7. Streaming & Multi-Turn Interactions (LangGraph Example)

The Helloworld example demonstrates the basic mechanics of A2A. For more advanced features like robust streaming, task state management, and multi-turn conversations powered by an LLM, we'll turn to the LangGraph example located in a2a-samples/samples/python/agents/langgraph/.

This example features a "Currency Agent" that uses the Gemini model via LangChain and LangGraph to answer currency conversion questions.

Setting up the LangGraph Example

1. Create a Gemini API Key, if you don't already have one.

2. Environment Variable:

   Create a .env file in the a2a-samples/samples/python/agents/langgraph/ directory:

   echo "GOOGLE_API_KEY=YOUR_API_KEY_HERE" > .env
   

   Replace YOUR_API_KEY_HERE with your actual Gemini API key.

3. Install Dependencies (if not already covered):

   The langgraph example has its own pyproject.toml which includes dependencies like langchain-google-genai and langgraph. When you installed the SDK from the a2a-samples root using pip install -e .[dev], this should have also installed the dependencies for the workspace examples, including langgraph-example. If you encounter import errors, ensure your primary SDK installation from the root directory was successful.

Running the LangGraph Server

Navigate to the a2a-samples/samples/python/agents/langgraph/ directory in your terminal and ensure your virtual environment (from the SDK root) is activated.

Start the LangGraph agent server:

python __main__.py

This will start the server, usually on http://localhost:10000.

Interacting with the LangGraph Agent

Open a new terminal window, activate your virtual environment, and navigate to a2a-samples/samples/python/agents/langgraph/.

Run its test client:

python test_client.py

Now, you can shut down the server by typing Ctrl+C in the terminal window where __main__.py is running.

Key Features Demonstrated

The langgraph example showcases several important A2A concepts:

1. LLM Integration:

   • agent.py defines CurrencyAgent. It uses ChatGoogleGenerativeAI and LangGraph's create_react_agent to process user queries.
   • This demonstrates how a real LLM can power the agent's logic.

2. Task State Management:

   • samples/langgraph/__main__.py initializes a DefaultRequestHandler with an InMemoryTaskStore.

     httpx_client = httpx.AsyncClient()
     request_handler = DefaultRequestHandler(
         agent_executor=CurrencyAgentExecutor(),
         task_store=InMemoryTaskStore(),
         push_notifier=InMemoryPushNotifier(httpx_client),
     )
     server = A2AStarletteApplication(
         agent_card=agent_card, http_handler=request_handler
     )
     
     uvicorn.run(server.build(), host=host, port=port)
     

   • The CurrencyAgentExecutor (in samples/langgraph/agent_executor.py), when its execute method is called by the DefaultRequestHandler, interacts with the RequestContext which contains the current task (if any).

   • For message/send, the DefaultRequestHandler uses the TaskStore to persist and retrieve task state across interactions. The response to message/send will be a full Task object if the agent's execution flow involves multiple steps or results in a persistent task.
   • The test_client.py's run_single_turn_test demonstrates getting a Task object back and then querying it using get_task.

3. Streaming with TaskStatusUpdateEvent and TaskArtifactUpdateEvent:

   • The execute method in CurrencyAgentExecutor is responsible for handling both non-streaming and streaming requests, orchestrated by the DefaultRequestHandler.
   • As the LangGraph agent processes the request (which might involve calling tools like get_exchange_rate), the CurrencyAgentExecutor enqueues different types of events onto the EventQueue:
     • TaskStatusUpdateEvent: For intermediate updates (e.g., "Looking up exchange rates...", "Processing the exchange rates.."). The final flag on these events is False.
     • TaskArtifactUpdateEvent: When the final answer is ready, it's enqueued as an artifact. The lastChunk flag is True.
     • A final TaskStatusUpdateEvent with state=TaskState.completed and final=True is sent to signify the end of the task for streaming.
   • The test_client.py's run_streaming_test function will print these individual event chunks as they are received from the server.

4. Multi-Turn Conversation (TaskState.input_required):

   • The CurrencyAgent can ask for clarification if a query is ambiguous (e.g., user asks "how much is 100 USD?").
   • When this happens, the CurrencyAgentExecutor will enqueue a TaskStatusUpdateEvent where status.state is TaskState.input_required and status.message contains the agent's question (e.g., "To which currency would you like to convert?"). This event will have final=True for the current interaction stream.
   • The test_client.py's run_multi_turn_test function demonstrates this:
     • It sends an initial ambiguous query.
     • The agent responds (via the DefaultRequestHandler processing the enqueued events) with a Task whose status is input_required.
     • The client then sends a second message, including the taskId and contextId from the first turn's Task response, to provide the missing information ("in GBP"). This continues the same task.

Exploring the Code

Take some time to look through these files:

• __main__.py: Server setup using A2AStarletteApplication and DefaultRequestHandler. Note the AgentCard definition includes capabilities.streaming=True.
• agent.py: The CurrencyAgent with LangGraph, LLM model, and tool definitions.
• agent_executor.py: The CurrencyAgentExecutor implementing the execute (and cancel) method. It uses the RequestContext to understand the ongoing task and the EventQueue to send back various events (TaskStatusUpdateEvent, TaskArtifactUpdateEvent, new Task object implicitly via the first event if no task exists).
• test_client.py: Demonstrates various interaction patterns, including retrieving task IDs and using them for multi-turn conversations.

This example provides a much richer illustration of how A2A facilitates complex, stateful, and asynchronous interactions between agents.