
+* [ ] Database connection pool (mock with `net.Conn`)
+* [ ] SIGTERM/SIGINT triggers graceful shutdown
+* [ ] Shutdown completes within deadline or forces exit
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+* [ ] **Single Context Tree**: Root `context.Context` passed to `Start()`, canceled on shutdown
+* [ ] **Channel Coordination**: Use `chan struct{}` for worker pool shutdown, not boolean flags
+* [ ] **Proper Ticker Cleanup**: `defer ticker.Stop()` with select in goroutine
+* [ ] **Dependency Order**: Shutdown in reverse order (stop accepting → drain workers → stop warmer → close DB)
+* [ ] **No `os.Exit()` in business logic**: Shutdown should be testable without process termination
+
+## 🧪 Self-Correction (Test Yourself)
+1. **The Sudden Death Test**:
+    - Send 100 requests, immediately send SIGTERM
+    - **Pass**: Server completes in-flight requests (not all 100), logs "shutting down", closes cleanly
+    - **Fail**: Server accepts new requests after signal, leaks goroutines, or crashes
+
+2. **The Slow Leak Test**:
+    - Run server for 5 minutes with 1 request/second
+    - Send SIGTERM, wait 15 seconds
+    - **Pass**: `go test` shows no goroutine leaks (use `runtime.NumGoroutine()`)
+    - **Fail**: Any increase in goroutine count from start to finish
+
+3. **The Timeout Test**:
+    - Start long-running request (sleep 20s)
+    - Send SIGTERM with 5s timeout context
+    - **Pass**: Forces shutdown after 5s, logs "shutdown timeout"
+    - **Fail**: Waits full 20s or deadlocks
+
+## 📚 Resources
+* [Go Blog: Context](https://go.dev/blog/context)
+* [Graceful Shutdown in Go](https://medium.com/honestbee-tw-engineer/gracefully-shutdown-in-go-http-server-5f5e6b83da5a)
+* [Signal Handling](https://medium.com/@marcus.olsson/writing-a-go-app-with-graceful-shutdown-5de1d2c6de96)
diff --git a/04-zero-allocation-json-parser/README.md b/04-zero-allocation-json-parser/README.md
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+# Kata 04: The Zero-Allocation JSON Parser
+
+**Target Idioms:** Performance Optimization, `json.RawMessage`, Streaming Parsers, Buffer Reuse
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why**
+Developers from dynamic languages often parse JSON by unmarshaling entire documents into `map[string]interface{}` or generic structs. In high-throughput Go services, this creates:
+1. Massive memory churn (GC pressure)
+2. Unnecessary allocations for unused fields
+3. Lost type safety
+
+The Go way: **Parse only what you need, reuse everything**. This kata teaches you to treat JSON as a stream, not a document.
+
+## 🎯 The Scenario
+You're processing **10MB/s of IoT sensor data** with JSON like:
+```json
+{"sensor_id": "temp-1", "timestamp": 1234567890, "readings": [22.1, 22.3, 22.0], "metadata": {...}}
+```
+You only need `sensor_id` and the first reading value. Traditional unmarshal would allocate for all fields and the entire readings array.
+
+## 🛠 The Challenge
+Implement `SensorParser` that extracts specific fields without full unmarshaling.
+
+### 1. Functional Requirements
+* [ ] Parse `sensor_id` (string) and first `readings` value (float64) from JSON stream
+* [ ] Process `io.Reader` input (could be HTTP body, file, or network stream)
+* [ ] Handle malformed JSON gracefully (skip bad records, continue parsing)
+* [ ] Benchmark under 100ns per object and 0 allocations per parse
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+* [ ] **NO `encoding/json.Unmarshal`**: Use `json.Decoder` with `Token()` streaming
+* [ ] **Reuse Buffers**: Use `sync.Pool` for `bytes.Buffer` or `json.Decoder`
+* [ ] **Early Exit**: Stop parsing once required fields are found
+* [ ] **Type Safety**: Return concrete struct `SensorData{sensorID string, value float64}`, not `interface{}`
+* [ ] **Memory Limit**: Process arbitrarily large streams in constant memory (<1MB heap)
+
+## 🧪 Self-Correction (Test Yourself)
+1. **The Allocation Test**:
+   ```go
+   go test -bench=. -benchmem -count=5
+   ```
+   **Pass**: `allocs/op` = 0 for parsing loop
+   **Fail**: Any allocations in hot path
+
+2. **The Stream Test**:
+    - Pipe 1GB of JSON through your parser (mock with repeating data)
+    - **Pass**: Memory usage flatlines after warm-up
+    - **Fail**: Memory grows linearly with input size
+
+3. **The Corruption Test**:
+    - Input: `{"sensor_id": "a"} {"bad json here` (malformed second object)
+    - **Pass**: Returns first object, logs/skips second, doesn't panic
+    - **Fail**: Parser crashes or stops processing entirely
+
+## 📚 Resources
+* [Go JSON Stream Parsing](https://ahmet.im/blog/golang-json-stream-parse/)
+* [json.RawMessage Tutorial](https://www.sohamkamani.com/golang/json/#raw-messages)
+* [Advanced JSON Techniques](https://eli.thegreenplace.net/2019/go-json-cookbook/)
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diff --git a/05-context-aware-error-propagator/README.md b/05-context-aware-error-propagator/README.md
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+# Kata 05: The Context-Aware Error Propagator
+**Target Idioms:** Error Wrapping, Context-Aware Errors, Custom Error Types
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+Developers from dynamic languages often treat errors as simple strings. Java developers wrap exceptions in layers of inheritance. **Go's error philosophy is different:** errors are values that should carry context and be inspectable without string parsing. The unidiomatic pattern is to `log.Printf("error: %v", err)` and return nil - this destroys debugging context. Idiomatic Go preserves the original error while adding layers of context.
+
+## 🎯 The Scenario
+You're building a **cloud storage gateway** that interacts with multiple services: authentication, metadata database, and blob storage. When a file upload fails, operators need to know exactly which layer failed and why - was it auth timeout? database deadlock? storage quota exceeded? Your error handling must preserve this information while being safe for logging.
