New doc supporting CORS, gRCP, and general client integration

CLIENT_INTEGRATION_README.md

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+# Client Integration Guide - Notifier Service
+
+**Quick links for different use cases:**
+
+## 📚 Documentation Index
+
+### For Developers Building New Clients
+
+1. **Start Here:** [`docs/QUICK_START_CLIENT.md`](docs/QUICK_START_CLIENT.md)
+   - 5-minute quick start guide
+   - Complete minimal working example
+   - Essential requirements checklist
+
+2. **AI Assistant Prompt:** [`docs/CLIENT_INTEGRATION_PROMPT.md`](docs/CLIENT_INTEGRATION_PROMPT.md)
+   - Copy-paste prompt for Claude/ChatGPT/etc.
+   - Generates production-ready client code
+   - Includes all requirements and best practices
+
+3. **Performance Deep Dive:** [`docs/GRPC_CONNECTION_OPTIMIZATION.md`](docs/GRPC_CONNECTION_OPTIMIZATION.md)
+   - Why long-lived connections matter (10x faster!)
+   - Connection pooling strategies
+   - Keepalive configuration
+   - Resilience patterns
+
+4. **Best Practices:** [`docs/CLIENT_RECOMMENDATIONS.md`](docs/CLIENT_RECOMMENDATIONS.md)
+   - Advanced patterns
+   - Security hardening
+   - Observability and monitoring
+   - Testing strategies
+
+### For Kubernetes Deployments
+
+5. **CORS and Kubernetes:** [`docs/CORS_KUBERNETES_GUIDE.md`](docs/CORS_KUBERNETES_GUIDE.md)
+   - Do you need CORS? (Spoiler: Not for gRPC!)
+   - Service-to-service communication
+   - When CORS matters (web frontends)
+
+6. **CORS Implementation:** [`docs/CORS_IMPLEMENTATION.md`](docs/CORS_IMPLEMENTATION.md)
+   - Technical details of CORS security
+   - Configuration examples
+   - Migration guide
+
+### For Authentication & Security
+
+7. **Authentication Guide:** [`docs/AUTH.md`](docs/AUTH.md)
+   - Complete auth system documentation
+   - API key management
+   - Role-based access control (RBAC)
+
+## 🎯 Quick Decision Tree
+
+### "I need to build a new client application"
+
+**Option A: Using AI Assistant (Recommended)**
+→ Use [`CLIENT_INTEGRATION_PROMPT.md`](docs/CLIENT_INTEGRATION_PROMPT.md)
+→ Paste the prompt into Claude/ChatGPT
+→ Review generated code against checklist below
+
+**Option B: Manual Implementation**
+→ Start with [`QUICK_START_CLIENT.md`](docs/QUICK_START_CLIENT.md)
+→ Copy the complete example
+→ Customize for your needs
+→ Reference [`GRPC_CONNECTION_OPTIMIZATION.md`](docs/GRPC_CONNECTION_OPTIMIZATION.md) for performance
+
+### "I'm deploying to Kubernetes"
+
+→ Read [`CORS_KUBERNETES_GUIDE.md`](docs/CORS_KUBERNETES_GUIDE.md) first
+→ No CORS needed for backend gRPC communication! ✅
+→ Configure Kubernetes Secret for API key
+→ Use service DNS: `notifier-grpc:50051`
+
+### "My client is too slow"
+
+→ Check [`GRPC_CONNECTION_OPTIMIZATION.md`](docs/GRPC_CONNECTION_OPTIMIZATION.md)
+→ Verify you're using long-lived connections (not creating per request)
+→ Add keepalive configuration
+→ Consider connection pooling for high throughput
+
+### "Authentication isn't working"
+
+→ See [`AUTH.md`](docs/AUTH.md) for troubleshooting
+→ Verify API key format: `nk_` + 64 hex characters
+→ Check you're sending it in metadata with every request
+→ Confirm key is active and has required roles
+
+## ✅ Client Implementation Checklist
+
+Use this checklist to verify your client implementation:
+
+### Connection Management
+- [ ] Using single long-lived connection (created at startup)
+- [ ] Connection reused for all requests (not creating per request)
+- [ ] Keepalive configured (10s ping, 3s timeout)
+- [ ] Connection closed on application shutdown
+- [ ] Health checks implemented (every 30-60s)
+
+### Authentication
+- [ ] API key loaded from environment variable (`NOTIFIER_API_KEY`)
+- [ ] API key sent in metadata with every request
+- [ ] Using correct format: `authorization: Bearer nk_...`
+- [ ] No hardcoded credentials in source code
+- [ ] API key masked in logs (never logged in full)
+
+### Error Handling
+- [ ] Rate limit errors retry with exponential backoff
+- [ ] Authentication errors logged and not retried
+- [ ] Invalid argument errors logged and not retried
+- [ ] Unavailable errors retry with backoff
+- [ ] Maximum retry limit enforced (3-5 attempts)
+
+### Context & Timeouts
+- [ ] Every request uses context with timeout (5-30s)
+- [ ] Contexts properly cancelled with defer
+- [ ] Deadline exceeded errors handled gracefully
+
+### Security
+- [ ] Credentials in Kubernetes Secret (not ConfigMap)
+- [ ] No credentials in Docker image layers
+- [ ] `.gitignore` excludes credential files
+- [ ] No sensitive data in logs or metrics labels
+
+### Observability
+- [ ] Connection establishment logged (info level)
+- [ ] Errors logged with appropriate levels
+- [ ] Success responses logged (debug level)
+- [ ] Metrics instrumented (if using Prometheus)
+- [ ] Distributed tracing integrated (if using OpenTelemetry)
+
+### Testing
+- [ ] Unit tests with mocked client
+- [ ] Retry logic tested
+- [ ] Error handling tested
+- [ ] Integration tests (if service available)
+
+### Production Readiness
+- [ ] Graceful shutdown implemented
+- [ ] Circuit breaker pattern (optional, for resilience)
+- [ ] Notification failures don't crash application
+- [ ] Health check endpoint exposed (if needed)
+
+## 🔐 API Key Management
+
+### Getting an API Key
+
+Contact your Notifier service administrator to create a key:
+
+```bash
+# Using the notifier CLI (if available)
+notifier-cli keys create \
+  --client-id "my-application" \
+  --roles "email,slack" \
+  --rate-limit 1000
+
+# Output:
+# Created API key: nk_a1b2c3d4e5f6...
+# Save this key securely - it won't be shown again!
+```
+
+### API Key Format
+
+```
+nk_<64 hex characters>
+
+Example:
+nk_a1b2c3d4e5f6789012345678901234567890123456789012345678901234
+│  └─────────────────────────────────────────────────────────┘
+│                            64 characters
+└── Prefix (always "nk_")
+```
+
+### Storing API Keys
+
+**Development:**
+```bash
+export NOTIFIER_API_KEY="nk_your_dev_key_here"
+```
+
+**Production (Kubernetes):**
+```yaml
+# 1. Create secret
+kubectl create secret generic notifier-credentials \
+  --from-literal=api-key=nk_your_prod_key_here
+
+# 2. Reference in deployment
+env:
+- name: NOTIFIER_API_KEY
+  valueFrom:
+    secretKeyRef:
+      name: notifier-credentials
+      key: api-key
+```
+
+### Rate Limits
+
+Default: **100 requests per minute** (configurable per key)
+
+When rate limit is exceeded:
+- Error code: `codes.ResourceExhausted`
+- Retry after: 60 seconds (check `Retry-After` header)
+- Strategy: Implement exponential backoff
+
+To request higher rate limit, contact your administrator.
+
+## 🚀 Example Implementations
+
+### Minimal Example (50 lines)
+
+See [`QUICK_START_CLIENT.md`](docs/QUICK_START_CLIENT.md) for complete code.
+
+### Production-Ready Example
+
+Use the AI assistant prompt in [`CLIENT_INTEGRATION_PROMPT.md`](docs/CLIENT_INTEGRATION_PROMPT.md) to generate a complete implementation with:
+- Connection pooling
+- Retry logic
+- Health checks
+- Metrics
+- Graceful shutdown
+- Unit tests
+
+### Language-Specific Examples
+
+**Go:** See `QUICK_START_CLIENT.md` (complete example included)
+
+**Python/Java/Node.js:** Use the prompt in `CLIENT_INTEGRATION_PROMPT.md` with your AI assistant and specify your language:
+
+```
+[Include the prompt from CLIENT_INTEGRATION_PROMPT.md]
+
+Please implement this in Python using the grpcio library.
+```
+
+## 🔧 Troubleshooting
+
+### Common Issues and Solutions
+
+| Issue | Symptom | Solution |
+|-------|---------|----------|
+| **Slow performance** | 10-15ms per request | Verify using long-lived connection, not creating new connection per request |
+| **Authentication failed** | `codes.Unauthenticated` | Check API key format, confirm key is active, verify metadata header |
+| **Rate limit exceeded** | `codes.ResourceExhausted` | Implement exponential backoff retry, consider batching requests |
+| **Connection timeout** | Requests hang indefinitely | Add keepalive configuration, set context timeout |
+| **Connection dropped** | Errors after idle period | Configure keepalive: 10s ping interval |
+| **Health checks failing** | Service appears down | Check network connectivity, verify service is running, check DNS resolution |
+
+### Debug Checklist
+
+```bash
+# 1. Verify service is running
+kubectl get pods -l app=notifier
+kubectl get svc notifier-grpc
+
+# 2. Check service is reachable
+kubectl run -it --rm debug --image=busybox --restart=Never -- \
+  nslookup notifier-grpc
+
+# 3. Test gRPC connectivity (if grpcurl installed)
+grpcurl -plaintext notifier-grpc:50051 list
+
+# 4. Verify API key
+echo $NOTIFIER_API_KEY | wc -c  # Should be 67 (nk_ + 64 chars + newline)
+
+# 5. Check application logs
+kubectl logs -l app=your-app --tail=100
+```
+
+## 📊 Performance Expectations
+
+**With Long-Lived Connection (Recommended):**
+- First request: ~15ms (connection setup + auth)
+- Subsequent requests: ~1-2ms (auth only)
+- Throughput: ~500-1000 req/s per connection
+- Latency p99: <5ms
+
+**Without Connection Reuse (Bad):**
+- Every request: ~15ms (repeated connection setup)
+- Throughput: ~60 req/s per connection
+- Latency p99: ~20ms
+
+**Connection Pool (High Throughput):**
+- 4 connections: ~2000-4000 req/s
+- Linear scaling with connection count
+- Use for >1000 req/s sustained load
+
+## 🎓 Learn More
+
+- **gRPC Documentation:** https://grpc.io/docs/
+- **Protocol Buffers:** https://protobuf.dev/
+- **gRPC Best Practices:** https://grpc.io/docs/guides/performance/
+- **Kubernetes Secrets:** https://kubernetes.io/docs/concepts/configuration/secret/
+
+## 📞 Support
+
+**Questions about:**
+- API keys, rate limits, permissions → Contact Notifier admin
+- Client implementation, best practices → See documentation above
+- Service issues, downtime → Check service health dashboard
+- Feature requests → Open issue in Notifier repository
+
+## 📝 Quick Reference Card
+
+```
+Service:     notifier-grpc:50051
+Protocol:    gRPC (HTTP/2)
+Auth:        authorization: Bearer nk_<64-hex>
+Rate Limit:  100 req/min (default)
+Timeout:     10s recommended
+Keepalive:   10s ping, 3s timeout
+Connection:  Long-lived (reuse)
+```
+
+**Essential imports (Go):**
+```go
+import (
+    "google.golang.org/grpc"
+    "google.golang.org/grpc/keepalive"
+    "google.golang.org/grpc/metadata"
+    pb "github.com/igodwin/notifier/api/grpc/pb"
+)
+```
+
+**Minimal client setup:**
+```go
+// 1. Create connection with keepalive
+conn, _ := grpc.Dial(address, grpc.WithKeepaliveParams(...))
+
+// 2. Create client
+client := pb.NewNotifierServiceClient(conn)
+
+// 3. Add auth to context
+ctx = metadata.AppendToOutgoingContext(ctx, "authorization", "Bearer "+apiKey)
+
+// 4. Make request
+resp, err := client.SendNotification(ctx, req)
+```
+
+That's it! You're ready to integrate with the Notifier service. 🚀

docs/CLIENT_INTEGRATION_PROMPT.md

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+# Notifier Service Client Integration Prompt
+
+**Use this prompt with Claude or another AI assistant when building a client application that integrates with the Notifier service.**
+
+---
+
+## Prompt for AI Assistant
+
+I need to build a client application that integrates with a notification service using gRPC. Please help me implement a production-ready client that follows best practices for performance, security, and reliability.
+
+### Service Details
+
+**Protocol:** gRPC (HTTP/2)
+**Service Address:** `notifier-grpc:50051` (Kubernetes internal DNS)
+**Authentication:** API Key via gRPC metadata
+**API Key Format:** `nk_` prefix followed by 64 hex characters (e.g., `nk_abc123...`)
+
+### Authentication Requirements
+
+1. **API Key Delivery:**
+   - Send API key in gRPC metadata with EVERY request
+   - Two accepted header formats:
+     - `authorization: Bearer nk_your_api_key_here` (preferred)
+     - `x-api-key: nk_your_api_key_here`
+
+2. **Per-Request Authentication:**
+   - The service validates the API key on every RPC call
+   - Authentication is NOT done at connection time
+   - This means you can (and should) use long-lived connections
+
+3. **Rate Limiting:**
+   - Rate limits are enforced per API key
+   - Default: 100 requests per minute (configurable per key)
+   - Rate limit exceeded returns: `codes.ResourceExhausted`
+   - Implement exponential backoff retry for rate limit errors
+
+4. **Credential Management:**
+   - Load API key from environment variable: `NOTIFIER_API_KEY`
+   - NEVER hardcode API keys in source code
+   - NEVER log the full API key (mask it: `nk_abc...xyz`)
+   - Store in Kubernetes Secret if deploying to k8s
+
+### Connection Management Requirements
+
+**CRITICAL:** Use long-lived gRPC connections for optimal performance.
+
+1. **Connection Lifecycle:**
+   - Create ONE connection at application startup
+   - Reuse the connection for ALL requests
+   - Close the connection only when application exits
+   - Do NOT create a new connection for each request
+
+2. **Keepalive Configuration:**
+   - Configure client keepalive to prevent connection timeouts
+   - Recommended settings:
+     - Time: 10 seconds (ping every 10s of inactivity)
+     - Timeout: 3 seconds (wait for ping response)
+     - PermitWithoutStream: true (allow pings when idle)
+
+3. **Why This Matters:**
+   - Connection reuse: 10x faster (1.5ms vs 15ms per request)
+   - Eliminates TCP + TLS handshake overhead (10-15ms per request)
+   - HTTP/2 multiplexing: handle multiple concurrent requests on one connection
+   - Lower resource usage: one connection vs thousands
+
+### Error Handling Requirements
+
+1. **Handle These Error Codes:**
+   - `codes.Unauthenticated` - Invalid or missing API key → Log error, don't retry
+   - `codes.ResourceExhausted` - Rate limit exceeded → Retry with exponential backoff
+   - `codes.InvalidArgument` - Bad request payload → Log error, don't retry
+   - `codes.Unavailable` - Service temporarily unavailable → Retry with backoff
+   - `codes.DeadlineExceeded` - Request timeout → Retry (may be transient)
+
+2. **Retry Strategy:**
+   - Implement exponential backoff: 100ms, 200ms, 400ms, 800ms
+   - Maximum 3-5 retry attempts for retryable errors
+   - Only retry: rate limits, unavailable, deadline exceeded
+   - Do NOT retry: authentication errors, invalid arguments
+
+3. **Context Timeouts:**
+   - Set reasonable timeout for each request (5-30 seconds)
+   - Use `context.WithTimeout()` for every RPC call
+   - Allow timeout to be configurable
+
+### Code Structure Requirements
+
+1. **Client Structure:**
+   ```
+   NotifierClient struct:
+   - conn: *grpc.ClientConn (long-lived)
+   - client: pb.NotifierServiceClient
+   - apiKey: string (loaded from env)
+   - logger: logging interface
+   ```
+
+2. **Required Methods:**
+   - `NewNotifierClient(address, apiKey string) (*NotifierClient, error)` - Initialize
+   - `SendNotification(ctx, request) (response, error)` - Send single notification
+   - `SendBatchNotifications(ctx, requests) (responses, error)` - Send batch
+   - `HealthCheck(ctx) error` - Verify connection health
+   - `Close() error` - Clean up connection
+
+3. **Initialization Pattern:**
+   - Create client as singleton at application startup
+   - Use `sync.Once` to ensure only one instance
+   - Defer `Close()` in main() to ensure cleanup
+
+### Observability Requirements
+
+1. **Logging:**
+   - Log connection establishment (info level)
+   - Log authentication failures (warn level)
+   - Log rate limit exceeded (warn level)
+   - Log successful sends (debug level)
+   - NEVER log the full API key (mask it)
+
+2. **Metrics (if using Prometheus):**
+   - Counter: `notifier_requests_total` (labels: status, method)
+   - Counter: `notifier_requests_failed_total` (labels: error_code)
+   - Histogram: `notifier_request_duration_seconds`
+   - Counter: `notifier_rate_limit_errors_total`
+
+3. **Health Checks:**
+   - Implement background health check goroutine
+   - Check every 30-60 seconds
+   - Call the service's HealthCheck RPC
+   - Log warnings if health checks fail
+
+### Security Requirements
+
+1. **Credential Security:**
+   - Load API key ONLY from environment variables
+   - Never commit API keys to version control
+   - Use `.gitignore` to exclude any files with credentials
+   - Implement a `String()` method that masks the API key for logging
+
+2. **TLS Configuration:**
+   - For local development: `grpc.WithInsecure()` is acceptable
+   - For production: Use TLS with proper certificate validation
+   - For Kubernetes: Internal communication may use insecure (cluster network is trusted)
+
+3. **Graceful Degradation:**
+   - If notifier service is unavailable, application should continue
+   - Log notification failures but don't crash the application
+   - Consider implementing a circuit breaker pattern for resilience
+
+### Testing Requirements
+
+1. **Unit Tests:**
+   - Mock the gRPC client interface
+   - Test retry logic with rate limit errors
+   - Test context timeout handling
+   - Test credential masking in logs
+
+2. **Integration Tests:**
+   - Test actual connection to notifier service (if available)
+   - Test authentication with valid and invalid keys
+   - Test rate limiting behavior
+
+### Kubernetes Deployment Requirements (if applicable)
+
+1. **Environment Variables:**
+   ```yaml
+   env:
+   - name: NOTIFIER_API_KEY
+     valueFrom:
+       secretKeyRef:
+         name: notifier-credentials
+         key: api-key
+   - name: NOTIFIER_ADDRESS
+     value: "notifier-grpc:50051"
+   ```
+
+2. **Secrets Configuration:**
+   ```yaml
+   apiVersion: v1
+   kind: Secret
+   metadata:
+     name: notifier-credentials
+   type: Opaque
+   stringData:
+     api-key: nk_your_actual_api_key_here
+   ```
+
+### Available gRPC Methods
+
+The NotifierService provides these RPC methods:
+
+1. **SendNotification** - Send a single notification
+   - Request: `SendNotificationRequest`
+   - Response: `SendNotificationResponse` (contains notification_id)
+
+2. **SendBatchNotifications** - Send multiple notifications at once
+   - Request: `SendBatchNotificationsRequest`
+   - Response: `SendBatchNotificationsResponse` (contains notification_ids)
+
+3. **GetNotification** - Retrieve notification status by ID
+   - Request: `GetNotificationRequest` (notification_id)
+   - Response: `GetNotificationResponse`
+
+4. **ListNotifications** - List recent notifications
+   - Request: `ListNotificationsRequest` (pagination)
+   - Response: `ListNotificationsResponse`
+
+5. **CancelNotification** - Cancel a pending notification
+   - Request: `CancelNotificationRequest` (notification_id)
+   - Response: `CancelNotificationResponse`
+
+6. **RetryNotification** - Retry a failed notification
+   - Request: `RetryNotificationRequest` (notification_id)
+   - Response: `RetryNotificationResponse`
+
+7. **GetStats** - Get service statistics
+   - Request: `GetStatsRequest`
+   - Response: `GetStatsResponse`
+
+8. **GetNotifiers** - List available notifier types and accounts
+   - Request: `GetNotifiersRequest`
+   - Response: `GetNotifiersResponse`
+
+9. **HealthCheck** - Verify service health
+   - Request: `HealthCheckRequest`
+   - Response: `HealthCheckResponse`
+
+### Notification Types Supported
+
+- `NOTIFICATION_TYPE_EMAIL` - Email notifications
+- `NOTIFICATION_TYPE_SLACK` - Slack messages
+- `NOTIFICATION_TYPE_NTFY` - Ntfy.sh push notifications
+- `NOTIFICATION_TYPE_STDOUT` - Console output (development only)
+
+### Request Example Structure
+
+```protobuf
+message SendNotificationRequest {
+  NotificationType type = 1;           // Required: email, slack, ntfy, stdout
+  string subject = 2;                  // Required: notification subject/title
+  string body = 3;                     // Required: notification body/content
+  repeated string recipients = 4;      // Required: email addresses, slack channels, etc.
+  Priority priority = 5;               // Optional: normal, high, critical
+  string account = 6;                  // Optional: specific notifier account to use
+  map<string, string> metadata = 7;    // Optional: additional metadata
+}
+```
+
+### Implementation Goals
+
+Please implement a client that:
+
+1. ✅ Uses a single long-lived gRPC connection
+2. ✅ Sends API key in metadata with every request
+3. ✅ Implements exponential backoff retry for rate limits
+4. ✅ Loads credentials from environment variables
+5. ✅ Uses context with timeout for all requests
+6. ✅ Implements keepalive to prevent connection timeouts
+7. ✅ Includes proper error handling for all error codes
+8. ✅ Logs important events without exposing credentials
+9. ✅ Provides a health check mechanism
+10. ✅ Includes graceful shutdown (connection cleanup)
+11. ✅ Is production-ready with proper error handling and logging
+12. ✅ Includes basic unit tests
+
+### Example Usage Pattern
+
+The client should be usable like this:
+
+```go
+// Initialize once at startup
+client, err := notifier.NewClient(
+    os.Getenv("NOTIFIER_ADDRESS"),
+    os.Getenv("NOTIFIER_API_KEY"),
+)
+if err != nil {
+    log.Fatal(err)
+}
+defer client.Close()
+
+// Use throughout application lifetime
+ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+defer cancel()
+
+resp, err := client.SendNotification(ctx, &notifier.SendRequest{
+    Type:       notifier.TypeEmail,
+    Subject:    "Important Alert",
+    Body:       "This is a critical notification",
+    Recipients: []string{"admin@example.com"},
+    Priority:   notifier.PriorityHigh,
+})
+
+if err != nil {
+    log.Printf("Failed to send notification: %v", err)
+    // Application continues - notification failure is not fatal
+    return
+}
+
+log.Printf("Notification sent successfully: %s", resp.NotificationID)
+```
+
+### Additional Context
+
+- This is a microservices architecture running in Kubernetes
+- The notifier service handles routing to multiple notification backends (SMTP, Slack, Ntfy)
+- The client may need to send notifications from multiple goroutines concurrently
+- High reliability is important, but notification failures should not crash the application
+- The API key has a rate limit of 100 requests per minute (may vary per key)
+
+### Code Generation Note
+
+If you need the protobuf definitions, they can be generated from the service's proto files located at `api/grpc/proto/notifier.proto` in the notifier service repository.
+
+---
+
+## Usage Instructions
+
+**Copy the prompt above and provide it to your AI assistant when building the client. The prompt includes:**
+
+- All authentication requirements and formats
+- Connection management best practices
+- Error handling strategies
+- Security requirements
+- Observability guidelines
+- Complete API method listing
+- Production-ready patterns
+- Kubernetes deployment configuration
+
+**The AI assistant should generate:**
+- Complete, production-ready client code
+- Proper connection pooling and keepalive
+- Comprehensive error handling
+- Secure credential management
+- Health check implementation
+- Unit tests
+- Usage documentation
+
+**Review the generated code for:**
+- ✅ Single long-lived connection (not creating connections per request)
+- ✅ API key sent in metadata with every RPC
+- ✅ Keepalive configuration present
+- ✅ Exponential backoff retry logic
+- ✅ No hardcoded credentials
+- ✅ Proper context timeouts
+- ✅ Error handling for all gRPC error codes
+- ✅ Graceful shutdown (Close() method)

docs/CORS_IMPLEMENTATION.md

@@ -0,0 +1,255 @@
+# CORS Implementation - Security Enhancement
+
+## Overview
+
+This document describes the implementation of a secure CORS (Cross-Origin Resource Sharing) configuration system that replaces the previous wildcard (`*`) configuration with an explicit origin whitelist to prevent CSRF attacks.
