# Step 20: Endpoint Discovery & Registration

**Phase 5: Microservice SDK**
**Estimated Complexity:** Medium
**Dependencies:** Step 19 (Microservice Host Builder)

---

## Overview

Endpoint discovery automatically finds and registers HTTP endpoints from microservice code using attributes and reflection. YAML configuration provides overrides for metadata like rate limits, authentication requirements, and versioning.

---

## Goals

1. Discover endpoints via reflection and attributes
2. Support YAML-based metadata overrides
3. Generate EndpointDescriptor for router registration
4. Support endpoint versioning and deprecation
5. Validate endpoint configurations at startup

---

## Endpoint Attributes

```csharp
namespace StellaOps.Microservice;

/// <summary>
/// Marks a class as containing Stella endpoints.
/// </summary>
[AttributeUsage(AttributeTargets.Class)]
public sealed class StellaEndpointAttribute : Attribute
{
    public string? BasePath { get; set; }
    public string? Version { get; set; }
    public string[]? Tags { get; set; }
}

/// <summary>
/// Marks a method as a Stella endpoint handler.
/// </summary>
[AttributeUsage(AttributeTargets.Method)]
public sealed class StellaRouteAttribute : Attribute
{
    public string Method { get; }
    public string Path { get; }
    public string? Name { get; set; }
    public string? Description { get; set; }

    public StellaRouteAttribute(string method, string path)
    {
        Method = method;
        Path = path;
    }
}

/// <summary>
/// Specifies authentication requirements for an endpoint.
/// </summary>
[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class)]
public sealed class StellaAuthAttribute : Attribute
{
    public bool Required { get; set; } = true;
    public string[]? RequiredClaims { get; set; }
    public string? Policy { get; set; }
}

/// <summary>
/// Specifies rate limiting for an endpoint.
/// </summary>
[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class)]
public sealed class StellaRateLimitAttribute : Attribute
{
    public int RequestsPerMinute { get; set; }
    public string? BucketKey { get; set; } // e.g., "sub", "ip", "path"
}

/// <summary>
/// Specifies timeout for an endpoint.
/// </summary>
[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class)]
public sealed class StellaTimeoutAttribute : Attribute
{
    public int TimeoutMs { get; }

    public StellaTimeoutAttribute(int timeoutMs)
    {
        TimeoutMs = timeoutMs;
    }
}

/// <summary>
/// Marks an endpoint as deprecated.
/// </summary>
[AttributeUsage(AttributeTargets.Method)]
public sealed class StellaDeprecatedAttribute : Attribute
{
    public string? Message { get; set; }
    public string? AlternativeEndpoint { get; set; }
    public string? SunsetDate { get; set; }
}

/// <summary>
/// Convenience attributes for common HTTP methods.
/// </summary>
public sealed class StellaGetAttribute : StellaRouteAttribute
{
    public StellaGetAttribute(string path) : base("GET", path) { }
}

public sealed class StellaPostAttribute : StellaRouteAttribute
{
    public StellaPostAttribute(string path) : base("POST", path) { }
}

public sealed class StellaPutAttribute : StellaRouteAttribute
{
    public StellaPutAttribute(string path) : base("PUT", path) { }
}

public sealed class StellaDeleteAttribute : StellaRouteAttribute
{
    public StellaDeleteAttribute(string path) : base("DELETE", path) { }
}

public sealed class StellaPatchAttribute : StellaRouteAttribute
{
    public StellaPatchAttribute(string path) : base("PATCH", path) { }
}
```

