# component_architecture_gateway.md — **Stella Ops Gateway** (Sprint 3600)

> Derived from Reference Architecture Advisory and Router Architecture Specification

> **Scope.** The Gateway WebService is the single HTTP ingress point for all external traffic. It authenticates requests via Authority (DPoP/mTLS), routes to microservices via the Router binary protocol, aggregates OpenAPI specifications, and enforces tenant isolation.
> **Ownership:** Platform Guild

---

## 0) Mission & Boundaries

### What Gateway Does

- **HTTP Ingress**: Single entry point for all external HTTP/HTTPS traffic
- **Authentication**: DPoP and mTLS token validation via Authority integration
- **Routing**: Routes HTTP requests to microservices via binary protocol (TCP/TLS)
- **OpenAPI Aggregation**: Combines endpoint specs from all registered microservices
- **Health Aggregation**: Provides unified health status from downstream services
- **Rate Limiting**: Per-tenant and per-identity request throttling
- **Tenant Propagation**: Extracts tenant context and propagates to microservices

### What Gateway Does NOT Do

- **Business Logic**: No domain logic; pure routing and auth
- **Data Storage**: Stateless; no persistent state beyond connection cache
- **Direct Database Access**: Never connects to PostgreSQL directly
- **SBOM/VEX Processing**: Delegates to Scanner, Excititor, etc.

---

## 1) Solution & Project Layout

```
src/Gateway/
├── StellaOps.Gateway.WebService/
│   ├── StellaOps.Gateway.WebService.csproj
│   ├── Program.cs                          # DI bootstrap, transport init
│   ├── Dockerfile
│   ├── appsettings.json
│   ├── appsettings.Development.json
│   ├── Configuration/
│   │   ├── GatewayOptions.cs               # All configuration options
│   │   └── TransportOptions.cs             # TCP/TLS transport config
│   ├── Middleware/
│   │   ├── TenantMiddleware.cs             # Tenant context extraction
│   │   ├── RequestRoutingMiddleware.cs     # HTTP → binary routing
│   │   ├── AuthenticationMiddleware.cs     # DPoP/mTLS validation
│   │   └── RateLimitingMiddleware.cs       # Per-tenant throttling
│   ├── Services/
│   │   ├── GatewayHostedService.cs         # Transport lifecycle
│   │   ├── OpenApiAggregationService.cs    # Spec aggregation
│   │   └── HealthAggregationService.cs     # Downstream health
│   └── Endpoints/
│       ├── HealthEndpoints.cs              # /health/*, /metrics
│       └── OpenApiEndpoints.cs             # /openapi.json, /openapi.yaml
```

### Dependencies

```xml
<ItemGroup>
  <ProjectReference Include="..\..\__Libraries\StellaOps.Router.Gateway\..." />
  <ProjectReference Include="..\..\__Libraries\StellaOps.Router.Transport.Tcp\..." />
  <ProjectReference Include="..\..\__Libraries\StellaOps.Router.Transport.Tls\..." />
  <ProjectReference Include="..\..\Auth\StellaOps.Auth.ServerIntegration\..." />
</ItemGroup>
```

---

## 2) External Dependencies

| Dependency | Purpose | Required |
|------------|---------|----------|
| **Authority** | OpTok validation, DPoP/mTLS | Yes |
| **Router.Gateway** | Routing state, endpoint discovery | Yes |
| **Router.Transport.Tcp** | Binary transport (dev) | Yes |
| **Router.Transport.Tls** | Binary transport (prod) | Yes |
| **Valkey/Redis** | Rate limiting state | Optional |

---

## 3) Contracts & Data Model

### Request Flow

```
┌──────────────┐     HTTPS      ┌─────────────────┐    Binary     ┌─────────────────┐
│   Client     │ ─────────────► │    Gateway      │ ────────────► │  Microservice   │
│  (CLI/UI)    │                │   WebService    │    Frame      │   (Scanner,     │
│              │ ◄───────────── │                 │ ◄──────────── │    Policy, etc) │
└──────────────┘     HTTPS      └─────────────────┘    Binary     └─────────────────┘
```

### Binary Frame Protocol

Gateway uses the Router binary protocol for internal communication:

| Frame Type | Purpose |
|------------|---------|
| HELLO | Microservice registration with endpoints |
| HEARTBEAT | Health check and latency measurement |
| REQUEST | HTTP request serialized to binary |
| RESPONSE | HTTP response serialized from binary |
| STREAM_DATA | Streaming response chunks |
| CANCEL | Request cancellation propagation |