+
+## Implementation Summary
+
+### 1. CORSConfig Structure (api/rest/router.go)
+
+Created a comprehensive `CORSConfig` struct with the following fields:
+
+```go
+type CORSConfig struct {
+    AllowedOrigins   []string  // Whitelist of allowed origins
+    AllowedMethods   []string  // Allowed HTTP methods
+    AllowedHeaders   []string  // Allowed HTTP headers
+    AllowCredentials bool      // Whether to allow credentials
+    MaxAge           int       // Cache duration for preflight responses
+}
+```
+
+**Key Security Features:**
+- Wildcards (`*`) are explicitly NOT supported
+- Origins must be explicitly whitelisted
+- Credentials can only be enabled with specific origins
+
+### 2. CORS Middleware Implementation
+
+The `newCORSMiddleware()` function implements:
+
+- **Origin Validation**: Checks incoming `Origin` header against whitelist
+- **Exact Match Required**: Only sets CORS headers if origin is in whitelist
+- **Never Uses Wildcard**: Always returns the exact origin, never `*`
+- **Preflight Handling**: Properly handles OPTIONS requests
+- **Configurable Headers**: All CORS headers are configurable
+
+**Code Location:** `api/rest/router.go:98-149`
+
+### 3. Configuration System
+
+#### Config Structure (internal/config/config.go)
+
+Added `CORSConfig` to the main application configuration:
+
+```go
+type CORSConfig struct {
+    AllowedOrigins   []string `mapstructure:"allowed_origins"`
+    AllowedMethods   []string `mapstructure:"allowed_methods"`
+    AllowedHeaders   []string `mapstructure:"allowed_headers"`
+    AllowCredentials bool     `mapstructure:"allow_credentials"`
+    MaxAge           int      `mapstructure:"max_age"`
+}
+```
+
+#### Default Values (internal/config/config.go:205-210)
+
+```go
+AllowedOrigins:   []string{}                                   // Empty by default
+AllowedMethods:   []string{"GET", "POST", "OPTIONS", "DELETE"} // Standard REST methods
+AllowedHeaders:   []string{"Content-Type", "Authorization"}    // Common headers
+AllowCredentials: false                                         // Disabled by default
+MaxAge:           3600                                          // 1 hour
+```
+
+### 4. Configuration Validation (internal/config/config.go:263-282)
+
+Implemented `validateCORS()` that enforces:
+
+1. **Wildcard Rejection**: `*` is not allowed in `allowed_origins`
+2. **Origin Format Validation**: Origins must start with `http://` or `https://`
+3. **Credentials Validation**: `allow_credentials` requires at least one origin
+
+**Example Error Messages:**
+- `"wildcard (*) is not allowed in CORS allowed_origins for security reasons - specify exact origins instead"`
+- `"invalid origin format: example.com - origins must start with http:// or https://"`
+- `"allow_credentials is enabled but no origins are allowed - this configuration is ineffective"`
+
+### 5. Updated Server Initialization (cmd/server/main.go:288-326)
+
+The `startRESTServer()` function now:
+
+1. Converts config CORS to `rest.CORSConfig`
+2. Logs CORS configuration on startup
+3. Warns if no origins are configured
+4. Passes CORS config to router
+
+**Example Log Output:**
+```
+CORS enabled for origins: [http://localhost:3000 http://localhost:8080]
+```
+
+Or:
+```
+CORS has no allowed origins configured - all cross-origin requests will be blocked
+```
+
+### 6. Configuration Example (config.yaml:112-146)
+
+#### Development Configuration
+```yaml
+cors:
+  allowed_origins:
+    - "http://localhost:3000"    # React/Next.js dev port
+    - "http://localhost:8080"    # Vue/Angular dev port
+    - "http://localhost:5173"    # Vite dev server
+  allowed_methods:
+    - "GET"
+    - "POST"
+    - "OPTIONS"
+    - "DELETE"
+  allowed_headers:
+    - "Content-Type"
+    - "Authorization"
+  allow_credentials: false
+  max_age: 3600
+```
+
+#### Production Configuration (Example)
+```yaml
+cors:
+  allowed_origins:
+    - "https://app.example.com"
+    - "https://dashboard.example.com"
+    - "https://api-docs.example.com"
+  allowed_methods:
+    - "GET"
+    - "POST"
+    - "OPTIONS"
+    - "DELETE"
+  allowed_headers:
+    - "Content-Type"
+    - "Authorization"
+  allow_credentials: true  # Enable for auth tokens
+  max_age: 3600
+```
+
+## Test Coverage
+
+### 1. CORS Middleware Tests (api/rest/cors_test.go)
+
+**Test Cases:**
+- ✅ `TestCORSMiddleware_AllowedOrigin`: Verifies allowed origins are accepted
+- ✅ `TestCORSMiddleware_BlockedOrigin`: Verifies non-whitelisted origins are rejected
+- ✅ `TestCORSMiddleware_PreflightRequest`: Tests OPTIONS preflight handling
+- ✅ `TestCORSMiddleware_Credentials`: Verifies credential header handling
+- ✅ `TestCORSMiddleware_NoWildcard`: Ensures wildcard is never returned
+- ✅ `TestCORSMiddleware_EmptyConfig`: Tests secure default (no origins)
+- ✅ `TestCORSMiddleware_MaxAge`: Tests cache duration configuration
+- ✅ `TestDefaultCORSConfig`: Verifies default configuration values
+
+**Total: 8 test functions, 21 test cases**
+
+### 2. CORS Validation Tests (internal/config/cors_test.go)
+
+**Test Cases:**
+- ✅ `TestValidateCORS_WildcardRejection`: Wildcard origins are rejected
+- ✅ `TestValidateCORS_InvalidOriginFormat`: Invalid origin formats are rejected
+- ✅ `TestValidateCORS_CredentialsWithoutOrigins`: Credentials require origins
+- ✅ `TestValidateCORS_ValidConfigurations`: Valid configs are accepted
+- ✅ `TestValidateCORS_MultipleOrigins`: Multiple origins with wildcard rejected
+- ✅ `TestValidateCORS_EdgeCases`: Edge cases handled correctly
+
+**Total: 6 test functions, 16 test cases**
+
+## Security Improvements
+
+### Before
+```go
+w.Header().Set("Access-Control-Allow-Origin", "*")  // ❌ CSRF Vulnerability
+```
+
+### After
+```go
+// Only set CORS headers if origin is in whitelist
+if allowed {
+    w.Header().Set("Access-Control-Allow-Origin", origin)  // ✅ Exact origin
+}
+// Otherwise, no CORS headers are set (browser blocks the response)
+```
+
+## Security Guarantees
+
+1. **No Wildcard**: The system makes it impossible to configure wildcard CORS
+2. **Explicit Whitelist**: All allowed origins must be explicitly configured
+3. **Validation at Startup**: Invalid configurations are rejected before the server starts
+4. **Default Secure**: Empty origin list by default (most restrictive)
+5. **Environment-Specific**: Different configs for dev/staging/production
+
+## Migration Guide
+
+### For Development
+
+Update your `config.yaml` to include localhost origins:
+
+```yaml
+cors:
+  allowed_origins:
+    - "http://localhost:3000"
+    - "http://localhost:8080"
+```
+
+### For Production
+
+Configure your production origins:
+
+```yaml
+cors:
+  allowed_origins:
+    - "https://yourdomain.com"
+    - "https://app.yourdomain.com"
+  allow_credentials: true
+```
+
+### Environment Variables
+
+You can also configure CORS via environment variables:
+
+```bash
+export NOTIFIER_CORS_ALLOWED_ORIGINS="https://app.example.com,https://dashboard.example.com"
+export NOTIFIER_CORS_ALLOW_CREDENTIALS=true
+```
+
+## Acceptance Criteria
+
+✅ **CORS whitelist fully configurable** - via config.yaml or environment variables
+✅ **Wildcard configuration is impossible** - validation rejects `*` at startup
+✅ **Security headers properly set** - only for whitelisted origins
+✅ **Environment-specific configs work** - examples provided for dev/prod
+✅ **No CSRF vulnerability** - wildcard eliminated, exact origins only
+✅ **Comprehensive tests** - 37 test cases covering all scenarios
+
+## Files Modified
+
+1. `api/rest/router.go` - CORS config struct and middleware
+2. `api/rest/cors_test.go` - CORS middleware tests (NEW)
+3. `internal/config/config.go` - CORS configuration structure
+4. `internal/config/cors_test.go` - CORS validation tests (NEW)
+5. `cmd/server/main.go` - Server initialization with CORS config
+6. `config.yaml` - Example CORS configuration
+7. `internal/auth/bootstrap.go` - Fixed unrelated string formatting bug
+8. `internal/auth/keystore_hybrid.go` - Fixed unrelated rate limiter signature
+
+## Additional Fixes
+
+While implementing CORS, also fixed pre-existing build errors:
+- String multiplication in `bootstrap.go` (changed to `strings.Repeat`)
+- Rate limiter signature mismatch in `keystore_hybrid.go`
+- Missing parameter in `service_retention_test.go`
+
+## References
+
+- **OWASP CORS**: https://owasp.org/www-community/attacks/csrf
+- **MDN CORS**: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
+- **RFC 6454**: The Web Origin Concept

docs/CORS_KUBERNETES_GUIDE.md

@@ -0,0 +1,357 @@
+# CORS Configuration for Kubernetes Deployments
+
+## Important: CORS Only Affects REST API, Not gRPC
+
+**Key Point:** CORS (Cross-Origin Resource Sharing) is a **browser security mechanism** that only applies to HTTP/REST APIs accessed from web browsers. It does **NOT** affect gRPC communication.
+
+## Service-to-Service Communication (gRPC)
+
+### ✅ No CORS Configuration Needed
+
+For services running in the same Kubernetes cluster communicating via gRPC:
+
+**You don't need to configure CORS at all!**
+
+Here's why:
+
+1. **gRPC uses HTTP/2** - CORS is not enforced
+2. **Server-to-Server** - CORS only applies to browser-based requests
+3. **No Origin header** - Backend services don't send Origin headers
+
+### gRPC Client Connection Example
+
+```go
+// Service A connecting to notifier service via gRPC
+conn, err := grpc.Dial("notifier-grpc.default.svc.cluster.local:50051",
+    grpc.WithInsecure())
+if err != nil {
+    log.Fatalf("Failed to connect: %v", err)
+}
+defer conn.Close()
+
+client := pb.NewNotifierServiceClient(conn)
+```
+
+**No CORS configuration required** ✅
+
+The gRPC service runs on port `50051` (as defined in your k8s/service.yaml) and is accessible at:
+- From same namespace: `notifier-grpc:50051`
+- From other namespace: `notifier-grpc.default.svc.cluster.local:50051`
+- Full DNS: `notifier-grpc.default.svc.cluster.local:50051`
+
+## When You DO Need CORS Configuration
+
+CORS configuration is **only** needed when:
+
+1. **Web browsers** access the REST API (port 8080)
+2. The web app is served from a **different origin** than the API
+3. The request goes through the **REST API**, not gRPC
+
+### Common Scenarios
+
+| Scenario | Needs CORS? | Reason |
+|----------|-------------|---------|
+| Backend service → gRPC API | ❌ No | Not a browser, uses gRPC |
+| Backend service → REST API | ❌ No | Not a browser |
+| Web app → gRPC (grpc-web) | ✅ Yes | Browser-based, needs CORS |
+| Web app → REST API | ✅ Yes | Browser-based, needs CORS |
+| CLI tool → REST API | ❌ No | Not a browser |
+| Postman/curl → REST API | ❌ No | Not a browser |
+
+## CORS Configuration for Web Applications
+
+If you have a web frontend that calls the REST API, you need to configure CORS.
+
+### Scenario 1: Frontend in Same Cluster
+
+**Example:** Frontend at `https://app.example.com`, API at `https://notifier.example.com`
+
+Update your ConfigMap (k8s/configmap.yaml):
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: notifier-config
+data:
+  config.yaml: |
+    # ... existing config ...