+
+## 🛠 The Challenge
+Create a service that uploads files to cloud storage with proper error handling.
+
+### 1. Functional Requirements
+* [ ] Implement three layers: `AuthService`, `MetadataService`, `StorageService`
+* [ ] Each layer can fail with specific error types
+* [ ] Return errors that expose the failure point and original cause
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+To pass this kata, you **must** strictly adhere to these rules:
+* [ ] **NO string-based error inspection:** You must use `%w` with `fmt.Errorf` for wrapping
+* [ ] **Custom Error Types:** Create specific error types for each service layer (e.g., `AuthError`, `StorageQuotaError`)
+* [ ] **Context-Aware Errors:** Errors must implement `Timeout()` and `Temporary()` methods where appropriate
+* [ ] **Safe Logging:** Errors must redact sensitive information (API keys, credentials) when logged
+* [ ] **Error Unwrapping:** Your errors must support `errors.Is()` and `errors.As()` for programmatic inspection
+
+## 🧪 Self-Correction (Test Yourself)
+Test your error handling with these scenarios:
+1.  **The "Sensitive Data Leak":**
+    * Force an auth error with a mock API key
+    * **Fail Condition:** If `fmt.Sprint(err)` contains the API key string
+2.  **The "Lost Context":**
+    * Wrap an `AuthError` three times through different layers
+    * **Fail Condition:** If `errors.As(err, &AuthError{})` returns false
+3.  **The "Timeout Confusion":**
+    * Create a timeout error in the storage layer
+    * **Fail Condition:** If `errors.Is(err, context.DeadlineExceeded)` returns false
+
+## 📚 Resources
+* [Go 1.13 Error Wrapping](https://go.dev/blog/go1.13-errors)
+* [Error Handling in Upspin](https://commandcenter.blogspot.com/2017/12/error-handling-in-upspin.html)
+* [Don't just check errors, handle them gracefully](https://dave.cheney.net/2016/04/27/dont-just-check-errors-handle-them-gracefully)
diff --git a/06-interface-based-middleware-chain/README.md b/06-interface-based-middleware-chain/README.md
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+# Kata 04: The Interface-Based Middleware Chain
+**Target Idioms:** Interface Design, Middleware Pattern, Composition over Inheritance
+**Difficulty:** 🔴 Advanced
+
+## 🧠 The "Why"
+Object-oriented developers often reach for class hierarchies and inheritance when building pipelines. In Go, **interfaces enable composition over inheritance**. The unidiomatic approach is to create a `BaseHandler` class with virtual methods. The idiomatic Go way uses small interfaces composed together. This pattern powers `http.Handler`, `io.Reader`, and many standard library patterns - but developers from other ecosystems struggle to see when to split interfaces.
+
+## 🎯 The Scenario
+You're building a **real-time analytics pipeline** for user events. Each event must pass through multiple processing stages: validation, enrichment, filtering, and finally storage. New stages will be added frequently. The pipeline must be:
+- Modular (add/remove stages without rewriting core logic)
+- Observable (track metrics at each stage)
+- Recoverable (continue processing after non-critical errors)
+
+## 🛠 The Challenge
+Create a middleware chain for processing user events.
+
+### 1. Functional Requirements
+* [ ] Process events through a configurable chain of middleware
+* [ ] Each middleware can modify, filter, or reject events
+* [ ] Provide metrics (counters, latencies) for each stage
+* [ ] Support graceful shutdown with context cancellation
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+To pass this kata, you **must** strictly adhere to these rules:
+* [ ] **Small Interfaces:** Define a `Processor` interface with a single method: `Process(context.Context, Event) ([]Event, error)`
+* [ ] **Middleware Composition:** Each middleware must implement the `Processor` interface and wrap another `Processor`
+* [ ] **Functional Options:** Configure middleware using functional options (e.g., `WithMetricsCollector()`)
+* [ ] **Context Propagation:** All middleware must respect context cancellation
+* [ ] **Zero Global State:** No package-level variables for configuration or state
+* [ ] **Testable by Design:** Each middleware must be unit-testable in isolation
+
+## 🧪 Self-Correction (Test Yourself)
+Test your implementation against these scenarios:
+1.  **The "Infinite Loop":**
+    * Create a middleware that generates 2 events from 1 input
+    * Chain it with a filtering middleware
+    * **Fail Condition:** If events multiply uncontrollably or memory usage grows exponentially
+2.  **The "Context Leak":**
+    * Add a middleware with a 10s timeout
+    * Cancel the context after 1s
+    * **Fail Condition:** If any middleware continues processing after context cancellation
+3.  **The "Interface Pollution":**
+    * Try to add a new middleware that needs access to database connections
+    * **Fail Condition:** If you had to modify the core `Processor` interface to add database methods
+
+## 📚 Resources
+* [Go Proverbs by Rob Pike](https://go-proverbs.github.io/)
+* [The Go Blog: Lexical Scanning in Go](https://blog.golang.org/lexical-scanning)
+* [Standard Library Inspiration: net/http.Handler](https://pkg.go.dev/net/http#Handler)
+* [Small Interfaces in the Standard Library](https://medium.com/@cep21/small-interfaces-in-go-1e912a7a7883)
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diff --git a/07-rate-limited-fanout/README.md b/07-rate-limited-fanout/README.md
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+# Kata 07: The Rate-Limited Fan-Out Client
+**Target Idioms:** Rate Limiting (`x/time/rate`), Bounded Concurrency (`x/sync/semaphore`), HTTP Client Hygiene, Context Cancellation  
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+In many ecosystems, you slap a “rate limit middleware” in front of a thread pool and call it a day. In Go, people often:
+- spawn too many goroutines (no backpressure),
+- forget per-request cancellation,
+- misuse `http.DefaultClient` (timeouts/transport reuse),
+- implement “sleep-based” rate limiting (jittery, wasteful).
+
+This kata forces **explicit control** over *rate*, *in-flight concurrency*, and *cancellation*.
+
+## 🎯 The Scenario
+You’re building an internal service that needs to fetch user widgets from a downstream API:
+- API allows **10 requests/sec** with bursts up to **20**
+- Your service must also cap concurrency at **max 8 in-flight** requests
+- If any request fails, cancel everything immediately (fail-fast), and return the first error.
