[Enhancement][A2A] Agent-to-Agent Gateway: REST API, SSE streaming, Task lifecycle, Java SDK

[qqeasonchen]

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Enhancement Request

Motivation

Apache EventMesh is evolving toward an Agent messaging bus. The A2A (Agent-to-Agent) protocol needs a runtime gateway that enables agents to discover each other, send tasks, and receive streaming responses — all through standard HTTP/SSE without requiring every agent to implement the EventMesh messaging protocol directly.

Current State

EventMesh already has basic A2A protocol plugin support (eventmesh-protocol-a2a) with AgentCard discovery and EnhancedA2AProtocolAdaptor. However, there is no:

• REST API gateway for external HTTP clients
• SSE (Server-Sent Events) streaming for real-time task status
• Task lifecycle management (create -> running -> completed/failed/cancelled)
• Typed Java SDK for agent developers
• Health check / task list / pagination endpoints

Describe the solution you'd like

Proposal

A2A Gateway Runtime (eventmesh-runtime)

Component	Description
A2AGatewayServer	Standalone HTTP server entry point
A2AGatewayService	Core gateway logic: task timeout, agent validation, cancel notification, concurrent-safe subscribers
A2AGatewayHttpHandler	Netty-based REST handler: CORS, SSE streaming, health check, pagination, precise error codes
TaskRegistry	In-memory task lifecycle with TTL cleanup, CopyOnWriteArrayList for concurrency
InMemoryA2AMessageTransport	Default transport implementation

A2A Protocol Plugin (eventmesh-protocol-a2a)

Component	Description
A2AClient	Typed Java SDK: sync/async task send, SSE streaming (streamTaskStatus), agent discovery, heartbeat
A2AMessageTransport	Transport abstraction interface
A2ATopicFactory	A2A topic convention standardization

REST API Endpoints

Method	Path	Description
POST	/a2a/tasks?mode=sync|async	Submit task
GET	/a2a/tasks/{taskId}	Query task status
GET	/a2a/tasks/{taskId}/stream	SSE streaming status
GET	/a2a/tasks	List tasks (paginated)
POST	/a2a/tasks/{taskId}/cancel	Cancel task
GET	/a2a/agents	List registered agents
GET	/a2a/agents/{agentName}/card	Get AgentCard
GET	/a2a/health	Health check

Key Engineering Decisions

• Task timeout: auto-fail non-terminal tasks after configurable threshold
• SSE: heartbeat mechanism + socketTimeout(0) for long-lived connections
• Concurrency: CopyOnWriteArrayList + ConcurrentHashMap throughout
• CORS: Access-Control-Allow-Origin on all A2A endpoints
• Agent validation: reject tasks for unregistered agents

Tests (all passing)

• A2AGatewayServiceTest: 15+ tests (timeout, agent validation, cancel, SSE, pagination)
• TaskRegistryTest: TTL, concurrent child creation, lifecycle transitions
• A2AClientServerIntegrationTest: full client-server integration (SSE, CORS, health, heartbeat)
• A2AGatewayEndToEndTest: end-to-end pipeline verification
• A2ATopicFactoryTest, InMemoryA2AMessageTransportTest

Documentation

7 docs updated: eventmesh-a2a-design.md, ARCHITECTURE.md, IMPLEMENTATION_SUMMARY.md (EN/CN), TEST_RESULTS.md, README.md (EN/CN)

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This issue tracks the A2A Gateway feature PR to upstream apache/eventmesh.

Are you willing to submit PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct *