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+# SOAR-0015: Error Handler Protocols for Client and Server
+
+Introduce `ClientErrorHandler` and `ServerErrorHandler` protocols for centralized error observation on both client and server sides.
+
+## Overview
+
+- Proposal: SOAR-0015
+- Author(s): [winnisx7](https://github.com/winnisx7)
+- Status: **Proposed**
+- Issue: [apple/swift-openapi-runtime#162](https://github.com/apple/swift-openapi-runtime/pull/162)
+- Implementation:
+    - [apple/swift-openapi-runtime#162](https://github.com/apple/swift-openapi-runtime/pull/162)
+- Affected components:
+    - runtime
+
+### Introduction
+
+This proposal introduces `ClientErrorHandler` and `ServerErrorHandler` protocols to provide extension points for centralized error observation on both client and server sides. These handlers are configured through the `Configuration` struct and are invoked after errors have been wrapped in `ClientError` or `ServerError`.
+
+### Motivation
+
+Currently, swift-openapi-runtime provides limited options for centralized error handling. Developers face the following challenges:
+
+**Problem 1: Scattered Error Handling**
+
+Errors must be handled individually at each API call site, leading to code duplication and inconsistent error handling.
+
+```swift
+// Current approach: individual handling at each call site
+do {
+    let response = try await client.getUser(...)
+} catch {
+    // Repeated error handling logic
+    logger.error("API error: \(error)")
+}
+
+do {
+    let response = try await client.getPosts(...)
+} catch {
+    // Same error handling logic repeated
+    logger.error("API error: \(error)")
+}
+```
+
+**Problem 2: Middleware Limitations**
+
+The existing `ClientMiddleware` operates at the HTTP request/response level, making it difficult to intercept decoding errors or runtime errors. There is still a lack of extension points for error **observation**.
+
+**Problem 3: Telemetry and Logging Complexity**
+
+To collect telemetry or implement centralized logging for all errors, developers currently need to modify every API call site.
+
+**Problem 4: Difficulty Utilizing Error Context**
+
+`ClientError` and `ServerError` contain rich context information such as `operationID`, `request`, and `response`, but centralized analysis using this information is difficult.
+
+### Proposed solution
+
+Introduce `ClientErrorHandler` and `ServerErrorHandler` protocols and add optional handler properties to the `Configuration` struct. These handlers are invoked **after** errors have been wrapped in `ClientError` or `ServerError`, allowing logging, monitoring, and analytics operations.
+
+```swift
+// Custom error handler with logging
+struct LoggingClientErrorHandler: ClientErrorHandler {
+    func handleClientError(_ error: ClientError) {
+        logger.error("Client error in \(error.operationID): \(error.causeDescription)")
+        analytics.track("client_error", metadata: [
+            "operation": error.operationID,
+            "status": error.response?.status.code
+        ])
+    }
+}
+
+let config = Configuration(
+    clientErrorHandler: LoggingClientErrorHandler()
+)
+let client = UniversalClient(configuration: config, transport: transport)
+```
+
+### Detailed design
+
+#### New Protocol Definitions
+
+```swift
+/// A protocol for handling client-side errors.
+///
+/// Implement this protocol to observe and react to errors that occur during
+/// client API calls. The handler is invoked after the error has been wrapped
+/// in a ``ClientError``.
+///
+/// Use this to add logging, monitoring, or analytics for client-side errors.
+public protocol ClientErrorHandler: Sendable {
+    /// Handles a client error.
+    ///
+    /// This method is called after an error has been wrapped in a ``ClientError``
+    /// but before it is thrown to the caller.
+    ///
+    /// - Parameter error: The client error that occurred, containing context such as
+    ///   the operation ID, request, response, and underlying cause.
+    func handleClientError(_ error: ClientError)
+}
+
+/// A protocol for handling server-side errors.
+///
+/// Implement this protocol to observe and react to errors that occur during
+/// server request handling. The handler is invoked after the error has been
+/// wrapped in a ``ServerError``.
+///
+/// Use this to add logging, monitoring, or analytics for server-side errors.
+public protocol ServerErrorHandler: Sendable {
+    /// Handles a server error.
+    ///
+    /// This method is called after an error has been wrapped in a ``ServerError``
+    /// but before it is thrown to the caller.
+    ///
+    /// - Parameter error: The server error that occurred, containing context such as
+    ///   the operation ID, request, and underlying cause.