+
+## 🛠 The Challenge
+Implement `FanOutClient` with:
+- `FetchAll(ctx context.Context, userIDs []int) (map[int][]byte, error)`
+
+### 1. Functional Requirements
+- [ ] Requests must respect a **QPS rate limit** + **burst**.
+- [ ] Requests must run concurrently but never exceed **MaxInFlight**.
+- [ ] Results returned as `map[userID]payload`.
+- [ ] On first error, cancel remaining work and return immediately.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Must** use `golang.org/x/time/rate.Limiter`.
+- [ ] **Must** use `golang.org/x/sync/semaphore.Weighted` (or equivalent semaphore pattern) for MaxInFlight.
+- [ ] **Must** use `http.NewRequestWithContext`.
+- [ ] **Must NOT** use `time.Sleep` for rate limiting.
+- [ ] **Must** reuse a single `http.Client` (with a configured `Transport` + `Timeout`).
+- [ ] Logging via `log/slog` (structured fields: userID, attempt, latency).
+
+## 🧪 Self-Correction (Test Yourself)
+- **If you spawn `len(userIDs)` goroutines:** you failed backpressure.
+- **If cancellation doesn’t stop waiting callers:** you failed context propagation.
+- **If QPS is enforced using `Sleep`:** you failed rate limiting.
+- **If you use `http.DefaultClient`:** you failed HTTP hygiene.
+
+## 📚 Resources
+- https://pkg.go.dev/golang.org/x/time/rate
+- https://pkg.go.dev/golang.org/x/sync/semaphore
+- https://go.dev/src/net/http/client.go
+- https://go.dev/src/net/http/transport.go
diff --git a/08-retry-backoff-policy/README.md b/08-retry-backoff-policy/README.md
new file mode 100644
index 0000000..f4219a2
--- /dev/null
+++ b/08-retry-backoff-policy/README.md
@@ -0,0 +1,47 @@
+# Kata 08: The Retry Policy That Respects Context
+**Target Idioms:** Retry Classification, Error Wrapping (`%w`), Timer Reuse, Context Deadlines  
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+In other languages, retries are often hidden in SDKs. In Go, it’s easy to write:
+- infinite retry loops,
+- retry-on-any-error (bad),
+- retry that ignores context cancellation (worse),
+- retry implemented with repeated `time.Sleep` (hard to test, wasteful).
+
+This kata makes you implement a **testable**, **context-aware** retry loop.
+
+## 🎯 The Scenario
+You call a flaky downstream service. You should retry only on **transient** failures:
+- `net.Error` with `Timeout() == true`
+- HTTP 429 / 503 (if you model HTTP)
+- sentinel `ErrTransient`
+
+Everything else must fail immediately.
+
+## 🛠 The Challenge
+Implement:
+- `type Retryer struct { ... }`
+- `func (r *Retryer) Do(ctx context.Context, fn func(context.Context) error) error`
+
+### 1. Functional Requirements
+- [ ] Retries up to `MaxAttempts`.
+- [ ] Uses exponential backoff: `base * 2^attempt` with a max cap.
+- [ ] Optional jitter (deterministic in tests).
+- [ ] Stops immediately on `ctx.Done()`.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Must NOT** call `time.Sleep` inside the retry loop.
+- [ ] **Must** use a `time.Timer` and `Reset` it (timer reuse).
+- [ ] **Must** wrap the final error with context (attempt count) using `%w`.
+- [ ] **Must** classify errors using `errors.Is` / `errors.As`.
+
+## 🧪 Self-Correction (Test Yourself)
+- **If context cancellation only stops after the sleep:** you failed.
+- **If you retry non-transient errors:** you failed classification.
+- **If you can’t test it without real time:** inject time/jitter sources.
+
+## 📚 Resources
+- https://go.dev/blog/go1.13-errors
+- https://pkg.go.dev/errors
+- https://pkg.go.dev/time
diff --git a/09-single-flight-ttl-cache/README.md b/09-single-flight-ttl-cache/README.md
new file mode 100644
index 0000000..11724af
--- /dev/null
+++ b/09-single-flight-ttl-cache/README.md
@@ -0,0 +1,42 @@
+# Kata 09: The Cache Stampede Shield
+**Target Idioms:** `singleflight`, TTL Cache, DoChan + Context Select, Lock Avoidance  
+**Difficulty:** 🔴 Advanced
+
+## 🧠 The "Why"
+In many stacks, caching is “just Redis”. In Go, an in-process cache is common, but people:
+- hold locks while calling the loader (deadly),
+- refresh the same key N times concurrently (stampede),
+- can’t cancel waiters cleanly.
+
+This kata is about **deduplicating in-flight loads** and making waiters **context-cancellable**.
+
+## 🎯 The Scenario
+You have expensive per-key loads (e.g., DB or remote API). If 200 goroutines ask for the same key at once:
+- loader must run **once**
+- others must wait (or return on ctx cancel)
+- TTL must be enforced
+
+## 🛠 The Challenge
+Implement:
+- `type Cache[K comparable, V any] struct { ... }`
+- `Get(ctx context.Context, key K, loader func(context.Context) (V, error)) (V, error)`
+
+### 1. Functional Requirements
+- [ ] Return cached value if not expired.
+- [ ] If expired/missing: load once, share result to all callers.
+- [ ] Callers must be able to stop waiting via `ctx.Done()`.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Must** use `golang.org/x/sync/singleflight.Group`.
+- [ ] **Must** use `DoChan` + `select` on `ctx.Done()` to cancel waiters.
+- [ ] **Must NOT** hold a mutex while calling `loader`.
+- [ ] Errors must be wrapped with key context using `%w`.
+
+## 🧪 Self-Correction (Test Yourself)
+- **If 200 goroutines trigger 200 loads:** you failed (no stampede protection).
+- **If a canceled context still blocks waiting:** you failed.
+- **If you lock around loader execution:** you failed (contention / deadlocks).
