# VAPORA Architecture

Comprehensive documentation of VAPORA's system architecture, design patterns, and implementation details.

## Architecture Layers

### 1. Protocol Layer

**A2A (Agent-to-Agent) Protocol**
- Standard protocol for agent-to-agent communication
- JSON-RPC 2.0 specification compliance
- Agent discovery via Agent Card
- Task dispatch and lifecycle tracking
- See: [ADR-0001: A2A Protocol Implementation](adr/0001-a2a-protocol-implementation.md)

**MCP (Model Context Protocol)**
- Real MCP transport with Stdio and SSE support
- 6 integrated tools for task/agent management
- Backend client integration
- Tool registry with JSON Schema validation

### 2. Server Layer

**vapora-a2a (A2A Server)**
- Axum-based HTTP server
- Endpoints: agent discovery, task dispatch, status query, health, metrics
- JSON-RPC 2.0 request/response handling
- **SurrealDB persistent storage** (production-ready)
- **NATS async coordination** for task lifecycle events
- **Prometheus metrics** (/metrics endpoint)
- Integration: AgentCoordinator, TaskManager, CoordinatorBridge
- Tasks survive server restarts (persistent)
- Background NATS listeners for TaskCompleted/TaskFailed

**vapora-a2a-client (A2A Client)**
- HTTP client library for A2A protocol
- Methods for discovery, dispatch, query
- **Exponential backoff retry** with jitter (100ms → 5s)
- Smart retry logic (5xx/network YES, 4xx NO)
- Timeout and error handling
- Full serialization support

### 3. Infrastructure Layer

**Kubernetes Deployment**
- StatefulSet-based deployment via Kustomize
- Environment-specific overlays (dev, prod)
- RBAC, resource quotas, anti-affinity
- ConfigMap-based A2A integration
- See: [ADR-0002: Kubernetes Deployment Strategy](adr/0002-kubernetes-deployment-strategy.md)

### 4. Integration Layer

**AgentCoordinator Integration**
- CoordinatorBridge maps A2A tasks to internal agents
- Task state management
- Background completion tracking

**Backend Integration**
- SurrealDB for persistent storage
- Multi-tenant scope isolation
- REST API endpoints

## Key Components

### A2A Protocol Types

| Type | Purpose | Location |
|------|---------|----------|
| `A2aTask` | Task request | protocol.rs |
| `A2aMessage` | Message with text/file parts | protocol.rs |
| `A2aTaskStatus` | Task state and result | protocol.rs |
| `A2aTaskResult` | Execution result with artifacts | protocol.rs |
| `AgentCard` | Agent capability advertisement | agent_card.rs |

### Error Handling

Two-layer strategy:
- **Domain Layer:** Type-safe Rust errors via `thiserror`
- **Protocol Layer:** JSON-RPC 2.0 error format
- See: [ADR-0003: Error Handling and JSON-RPC 2.0 Compliance](adr/0003-error-handling-and-json-rpc-compliance.md)

## Crate Dependencies

```
vapora-a2a (A2A Server)
├── vapora-agents
├── vapora-shared
├── axum
├── tokio
└── serde/serde_json

vapora-a2a-client (A2A Client)
├── vapora-a2a
├── vapora-shared
├── reqwest
├── tokio
└── serde/serde_json

vapora-mcp-server (MCP Transport)
├── vapora-agents
├── vapora-shared
├── axum
├── reqwest
└── serde/serde_json
```

## LLM Routing & Cost Management

**NEW: Two comprehensive guides for working with LLM providers (Claude, OpenAI, Gemini, Ollama)**

### For Developers

- **[LLM Provider Patterns](llm-provider-patterns.md)** — Four implementation approaches:
  1. **Mocks** — Zero-cost development without API subscriptions
  2. **SDK Direct** — Full integration with official APIs
  3. **Add Provider** — Extending VAPORA with custom providers
  4. **End-to-End** — Complete request-to-response flow

- **[LLM Provider Implementation Guide](llm-provider-implementation.md)** — How VAPORA implements it today:
  - LLMClient trait abstraction (Claude, OpenAI, Gemini, Ollama)
  - Hybrid routing engine (rules + dynamic + manual override)
  - Cost tracking & token accounting
  - Three-tier budget enforcement
  - Automatic fallback chains
  - Production code examples

### Key Insights

| Scenario | Pattern | Cost |
|----------|---------|------|
| Local development | Mocks | $0 |
| CI/integration tests | SDK + mocks | $0 |
| Staging/production | SDK real | $varies |
| Privacy-critical | Ollama local | $0 |
| Cost-optimized | Gemini + Ollama fallback | $0.005/1k |

**Start here**: [llm-provider-patterns.md](llm-provider-patterns.md) for patterns without subscriptions.

---

## Related Documentation

- [ADR Index](adr/README.md) - Architecture decision records
- [multi-ia-router.md](multi-ia-router.md) - Detailed LLM router specification
- Related ADRs:
  - [ADR-0007: Multi-Provider LLM Support](../adrs/0007-multi-provider-llm.md)
  - [ADR-0012: Three-Tier LLM Routing](../adrs/0012-llm-routing-tiers.md)
  - [ADR-0015: Budget Enforcement](../adrs/0015-budget-enforcement.md)
  - [ADR-0016: Cost Efficiency Ranking](../adrs/0016-cost-efficiency-ranking.md)