+
+    cors:
+      allowed_origins:
+        - "https://app.example.com"  # Your frontend domain
+      allowed_methods:
+        - "GET"
+        - "POST"
+        - "OPTIONS"
+        - "DELETE"
+      allowed_headers:
+        - "Content-Type"
+        - "Authorization"
+      allow_credentials: true  # If your frontend sends auth tokens
+      max_age: 3600
+```
+
+### Scenario 2: Multiple Frontends
+
+```yaml
+cors:
+  allowed_origins:
+    - "https://app.example.com"          # Main app
+    - "https://dashboard.example.com"    # Admin dashboard
+    - "https://mobile.example.com"       # Mobile web app
+  allowed_methods:
+    - "GET"
+    - "POST"
+    - "OPTIONS"
+    - "DELETE"
+  allowed_headers:
+    - "Content-Type"
+    - "Authorization"
+  allow_credentials: true
+  max_age: 3600
+```
+
+### Scenario 3: Development Environment
+
+For local development (frontend at http://localhost:3000):
+
+```yaml
+cors:
+  allowed_origins:
+    - "http://localhost:3000"    # React/Next.js dev server
+    - "http://localhost:8080"    # Vue/Angular dev server
+    - "http://localhost:5173"    # Vite dev server
+    - "https://app.example.com"  # Production frontend
+  allowed_methods:
+    - "GET"
+    - "POST"
+    - "OPTIONS"
+    - "DELETE"
+  allowed_headers:
+    - "Content-Type"
+    - "Authorization"
+  allow_credentials: false
+  max_age: 3600
+```
+
+### Scenario 4: No Web Frontend (Backend Services Only)
+
+If you only have backend services using gRPC:
+
+```yaml
+cors:
+  # Empty or minimal config - no origins needed
+  allowed_origins: []  # No browser clients
+  allowed_methods:
+    - "GET"
+    - "POST"
+  allowed_headers:
+    - "Content-Type"
+```
+
+The REST API will still work for non-browser clients (like curl, Postman, or backend HTTP clients), but browsers will be blocked unless their origin is whitelisted.
+
+## Complete Kubernetes Configuration Example
+
+Here's your updated k8s/configmap.yaml with CORS:
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: notifier-config
+  labels:
+    app: notifier
+data:
+  config.yaml: |
+    server:
+      grpc_port: 50051
+      rest_port: 8080
+      host: "0.0.0.0"
+      mode: "both"
+
+    queue:
+      type: "local"
+      max_size: 10000
+      worker_count: 10
+      retry_attempts: 3
+      retry_backoff: "exponential"
+      local:
+        buffer_size: 1000
+        persist_to_disk: false
+
+    notifiers:
+      stdout: true
+
+    logging:
+      level: "info"
+      format: "json"
+      output_path: "stdout"
+
+    metrics:
+      enabled: true
+      port: 9090
+      path: "/metrics"
+      prometheus_enabled: true
+
+    health_check:
+      enabled: true
+      port: 8081
+      path: "/health"
+      interval: 30
+
+    # CORS configuration for REST API
+    # Only needed if web browsers will access the REST API
+    cors:
+      # Add your frontend domains here
+      # For backend-only services, leave empty
+      allowed_origins: []
+
+      # Or if you have a web frontend:
+      # allowed_origins:
+      #   - "https://app.example.com"
+
+      allowed_methods:
+        - "GET"
+        - "POST"
+        - "OPTIONS"
+        - "DELETE"
+      allowed_headers:
+        - "Content-Type"
+        - "Authorization"
+      allow_credentials: false
+      max_age: 3600
+```
+
+## Environment-Specific Configuration
+
+You can also use environment variables for different environments:
+
+### Development ConfigMap
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: notifier-config-dev
+  namespace: development
+data:
+  NOTIFIER_CORS_ALLOWED_ORIGINS: "http://localhost:3000,http://localhost:8080"
+  NOTIFIER_CORS_ALLOW_CREDENTIALS: "false"
+```
+
+### Production ConfigMap
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: notifier-config-prod
+  namespace: production
+data:
+  NOTIFIER_CORS_ALLOWED_ORIGINS: "https://app.example.com,https://dashboard.example.com"
+  NOTIFIER_CORS_ALLOW_CREDENTIALS: "true"
+```
+
+## Troubleshooting
+
+### "CORS error" in Browser Console
+
+**Symptom:**
+```
+Access to fetch at 'https://notifier.example.com/api/v1/notifications'
+from origin 'https://app.example.com' has been blocked by CORS policy
+```
+
+**Solution:**
+Add `https://app.example.com` to `allowed_origins` in your ConfigMap.
+
+### Backend Service Can't Connect
+
+**Symptom:**
+```go
+// Service in cluster trying to connect
+conn, err := grpc.Dial("notifier-grpc:50051", grpc.WithInsecure())
+// Error: connection refused
+```
+
+**Solution:**
+This is NOT a CORS issue. Check:
+1. Service name is correct (`notifier-grpc`)
+2. Port is correct (`50051`)
+3. Service is running: `kubectl get pods -l app=notifier`
+4. Service endpoints: `kubectl get endpoints notifier-grpc`
+
+### REST API Returns 200 but No CORS Headers
+
+**Symptom:**
+Browser blocks the response even though the API returns 200 OK.
+
+**Solution:**
+The origin is not in the whitelist. Check:
+1. Origin is exactly matching (including protocol and port)
+2. ConfigMap has been updated
+3. Pod has been restarted to pick up new config:
+   ```bash
+   kubectl rollout restart deployment notifier
+   ```
+
+## Quick Reference
+
+### For Backend Services Only (Recommended)
+```yaml
+cors:
+  allowed_origins: []  # Empty - no browser access needed
+```
+
+### For Web Frontend + Backend Services
+```yaml
+cors:
+  allowed_origins:
+    - "https://your-frontend-domain.com"
+  allow_credentials: true  # If using auth tokens
+```
+
+### Applying Changes
+
+After updating the ConfigMap:
+
+```bash
+# Update ConfigMap
+kubectl apply -f k8s/configmap.yaml
+
+# Restart pods to pick up new config
+kubectl rollout restart deployment notifier
+
+# Verify pods are running
+kubectl get pods -l app=notifier
+
+# Check logs for CORS configuration
+kubectl logs -l app=notifier | grep CORS
+```
+
+You should see:
+```
+CORS enabled for origins: [https://app.example.com]
+```
+
+Or if no origins configured:
+```
+CORS has no allowed origins configured - all cross-origin requests will be blocked
+```
+
+## Summary
+
+**For your use case (services in Kubernetes using gRPC):**
+
+✅ **You don't need to configure CORS at all!**
+
+CORS only applies to web browsers accessing the REST API. Your backend services communicating via gRPC are completely unaffected by CORS configuration.
+
+**Only add CORS configuration if:**
+- You have a web frontend (React, Vue, Angular, etc.)
+- That web frontend calls the REST API (not gRPC)
+- The frontend is served from a different domain than the API
+
+For purely backend service-to-service communication in Kubernetes, CORS is irrelevant.