+
+## 📚 Resources
+- https://pkg.go.dev/golang.org/x/sync/singleflight
+- https://go.dev/blog/go1.13-errors
diff --git a/10-worker-pool-errors-join/README.md b/10-worker-pool-errors-join/README.md
new file mode 100644
index 0000000..06d084b
--- /dev/null
+++ b/10-worker-pool-errors-join/README.md
@@ -0,0 +1,45 @@
+# Kata 10: The Worker Pool With Backpressure and Joined Errors
+**Target Idioms:** Worker Pools, Channel Ownership, `errors.Join`, Context Cancellation  
+**Difficulty:** 🔴 Advanced
+
+## 🧠 The "Why"
+Many devs bring “thread pool” instincts and end up with:
+- goroutines that never exit,
+- unbounded queues,
+- “first error wins” even when you want a summary,
+- ad-hoc error channels without cleanup.
+
+This kata forces correctness: **bounded work**, **clean shutdown**, and **error aggregation**.
+
+## 🎯 The Scenario
+You process a stream of jobs (e.g., image resizing). You want:
+- fixed number of workers
+- bounded queue (backpressure)
+- either fail-fast OR collect all errors (configurable)
+
+## 🛠 The Challenge
+Implement:
+- `type Pool struct { ... }`
+- `Run(ctx context.Context, jobs <-chan Job) error`
+
+Where `Job` is `func(context.Context) error`.
+
+### 1. Functional Requirements
+- [ ] `N` workers process from `jobs`.
+- [ ] Optional `StopOnFirstError`.
+- [ ] If not fail-fast: return `errors.Join(errs...)` after draining.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Must** use `errors.Join` for aggregation.
+- [ ] **Must** respect `ctx.Done()` (workers exit).
+- [ ] **Must** close internal channels from the sender side only.
+- [ ] **Must** guarantee no goroutine leak when `jobs` closes early or ctx cancels.
+
+## 🧪 Self-Correction (Test Yourself)
+- **If workers keep running after ctx cancel:** failed.
+- **If you can deadlock by closing channels from the wrong side:** failed.
+- **If you return before draining in non-fail-fast mode:** failed.
+
+## 📚 Resources
+- https://go.dev/doc/go1.20
+- https://go.dev/src/errors/join.go
diff --git a/11-ndjson-stream-reader/README.md b/11-ndjson-stream-reader/README.md
new file mode 100644
index 0000000..a880e0d
--- /dev/null
+++ b/11-ndjson-stream-reader/README.md
@@ -0,0 +1,36 @@
+# Kata 11: The NDJSON Reader That Survives Long Lines
+**Target Idioms:** Streaming I/O (`io.Reader`), `bufio.Reader` vs `Scanner`, Handling `ErrBufferFull`, Low Allocation  
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+Seasoned devs reach for `bufio.Scanner` and it “works”… until production sends a line > 64K and you get:
+`bufio.Scanner: token too long`.
+
+This kata forces you to implement a streaming reader that can handle **arbitrarily large lines** without falling over.
+
+## 🎯 The Scenario
+You ingest NDJSON logs from stdin or a file. Lines can be huge (hundreds of KB). You must process line-by-line.
+
+## 🛠 The Challenge
+Implement:
+- `func ReadNDJSON(ctx context.Context, r io.Reader, handle func([]byte) error) error`
+
+### 1. Functional Requirements
+- [ ] Call `handle(line)` for each line (without the trailing newline).
+- [ ] Stop immediately on `handle` error.
+- [ ] Stop immediately on `ctx.Done()`.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Must NOT** rely on default `bufio.Scanner` behavior.
+- [ ] **Must** use `bufio.Reader` and correctly handle `ReadSlice('\n')` returning `ErrBufferFull`.
+- [ ] **Must** avoid per-line allocations where possible (reuse buffers).
+- [ ] Wrap errors with line number context using `%w`.
+
+## 🧪 Self-Correction (Test Yourself)
+- **If a 200KB line crashes with “token too long”:** you failed.
+- **If cancellation doesn’t stop promptly:** you failed.
+- **If you allocate a new buffer each line:** you failed the low-allocation goal.
+
+## 📚 Resources
+- https://pkg.go.dev/bufio
+- https://pkg.go.dev/io
diff --git a/12-sync-pool-buffer-middleware/README.md b/12-sync-pool-buffer-middleware/README.md
new file mode 100644
index 0000000..55227a9
--- /dev/null
+++ b/12-sync-pool-buffer-middleware/README.md
@@ -0,0 +1,42 @@
+# Kata 12: The sync.Pool Buffer Middleware
+**Target Idioms:** `sync.Pool`, Avoiding GC Pressure, `bytes.Buffer` Reset, Benchmarks (`-benchmem`)  
+**Difficulty:** 🔴 Advanced
+
+## 🧠 The "Why"
+In Go, performance regressions often come from allocation/GC churn, not “slow CPU”.
+People use `sync.Pool` incorrectly:
+- pooling long-lived objects (wrong),
+- forgetting to reset buffers (data leak),
+- storing huge buffers back into the pool (memory bloat).
+
+This kata is about **safe pooling** for high-throughput handlers.
+
+## 🎯 The Scenario
+You’re writing an HTTP middleware that:
+- reads up to 16KB of request body for audit logging
+- must not allocate per-request in the hot path
+
+## 🛠 The Challenge
+Implement a middleware:
+- `func AuditBody(max int, next http.Handler) http.Handler`
+
+### 1. Functional Requirements
+- [ ] Read up to `max` bytes of request body (do not consume beyond `max`).
+- [ ] Log the captured bytes with `slog` fields.
+- [ ] Pass the request downstream intact (body still readable).
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Must** use `sync.Pool` to reuse buffers.
+- [ ] **Must** `Reset()`/clear buffers before putting back.
+- [ ] **Must** bound memory: never keep buffers larger than `max` in the pool.
+- [ ] Provide a benchmark showing reduced allocations (`go test -bench . -benchmem`).
+
+## 🧪 Self-Correction (Test Yourself)
+- **If a request leaks previous request content:** you failed (no reset).
+- **If allocations are ~O(requests):** you failed pooling.
+- **If buffers grow unbounded and stay in pool:** you failed memory bounds.