---

## Endpoint Descriptor

```csharp
namespace StellaOps.Microservice;

/// <summary>
/// Describes an endpoint for router registration.
/// </summary>
public sealed class EndpointDescriptor
{
    /// <summary>HTTP method (GET, POST, etc.).</summary>
    public required string Method { get; init; }

    /// <summary>Path pattern (may include parameters like {id}).</summary>
    public required string Path { get; init; }

    /// <summary>Unique endpoint name.</summary>
    public string? Name { get; init; }

    /// <summary>Endpoint description for documentation.</summary>
    public string? Description { get; init; }

    /// <summary>API version.</summary>
    public string? Version { get; init; }

    /// <summary>Tags for grouping/filtering.</summary>
    public string[]? Tags { get; init; }

    /// <summary>Whether authentication is required.</summary>
    public bool RequiresAuth { get; init; } = true;

    /// <summary>Required claims for access.</summary>
    public string[]? RequiredClaims { get; init; }

    /// <summary>Authentication policy name.</summary>
    public string? AuthPolicy { get; init; }

    /// <summary>Rate limit configuration.</summary>
    public RateLimitDescriptor? RateLimit { get; init; }

    /// <summary>Request timeout in milliseconds.</summary>
    public int? TimeoutMs { get; init; }

    /// <summary>Deprecation information.</summary>
    public DeprecationDescriptor? Deprecation { get; init; }

    /// <summary>Custom metadata.</summary>
    public Dictionary<string, string>? Metadata { get; init; }
}

public sealed class RateLimitDescriptor
{
    public int RequestsPerMinute { get; init; }
    public string BucketKey { get; init; } = "sub";
}

public sealed class DeprecationDescriptor
{
    public string? Message { get; init; }
    public string? AlternativeEndpoint { get; init; }
    public DateOnly? SunsetDate { get; init; }
}
```

---

## Endpoint Discovery Interface

```csharp
namespace StellaOps.Microservice;

public interface IEndpointDiscovery
{
    /// <summary>
    /// Discovers endpoints from configured assemblies.
    /// </summary>
    Task<IReadOnlyList<DiscoveredEndpoint>> DiscoverAsync(CancellationToken cancellationToken);
}

public sealed class DiscoveredEndpoint
{
    public required EndpointDescriptor Descriptor { get; init; }
    public required Type HandlerType { get; init; }
    public required MethodInfo HandlerMethod { get; init; }
}
```

---

## Reflection-Based Discovery

```csharp
namespace StellaOps.Microservice;

public sealed class ReflectionEndpointDiscovery : IEndpointDiscovery
{
    private readonly EndpointDiscoveryConfig _config;
    private readonly ILogger<ReflectionEndpointDiscovery> _logger;

    public ReflectionEndpointDiscovery(
        StellaMicroserviceOptions options,
        ILogger<ReflectionEndpointDiscovery> logger)
    {
        _config = options.Discovery;
        _logger = logger;
    }

    public Task<IReadOnlyList<DiscoveredEndpoint>> DiscoverAsync(CancellationToken cancellationToken)
    {
        var endpoints = new List<DiscoveredEndpoint>();
        var assemblies = GetAssembliesToScan();

        foreach (var assembly in assemblies)
        {
            foreach (var type in assembly.GetExportedTypes())
            {
                var classAttr = type.GetCustomAttribute<StellaEndpointAttribute>();
                if (classAttr == null)
                    continue;

                var classAuth = type.GetCustomAttribute<StellaAuthAttribute>();
                var classRateLimit = type.GetCustomAttribute<StellaRateLimitAttribute>();
                var classTimeout = type.GetCustomAttribute<StellaTimeoutAttribute>();

                foreach (var method in type.GetMethods(BindingFlags.Public | BindingFlags.Instance))
                {
                    var routeAttr = method.GetCustomAttribute<StellaRouteAttribute>();
                    if (routeAttr == null)
                        continue;

                    var endpoint = BuildEndpoint(
                        type, method, classAttr, routeAttr,
                        classAuth, classRateLimit, classTimeout);

                    endpoints.Add(endpoint);

                    _logger.LogDebug(
                        "Discovered endpoint: {Method} {Path}",
                        endpoint.Descriptor.Method, endpoint.Descriptor.Path);
                }
            }
        }

        _logger.LogInformation("Discovered {Count} endpoints", endpoints.Count);
        return Task.FromResult<IReadOnlyList<DiscoveredEndpoint>>(endpoints);
    }

    private IEnumerable<Assembly> GetAssembliesToScan()
    {
        if (_config.ScanAssemblies.Any())
        {
            return _config.ScanAssemblies.Select(Assembly.Load);
        }

        // Default: scan entry assembly and referenced assemblies
        var entry = Assembly.GetEntryAssembly();
        if (entry == null)
            return Enumerable.Empty<Assembly>();

        return new[] { entry }
            .Concat(entry.GetReferencedAssemblies().Select(Assembly.Load));
    }

    private DiscoveredEndpoint BuildEndpoint(
        Type handlerType,
        MethodInfo method,
        StellaEndpointAttribute classAttr,
        StellaRouteAttribute routeAttr,
        StellaAuthAttribute? classAuth,
        StellaRateLimitAttribute? classRateLimit,
        StellaTimeoutAttribute? classTimeout)
    {
        // Method-level attributes override class-level
        var methodAuth = method.GetCustomAttribute<StellaAuthAttribute>() ?? classAuth;
        var methodRateLimit = method.GetCustomAttribute<StellaRateLimitAttribute>() ?? classRateLimit;
        var methodTimeout = method.GetCustomAttribute<StellaTimeoutAttribute>() ?? classTimeout;
        var deprecatedAttr = method.GetCustomAttribute<StellaDeprecatedAttribute>();

        // Build full path
        var basePath = classAttr.BasePath?.TrimEnd('/') ?? "";
        if (!string.IsNullOrEmpty(_config.BasePath))
        {
            basePath = _config.BasePath.TrimEnd('/') + basePath;
        }
        var fullPath = basePath + "/" + routeAttr.Path.TrimStart('/');

        var descriptor = new EndpointDescriptor
        {
            Method = routeAttr.Method,
            Path = fullPath,
            Name = routeAttr.Name ?? $"{handlerType.Name}.{method.Name}",
            Description = routeAttr.Description,
            Version = classAttr.Version,
            Tags = classAttr.Tags,
            RequiresAuth = methodAuth?.Required ?? true,
            RequiredClaims = methodAuth?.RequiredClaims,
            AuthPolicy = methodAuth?.Policy,
            RateLimit = methodRateLimit != null ? new RateLimitDescriptor
            {
                RequestsPerMinute = methodRateLimit.RequestsPerMinute,
                BucketKey = methodRateLimit.BucketKey ?? "sub"
            } : null,
            TimeoutMs = methodTimeout?.TimeoutMs,
            Deprecation = deprecatedAttr != null ? new DeprecationDescriptor
            {
                Message = deprecatedAttr.Message,
                AlternativeEndpoint = deprecatedAttr.AlternativeEndpoint,
                SunsetDate = DateOnly.TryParse(deprecatedAttr.SunsetDate, out var date) ? date : null
            } : null
        };

        return new DiscoveredEndpoint
        {
            Descriptor = descriptor,
            HandlerType = handlerType,
            HandlerMethod = method
        };
    }
}
```