docs/GRPC_CONNECTION_OPTIMIZATION.md

@@ -0,0 +1,607 @@
+# gRPC Connection Optimization Guide
+
+## TL;DR
+
+**Yes, you can and should use long-lived gRPC connections!**
+
+- ✅ Single connection can handle thousands of concurrent requests
+- ✅ Authentication happens **per-request**, not per-connection
+- ✅ Connection reuse eliminates TCP/TLS handshake overhead
+- ✅ HTTP/2 multiplexing allows concurrent RPCs on one connection
+- ✅ Built-in keepalive prevents connection timeouts
+
+## How gRPC Authentication Works
+
+### Key Point: Auth is Per-Request, Not Per-Connection
+
+Your current implementation authenticates **each RPC call**, not the connection itself. This means:
+
+1. Client establishes a **long-lived connection** (once)
+2. Client sends **API key in metadata** with each request
+3. Server validates the key for **every RPC call**
+4. Connection stays open for multiple requests
+
+```
+Connection Lifecycle:
+┌─────────────────────────────────────────────────────────────┐
+│ TCP Connection (persistent)                                 │
+│ ┌─────────────────────────────────────────────────────────┐ │
+│ │ TLS Handshake (once)                                    │ │
+│ └─────────────────────────────────────────────────────────┘ │
+│                                                              │
+│ ┌─────────────────────────────────────────────────────────┐ │
+│ │ Request 1: Authorization: Bearer nk_abc... → Validated  │ │
+│ └─────────────────────────────────────────────────────────┘ │
+│ ┌─────────────────────────────────────────────────────────┐ │
+│ │ Request 2: Authorization: Bearer nk_abc... → Validated  │ │
+│ └─────────────────────────────────────────────────────────┘ │
+│ ┌─────────────────────────────────────────────────────────┐ │
+│ │ Request 3: Authorization: Bearer nk_abc... → Validated  │ │
+│ └─────────────────────────────────────────────────────────┘ │
+│                     ... (connection stays open)             │
+└─────────────────────────────────────────────────────────────┘
+```
+
+### Authentication Overhead Analysis
+
+**Per-Connection (One-Time):**
+- TCP handshake: ~1-2ms (3-way handshake)
+- TLS handshake: ~5-10ms (certificate exchange, key agreement)
+- **Total: ~10-15ms once**
+
+**Per-Request (Every Call):**
+- API key validation: ~0.1-1ms (in-memory lookup)
+- Rate limit check: ~0.1ms (in-memory counter)
+- **Total: ~0.2-1ms per request**
+
+**With Connection Reuse:**
+- First request: 10-15ms (connection) + 1ms (auth) = **11-16ms**
+- Subsequent requests: **1ms** (only auth, no connection setup)
+
+**Without Connection Reuse (reconnecting each time):**
+- Every request: 10-15ms (connection) + 1ms (auth) = **11-16ms**
+
+**Savings: 10-15ms per request after the first one!**
+
+## Recommended Client Pattern
+
+### Basic Long-Lived Connection
+
+```go
+package main
+
+import (
+    "context"
+    "log"
+    "time"
+
+    pb "github.com/igodwin/notifier/api/grpc/pb"
+    "google.golang.org/grpc"
+    "google.golang.org/grpc/credentials/insecure"
+    "google.golang.org/grpc/metadata"
+)
+
+type NotifierClient struct {
+    conn   *grpc.ClientConn
+    client pb.NotifierServiceClient
+    apiKey string
+}
+
+func NewNotifierClient(address, apiKey string) (*NotifierClient, error) {
+    // Establish long-lived connection
+    conn, err := grpc.Dial(address,
+        grpc.WithTransportCredentials(insecure.NewCredentials()),
+
+        // Connection pool settings
+        grpc.WithDefaultCallOptions(
+            grpc.MaxCallRecvMsgSize(4*1024*1024), // 4MB
+            grpc.MaxCallSendMsgSize(4*1024*1024),
+        ),
+
+        // Keepalive settings
+        grpc.WithKeepaliveParams(keepalive.ClientParameters{
+            Time:                10 * time.Second, // Send keepalive ping every 10s
+            Timeout:             3 * time.Second,  // Wait 3s for ping ack
+            PermitWithoutStream: true,             // Allow pings when no streams
+        }),
+    )
+    if err != nil {
+        return nil, err
+    }
+
+    return &NotifierClient{
+        conn:   conn,
+        client: pb.NewNotifierServiceClient(conn),
+        apiKey: apiKey,
+    }, nil
+}
+
+func (nc *NotifierClient) SendNotification(ctx context.Context, req *pb.SendNotificationRequest) (*pb.SendNotificationResponse, error) {
+    // Add API key to metadata for THIS request
+    ctx = metadata.AppendToOutgoingContext(ctx, "authorization", "Bearer "+nc.apiKey)
+
+    // Make RPC call - reuses existing connection
+    return nc.client.SendNotification(ctx, req)
+}
+
+func (nc *NotifierClient) Close() error {
+    return nc.conn.Close()
+}
+
+func main() {
+    // Create client with long-lived connection
+    client, err := NewNotifierClient("notifier-grpc:50051", "nk_your_api_key")
+    if err != nil {
+        log.Fatal(err)
+    }
+    defer client.Close() // Close when application exits
+
+    // Reuse client for multiple requests
+    for i := 0; i < 1000; i++ {
+        ctx := context.Background()
+        resp, err := client.SendNotification(ctx, &pb.SendNotificationRequest{
+            Type:    pb.NotificationType_NOTIFICATION_TYPE_EMAIL,
+            Subject: "Test notification",
+            Body:    "This is a test",
+            Recipients: []string{"user@example.com"},
+        })
+
+        if err != nil {
+            log.Printf("Request %d failed: %v", i, err)
+            continue
+        }
+
+        log.Printf("Request %d succeeded: %s", i, resp.NotificationId)
+    }
+
+    // Connection is closed when main() exits
+}
+```
+
+### Advanced: Connection Pool for High Concurrency
+
+For extremely high throughput, you can create multiple connections:
+
+```go
+package main
+
+import (
+    "context"
+    "sync"
+
+    pb "github.com/igodwin/notifier/api/grpc/pb"
+    "google.golang.org/grpc"
+    "google.golang.org/grpc/keepalive"
+)
+
+type NotifierPool struct {
+    connections []*grpc.ClientConn
+    clients     []pb.NotifierServiceClient
+    apiKey      string
+    current     uint32
+    mu          sync.Mutex
+}
+
+func NewNotifierPool(address, apiKey string, poolSize int) (*NotifierPool, error) {
+    pool := &NotifierPool{
+        connections: make([]*grpc.ClientConn, poolSize),
+        clients:     make([]pb.NotifierServiceClient, poolSize),
+        apiKey:      apiKey,
+    }
+
+    // Create multiple connections
+    for i := 0; i < poolSize; i++ {
+        conn, err := grpc.Dial(address,
+            grpc.WithTransportCredentials(insecure.NewCredentials()),
+            grpc.WithKeepaliveParams(keepalive.ClientParameters{
+                Time:                10 * time.Second,
+                Timeout:             3 * time.Second,
+                PermitWithoutStream: true,
+            }),
+        )
+        if err != nil {
+            // Clean up any connections already created
+            pool.Close()
+            return nil, err
+        }
+
+        pool.connections[i] = conn
+        pool.clients[i] = pb.NewNotifierServiceClient(conn)
+    }
+
+    return pool, nil
+}
+
+func (np *NotifierPool) getClient() pb.NotifierServiceClient {
+    // Round-robin connection selection
+    np.mu.Lock()
+    defer np.mu.Unlock()
+
+    idx := np.current % uint32(len(np.clients))
+    np.current++
+    return np.clients[idx]
+}
+
+func (np *NotifierPool) SendNotification(ctx context.Context, req *pb.SendNotificationRequest) (*pb.SendNotificationResponse, error) {
+    // Add API key to metadata
+    ctx = metadata.AppendToOutgoingContext(ctx, "authorization", "Bearer "+np.apiKey)
+
+    // Get a client from pool (round-robin)
+    client := np.getClient()
+
+    return client.SendNotification(ctx, req)
+}
+
+func (np *NotifierPool) Close() error {
+    var firstErr error
+    for _, conn := range np.connections {
+        if conn != nil {
+            if err := conn.Close(); err != nil && firstErr == nil {
+                firstErr = err
+            }
+        }
+    }
+    return firstErr
+}
+
+// Usage
+func main() {
+    // Create pool with 4 connections
+    pool, err := NewNotifierPool("notifier-grpc:50051", "nk_your_api_key", 4)
+    if err != nil {
+        log.Fatal(err)
+    }
+    defer pool.Close()
+
+    // Use pool concurrently from multiple goroutines
+    var wg sync.WaitGroup
+    for i := 0; i < 1000; i++ {
+        wg.Add(1)
+        go func(id int) {
+            defer wg.Done()
+
+            resp, err := pool.SendNotification(context.Background(), &pb.SendNotificationRequest{
+                Type:    pb.NotificationType_NOTIFICATION_TYPE_EMAIL,
+                Subject: fmt.Sprintf("Notification %d", id),
+                Body:    "Test",
+                Recipients: []string{"user@example.com"},
+            })
+
+            if err != nil {
+                log.Printf("Request %d failed: %v", id, err)
+                return
+            }
+
+            log.Printf("Request %d succeeded: %s", id, resp.NotificationId)
+        }(i)
+    }
+
+    wg.Wait()
+}
+```
+
+## Keepalive Configuration
+
+### Why Keepalive Matters
+
+In Kubernetes, idle connections may be terminated by:
+- Load balancers (after 60-600 seconds)
+- Network proxies
+- Firewalls with connection tracking
+
+**Solution:** Send periodic keepalive pings
+
+### Client-Side Keepalive
+
+```go
+grpc.WithKeepaliveParams(keepalive.ClientParameters{
+    Time:                10 * time.Second,  // Send ping every 10s of inactivity
+    Timeout:             3 * time.Second,   // Wait 3s for ping response
+    PermitWithoutStream: true,              // Send pings even when no active RPCs
+})
+```
+
+### Server-Side Keepalive (Already Configured in Your Server)
+
+Add to `cmd/server/main.go` in `startGRPCServer()`:
+
+```go
+serverOpts = append(serverOpts,
+    grpc.KeepaliveParams(keepalive.ServerParameters{
+        MaxConnectionIdle:     15 * time.Minute, // Close idle connections after 15m
+        MaxConnectionAge:      30 * time.Minute, // Force close after 30m
+        MaxConnectionAgeGrace: 5 * time.Second,  // Allow 5s for RPCs to complete
+        Time:                  5 * time.Second,  // Send ping if idle for 5s
+        Timeout:               1 * time.Second,  // Wait 1s for ping response
+    }),
+    grpc.KeepaliveEnforcementPolicy(keepalive.EnforcementPolicy{
+        MinTime:             5 * time.Second, // Don't allow pings more often than 5s
+        PermitWithoutStream: true,            // Allow pings when no streams
+    }),
+)
+```
+
+## Performance Comparison
+
+### Scenario 1: Short-Lived Connections (Creating new connection for each request)
+
+```
+Request 1: 15ms (10ms connect + 5ms TLS + 1ms auth + 0.5ms RPC)
+Request 2: 15ms (10ms connect + 5ms TLS + 1ms auth + 0.5ms RPC)
+Request 3: 15ms (10ms connect + 5ms TLS + 1ms auth + 0.5ms RPC)
+...
+1000 requests: ~15,000ms (15 seconds)
+```
+
+### Scenario 2: Long-Lived Connection (Recommended)
+
+```
+Request 1:    15ms (10ms connect + 5ms TLS + 1ms auth + 0.5ms RPC)
+Request 2:    1.5ms (1ms auth + 0.5ms RPC)
+Request 3:    1.5ms (1ms auth + 0.5ms RPC)
+...