+
+## 📚 Resources
+- https://pkg.go.dev/sync
+- https://go.dev/doc/gc-guide
+- https://go.dev/blog/pprof
diff --git a/13-iofs-config-loader/README.md b/13-iofs-config-loader/README.md
new file mode 100644
index 0000000..8d6d2fb
--- /dev/null
+++ b/13-iofs-config-loader/README.md
@@ -0,0 +1,37 @@
+# Kata 13: The Filesystem-Agnostic Config Loader
+**Target Idioms:** `io/fs` abstraction, `fs.WalkDir`, Testability via `fstest.MapFS`, `embed` readiness  
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+In Go, passing `"/etc/app/config"` all over the place hard-couples your logic to the OS.
+Idiomatic Go uses `fs.FS` so you can:
+- load from disk,
+- load from embedded files,
+- load from a ZIP filesystem,
+- unit test without touching the real filesystem.
+
+## 🎯 The Scenario
+Your CLI loads configuration fragments from a directory tree, merges them, and prints a final config report.
+
+## 🛠 The Challenge
+Implement:
+- `func LoadConfigs(fsys fs.FS, root string) (map[string][]byte, error)`
+
+### 1. Functional Requirements
+- [ ] Walk `root` recursively and read all `*.conf` files.
+- [ ] Return a map of `path -> content`.
+- [ ] Reject invalid paths cleanly.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Must** accept `fs.FS` (not `os` paths) in the core API.
+- [ ] **Must** use `fs.WalkDir` and `fs.ReadFile`.
+- [ ] **Must NOT** use `os.Open` / `filepath.Walk` inside the core loader.
+- [ ] Unit tests must use `testing/fstest.MapFS`.
+
+## 🧪 Self-Correction (Test Yourself)
+- **If you can’t test without real files:** you failed.
+- **If your loader only works on disk:** you failed the abstraction goal.
+
+## 📚 Resources
+- https://pkg.go.dev/io/fs
+- https://go.dev/src/embed/embed.go
diff --git a/14-leak-free-scheduler/README.md b/14-leak-free-scheduler/README.md
new file mode 100644
index 0000000..f367666
--- /dev/null
+++ b/14-leak-free-scheduler/README.md
@@ -0,0 +1,43 @@
+# Kata 14: The Leak-Free Scheduler
+**Target Idioms:** `time.Timer`/`time.Ticker`, Stop/Reset patterns, Jitter, Context Cancellation  
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+Scheduling in Go is deceptively easy until you ship:
+- goroutines that never stop,
+- overlapping executions,
+- ticker drift and backlog,
+- resource retention from careless timer usage.
+
+This kata makes you build a scheduler that is **predictable** and **stoppable**.
+
+## 🎯 The Scenario
+You need to periodically refresh a local cache:
+- every 5s, with ±10% jitter
+- do not overlap refreshes
+- stop immediately on shutdown
+
+## 🛠 The Challenge
+Implement:
+- `type Scheduler struct { ... }`
+- `func (s *Scheduler) Run(ctx context.Context, job func(context.Context) error) error`
+
+### 1. Functional Requirements
+- [ ] Run `job` periodically (interval + jitter).
+- [ ] Never run `job` concurrently with itself.
+- [ ] Exit on `ctx.Done()`.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Must NOT** use `time.Tick` (no stop control).
+- [ ] **Must** use a `time.Timer` or `time.Ticker` with correct stop/reset.
+- [ ] **Must** propagate context into `job`.
+- [ ] Log job duration and errors via `slog`.
+
+## 🧪 Self-Correction (Test Yourself)
+- **If `job` overlap occurs:** you failed.
+- **If cancel doesn’t stop quickly:** you failed.
+- **If goroutines remain after exit:** you failed.
+
+## 📚 Resources
+- https://pkg.go.dev/time
+- https://go.dev/wiki/Go123Timer
diff --git a/15-testing-parallel-fuzz-harness/README.md b/15-testing-parallel-fuzz-harness/README.md
new file mode 100644
index 0000000..078dac0
--- /dev/null
+++ b/15-testing-parallel-fuzz-harness/README.md
@@ -0,0 +1,51 @@
+# Kata 15: The Go Test Harness (Subtests, Parallel, Fuzz)
+**Target Idioms:** Table-Driven Tests, `t.Run`, `t.Parallel`, Fuzzing (`go test -fuzz`)  
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+Developers often write:
+- one-off tests with repetition,
+- unsafe parallel subtests (loop variable capture),
+- no fuzz testing for parsers/sanitizers.
+
+Idiomatic Go testing is:
+- table-driven,
+- readable failures,
+- parallel where safe,
+- fuzzed for edge cases.
+
+## 🎯 The Scenario
+You’re implementing a sanitizer:
+- `func NormalizeHeaderKey(s string) (string, error)`
+  Rules:
+- only ASCII letters/digits/hyphen allowed
+- normalize to canonical header form (e.g., `content-type` -> `Content-Type`)
+- reject invalid input
+
+## 🛠 The Challenge
+Write:
+1) The implementation, and
+2) A test suite that proves it’s solid.
+
+### 1. Functional Requirements
+- [ ] Canonicalize valid inputs.
+- [ ] Reject invalid characters.
+- [ ] Stable behavior (same input => same output).
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] Tests must be **table-driven** with `t.Run`.
+- [ ] Use **parallel subtests** correctly (no loop var capture bugs).
+- [ ] Include a **fuzz test** that:
+    - never panics,
+    - never returns a string containing invalid characters,
+    - roundtrips canonical form (calling Normalize twice is idempotent).
+
+## 🧪 Self-Correction (Test Yourself)
+- **If parallel subtests flake:** you likely captured the loop variable.
+- **If fuzzing finds panics:** you missed an edge case.
