A2A Executor Patterns

Purpose: Provide production-ready executor patterns for implementing Agent-to-Agent (A2A) protocol task handlers with proper error handling, retry logic, and execution flows.

Activation Triggers:

• Implementing A2A protocol executors
• Building task handler functions
• Creating agent execution flows
• Managing task lifecycle and state
• Implementing retry and error recovery
• Building executor middleware
• Task validation and sanitization

Key Resources:

• templates/basic-executor.ts - Simple synchronous executor
• templates/basic-executor.py - Python synchronous executor
• templates/async-executor.ts - Asynchronous task executor
• templates/async-executor.py - Python async executor
• templates/streaming-executor.ts - Streaming result executor
• templates/streaming-executor.py - Python streaming executor
• templates/batch-executor.ts - Batch task processing
• scripts/validate-executor.sh - Validate executor implementation
• scripts/test-executor.sh - Test executor against A2A spec
• examples/ - Production executor implementations

Core Executor Patterns

1. Basic Executor (Synchronous)

When to use: Simple, fast tasks with immediate results

Template: templates/basic-executor.ts or templates/basic-executor.py

Pattern:

async function executeTask(task: A2ATask): Promise<A2AResult> {
  // 1. Validate input
  validateTask(task)

  // 2. Execute task
  const result = await processTask(task)

  // 3. Return result
  return {
    status: 'completed',
    result,
    taskId: task.id
  }
}

Best for: Quick operations, validation tasks, simple transformations

2. Async Executor (Long-Running)

When to use: Tasks that take time and need status updates

Template: templates/async-executor.ts or templates/async-executor.py

Pattern:

• Accept task and return task ID immediately
• Process task asynchronously
• Provide status endpoint
• Send completion callback

Best for: LLM inference, file processing, data analysis

3. Streaming Executor

When to use: Results should be delivered incrementally

Template: templates/streaming-executor.ts or templates/streaming-executor.py

Pattern:

• Open stream connection
• Send partial results as available
• Close stream on completion
• Handle backpressure

Best for: Text generation, real-time data, progressive results

4. Batch Executor

When to use: Processing multiple related tasks efficiently

Template: templates/batch-executor.ts

Pattern:

• Accept multiple tasks
• Group by similarity
• Process in parallel batches
• Return aggregated results

Best for: Bulk operations, parallel processing, resource optimization

Execution Flow Components

1. Task Validation

function validateTask(task: A2ATask): void {
  // Validate required fields
  if (!task.id) throw new ValidationError('Task ID required')
  if (!task.type) throw new ValidationError('Task type required')

  // Validate task parameters
  validateParameters(task.parameters)

  // Check executor capabilities
  if (!supportsTaskType(task.type)) {
    throw new UnsupportedTaskError(task.type)
  }
}

Purpose: Catch errors early, provide clear feedback

2. Error Handling

async function executeWithErrorHandling(task: A2ATask) {
  try {
    return await executeTask(task)
  } catch (error) {
    if (error instanceof ValidationError) {
      return { status: 'failed', error: error.message }
    }
    if (error instanceof RetryableError) {
      return scheduleRetry(task)
    }
    // Log and return generic error
    logger.error('Task execution failed', { taskId: task.id, error })
    return { status: 'failed', error: 'Internal error' }
  }
}

Error Types:

• ValidationError - Invalid input, don't retry
• RetryableError - Temporary failure, safe to retry
• FatalError - Permanent failure, abort

3. Retry Logic

const retryConfig = {
  maxAttempts: 3,
  backoff: 'exponential', // or 'linear', 'fixed'
  initialDelay: 1000, // ms
  maxDelay: 30000
}

async function executeWithRetry(
  task: A2ATask,
  attempt: number = 1
): Promise<A2AResult> {
  try {
    return await executeTask(task)
  } catch (error) {
    if (attempt >= retryConfig.maxAttempts) {
      throw new MaxRetriesExceededError(task.id)
    }

    if (error instanceof RetryableError) {
      const delay = calculateBackoff(attempt)
      await sleep(delay)
      return executeWithRetry(task, attempt + 1)
    }

    throw error
  }
}

Retry Strategies:

• Exponential backoff: delay = initialDelay * (2 ^ attempt)
• Linear backoff: delay = initialDelay * attempt
• Fixed delay: delay = initialDelay