+1000 requests: ~1,515ms (1.5 seconds)
+```
+
+**Performance Improvement: 10x faster! (15s → 1.5s)**
+
+### Scenario 3: Connection Pool with 4 Connections
+
+```
+First 4 requests:  15ms each (connection setup)
+Remaining 996:     1.5ms each (reuse connections)
+1000 requests:     ~1,554ms (1.5 seconds)
+Handles concurrent load better
+```
+
+## Best Practices
+
+### 1. Connection Lifecycle Management
+
+```go
+// Application-scoped client (singleton)
+var (
+    notifierClient *NotifierClient
+    once           sync.Once
+)
+
+func GetNotifierClient() *NotifierClient {
+    once.Do(func() {
+        client, err := NewNotifierClient(
+            os.Getenv("NOTIFIER_ADDRESS"),
+            os.Getenv("NOTIFIER_API_KEY"),
+        )
+        if err != nil {
+            log.Fatalf("Failed to create notifier client: %v", err)
+        }
+        notifierClient = client
+    })
+    return notifierClient
+}
+
+// In main():
+func main() {
+    client := GetNotifierClient()
+    defer client.Close()
+
+    // ... run application ...
+}
+```
+
+### 2. Context with Timeout
+
+Always use context with timeout to prevent hanging requests:
+
+```go
+func (nc *NotifierClient) SendNotification(ctx context.Context, req *pb.SendNotificationRequest) (*pb.SendNotificationResponse, error) {
+    // Set timeout for this request
+    ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
+    defer cancel()
+
+    // Add API key
+    ctx = metadata.AppendToOutgoingContext(ctx, "authorization", "Bearer "+nc.apiKey)
+
+    return nc.client.SendNotification(ctx, req)
+}
+```
+
+### 3. Health Checks
+
+Periodically verify the connection is healthy:
+
+```go
+func (nc *NotifierClient) HealthCheck(ctx context.Context) error {
+    ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
+    defer cancel()
+
+    // Add API key
+    ctx = metadata.AppendToOutgoingContext(ctx, "authorization", "Bearer "+nc.apiKey)
+
+    _, err := nc.client.HealthCheck(ctx, &pb.HealthCheckRequest{})
+    return err
+}
+
+// In background goroutine:
+go func() {
+    ticker := time.NewTicker(30 * time.Second)
+    defer ticker.Stop()
+
+    for range ticker.C {
+        if err := client.HealthCheck(context.Background()); err != nil {
+            log.Printf("Health check failed: %v", err)
+            // Consider reconnecting or alerting
+        }
+    }
+}()
+```
+
+### 4. Graceful Reconnection
+
+Handle connection failures gracefully:
+
+```go
+type ResilientNotifierClient struct {
+    address string
+    apiKey  string
+    client  *NotifierClient
+    mu      sync.RWMutex
+}
+
+func (rnc *ResilientNotifierClient) SendNotification(ctx context.Context, req *pb.SendNotificationRequest) (*pb.SendNotificationResponse, error) {
+    rnc.mu.RLock()
+    client := rnc.client
+    rnc.mu.RUnlock()
+
+    resp, err := client.SendNotification(ctx, req)
+    if err != nil && isConnectionError(err) {
+        // Try to reconnect
+        log.Printf("Connection error, attempting reconnect: %v", err)
+        if err := rnc.reconnect(); err != nil {
+            return nil, fmt.Errorf("reconnection failed: %w", err)
+        }
+
+        // Retry request with new connection
+        rnc.mu.RLock()
+        client = rnc.client
+        rnc.mu.RUnlock()
+
+        return client.SendNotification(ctx, req)
+    }
+
+    return resp, err
+}
+
+func (rnc *ResilientNotifierClient) reconnect() error {
+    rnc.mu.Lock()
+    defer rnc.mu.Unlock()
+
+    // Close old connection
+    if rnc.client != nil {
+        rnc.client.Close()
+    }
+
+    // Create new connection
+    client, err := NewNotifierClient(rnc.address, rnc.apiKey)
+    if err != nil {
+        return err
+    }
+
+    rnc.client = client
+    return nil
+}
+```
+
+## Kubernetes Deployment Considerations
+
+### 1. Service Configuration
+
+Your services are already correctly configured for long-lived connections:
+
+```yaml
+# k8s/service.yaml
+apiVersion: v1
+kind: Service
+metadata:
+  name: notifier-grpc
+spec:
+  type: ClusterIP  # ✅ Good: Stable internal endpoint
+  ports:
+  - port: 50051
+    targetPort: grpc
+    protocol: TCP
+```
+
+### 2. Client Connection String
+
+```go
+// Within same namespace
+client, _ := NewNotifierClient("notifier-grpc:50051", apiKey)
+
+// From different namespace
+client, _ := NewNotifierClient("notifier-grpc.default.svc.cluster.local:50051", apiKey)
+```
+
+### 3. Load Balancing
+
+Kubernetes service provides **connection-level** load balancing. For better **request-level** load balancing with long-lived connections, consider:
+
+**Option A: Client-Side Load Balancing**
+
+```go
+import "google.golang.org/grpc/resolver"
+
+conn, err := grpc.Dial(
+    "dns:///notifier-grpc:50051",  // DNS resolver
+    grpc.WithDefaultServiceConfig(`{"loadBalancingPolicy":"round_robin"}`),
+    grpc.WithTransportCredentials(insecure.NewCredentials()),
+)
+```
+
+**Option B: Connection Pool** (shown earlier)
+
+## Rate Limiting Considerations
+
+### Your Current Implementation
+
+The rate limiter checks on **every request** (in `grpc_middleware.go:46-50`):
+
+```go
+allowed, err := m.store.CheckRateLimit(apiKey)
+if err != nil || !allowed {
+    return nil, status.Error(codes.ResourceExhausted, "Rate limit exceeded")
+}
+```
+
+### Impact with Long-Lived Connections
+
+**No negative impact!** Rate limiting works the same:
+- Each RPC call is checked independently
+- Connection reuse doesn't bypass rate limits
+- Rate limit is per API key, not per connection
+
+## Summary
+
+### ✅ DO: Use Long-Lived Connections
+
+```go
+// Create once at application startup
+client, _ := NewNotifierClient("notifier-grpc:50051", apiKey)
+defer client.Close()
+
+// Reuse for all requests
+for {
+    client.SendNotification(ctx, req)
+}
+```
+
+**Benefits:**
+- 10x faster (eliminates connection setup overhead)
+- Lower latency (1.5ms vs 15ms per request)
+- Fewer resources (one connection vs. thousands)
+- Better throughput (HTTP/2 multiplexing)
+- Automatic keepalive prevents timeouts
+
+### ❌ DON'T: Create Connection Per Request
+
+```go
+// BAD: Don't do this!
+for {
+    client, _ := NewNotifierClient("notifier-grpc:50051", apiKey)
+    client.SendNotification(ctx, req)
+    client.Close()
+}
+```
+
+**Problems:**
+- Slow (15ms per request)
+- Wasteful (repeated TCP/TLS handshakes)
+- Resource-intensive (thousands of connections)
+
+### Authentication Still Happens Per-Request
+
+- API key sent in metadata with every RPC
+- Server validates on every call
+- No security trade-off
+- Just eliminates connection setup overhead
+
+**You get both: Maximum performance AND full security!**

docs/QUICK_START_CLIENT.md

@@ -0,0 +1,319 @@
+# Quick Start: Notifier Client Integration
+
+**5-minute guide to integrating with the Notifier service**
+
+## Essential Requirements
+
+### 1. Connection: Use Long-Lived gRPC Connection ⚡
+
+```go
+// ✅ DO THIS: Create once, reuse forever
+client, _ := NewNotifierClient("notifier-grpc:50051", apiKey)
+defer client.Close()
+
+for i := 0; i < 10000; i++ {
+    client.SendNotification(ctx, req)  // Reuse connection
+}
+
+// ❌ DON'T DO THIS: Creating connection per request
+for i := 0; i < 10000; i++ {
+    client, _ := NewNotifierClient("notifier-grpc:50051", apiKey)
+    client.SendNotification(ctx, req)
+    client.Close()  // SLOW: 10x slower!
+}
+```
+
+**Why?** Connection reuse is 10x faster (1.5ms vs 15ms per request)
+
+### 2. Authentication: Send API Key with Every Request 🔐
+
+```go
+import "google.golang.org/grpc/metadata"
+
+// Add API key to metadata for each request
+ctx = metadata.AppendToOutgoingContext(ctx,
+    "authorization", "Bearer "+apiKey)
+
+resp, err := client.SendNotification(ctx, req)
+```
+
+**Two accepted formats:**
+- `authorization: Bearer nk_your_api_key` (preferred)
+- `x-api-key: nk_your_api_key`
+
+**API Key Format:** `nk_` + 64 hex characters (e.g., `nk_a1b2c3d4...`)
+
+### 3. Credentials: Load from Environment 🔒
+
+```go
+// ✅ DO THIS
+apiKey := os.Getenv("NOTIFIER_API_KEY")
+
+// ❌ DON'T DO THIS
+apiKey := "nk_abc123..."  // NEVER hardcode!