+
+## 📚 Resources
+- https://go.dev/blog/subtests
+- https://go.dev/wiki/TableDrivenTests
+- https://go.dev/doc/security/fuzz/
+- https://go.dev/doc/tutorial/fuzz
diff --git a/16-http-client-hygiene/README.md b/16-http-client-hygiene/README.md
new file mode 100644
index 0000000..d0aac86
--- /dev/null
+++ b/16-http-client-hygiene/README.md
@@ -0,0 +1,48 @@
+# Kata 16: The HTTP Client Hygiene Wrapper
+**Target Idioms:** `net/http` Transport Reuse, Timeouts, Context-First APIs, Response Body Draining  
+**Difficulty:** 🔴 Advanced
+
+## 🧠 The "Why"
+“Works locally” HTTP code in Go often fails in prod because people:
+- use `http.DefaultClient` with no timeouts,
+- create a new client/transport per request (connection churn),
+- forget to close bodies (leaks + no keep-alive reuse),
+- don’t drain bodies (prevents connection reuse).
+
+This kata is about building a small internal SDK the **Go way**.
+
+## 🎯 The Scenario
+Your service calls a downstream API that sometimes returns large error bodies and sometimes hangs.
+You need:
+- strict timeouts,
+- proper cancellation,
+- safe connection reuse,
+- structured logs.
+
+## 🛠 The Challenge
+Implement:
+- `type APIClient struct { ... }`
+- `func (c *APIClient) GetJSON(ctx context.Context, url string, out any) error`
+
+### 1. Functional Requirements
+- [ ] Use `http.NewRequestWithContext`.
+- [ ] Decode JSON on 2xx responses into `out`.
+- [ ] On non-2xx: read up to N bytes of body and return an error including status code.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Must NOT** use `http.DefaultClient`.
+- [ ] **Must** configure timeouts (`Client.Timeout` and/or transport-level timeouts).
+- [ ] **Must** reuse a single `Transport` (connection pooling).
+- [ ] **Must** `defer resp.Body.Close()`.
+- [ ] **Must** drain (at least partially) error bodies to allow connection reuse.
+- [ ] Use `slog` with fields: method, url, status, latency.
+
+## 🧪 Self-Correction (Test Yourself)
+- **If connections spike under load:** you probably rebuild transports.
+- **If keep-alives don’t work:** you likely didn’t drain/close body.
+- **If hangs occur:** you likely lack correct timeout configuration.
+
+## 📚 Resources
+- https://go.dev/src/net/http/client.go
+- https://go.dev/src/net/http/transport.go
+- https://blog.cloudflare.com/the-complete-guide-to-golang-net-http-timeouts/
diff --git a/17-context-aware-channel-sender/README.md b/17-context-aware-channel-sender/README.md
new file mode 100644
index 0000000..d61e434
--- /dev/null
+++ b/17-context-aware-channel-sender/README.md
@@ -0,0 +1,56 @@
+# Kata 17: The Context-Aware Channel Sender (No Leaked Producers)
+**Target Idioms:** Pipeline Cancellation, Select-on-Send, Channel Ownership, Goroutine Leak Prevention  
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+A goroutine sending on a channel blocks until a receiver is ready (unless buffered space is available). If the receiver exits early (timeout, HTTP cancel, upstream error), producers can block forever and leak.
+
+Idiomatic Go fixes this by:
+- threading `context.Context` through the pipeline
+- **selecting on every send** (`case out <- v` vs `case <-ctx.Done()`), as recommended in Go’s pipeline cancellation patterns and real leak writeups.
+
+## 🎯 The Scenario
+You’re building a data pipeline step that fetches N URLs concurrently and streams results downstream. If the request is canceled (client disconnect, global timeout), **all fetchers must stop immediately** and no goroutine may remain blocked on `out <- result`.
+
+## 🛠 The Challenge
+Implement:
+- `type DataFetcher struct { ... }`
+- `func (f *DataFetcher) Fetch(ctx context.Context, urls []string) <-chan Result`
+
+Where:
+- `Result` contains `URL`, `Body []byte`, `Err error` (or similar).
+
+### 1. Functional Requirements
+- [ ] Start concurrent fetchers for all URLs (or bounded concurrency if you choose).
+- [ ] Send results as they complete (order doesn’t matter).
+- [ ] Stop promptly on `ctx.Done()`.
+- [ ] Close the output channel exactly once after all producers exit.
+- [ ] Return partial results that already completed before cancellation.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Every send uses select:** no bare `out <- x`.
+- [ ] **Channel ownership:** only the producer side closes `out`.
+- [ ] **No goroutine leaks:** all goroutines exit when ctx is canceled.
+- [ ] **No double close:** prove it structurally (single closer goroutine). Avoid `sync.Once` unless you can justify it.
+- [ ] **Buffer choice is intentional:** if you buffer, document why and how you chose the size.
+
+### 3. Hints (Allowed Tools)
+- You may use `errgroup` or a simple worker pattern, but the key is: **send must be cancel-aware**.
+- If you do bounded concurrency, prefer `x/sync/semaphore` or a worker pool (but don’t turn this kata into a rate-limiter kata).
+
+## 🧪 Self-Correction (Test Yourself)
+1. **Forgotten Sender**
+    - Start 50 fetchers, consume only 1 result, then cancel.
+    - **Pass:** goroutine count returns near baseline quickly (use `runtime.NumGoroutine()` as a sanity check).
+
+2. **Cancellation Before First Receive**
+    - Cancel ctx immediately after calling `Fetch`.
+    - **Pass:** no goroutine blocks trying to send.
+
+3. **Close Discipline**
+    - Cancel ctx from multiple places.
+    - **Pass:** no `panic: close of closed channel`.
+
+## 📚 Resources
+- https://go.dev/blog/pipelines
+- https://www.ardanlabs.com/blog/2018/11/goroutine-leaks-the-forgotten-sender.html
diff --git a/18-embedfs-dev-prod-switch/README.md b/18-embedfs-dev-prod-switch/README.md
new file mode 100644
index 0000000..c02e602
--- /dev/null
+++ b/18-embedfs-dev-prod-switch/README.md
@@ -0,0 +1,54 @@
+# Kata 18: embed.FS Dev/Prod Switch Without Handler Forks
+**Target Idioms:** `embed`, `io/fs`, Build Tags, `fs.Sub`, Same Handler Code Path  
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+Embedding assets is great for production (single binary), but terrible for frontend iteration if every CSS tweak needs a rebuild.