---

## YAML Override Provider

```csharp
namespace StellaOps.Microservice;

public interface IEndpointOverrideProvider
{
    /// <summary>
    /// Applies overrides to discovered endpoints.
    /// </summary>
    void ApplyOverrides(IList<DiscoveredEndpoint> endpoints);
}

public sealed class YamlEndpointOverrideProvider : IEndpointOverrideProvider
{
    private readonly EndpointDiscoveryConfig _config;
    private readonly ILogger<YamlEndpointOverrideProvider> _logger;
    private readonly Dictionary<string, EndpointOverride> _overrides = new();

    public YamlEndpointOverrideProvider(
        StellaMicroserviceOptions options,
        ILogger<YamlEndpointOverrideProvider> logger)
    {
        _config = options.Discovery;
        _logger = logger;

        LoadOverrides();
    }

    private void LoadOverrides()
    {
        if (string.IsNullOrEmpty(_config.ConfigFilePath))
            return;

        if (!File.Exists(_config.ConfigFilePath))
        {
            _logger.LogWarning("Endpoint config file not found: {Path}", _config.ConfigFilePath);
            return;
        }

        var yaml = File.ReadAllText(_config.ConfigFilePath);
        var deserializer = new DeserializerBuilder()
            .WithNamingConvention(CamelCaseNamingConvention.Instance)
            .Build();

        var config = deserializer.Deserialize<EndpointOverrideConfig>(yaml);

        if (config?.Endpoints != null)
        {
            foreach (var (key, value) in config.Endpoints)
            {
                _overrides[key] = value;
            }
        }

        _logger.LogInformation("Loaded {Count} endpoint overrides", _overrides.Count);
    }

    public void ApplyOverrides(IList<DiscoveredEndpoint> endpoints)
    {
        foreach (var endpoint in endpoints)
        {
            var key = $"{endpoint.Descriptor.Method} {endpoint.Descriptor.Path}";

            if (_overrides.TryGetValue(key, out var over) ||
                _overrides.TryGetValue(endpoint.Descriptor.Path, out over) ||
                (endpoint.Descriptor.Name != null && _overrides.TryGetValue(endpoint.Descriptor.Name, out over)))
            {
                ApplyOverride(endpoint, over);
            }
        }
    }

    private void ApplyOverride(DiscoveredEndpoint endpoint, EndpointOverride over)
    {
        // Create new descriptor with overrides applied
        var original = endpoint.Descriptor;

        var updated = new EndpointDescriptor
        {
            Method = original.Method,
            Path = original.Path,
            Name = over.Name ?? original.Name,
            Description = over.Description ?? original.Description,
            Version = over.Version ?? original.Version,
            Tags = over.Tags ?? original.Tags,
            RequiresAuth = over.RequiresAuth ?? original.RequiresAuth,
            RequiredClaims = over.RequiredClaims ?? original.RequiredClaims,
            AuthPolicy = over.AuthPolicy ?? original.AuthPolicy,
            RateLimit = over.RateLimit != null ? new RateLimitDescriptor
            {
                RequestsPerMinute = over.RateLimit.RequestsPerMinute,
                BucketKey = over.RateLimit.BucketKey ?? "sub"
            } : original.RateLimit,
            TimeoutMs = over.TimeoutMs ?? original.TimeoutMs,
            Deprecation = original.Deprecation, // Keep original deprecation
            Metadata = MergeMetadata(original.Metadata, over.Metadata)
        };

        // Replace descriptor (need mutable property or rebuild)
        // In real implementation, use record with 'with' expression
        _logger.LogDebug("Applied override to endpoint {Path}", original.Path);
    }

    private Dictionary<string, string>? MergeMetadata(
        Dictionary<string, string>? original,
        Dictionary<string, string>? over)
    {
        if (original == null && over == null)
            return null;

        var result = new Dictionary<string, string>(original ?? new());
        if (over != null)
        {
            foreach (var (key, value) in over)
            {
                result[key] = value;
            }
        }
        return result;
    }
}

internal class EndpointOverrideConfig
{
    public Dictionary<string, EndpointOverride>? Endpoints { get; set; }
}

internal class EndpointOverride
{
    public string? Name { get; set; }
    public string? Description { get; set; }
    public string? Version { get; set; }
    public string[]? Tags { get; set; }
    public bool? RequiresAuth { get; set; }
    public string[]? RequiredClaims { get; set; }
    public string? AuthPolicy { get; set; }
    public RateLimitOverride? RateLimit { get; set; }
    public int? TimeoutMs { get; set; }
    public Dictionary<string, string>? Metadata { get; set; }
}

internal class RateLimitOverride
{
    public int RequestsPerMinute { get; set; }
    public string? BucketKey { get; set; }
}
```

---

## Endpoint Registry

```csharp
namespace StellaOps.Microservice;

public interface IEndpointRegistry
{
    Task<EndpointDescriptor[]> DiscoverEndpointsAsync(CancellationToken cancellationToken);
    DiscoveredEndpoint? FindEndpoint(string method, string path);
}

public sealed class EndpointRegistry : IEndpointRegistry
{
    private readonly IEndpointDiscovery _discovery;
    private readonly IEndpointOverrideProvider? _overrideProvider;
    private readonly ILogger<EndpointRegistry> _logger;
    private IReadOnlyList<DiscoveredEndpoint>? _endpoints;
    private readonly Dictionary<string, DiscoveredEndpoint> _endpointLookup = new();

    public EndpointRegistry(
        IEndpointDiscovery discovery,
        IEndpointOverrideProvider? overrideProvider,
        ILogger<EndpointRegistry> logger)
    {
        _discovery = discovery;
        _overrideProvider = overrideProvider;
        _logger = logger;
    }

    public async Task<EndpointDescriptor[]> DiscoverEndpointsAsync(CancellationToken cancellationToken)
    {
        _endpoints = await _discovery.DiscoverAsync(cancellationToken);

        if (_overrideProvider != null)
        {
            var mutableList = _endpoints.ToList();
            _overrideProvider.ApplyOverrides(mutableList);
            _endpoints = mutableList;
        }

        // Build lookup table
        _endpointLookup.Clear();
        foreach (var endpoint in _endpoints)
        {
            var key = $"{endpoint.Descriptor.Method}:{endpoint.Descriptor.Path}";
            _endpointLookup[key] = endpoint;
        }

        // Validate endpoints
        ValidateEndpoints(_endpoints);

        return _endpoints.Select(e => e.Descriptor).ToArray();
    }

    public DiscoveredEndpoint? FindEndpoint(string method, string path)
    {
        // Exact match
        var key = $"{method}:{path}";
        if (_endpointLookup.TryGetValue(key, out var endpoint))
            return endpoint;

        // Pattern match for path parameters
        foreach (var ep in _endpoints ?? Enumerable.Empty<DiscoveredEndpoint>())
        {
            if (ep.Descriptor.Method != method)
                continue;

            if (IsPathMatch(path, ep.Descriptor.Path))
                return ep;
        }

        return null;
    }

    private bool IsPathMatch(string requestPath, string pattern)
    {
        var patternSegments = pattern.Split('/', StringSplitOptions.RemoveEmptyEntries);
        var pathSegments = requestPath.Split('/', StringSplitOptions.RemoveEmptyEntries);

        if (patternSegments.Length != pathSegments.Length)
            return false;

        for (int i = 0; i < patternSegments.Length; i++)
        {
            var patternSeg = patternSegments[i];
            var pathSeg = pathSegments[i];

            // Check for path parameter
            if (patternSeg.StartsWith('{') && patternSeg.EndsWith('}'))
                continue;

            if (!string.Equals(patternSeg, pathSeg, StringComparison.OrdinalIgnoreCase))
                return false;
        }

        return true;
    }

    private void ValidateEndpoints(IReadOnlyList<DiscoveredEndpoint> endpoints)
    {
        var duplicates = endpoints
            .GroupBy(e => $"{e.Descriptor.Method}:{e.Descriptor.Path}")
            .Where(g => g.Count() > 1)
            .Select(g => g.Key)
            .ToList();

        if (duplicates.Any())
        {
            throw new InvalidOperationException(
                $"Duplicate endpoints detected: {string.Join(", ", duplicates)}");
        }

        // Validate handler method signatures
        foreach (var endpoint in endpoints)
        {
            ValidateHandlerMethod(endpoint);
        }
    }

    private void ValidateHandlerMethod(DiscoveredEndpoint endpoint)
    {
        var method = endpoint.HandlerMethod;
        var returnType = method.ReturnType;

        // Must return Task<ResponsePayload> or Task<T> where T can be serialized
        if (!typeof(Task).IsAssignableFrom(returnType))
        {
            throw new InvalidOperationException(
                $"Handler {method.Name} must return Task or Task<T>");
        }
    }
}
```

---

## YAML Configuration Example

```yaml
# endpoints.yaml - Endpoint overrides

Endpoints:
  # Override by path
  "GET /billing/invoices":
    RateLimit:
      RequestsPerMinute: 100
      BucketKey: "sub"
    TimeoutMs: 30000

  # Override by name
  "InvoiceHandler.GetInvoice":
    RequiredClaims:
      - "billing:read"
    AuthPolicy: "billing-read"

  # Override by method + path
  "POST /billing/invoices":
    RequiredClaims:
      - "billing:write"
    RateLimit:
      RequestsPerMinute: 10
      BucketKey: "sub"
    Metadata:
      audit: "required"
```

---

## Deliverables

1. `StellaOps.Microservice/Attributes/*.cs` (all endpoint attributes)
2. `StellaOps.Microservice/EndpointDescriptor.cs`
3. `StellaOps.Microservice/IEndpointDiscovery.cs`
4. `StellaOps.Microservice/ReflectionEndpointDiscovery.cs`
5. `StellaOps.Microservice/IEndpointOverrideProvider.cs`
6. `StellaOps.Microservice/YamlEndpointOverrideProvider.cs`
7. `StellaOps.Microservice/IEndpointRegistry.cs`
8. `StellaOps.Microservice/EndpointRegistry.cs`
9. Attribute parsing tests
10. YAML override tests
11. Path matching tests

---

## Next Step

Proceed to [Step 21: Request/Response Context](21-Step.md) to implement the request handling context.