4. Task State Management

interface TaskState {
  id: string
  status: 'pending' | 'running' | 'completed' | 'failed'
  result?: any
  error?: string
  startTime: Date
  endTime?: Date
  attempts: number
}

class TaskStore {
  private tasks = new Map<string, TaskState>()

  createTask(id: string): TaskState {
    const state: TaskState = {
      id,
      status: 'pending',
      startTime: new Date(),
      attempts: 0
    }
    this.tasks.set(id, state)
    return state
  }

  updateTask(id: string, update: Partial<TaskState>): void {
    const state = this.tasks.get(id)
    if (state) {
      Object.assign(state, update)
    }
  }

  getTask(id: string): TaskState | undefined {
    return this.tasks.get(id)
  }
}

Executor Middleware

1. Logging Middleware

function loggingMiddleware(
  executor: Executor
): Executor {
  return async (task) => {
    logger.info('Task started', { taskId: task.id })
    const start = Date.now()

    try {
      const result = await executor(task)
      const duration = Date.now() - start
      logger.info('Task completed', { taskId: task.id, duration })
      return result
    } catch (error) {
      const duration = Date.now() - start
      logger.error('Task failed', { taskId: task.id, duration, error })
      throw error
    }
  }
}

2. Metrics Middleware

function metricsMiddleware(
  executor: Executor
): Executor {
  return async (task) => {
    metrics.increment('tasks.started', { type: task.type })
    const start = Date.now()

    try {
      const result = await executor(task)
      const duration = Date.now() - start
      metrics.timing('tasks.duration', duration, { type: task.type })
      metrics.increment('tasks.completed', { type: task.type })
      return result
    } catch (error) {
      metrics.increment('tasks.failed', { type: task.type })
      throw error
    }
  }
}

3. Rate Limiting Middleware

function rateLimitMiddleware(
  executor: Executor,
  limit: { requests: number, window: number }
): Executor {
  const limiter = new RateLimiter(limit.requests, limit.window)

  return async (task) => {
    await limiter.acquire()
    try {
      return await executor(task)
    } finally {
      limiter.release()
    }
  }
}

Production Best Practices

1. Timeouts

async function executeWithTimeout(
  task: A2ATask,
  timeoutMs: number
): Promise<A2AResult> {
  return Promise.race([
    executeTask(task),
    new Promise((_, reject) =>
      setTimeout(() => reject(new TimeoutError()), timeoutMs)
    )
  ])
}

2. Resource Cleanup

async function executeWithCleanup(task: A2ATask) {
  const resources = []

  try {
    const resource = await allocateResource()
    resources.push(resource)

    return await executeTask(task, resource)
  } finally {
    // Always cleanup, even on error
    await Promise.all(
      resources.map(r => r.cleanup())
    )
  }
}

3. Graceful Shutdown

class GracefulExecutor {
  private activeTasks = new Set<string>()
  private shuttingDown = false

  async execute(task: A2ATask): Promise<A2AResult> {
    if (this.shuttingDown) {
      throw new Error('Executor is shutting down')
    }

    this.activeTasks.add(task.id)

    try {
      return await executeTask(task)
    } finally {
      this.activeTasks.delete(task.id)
    }
  }

  async shutdown(): Promise<void> {
    this.shuttingDown = true

    // Wait for active tasks to complete
    while (this.activeTasks.size > 0) {
      await sleep(100)
    }
  }
}

Common Executor Types

1. LLM Executor

Example: examples/llm-executor.ts

Executes LLM inference tasks with streaming

2. Function Executor

Example: examples/function-executor.ts

Calls functions/tools and returns results

3. Workflow Executor

Example: examples/workflow-executor.ts

Orchestrates multi-step workflows

4. Validation Executor

Example: examples/validation-executor.ts

Validates data and returns compliance results

Validation and Testing

Scripts:

• scripts/validate-executor.sh - Validate executor structure
• scripts/test-executor.sh - Test against A2A spec

Run validation:

bash scripts/validate-executor.sh your-executor.ts

Run tests:

bash scripts/test-executor.sh your-executor.ts

Resources

TypeScript Templates:

• basic-executor.ts - Simple sync executor
• async-executor.ts - Async with status tracking
• streaming-executor.ts - Streaming results
• batch-executor.ts - Batch processing

Python Templates:

• basic-executor.py - Simple sync executor
• async-executor.py - Async with status tracking
• streaming-executor.py - Streaming results

Scripts:

• validate-executor.sh - Structure validation
• test-executor.sh - A2A spec compliance

Examples:

• llm-executor.ts - LLM inference executor
• function-executor.ts - Function calling executor
• workflow-executor.ts - Multi-step workflows
• validation-executor.ts - Data validation

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Protocol Version: A2A Protocol v1.0 Runtime: Node.js 18+, Python 3.9+

Best Practice: Start with basic executor, add complexity (async, streaming, batching) only as needed