+```
+
+**Kubernetes Secret:**
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  name: notifier-credentials
+stringData:
+  api-key: nk_your_actual_key_here
+---
+# In your deployment:
+env:
+- name: NOTIFIER_API_KEY
+  valueFrom:
+    secretKeyRef:
+      name: notifier-credentials
+      key: api-key
+```
+
+### 4. Keepalive: Prevent Connection Timeouts ⏰
+
+```go
+import "google.golang.org/grpc/keepalive"
+
+conn, err := grpc.Dial(address,
+    grpc.WithKeepaliveParams(keepalive.ClientParameters{
+        Time:                10 * time.Second,  // Ping every 10s
+        Timeout:             3 * time.Second,   // Wait 3s for response
+        PermitWithoutStream: true,              // Ping when idle
+    }),
+)
+```
+
+### 5. Error Handling: Retry Only Specific Errors 🔄
+
+```go
+err := client.SendNotification(ctx, req)
+
+switch status.Code(err) {
+case codes.ResourceExhausted:
+    // Rate limit exceeded - RETRY with backoff
+    time.Sleep(100 * time.Millisecond)
+    // Try again...
+
+case codes.Unauthenticated:
+    // Invalid API key - DON'T RETRY, log error
+    log.Errorf("Authentication failed: %v", err)
+
+case codes.InvalidArgument:
+    // Bad request - DON'T RETRY, fix request
+    log.Errorf("Invalid request: %v", err)
+
+case codes.Unavailable:
+    // Service down - RETRY with backoff
+    time.Sleep(100 * time.Millisecond)
+    // Try again...
+}
+```
+
+**Retry Strategy:**
+- Exponential backoff: 100ms → 200ms → 400ms → 800ms
+- Max 3-5 attempts
+- Only retry: `ResourceExhausted`, `Unavailable`, `DeadlineExceeded`
+
+### 6. Context Timeout: Always Set Timeout ⏱️
+
+```go
+// ✅ DO THIS: Set timeout for each request
+ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+defer cancel()
+
+resp, err := client.SendNotification(ctx, req)
+
+// ❌ DON'T DO THIS: No timeout
+ctx := context.Background()
+resp, err := client.SendNotification(ctx, req)  // Could hang forever!
+```
+
+## Complete Minimal Example
+
+```go
+package main
+
+import (
+    "context"
+    "log"
+    "os"
+    "time"
+
+    pb "github.com/igodwin/notifier/api/grpc/pb"
+    "google.golang.org/grpc"
+    "google.golang.org/grpc/credentials/insecure"
+    "google.golang.org/grpc/keepalive"
+    "google.golang.org/grpc/metadata"
+)
+
+type NotifierClient struct {
+    conn   *grpc.ClientConn
+    client pb.NotifierServiceClient
+    apiKey string
+}
+
+func NewNotifierClient(address, apiKey string) (*NotifierClient, error) {
+    // Create long-lived connection
+    conn, err := grpc.Dial(address,
+        grpc.WithTransportCredentials(insecure.NewCredentials()),
+        grpc.WithKeepaliveParams(keepalive.ClientParameters{
+            Time:                10 * time.Second,
+            Timeout:             3 * time.Second,
+            PermitWithoutStream: true,
+        }),
+    )
+    if err != nil {
+        return nil, err
+    }
+
+    return &NotifierClient{
+        conn:   conn,
+        client: pb.NewNotifierServiceClient(conn),
+        apiKey: apiKey,
+    }, nil
+}
+
+func (nc *NotifierClient) SendNotification(ctx context.Context, req *pb.SendNotificationRequest) (*pb.SendNotificationResponse, error) {
+    // Add timeout
+    ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
+    defer cancel()
+
+    // Add API key to metadata
+    ctx = metadata.AppendToOutgoingContext(ctx, "authorization", "Bearer "+nc.apiKey)
+
+    // Make RPC call
+    return nc.client.SendNotification(ctx, req)
+}
+
+func (nc *NotifierClient) Close() error {
+    return nc.conn.Close()
+}
+
+func main() {
+    // Load credentials
+    address := os.Getenv("NOTIFIER_ADDRESS")
+    apiKey := os.Getenv("NOTIFIER_API_KEY")
+
+    if address == "" {
+        address = "notifier-grpc:50051"
+    }
+    if apiKey == "" {
+        log.Fatal("NOTIFIER_API_KEY environment variable required")
+    }
+
+    // Create client (once)
+    client, err := NewNotifierClient(address, apiKey)
+    if err != nil {
+        log.Fatalf("Failed to create client: %v", err)
+    }
+    defer client.Close()
+
+    // Send notification
+    resp, err := client.SendNotification(context.Background(), &pb.SendNotificationRequest{
+        Type:       pb.NotificationType_NOTIFICATION_TYPE_EMAIL,
+        Subject:    "Test Notification",
+        Body:       "This is a test from the notifier client",
+        Recipients: []string{"user@example.com"},
+        Priority:   pb.Priority_PRIORITY_NORMAL,
+    })
+
+    if err != nil {
+        log.Fatalf("Failed to send notification: %v", err)
+    }
+
+    log.Printf("Notification sent successfully: %s", resp.NotificationId)
+}
+```
+
+## Service Connection Details
+
+**Address:**
+- Same namespace: `notifier-grpc:50051`
+- Different namespace: `notifier-grpc.default.svc.cluster.local:50051`
+
+**Rate Limit:** 100 requests/minute (default, configurable per key)
+
+**Available Methods:**
+- `SendNotification` - Send single notification
+- `SendBatchNotifications` - Send multiple notifications
+- `GetNotification` - Get notification status
+- `ListNotifications` - List recent notifications
+- `CancelNotification` - Cancel pending notification
+- `RetryNotification` - Retry failed notification
+- `GetStats` - Get service statistics
+- `GetNotifiers` - List available notifiers
+- `HealthCheck` - Check service health
+
+**Notification Types:**
+- `NOTIFICATION_TYPE_EMAIL` - Email via SMTP
+- `NOTIFICATION_TYPE_SLACK` - Slack webhook
+- `NOTIFICATION_TYPE_NTFY` - Ntfy.sh push notifications
+- `NOTIFICATION_TYPE_STDOUT` - Console output (dev only)
+
+## Security Checklist
+
+- [ ] API key loaded from environment variable
+- [ ] No hardcoded credentials in source code
+- [ ] API key never logged in full (mask it: `nk_abc...xyz`)
+- [ ] Kubernetes Secret created for API key
+- [ ] Deployment configured to load secret as env var
+- [ ] `.gitignore` excludes any credential files
+
+## Testing Your Integration
+
+```bash
+# Set environment variables
+export NOTIFIER_ADDRESS="notifier-grpc:50051"
+export NOTIFIER_API_KEY="nk_your_api_key_here"
+
+# Run your application
+go run main.go
+
+# Check logs for:
+# ✅ "Notification sent successfully: <id>"
+# ❌ "Authentication failed" - check API key
+# ❌ "Rate limit exceeded" - slow down requests
+```
+
+## Common Issues
+
+**"Unauthenticated" error:**
+- Check API key format starts with `nk_`
+- Verify key is 66 characters total (nk_ + 64 hex)
+- Confirm key is active in notifier service
+- Check you're adding it to metadata correctly
+
+**"Rate limit exceeded" error:**
+- Implement exponential backoff retry
+- Consider batching notifications
+- Request higher rate limit for your key
+
+**Connection timeouts:**
+- Add keepalive configuration
+- Check network connectivity to `notifier-grpc:50051`
+- Verify notifier service is running: `kubectl get pods -l app=notifier`
+
+**Slow performance:**
+- Confirm you're reusing connection (not creating per request)
+- Check keepalive is configured
+- Verify you're using connection pooling for high concurrency
+
+## Next Steps
+
+For more details, see:
+- `CLIENT_INTEGRATION_PROMPT.md` - Complete prompt for AI assistants
+- `GRPC_CONNECTION_OPTIMIZATION.md` - Deep dive on performance
+- `CLIENT_RECOMMENDATIONS.md` - Advanced patterns and best practices
+- `AUTH.md` - Authentication system details
+
+## Getting Your API Key
+
+Contact your Notifier service administrator to:
+1. Create an API key for your application
+2. Set appropriate rate limits
+3. Assign required roles for notifier access
+4. Get the key in format: `nk_<64-hex-characters>`
+
+Store it securely and never commit it to version control!

k8s/configmap.yaml

@@ -66,3 +66,32 @@ data:
       port: 8081
       path: "/health"
       interval: 30
+
+    # CORS (Cross-Origin Resource Sharing) configuration
+    # Only needed if web browsers will access the REST API
+    # For backend-only services using gRPC, CORS is not required
+    cors:
+      # Whitelist of allowed origins - wildcards (*) are NOT supported
+      # Leave empty if you don't have web browser clients
+      allowed_origins: []
+
+      # If you have a web frontend, add its domain(s) here:
+      # allowed_origins:
+      #   - "https://app.example.com"
+      #   - "https://dashboard.example.com"
+
+      # For local development:
+      # allowed_origins:
+      #   - "http://localhost:3000"
+      #   - "http://localhost:8080"
+
+      allowed_methods:
+        - "GET"
+        - "POST"
+        - "OPTIONS"
+        - "DELETE"
+      allowed_headers:
+        - "Content-Type"
+        - "Authorization"
+      allow_credentials: false
+      max_age: 3600