+Idiomatic Go solves this with:
+- compile-time selection via build tags
+- a shared `fs.FS` abstraction so handler code doesn’t branch on “dev/prod”.
+
+## 🎯 The Scenario
+You run a small internal dashboard:
+- Prod: ship a single binary (assets embedded).
+- Dev: designers update `static/` and `templates/` live without recompiling.
+
+## 🛠 The Challenge
+Create a server that serves:
+- templates from `templates/`
+- static assets from `static/`
+
+### 1. Functional Requirements
+- [ ] `GET /` renders an HTML template.
+- [ ] `GET /static/...` serves static files.
+- [ ] Dev mode serves from disk; prod mode serves embedded.
+- [ ] Handler code is identical in both modes.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Build tags:** two files:
+    - `assets_dev.go` with `//go:build dev`
+    - `assets_prod.go` with `//go:build !dev`
+- [ ] **Return `fs.FS`:** `func Assets() (templates fs.FS, static fs.FS, err error)`
+- [ ] **Use `fs.Sub`:** exported FS must have *clean roots* (no `static/static/...` path bugs).
+- [ ] **No runtime env checks in handlers:** mode selection must be compile-time.
+- [ ] **Single `http.FileServer` setup:** no duplicated handler logic for dev vs prod.
+
+## 🧪 Self-Correction (Test Yourself)
+1. **Live Reload**
+    - Build with `-tags dev`.
+    - Modify a CSS file and refresh.
+    - **Pass:** change shows without rebuild.
+
+2. **Binary Portability**
+    - Build without tags.
+    - Delete `static/` and `templates/` from disk.
+    - **Pass:** server still serves assets/templates.
+
+3. **Prefix Correctness**
+    - Request `/static/app.css`.
+    - **Pass:** works in both modes (no 404 due to prefix mismatch).
+
+## 📚 Resources
+- https://pkg.go.dev/embed
+- https://pkg.go.dev/io/fs
+- https://pkg.go.dev/io/fs#Sub
diff --git a/19-defer-cleanup-chain/README.md b/19-defer-cleanup-chain/README.md
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+++ b/19-defer-cleanup-chain/README.md
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+# Kata 19: The Cleanup Chain (defer + LIFO + Error Preservation)
+**Target Idioms:** `defer` Discipline, Named Returns, Error Composition (`errors.Join`), Close/Rollback Ordering  
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+`defer` is easy to misuse:
+- deferring in loops (resource spikes),
+- ignoring `Close()` / `Rollback()` errors,
+- losing the original failure when cleanup also fails,
+- wrong cleanup ordering (commit then rollback nonsense).
+
+Idiomatic Go keeps cleanup local, ordered, and preserves important errors.
+
+## 🎯 The Scenario
+You implement `BackupDatabase`:
+- open output file
+- connect DB
+- begin transaction
+- stream rows to file
+- commit
+  If anything fails, you must close/rollback what was already acquired.
+
+## 🛠 The Challenge
+Implement:
+- `func BackupDatabase(ctx context.Context, dbURL, filename string) (err error)`
+
+Use mock interfaces for DB + Tx + Rows if you want (recommended).
+
+### 1. Functional Requirements
+- [ ] Open file for writing.
+- [ ] Connect to DB.
+- [ ] Begin Tx.
+- [ ] Write data (simulate streaming).
+- [ ] Commit on success.
+- [ ] On failure: rollback + close resources in correct order.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Defer cleanup immediately after acquisition.**
+- [ ] **No manual cleanup paths** except by controlling flags (e.g., `committed bool`) used by deferred funcs.
+- [ ] **Preserve both errors:** if main operation fails and cleanup fails too, return a combined error (`errors.Join`).
+- [ ] **Named return `err`** so defers can amend it safely.
+- [ ] **No defer-in-loop for per-row resources:** if your mock has per-row closers, show the correct pattern.
+
+## 🧪 Self-Correction (Test Yourself)
+1. **Tx Begin Fails**
+    - Make `Begin()` error.
+    - **Pass:** file + db connection still close.
+
+2. **Commit Fails + Close Fails**
+    - Make `Commit()` return error and also make `file.Close()` return error.
+    - **Pass:** returned error clearly contains both (use `errors.Join`).
+
+3. **No FD Leak**
+    - Run 1000 times.
+    - **Pass:** file descriptors don’t grow.
+
+## 📚 Resources
+- https://go.dev/blog/defer-panic-and-recover
+- https://go.dev/doc/go1.20 (errors.Join)
diff --git a/20-nil-interface-gotcha/README.md b/20-nil-interface-gotcha/README.md
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+++ b/20-nil-interface-gotcha/README.md
@@ -0,0 +1,54 @@
+# Kata 20: The “nil != nil” Interface Trap (Typed nil Errors)
+**Target Idioms:** Interface Semantics, Typed nil Pitfall, Safe Error Returns, `errors.As`  
+**Difficulty:** 🔴 Advanced
+
+## 🧠 The "Why"
+In Go, an interface value is only nil when **both** its dynamic type and value are nil.
+If you return a **typed nil pointer** (e.g., `(*MyError)(nil)`) as an `error`, the interface has a non-nil type, so `err != nil` becomes true even though the pointer inside is nil.
+
+This bites real code in production (especially custom error types and factories).
+
+## 🎯 The Scenario
+A function returns `error`. Sometimes it returns a typed nil pointer.
+Your caller checks `if err != nil` and takes an error path, logs misleading failures, or even panics when accessing fields/methods.
+
+## 🛠 The Challenge
+Write a minimal package that:
+1) demonstrates the bug, and
+2) fixes it with an idiomatic pattern.
+
+### 1. Functional Requirements
+- [ ] Implement `type MyError struct { Op string }` (or similar).
+- [ ] Implement a function `DoThing(...) error` that **sometimes returns** `(*MyError)(nil)` as `error`.
+- [ ] Demonstrate:
+    - `err != nil` is true
+    - `fmt.Printf("%T %#v\n", err, err)` shows the typed nil behavior
+- [ ] Provide a corrected version that returns a true nil interface when there is no error.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Must show the failing behavior** in a test (`go test`).
+- [ ] **Must show the fix** in a test.
+- [ ] **Must not “fix” by panicking or by sentinel errors.**
+- [ ] Use one of these idiomatic fixes:
+    - return `nil` explicitly when the pointer is nil
+    - or return `error(nil)` in the relevant branch
+- [ ] Demonstrate safe extraction using:
+    - `var me *MyError; errors.As(err, &me)` and check `me != nil`
+
+## 🧪 Self-Correction (Test Yourself)
+1. **The Trap Repro**
+    - Make `DoThing()` return `var e *MyError = nil; return e`
+    - **Pass:** your test proves `err != nil` is true.
+
+2. **The Fix**
+    - If internal pointer is nil, return literal `nil`.
+    - **Pass:** `err == nil` works, callers behave correctly.
+
+3. **Extraction Safety**
+    - Wrap the error and still extract with `errors.As`.
+    - **Pass:** extraction works through wrapping layers.
+
+## 📚 Resources
+- https://go.dev/blog/laws-of-reflection (interface basics)
+- https://go.dev/blog/go1.13-errors (errors.As)
+- https://forum.golangbridge.org/t/logic-behind-failing-nil-check/16331
diff --git a/21-my-very-creative-challange/README.md b/21-my-very-creative-challange/README.md
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index 0000000..e69de29
diff --git a/README.md b/README.md
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--- /dev/null
+++ b/README.md
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+# 🥋 Go Katas 🥋
+
+>  "I fear not the man who has practiced 10,000 kicks once, but I fear the man who has practiced one kick 10,000 times."
+(Bruce Lee)
+
+## What should it be?
+- Go is simple to learn, but nuanced to master. The difference between "working code" and "idiomatic code" often lies in details such as safety, memory efficiency, and concurrency control.
+
+- This repository is a collection of **Daily Katas**: small, standalone coding challenges designed to drill specific Go patterns into your muscle memory.
+
+## What should it NOT be? 
+
+- This is not intended to teach coding, having Go as the programming mean. Not even intended to teach you Go **in general**
+- The focus should be as much as possible challenging oneself to solve common software engineering problems **the Go way**. 
+- Several seasoned developers spent years learning and applying best-practices at prod-grade context. Once they decide to switch to go, they would face two challanges:
+  - Is there a window of knowledge transform here, so that I don't have to through years of my career from the window at start from zero?
+  - If yes, the which parts should I focus on to recognize the mismatches and use them the expected way in the Go land?
+
+## How to Use This Repo
+1.  **Pick a Kata:** Navigate to any `XX-kata-yy` folder.
+2.  **Read the Challenge:** Open the `README.md` inside that folder. It defines the Goal, the Constraints, and the "Idiomatic Patterns" you must use.
+3.  **Solve It:** Initialize a module inside the folder and write your solution.
+4.  **Reflect:** Compare your solution with the provided "Reference Implementation" (if available) or the core patterns listed.
+
+## Contribution Guidelines
+
+### Have a favorite Go pattern?
+1. Create a new folder `XX-your-topic`. (`XX` is an ordinal number)
+2. Copy the [README_TEMPLATE.md](./README_TEMPLATE.md) to the new folder as `README.md`
+3. Define the challenge: focus on **real-world scenarios** (e.g., handling timeouts, zero-allocation sets), and **idiomatic Go**, not just algorithmic puzzles.
+4. **Optionally**, create a `main.go` or any other relevant files under the project containing blueprint of the implementation, **as long as you think it reduces confusion and keeps the implementation  focused**
+5. Submit a PR.
+
+### Using the script
+
+You can use the shorthand script to add a new challenge, it will create a new folder and a new README.md file under it:
+```bash
+./add.sh my-very-creative-challange
+```
+This will create a new folder `21-my-very-creative-challange` (in case the latest challange was under the folder name `20-latest-name-here`) and add a `README.md` under it
+
+```bash
+medunes@medunes:~/projects/go-kata$  ls 21-my-very-creative-challange/
+README.md
+```
diff --git a/README_TEMPLATE.md b/README_TEMPLATE.md
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index 0000000..da8ede0
--- /dev/null
+++ b/README_TEMPLATE.md
@@ -0,0 +1,30 @@
+# Kata [ID]: [Title of the Challenge]
+
+**Target Idioms:** [e.g. Concurrency Patterns, Interface Pollution, Error Wrapping]
+**Difficulty:** [🟢 Beginner / 🟡 Intermediate / 🔴 Advanced]
+
+## 🧠 The "Why"
+*Briefly explain the mismatch seasoned devs would face here. Why would the "Java way" or "Python way" fail in Go for this specific problem?*
+
+## 🎯 The Scenario
+*[Describe a realistic production problem. Example: "You are building a CLI that parses 1GB logs..."]*
+
+## 🛠 The Challenge
+Implement a solution that satisfies the following requirements.
+
+### 1. Functional Requirements
+* [ ] [Requirement A]
+* [ ] [Requirement B]
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+*To pass this kata, you **must** use the following patterns:*
+* [ ] **Constraint 1:** [e.g. Do not allocate new memory inside the loop.]
+* [ ] **Constraint 2:** [e.g. Use `functional options` for configuration.]
+* [ ] **Constraint 3:** [e.g. Return wrapped errors.]
+
+## 🧪 Self-Correction (Test Yourself)
+* **If you did X:** [Explain why this is "un-idiomatic" or dangerous in Go]
+* **Instead, ensure Y:** [Explain the idiomatic fix]
+
+## 📚 Resources
+* [Link to Go Blog or Spec]
\ No newline at end of file
diff --git a/add.sh b/add.sh
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--- /dev/null
+++ b/add.sh
@@ -0,0 +1,12 @@
+#!/bin/bash
+name="${1}"
+if [[ -z "${name}" ]]; then
+  echo "please provide a 'kebab-case' name of the  new challenge."
+  exit 1
+fi
+number=$(ls -d */ | wc -l)
+number=$((10#$number + 1))
+number=$(printf "%02d" $number)
+folder="$number-${1}"
+mkdir $folder
+touch "$folder/README.md"