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a2a-spring-boot-samples

Spring Boot samples demonstrating the Agent-to-Agent (A2A) Protocol for microagent coordination. An LLM-powered client (using Spring AI + ChatClient) handles natural language queries, intelligently routes them to specialized agents that communicate over the A2A Protocol, and maintains multi-turn conversation context.

Architecture Overview

┌─────────────────────────────────────────────────────────────────┐
│                      User (LLM Chat Interface)                  │
└────────────────────────────┬────────────────────────────────────┘
                             │ Natural Language Query
                             ↓
┌─────────────────────────────────────────────────────────────────┐
│              A2A Client (Port 8080, Spring AI)                  │
│  • ChatClient + Tool-calling for agent routing                 │
│  • Session-based conversation memory (20 message window)        │
│  • Google Gemini 2.5-flash-lite (default LLM)                  │
└────────────────────────────┬────────────────────────────────────┘
                             │ A2A Protocol (JSON-RPC)
                  ┌──────────┴──────────┐
                  ↓                     ↓
    ┌──────────────────────┐  ┌──────────────────────┐
    │ Order Agent (8081)   │  │ Delivery Agent (8082)│
    │ • Order List         │  │ • Track Delivery     │
    │ • Cancel Check       │  │ + Query Order Info   │
    │ (Parallel calls)     │  │                      │
    └──────────┬───────────┘  └──────────┬───────────┘
               │                         │
               ↓ A2A (ROLE_AGENT)        │
         ┌──────────────────┐            │
         │ Payment Agent    │ ←──────────┘
         │ (8083)           │
         │ • Refund Status  │
         │ (Internal only)  │
         └──────────────────┘

Module Structure & Ports

Module Port Responsibility
a2a-client 8080 Entry point: Spring AI ChatClient, LLM tool-calling, session-based chat
a2a-order-server 8081 Order management: list, cancellation eligibility check (with parallel delivery/payment calls)
a2a-delivery-server 8082 Shipping: delivery tracking, order info enrichment
a2a-payment-server 8083 Payment/refund status (internal agent-to-agent calls only)

Key Communication Patterns

A2A Protocol (Agent-to-Agent)

Agents communicate exclusively via A2A Protocol (JSON-RPC over HTTP):

  • External calls (Client → Agent): Message.Role.ROLE_USER with skill ID in message metadata
  • Internal calls (Agent → Agent): Message.Role.ROLE_AGENT with skill ID in message metadata
    • Each SkillExecutor declares skillId() and requiredRole() — the server routes by skill ID and enforces role-based access control
    • Skill ID is carried in message.metadata() under the key "skillId" (A2aMetadataKeys.SKILL_ID)

Supported JSON-RPC methods (POST /a2a):

Method Description
message/send Send a message; returns a task or message event
tasks/get Retrieve task by ID
tasks/cancel Cancel a task
tasks/list List tasks
tasks/pushNotification/create Register push notification config for a task
tasks/pushNotification/get Get push notification config
tasks/pushNotification/delete Delete push notification config

Streaming requests and unknown types return an UnsupportedOperationError. JSON-RPC error responses use HTTP 500.

Parallel Agent Coordination (Order Cancellation Check)

When checking order cancellation eligibility, Order Agent initiates concurrent calls:

  1. Delivery Agent (with tracking number)

    • Returns delivery status (배송중, 배송완료, etc.)
    • ✗ Blocks cancellation if shipping in progress or completed

  2. Payment Agent (with order number)

    • Returns refund eligibility (refundEligible:true/false)
    • ✗ Blocks cancellation if refund not allowed

Order Agent waits for both responses (timeout: a2a.client.timeout-seconds, default 15s) and combines results to determine final cancellation eligibility.

Session-Based Chat API

The client provides a stateful chat interface with conversation memory.

Endpoint: POST /chat

Request:
{
  "message": "ORD-1001 취소 가능해?",
  "sessionId": "optional-for-continuation",
  "memberId": "optional-user-context"
}

Response:
{
  "sessionId": "session-uuid",
  "response": "주문 ORD-1001은 취소 가능합니다..."
}

Same sessionId maintains multi-turn conversation context (up to 20 messages per session).

Quick Start

1. Set Environment Variables

export GOOGLE_API_KEY="your-google-ai-api-key"  # Required for client
# Optional:
export APP_CHAT_PROVIDER="google-genai"  # default
export GOOGLE_GENAI_MODEL="gemini-2.5-flash-lite"  # default

2. Run All Servers (in separate terminals)

# Terminal 1: Order Agent
./gradlew :a2a-server:a2a-order-server:bootRun

# Terminal 2: Delivery Agent
./gradlew :a2a-server:a2a-delivery-server:bootRun

# Terminal 3: Payment Agent
./gradlew :a2a-server:a2a-payment-server:bootRun

# Terminal 4: A2A Client (after other servers are ready)
./gradlew :a2a-client:bootRun

3. Chat Examples

# New session - Ask about order cancellation
curl -X POST http://localhost:8080/chat \
  -H "Content-Type: application/json" \
  -d '{
    "message": "ORD-1001 취소 가능해?",
    "memberId": "user123"
  }'

# Continuing same session - Track delivery
curl -X POST http://localhost:8080/chat \
  -H "Content-Type: application/json" \
  -d '{
    "message": "TRACK-1001 배송 어디쯤이야?",
    "sessionId": "<sessionId-from-previous-response>"
  }'

# Freeform query - Let LLM decide
curl -X POST http://localhost:8080/chat \
  -H "Content-Type: application/json" \
  -d '{
    "message": "주문 상태 확인하고 취소할 수 있으면 취소해줄 수 있어?"
  }'

Build & Test

# Build all modules
./gradlew build

# Run tests
./gradlew test

# Test specific module
./gradlew :a2a-client:test

# Validate JavaDoc (included in build)
./gradlew javadoc

Documentation

All modules include comprehensive English JavaDoc documentation:

  • Class-level documentation explaining purpose and role
  • Method documentation with @param and @return tags
  • Record documentation with parameter descriptions
  • Interface method documentation

View generated docs after build: build/docs/javadoc/index.html in each module.

Technology Stack

  • Java 17, Spring Boot 3.3.5, Gradle
  • Spring AI 1.1.2 — LLM integration via ChatClient (Google Gemini by default)
  • A2A Java SDK 1.0.0.Alpha3 — Agent-to-Agent protocol (JSON-RPC)
  • Gson 2.13.2 — JSON parsing
  • Lombok — boilerplate reduction