+    func handleServerError(_ error: ServerError)
+}
+```
+
+#### Configuration Struct Changes
+
+```swift
+public struct Configuration: Sendable {
+    // ... existing properties ...
+    
+    /// Custom XML coder for encoding and decoding xml bodies.
+    public var xmlCoder: (any CustomCoder)?
+    
+    /// The handler for client-side errors.
+    ///
+    /// This handler is invoked after a client error has been wrapped in a ``ClientError``.
+    /// Use this to add logging, monitoring, or analytics for client-side errors.
+    /// If `nil`, errors are thrown without additional handling.
+    public var clientErrorHandler: (any ClientErrorHandler)?
+    
+    /// The handler for server-side errors.
+    ///
+    /// This handler is invoked after a server error has been wrapped in a ``ServerError``.
+    /// Use this to add logging, monitoring, or analytics for server-side errors.
+    /// If `nil`, errors are thrown without additional handling.
+    public var serverErrorHandler: (any ServerErrorHandler)?
+    
+    /// Creates a new configuration with the specified values.
+    ///
+    /// - Parameters:
+    ///   - dateTranscoder: The transcoder for date/time conversions.
+    ///   - multipartBoundaryGenerator: The generator for multipart boundaries.
+    ///   - xmlCoder: Custom XML coder for encoding and decoding xml bodies.
+    ///   - clientErrorHandler: Optional handler for observing client-side errors. Defaults to `nil`.
+    ///   - serverErrorHandler: Optional handler for observing server-side errors. Defaults to `nil`.
+    public init(
+        dateTranscoder: any DateTranscoder = .iso8601,
+        multipartBoundaryGenerator: any MultipartBoundaryGenerator = .random,
+        xmlCoder: (any CustomCoder)? = nil,
+        clientErrorHandler: (any ClientErrorHandler)? = nil,
+        serverErrorHandler: (any ServerErrorHandler)? = nil
+    ) {
+        self.dateTranscoder = dateTranscoder
+        self.multipartBoundaryGenerator = multipartBoundaryGenerator
+        self.xmlCoder = xmlCoder
+        self.clientErrorHandler = clientErrorHandler
+        self.serverErrorHandler = serverErrorHandler
+    }
+}
+```
+
+#### UniversalClient Changes
+
+In `UniversalClient`, the configured handler is called after the error has been wrapped in `ClientError`:
+
+```swift
+// Inside UniversalClient (pseudocode)
+do {
+    // API call logic
+} catch {
+    let clientError = ClientError(
+        operationID: operationID,
+        request: request,
+        response: response,
+        underlyingError: error
+    )
+    
+    // Call handler if configured
+    configuration.clientErrorHandler?.handleClientError(clientError)
+    
+    throw clientError
+}
+```
+
+#### UniversalServer Changes
+
+Similarly, `UniversalServer` calls the handler after wrapping the error in `ServerError`.
+
+#### Error Handling Flow
+
+```
+┌──────────────────────┐
+│   API Call/Handle    │
+└──────────┬───────────┘
+           │
+           ▼ (error occurs)
+┌──────────────────────┐
+│ Wrap in ClientError/ │
+│ ServerError          │
+└──────────┬───────────┘
+           │
+           ▼
+┌──────────────────────┐
+│ errorHandler         │──────┐
+│ configured?          │      │ No
+└──────────┬───────────┘      │
+      Yes  │                  │
+           ▼                  │
+┌──────────────────────┐      │
+│ handleClientError/   │      │
+│ handleServerError    │      │
+│ called (observe)     │      │
+└──────────┬───────────┘      │
+           │                  │
+           ▼                  ▼
+┌─────────────────────────────────┐
+│ Original ClientError/ServerError│
+│ thrown                          │
+└─────────────────────────────────┘
+```
+
+> **Important:** Handlers only **observe** errors; they do not transform or suppress them. The original error is always thrown.
+
+#### Usage Examples
+
+**Basic Usage: Logging**
+
+```swift
+struct LoggingClientErrorHandler: ClientErrorHandler {
+    func handleClientError(_ error: ClientError) {
+        print("🚨 Client error in \(error.operationID): \(error.causeDescription)")
+    }
+}
+
+let config = Configuration(
+    clientErrorHandler: LoggingClientErrorHandler()
+)
+```
+
+**Telemetry Integration**
+
+```swift
+struct AnalyticsClientErrorHandler: ClientErrorHandler {
+    let analytics: AnalyticsService
+    
+    func handleClientError(_ error: ClientError) {
+        analytics.track("client_error", metadata: [
+            "operation": error.operationID,
+            "status": error.response?.status.code as Any,
+            "cause": error.causeDescription,
+            "timestamp": Date().ISO8601Format()
+        ])
+    }
+}
+```
+
+**Conditional Logging (Operation ID Based)**
+
+```swift
+struct SelectiveLoggingHandler: ClientErrorHandler {
+    let criticalOperations: Set<String>
+    
+    func handleClientError(_ error: ClientError) {
+        if criticalOperations.contains(error.operationID) {
+            // Send immediate alert for critical operations
+            alertService.sendAlert(
+                message: "Critical operation failed: \(error.operationID)",
+                severity: .high
+            )
+        }
+        
+        // Log all errors
+        logger.error("[\(error.operationID)] \(error.causeDescription)")
+    }
+}
+```
+
+**Server-Side Error Handler**
+
+```swift
+struct ServerErrorLoggingHandler: ServerErrorHandler {
+    func handleServerError(_ error: ServerError) {
+        logger.error("""
+            Server error:
+            - Operation: \(error.operationID)
+            - Request: \(error.request)
+            - Cause: \(error.underlyingError)
+            """)
+    }
+}
+
+let config = Configuration(
+    serverErrorHandler: ServerErrorLoggingHandler()
+)
+```
+
+### API stability
+
+This change maintains **full backward compatibility**:
+
+- The `clientErrorHandler` and `serverErrorHandler` parameters default to `nil`, so existing code works without modification.
+- Existing `Configuration` initialization code continues to work unchanged.
+
+```swift
+// Existing code - works without changes
+let config = Configuration()
+let config = Configuration(dateTranscoder: .iso8601)
+
+// Using new features
+let config = Configuration(
+    clientErrorHandler: LoggingClientErrorHandler()
+)
+let config = Configuration(
+    clientErrorHandler: LoggingClientErrorHandler(),
+    serverErrorHandler: ServerErrorLoggingHandler()
+)
+```
+
+### Test plan
+
+**Unit Tests**
+
+1. **Default Behavior Tests**
+   - Verify errors are thrown normally when `clientErrorHandler` is `nil`
+   - Verify errors are thrown normally when `serverErrorHandler` is `nil`
+
+2. **Handler Invocation Tests**
+   - Verify `handleClientError` is called when `ClientError` occurs
+   - Verify `handleServerError` is called when `ServerError` occurs
+   - Verify original error is thrown after handler invocation
+
+3. **Sendable Conformance Tests**
+   - Verify handler protocols properly conform to `Sendable`
+
+4. **Error Context Tests**
+   - Verify `ClientError`/`ServerError` passed to handlers contains correct context
+
+**Integration Tests**
+
+1. **Real API Call Scenarios**
+   - Verify handlers are called for various error situations including network errors and decoding errors
+
+2. **Performance Tests**
+   - Verify error handler addition has minimal performance impact
+
+### Future directions
+
+- **Async handler methods**: The current design uses synchronous handler methods. A future enhancement could introduce async variants for handlers that need to perform asynchronous operations like remote logging.
+
+- **Error transformation**: While this proposal focuses on error observation, a future proposal could introduce error transformation capabilities, allowing handlers to modify or replace errors before they are thrown.
+
+- **Built-in handler implementations**: The runtime could provide common handler implementations out of the box, such as a `LoggingErrorHandler` that integrates with swift-log.
+
+### Alternatives considered
+
+**Using middleware for error handling**
+
+One alternative considered was extending the existing `ClientMiddleware` and `ServerMiddleware` protocols to handle errors. However, middleware operates at the HTTP request/response level and cannot intercept errors that occur during response decoding or other runtime operations. The error handler approach provides a more comprehensive solution for error observation.
+
+**Closure-based handlers instead of protocols**
+
+Instead of defining `ClientErrorHandler` and `ServerErrorHandler` protocols, we could use closure properties directly:
+
+```swift
+public var onClientError: ((ClientError) -> Void)?
+```
+
+While this approach is simpler, the protocol-based design was chosen because:
+- It allows for more complex handler implementations with internal state
+- It provides better documentation through protocol requirements
+- It follows the existing patterns in swift-openapi-runtime (e.g., `DateTranscoder`, `MultipartBoundaryGenerator`)