feat: add OnStart hook for synchronous startup jobs

docs/advanced-guide/startup-hooks/page.md

@@ -0,0 +1,55 @@
+# Startup Hooks
+
+GoFr provides a way to run synchronous jobs when your application starts, before any servers begin handling requests. This is useful for tasks like seeding a database, warming up a cache, or performing other critical setup procedures.
+
+## `OnStart`
+
+You can register a startup hook using the `a.OnStart()` method on your `app` instance.
+
+### Usage
+
+The method accepts a function with the signature `func(ctx *gofr.Context) error`.
+
+- The `*gofr.Context` passed to the hook is fully initialized and provides access to all dependency-injection-managed services (e.g., `ctx.Container.SQL`, `ctx.Container.Redis`).
+- If any `OnStart` hook returns an error, the application will log the error and refuse to start.
+
+### Example: Warming up a Cache
+
+Here is an example of using `OnStart` to set an initial value in a Redis cache when the application starts.
+
+```go
+package main
+
+import (
+    "gofr.dev/pkg/gofr"
+)
+
+func main() {
+    a := gofr.New()
+
+    // Register an OnStart hook to warm up a cache.
+    a.OnStart(func(ctx *gofr.Context) error {
+        ctx.Logger.Info("Warming up the cache...")
+
+        // In a real app, this might come from a database or another service.
+        cacheKey := "initial-data"
+        cacheValue := "This is some data cached at startup."
+
+        err := ctx.Redis.Set(ctx, cacheKey, cacheValue, 0).Err()
+        if err != nil {
+            ctx.Logger.Errorf("Failed to warm up cache: %v", err)
+            return err // Return the error to halt startup if caching fails.
+        }
+
+        ctx.Logger.Info("Cache warmed up successfully!")
+
+        return nil
+    })
+
+    // ... register your routes
+
+    a.Run()
+}
+```
+
+This ensures that critical startup tasks are completed successfully before the application begins accepting traffic.

examples/http-server/main.go

@@ -21,6 +21,27 @@ func main() {
 	//HTTP service with default health check endpoint
 	a.AddHTTPService("anotherService", "http://localhost:9000")
 
+	// Register an OnStart hook to warm up a cache.
+	// This runs before route registration as intended.
+	a.OnStart(func(ctx *gofr.Context) error {
+		ctx.Logger.Info("Warming up the cache...")
+
+		// Example: Fetch some data and store it in Redis.
+		// In a real app, this might come from a database or another service.
+		cacheKey := "initial-data"
+		cacheValue := "This is some data cached at startup."
+
+		err := ctx.Redis.Set(ctx, cacheKey, cacheValue, 0).Err()
+		if err != nil {
+			ctx.Logger.Errorf("Failed to warm up cache: %v", err)
+			return err // Return the error to halt startup if caching fails.
+		}
+
+		ctx.Logger.Info("Cache warmed up successfully!")
+
+		return nil
+	})
+
 	// Add all the routes
 	a.GET("/hello", HelloHandler)
 	a.GET("/error", ErrorHandler)