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5. Starting the Server

Now that we have an Agent Card and an Agent Executor, we can set up and start the A2A server.

The A2A Python SDK provides an A2AStarletteApplication class that simplifies running an A2A-compliant HTTP server. It uses Starlette for the web framework and is typically run with an ASGI server like Uvicorn.

Server Setup in Helloworld

Let's look at examples/helloworld/__main__.py again to see how the server is initialized and started.

# examples/helloworld/__main__.py
from agent_executor import HelloWorldAgentExecutor

from a2a.server.apps import A2AStarletteApplication
from a2a.server.request_handlers import DefaultRequestHandler
from a2a.server.tasks import InMemoryTaskStore # For task state management
from a2a.types import (
    # ... other imports ...
    AgentCard,
    AgentSkill,
    AgentCapabilities,
    AgentAuthentication,
    # ...
)
import uvicorn

if __name__ == '__main__':
    # ... AgentSkill and AgentCard definition from previous steps ...
    skill = AgentSkill(
        id='hello_world',
        name='Returns hello world',
        description='just returns hello world',
        tags=['hello world'],
        examples=['hi', 'hello world'],
    )

    agent_card = AgentCard(
        name='Hello World Agent',
        description='Just a hello world agent',
        url='http://localhost:9999/',
        version='1.0.0',
        defaultInputModes=['text'],
        defaultOutputModes=['text'],
        capabilities=AgentCapabilities(),
        skills=[skill],
        authentication=AgentAuthentication(schemes=['public']),
    )

    # 1. Request Handler
    request_handler = DefaultRequestHandler(
        agent_executor=HelloWorldAgentExecutor(),
        task_store=InMemoryTaskStore(), # Provide a task store
    )

    # 2. A2A Starlette Application
    server_app_builder = A2AStarletteApplication(
        agent_card=agent_card, http_handler=request_handler
    )

    # 3. Start Server using Uvicorn
    uvicorn.run(server_app_builder.build(), host='0.0.0.0', port=9999)

Let's break this down:

  1. DefaultRequestHandler:

    • The SDK provides DefaultRequestHandler. This handler takes your AgentExecutor implementation (here, HelloWorldAgentExecutor) and a TaskStore (here, InMemoryTaskStore).
    • It routes incoming A2A RPC calls to the appropriate methods on your executor (like execute or cancel).
    • The TaskStore is used by the DefaultRequestHandler to manage the lifecycle of tasks, especially for stateful interactions, streaming, and resubscription. Even if your agent executor is simple, the handler needs a task store.

  2. A2AStarletteApplication:

    • The A2AStarletteApplication class is instantiated with the agent_card and the request_handler (referred to as http_handler in its constructor).
    • The agent_card is crucial because the server will expose it at the /.well-known/agent.json endpoint (by default).
    • The request_handler is responsible for processing all incoming A2A method calls by interacting with your AgentExecutor.

  3. uvicorn.run(server_app_builder.build(), ...):

    • The A2AStarletteApplication has a build() method that constructs the actual Starlette application.
    • This application is then run using uvicorn.run(), making your agent accessible over HTTP.
    • host='0.0.0.0' makes the server accessible on all network interfaces on your machine.
    • port=9999 specifies the port to listen on. This matches the url in the AgentCard.

Running the Helloworld Server

Navigate to the a2a-python directory in your terminal (if you're not already there) and ensure your virtual environment is activated.

To run the Helloworld server:

# from the a2a-python directory
python examples/helloworld/__main__.py

You should see output similar to this, indicating the server is running:

INFO:     Started server process [xxxxx]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:9999 (Press CTRL+C to quit)

Your A2A Helloworld agent is now live and listening for requests! In the next step, we'll interact with it.