A2A Protocol Technical Documentation
Core Concepts
Core Concepts Overview

Core Concepts Overview

This document will help you understand the fundamental concepts and design philosophy of A2A Protocol.

Design Goals

A2A Protocol’s core objectives are:

  1. Standardized Communication

    • Unified agent communication protocol
    • Standardized message format definitions
    • Extensible protocol architecture

  2. High Interoperability

    • Cross-platform collaboration support
    • Compatibility with multiple agent frameworks
    • Unified capability description

  3. Enterprise Features

    • Secure and reliable communication
    • Flexible extension mechanisms
    • Comprehensive monitoring system

Basic Concepts

1. Agent

Agents are the core entities in the protocol with the following characteristics:

  • Unique Identifier: Each agent has a globally unique ID
  • Capability Description: Functionality described in standard format
  • State Management: Maintains agent runtime state
  • Lifecycle: Includes creation, startup, shutdown phases

Capability Description Example 

{
  "id": "chat-assistant-001",
  "name": "Customer Service Assistant",
  "version": "1.0.0",
  "capabilities": [
    {
      "type": "chat",
      "description": "Multi-turn dialogue support",
      "inputs": ["message"],
      "outputs": ["reply"]
    },
    {
      "type": "kb-search",
      "description": "Knowledge base search",
      "inputs": ["query"],
      "outputs": ["results"]
    }
  ],
  "metadata": {
    "vendor": "google",
    "model": "llm-v1",
    "language": ["en", "es"]
  }
}

2. Task

Tasks are specific work units executed by agents:

  • Task Type: Identifies how the task should be processed
  • Input/Output: Defines task data formats
  • Status Tracking: Records task execution progress
  • Timeout Control: Manages task execution time

Task Lifecycle 

  graph LR
    A[Created] --> B[Waiting]
    B --> C[Executing]
    C --> D[Completed]
    C --> E[Failed]
    B --> F[Cancelled]

3. Communication Model

A2A Protocol supports multiple communication modes:

Synchronous Communication 

  sequenceDiagram
    Client->>Agent: Request
    Agent->>Client: Response

Asynchronous Communication 

  sequenceDiagram
    Client->>Agent: Submit Task
    Agent-->>Client: Return Task ID
    Note over Agent: Process Task
    Agent->>Client: Push Result

Streaming 

  sequenceDiagram
    Client->>Agent: Establish Connection
    loop Data Stream
        Agent->>Client: Send Data Chunk
    end
    Agent->>Client: Completion Mark

4. Message Format

All communications use a standard message format:

{
  "type": "message",
  "version": "2025.1",
  "id": "msg-123",
  "timestamp": "2025-04-12T10:30:00Z",
  "source": "agent-001",
  "target": "agent-002",
  "payload": {
    "content": "Message content",
    "format": "text"
  },
  "metadata": {
    "trace_id": "trace-123",
    "priority": "normal"
  }
}

Architecture Design

1. Layered Architecture

A2A Protocol uses a four-layer architecture:

  1. Transport Layer

    • TCP/IP protocol
    • WebSocket protocol
    • HTTP/2 protocol

  2. Message Layer

    • Message encoding/decoding
    • Serialization/deserialization
    • Compression/decompression

  3. Protocol Layer

    • Message routing
    • Flow control
    • Error handling

  4. Application Layer

    • Business logic
    • State management
    • Plugin extensions

2. Plugin System

Extend functionality through plugins:

from a2a.plugin import Plugin

class CustomPlugin(Plugin):
    def on_message(self, message):
        # Process message
        pass
        
    def on_task(self, task):
        # Process task
        pass

Security Mechanisms

1. Authentication & Authorization

Supports multiple authentication methods:

  • API key authentication
  • OAuth2 authentication
  • Certificate authentication
  • Token authentication

2. Access Control

Based on RBAC model:

{
  "role": "operator",
  "permissions": [
    "agent:read",
    "task:create",
    "task:read"
  ],
  "resources": [
    "agent-group-1/*",
    "task-type-a/*"
  ]
}

3. Data Security

  • Transport encryption (TLS)
  • End-to-end encryption
  • Data masking
  • Audit logging

Best Practices

1. Agent Design

  • Single responsibility principle
  • Clear capability boundaries
  • Testable interfaces
  • Graceful degradation

2. Error Handling

  • Use standard error codes
  • Provide detailed error messages
  • Implement retry mechanisms
  • Log error information

3. Performance Optimization

  • Use connection pools
  • Enable compression
  • Implement local caching
  • Process requests asynchronously

Advanced Concepts

1. Agent Orchestration

Define orchestration logic using DSL:

agents:
  - id: parser
    type: text-parser
  - id: translator
    type: translator
  - id: formatter
    type: text-formatter

workflow:
  - name: process-text
    steps:
      - agent: parser
        action: parse
      - agent: translator
        action: translate
      - agent: formatter
        action: format

2. State Management

Agent state machine:

  stateDiagram-v2
    [*] --> Initialized
    Initialized --> Ready
    Ready --> Processing
    Processing --> Ready
    Processing --> Error
    Error --> Ready
    Ready --> Stopped
    Stopped --> [*]

Resources

💡 Learning Tips:

  1. Start with basic concepts
  2. Practice with examples
  3. Explore advanced topics
  4. Reference best practices
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