### Endpoint Descriptor

```csharp
public sealed class EndpointDescriptor
{
    public required string Method { get; init; }      // GET, POST, etc.
    public required string Path { get; init; }        // /api/v1/scans/{id}
    public required string ServiceName { get; init; } // scanner
    public required string Version { get; init; }     // 1.0.0
    public TimeSpan DefaultTimeout { get; init; }     // 30s
    public bool SupportsStreaming { get; init; }      // true for large responses
    public IReadOnlyList<ClaimRequirement> RequiringClaims { get; init; }
}
```

### Routing State

```csharp
public interface IRoutingStateManager
{
    ValueTask RegisterEndpointsAsync(ConnectionState conn, HelloPayload hello);
    ValueTask<InstanceSelection?> SelectInstanceAsync(string method, string path);
    ValueTask UpdateHealthAsync(ConnectionState conn, HeartbeatPayload heartbeat);
    ValueTask DrainConnectionAsync(string connectionId);
}
```

---

## 4) REST API

Gateway exposes minimal management endpoints; all business APIs are routed to microservices.

### Health Endpoints

| Endpoint | Auth | Description |
|----------|------|-------------|
| `GET /health/live` | None | Liveness probe |
| `GET /health/ready` | None | Readiness probe |
| `GET /health/startup` | None | Startup probe |
| `GET /metrics` | None | Prometheus metrics |

### OpenAPI Endpoints

| Endpoint | Auth | Description |
|----------|------|-------------|
| `GET /openapi.json` | None | Aggregated OpenAPI 3.1.0 spec |
| `GET /openapi.yaml` | None | YAML format spec |

---

## 5) Execution Flow

### Request Routing

```mermaid
sequenceDiagram
    participant C as Client
    participant G as Gateway
    participant A as Authority
    participant M as Microservice

    C->>G: HTTPS Request + DPoP Token
    G->>A: Validate Token
    A-->>G: Claims (sub, tid, scope)
    G->>G: Select Instance (Method, Path)
    G->>M: Binary REQUEST Frame
    M-->>G: Binary RESPONSE Frame
    G-->>C: HTTPS Response
```

### Microservice Registration

```mermaid
sequenceDiagram
    participant M as Microservice
    participant G as Gateway

    M->>G: TCP/TLS Connect
    M->>G: HELLO (ServiceName, Version, Endpoints)
    G->>G: Register Endpoints
    G-->>M: HELLO ACK

    loop Every 10s
        G->>M: HEARTBEAT
        M-->>G: HEARTBEAT (latency, health)
        G->>G: Update Health State
    end
```

---

## 6) Instance Selection Algorithm

```csharp
public ValueTask<InstanceSelection?> SelectInstanceAsync(string method, string path)
{
    // 1. Find all endpoints matching (method, path)
    var candidates = _endpoints
        .Where(e => e.Method == method && MatchPath(e.Path, path))
        .ToList();

    // 2. Filter by health
    candidates = candidates
        .Where(c => c.Health is InstanceHealthStatus.Healthy or InstanceHealthStatus.Degraded)
        .ToList();

    // 3. Region preference
    var localRegion = candidates.Where(c => c.Region == _config.Region).ToList();
    var neighborRegions = candidates.Where(c => _config.NeighborRegions.Contains(c.Region)).ToList();
    var otherRegions = candidates.Except(localRegion).Except(neighborRegions).ToList();

    var preferred = localRegion.Any() ? localRegion
                  : neighborRegions.Any() ? neighborRegions
                  : otherRegions;

    // 4. Within tier: prefer lower latency, then most recent heartbeat
    return preferred
        .OrderBy(c => c.AveragePingMs)
        .ThenByDescending(c => c.LastHeartbeatUtc)
        .FirstOrDefault();
}
```

---

## 7) Configuration

```yaml
gateway:
  node:
    region: "eu1"
    nodeId: "gw-eu1-01"
    environment: "prod"

  transports:
    tcp:
      enabled: true
      port: 9100
      maxConnections: 1000
      receiveBufferSize: 65536
      sendBufferSize: 65536
    tls:
      enabled: true
      port: 9443
      certificatePath: "/certs/gateway.pfx"
      certificatePassword: "${GATEWAY_CERT_PASSWORD}"
      clientCertificateMode: "RequireCertificate"
      allowedClientCertificateThumbprints: []

  routing:
    defaultTimeout: "30s"
    maxRequestBodySize: "100MB"
    streamingEnabled: true
    streamingBufferSize: 16384
    neighborRegions: ["eu2", "us1"]

  auth:
    dpopEnabled: true
    dpopMaxClockSkew: "60s"
    mtlsEnabled: true
    rateLimiting:
      enabled: true
      requestsPerMinute: 1000
      burstSize: 100
      redisConnectionString: "${REDIS_URL}"

  openapi:
    enabled: true
    cacheTtlSeconds: 300
    title: "Stella Ops API"
    version: "1.0.0"

  health:
    heartbeatIntervalSeconds: 10
    heartbeatTimeoutSeconds: 30
    unhealthyThreshold: 3
```

---

## 8) Scale & Performance

| Metric | Target | Notes |
|--------|--------|-------|
| Routing latency (P50) | <2ms | Overhead only; excludes downstream |
| Routing latency (P99) | <5ms | Under normal load |
| Concurrent connections | 10,000 | Per gateway instance |
| Requests/second | 50,000 | Per gateway instance |
| Memory footprint | <512MB | Base; scales with connections |

### Scaling Strategy

- Horizontal scaling behind load balancer
- Sticky sessions NOT required (stateless)
- Regional deployment for latency optimization
- Rate limiting via distributed Valkey/Redis

---

## 9) Security Posture

### Authentication

| Method | Description |
|--------|-------------|
| DPoP | Proof-of-possession tokens from Authority |
| mTLS | Certificate-bound tokens for machine clients |

### Authorization

- Claims-based authorization per endpoint
- Required claims defined in endpoint descriptors
- Tenant isolation via `tid` claim

### Transport Security

| Component | Encryption |
|-----------|------------|
| Client → Gateway | TLS 1.3 (HTTPS) |
| Gateway → Microservices | TLS (prod), TCP (dev only) |

### Rate Limiting

- Per-tenant: Configurable requests/minute
- Per-identity: Burst protection
- Global: Circuit breaker for overload

---

## 10) Observability & Audit

### Metrics (Prometheus)

```
gateway_requests_total{service,method,path,status}
gateway_request_duration_seconds{service,method,path,quantile}
gateway_active_connections{service}
gateway_transport_frames_total{type}
gateway_auth_failures_total{reason}
gateway_rate_limit_exceeded_total{tenant}
```

### Traces (OpenTelemetry)

- Span per request: `gateway.route`
- Child span: `gateway.auth.validate`
- Child span: `gateway.transport.send`

### Logs (Structured)

```json
{
  "timestamp": "2025-12-21T10:00:00Z",
  "level": "info",
  "message": "Request routed",
  "correlationId": "abc123",
  "tenantId": "tenant-1",
  "method": "GET",
  "path": "/api/v1/scans/xyz",
  "service": "scanner",
  "durationMs": 45,
  "status": 200
}
```

---

## 11) Testing Matrix

| Test Type | Scope | Coverage Target |
|-----------|-------|-----------------|
| Unit | Routing algorithm, auth validation | 90% |
| Integration | Transport + routing flow | 80% |
| E2E | Full request path with mock services | Key flows |
| Performance | Latency, throughput, connection limits | SLO targets |
| Chaos | Connection failures, microservice crashes | Resilience |

### Test Fixtures

- `StellaOps.Router.Transport.InMemory` for transport mocking
- Mock Authority for auth testing
- `WebApplicationFactory` for integration tests

---

## 12) DevOps & Operations

### Deployment

```yaml
# Kubernetes deployment excerpt
apiVersion: apps/v1
kind: Deployment
metadata:
  name: gateway
spec:
  replicas: 3
  template:
    spec:
      containers:
      - name: gateway
        image: stellaops/gateway:1.0.0
        ports:
        - containerPort: 8080   # HTTPS
        - containerPort: 9443   # TLS (microservices)
        resources:
          requests:
            memory: "256Mi"
            cpu: "250m"
          limits:
            memory: "512Mi"
            cpu: "1000m"
        livenessProbe:
          httpGet:
            path: /health/live
            port: 8080
        readinessProbe:
          httpGet:
            path: /health/ready
            port: 8080
```

### SLOs

| SLO | Target | Measurement |
|-----|--------|-------------|
| Availability | 99.9% | Uptime over 30 days |
| Latency P99 | <50ms | Includes downstream |
| Error rate | <0.1% | 5xx responses |

---

## 13) Roadmap

| Feature | Sprint | Status |
|---------|--------|--------|
| Core implementation | 3600.0001.0001 | TODO |
| WebSocket support | Future | Planned |
| gRPC passthrough | Future | Planned |
| GraphQL aggregation | Future | Exploration |

---

## 14) References

- Router Architecture: `docs/modules/router/architecture.md`
- OpenAPI Aggregation: `docs/modules/gateway/openapi.md`
- Authority Integration: `docs/modules/authority/architecture.md`
- Reference Architecture: `docs/product-advisories/archived/2025-12-21-reference-architecture/`

---

**Last Updated**: 2025-12-21 (Sprint 3600)