add readme file

Author: spdeepak

README.md

@@ -1 +1,149 @@
-# Capsule Cache
+## Capsule Cache
+
+A lightweight, memory-quota-aware HTTP output cache middleware for Go web frameworks like Gin, Chi, and Fiber. This package addresses the challenge of safely deploying in-memory caches in containerized environments (like Kubernetes) by enforcing a strict memory limit.
+
+---
+
+### Key Features
+
+*   **Hard Memory Limit (Quota):** Prevents Out-of-Memory (OOM) issues in containerized environments by evicting Least Recently Used (LRU) items when a configurable MB limit is hit. This ensures stability in memory-constrained setups.
+*   **Framework Agnostic:** Implemented as a standard `net/http` middleware, ensuring compatibility with all major Go routers (Gin, Chi, Fiber via their adapter methods).
+*   **Stale-While-Revalidate (SWR):** Serves stale content instantly while triggering a non-blocking background refresh to update the cache entry.
+*   **Time-to-Live (TTL):** Standard freshness control for cached entries.
+*   **Pluggable Backend:** Ships with a memory-safe LRU store and an interface for easy integration with external services like Redis.
+*   **Flexible Key Strategy:** Supports cache keys based on URL, headers, and request body hash for varied API use cases (e.g., translation services, API throttling).
+
+---
+
+### Installation
+
+```bash
+go get github.com/spdeepak/capsulecache
+```
+
+---
+
+### Usage
+
+The core of the package is the `NewCacheMiddleware` function, which requires a `Store` implementation and a `Config`.
+
+#### 1. Setup the Cache Store (In-Memory LRU with Quota)
+
+The example below sets a hard memory limit of **100 Megabytes (MB)**.
+
+```go
+package main
+
+import (
+    "log"
+    "net/http"
+    "time"
+    
+    "github.com/go-chi/chi/v5"
+    "github.com/spdeepak/capsulecache"
+)
+
+// Global store for access in other handlers (e.g., for manual invalidation)
+var store cache.CacheStore 
+
+func main() {
+    // Initialize the In-Memory Cache Store with a hard limit of 100 MB
+    const maxMemoryMB = 100
+    store = cache.NewInMemoryQuotaLRU(maxMemoryMB)
+    defer store.Close()
+
+    // Configure the Middleware
+    cfg := &cache.Config{
+        DefaultTTL: 1 * time.Minute,
+        DefaultSWR: 15 * time.Second, // Allow stale content for 15s while refreshing
+    }
+
+    // Initialize the Middleware
+    cacheMiddleware := cache.NewCacheMiddleware(store, cfg)
+    
+    // Setup Chi Router
+    r := chi.NewRouter()
+
+    // Apply the cache middleware to a specific route group or handler
+    r.Group(func(r chi.Router) {
+        r.Use(cacheMiddleware) // Apply cache to handlers in this group
+
+        r.Get("/api/products/{id}", productHandler)
+        r.Get("/public/data", publicDataHandler)
+    })
+    
+    // Non-cached route
+    r.Post("/api/products", createProductHandler)
+
+    log.Println("Server starting on :8080")
+    http.ListenAndServe(":8080", r)
+}
+
+// Example Handler
+func publicDataHandler(w http.ResponseWriter, r *http.Request) {
+    // This expensive operation will only run once per cache TTL
+    time.Sleep(500 * time.Millisecond) 
+    w.Write([]byte("Cached response: " + time.Now().Format(time.RFC3339)))
+}
+```
+
+#### 2. Integration with Frameworks
+
+| Framework | Integration Method | Example |
+| :--- | :--- | :--- |
+| **Chi** | Standard `r.Use(middleware)` | See example above (`r.Use(cacheMiddleware)`) |
+| **Gin** | Use `gin.WrapH()` | `r.GET("/path", gin.WrapH(cacheMiddleware(http.HandlerFunc(myHandler))))` |
+| **Fiber** | Use the standard `net/http` adapter (e.g., `adaptor.HTTPHandlerFunc`) | `app.Use(adaptor.HTTPHandlerFunc(cacheMiddleware(http.HandlerFunc(myHandler))))` |
+
+---
+
+### Cache Invalidation
+
+For compliance or immediate content updates, the `Delete` method on the `CacheStore` can be used to manually purge an entry.
+
+```go
+// Example: Purging a cache entry after a POST request
+func createProductHandler(w http.ResponseWriter, r *http.Request) {
+    // ... logic to create product ...
+    
+    productID := "123" // Assume product ID is known
+    productKey := "GET:/api/products/" + productID // Must match the KeyGenerator logic
+
+    // Invalidate the cached GET response for the product
+    if err := store.Delete(productKey); err != nil {
+        log.Printf("Failed to purge cache for key %s: %v", productKey, err)
+    }
+
+    w.WriteHeader(http.StatusCreated)
+}
+```
+
+---
+
+### Custom Cache Key Strategy
+
+You can define a custom function to include specific headers (e.g., `Accept-Language` for translation services) in the cache key.
+
+```go
+// Example: Caching based on URL AND Accept-Language header
+func KeyWithLanguage(r *http.Request) string {
+    // Assuming DefaultKeyGenerator is also exported from capsulecache/cache
+    baseKey := cache.DefaultKeyGenerator(r) 
+
+    lang := r.Header.Get("Accept-Language")
+    if lang != "" {
+        // Simple extraction for the primary language tag (e.g., "en-US,en;q=0.9" -> "en-US")
+        if len(lang) > 5 { 
+            lang = lang[:5] 
+        }
+        return baseKey + ":lang=" + lang
+    }
+    return baseKey
+}
+
+// In main():
+// cfg := &cache.Config{
+//     DefaultTTL: 1 * time.Minute,
+//     KeyGenerator: KeyWithLanguage,
+// }
+```