Chapter 5: Asynchronous Processing (AsyncNode, AsyncFlow)

In Chapter 4: Flow (Flow, AsyncFlow), we learned how Flow and AsyncFlow orchestrate sequences of Nodes (BaseNode, Node, AsyncNode) to create complete applications. Now, we’re going to zoom in on a powerful feature that AsyncFlow enables: Asynchronous Processing. This is key to building AI applications that feel responsive and can handle tasks that involve waiting, like calling web APIs or interacting with users.

The Problem: Waiting Can Be Wasteful!

Imagine you’re building an AI assistant that needs to:

  1. Ask the user for a city name.
  2. Fetch the current weather for that city from an online weather service (this involves a network request, which can take a few seconds).
  3. Tell the user the weather.

If we build this “synchronously” (one step strictly after the other, waiting for each to finish), your application would freeze while it’s waiting for the weather service. The user can’t do anything else; the app just hangs. This isn’t a great experience!

This is where asynchronous processing helps. It’s like a skilled chef in a busy kitchen.

  • A synchronous chef would prepare one dish from start to finish: chop vegetables, put it on the stove, wait for it to simmer, then plate it. Only after that one dish is completely done would they start the next. If simmering takes 20 minutes, they’re just standing there waiting!
  • An asynchronous chef is much more efficient! They can start chopping vegetables for dish A, put it on the stove to simmer, and while it’s simmering (a waiting period), they can start preparing dish B, or perhaps clean up. They don’t idly wait; they switch to other tasks that can be done.

PocketFlow’s AsyncNode and AsyncFlow let your AI application be like that efficient, asynchronous chef.

What is Asynchronous Processing?

Asynchronous processing allows your program to start a potentially long-running task (like an API call or waiting for user input) and then, instead of freezing and waiting for it to complete, it can switch to doing other work. When the long-running task eventually finishes, the program can pick up where it left off with that task.

This is especially crucial for I/O-bound tasks. “I/O” stands for Input/Output, like:

  • Reading/writing files from a disk.
  • Making requests over a network (e.g., to an LLM API, a database, or a web service).
  • Waiting for user input.

These tasks often involve waiting for something external to the program itself. Asynchronous processing ensures your application remains responsive and can handle multiple things (seemingly) at once, improving overall throughput and user experience.

In Python, this is often achieved using the async and await keywords.

  • async def is used to define an asynchronous function (also called a “coroutine”).
  • await is used inside an async function to pause its execution until an awaited task (another coroutine or an I/O operation) completes. While paused, other asynchronous tasks can run.

Meet AsyncNode: The Asynchronous Worker

In PocketFlow, an AsyncNode is a special type of Node (BaseNode, Node, AsyncNode) designed for asynchronous operations. It looks very similar to a regular Node, but its core methods (prep, exec, post) are defined as async functions:

  • async def prep_async(self, shared)
  • async def exec_async(self, prep_res)
  • async def post_async(self, shared, prep_res, exec_res)

Inside these methods, you can use await to call other asynchronous functions or perform non-blocking I/O operations.

Let’s create a simple AsyncNode that simulates fetching data from a website. We’ll use asyncio.sleep() to mimic the delay of a network request.

import asyncio
from pocketflow import AsyncNode

class WeatherFetcherNode(AsyncNode):
    async def prep_async(self, shared):
        city = shared.get("city_name", "Unknown city")
        print(f"WeatherFetcherNode: Preparing to fetch weather for {city}.")
        return city

    async def exec_async(self, city):
        print(f"WeatherFetcherNode: Calling weather API for {city}...")
        await asyncio.sleep(2) # Simulate a 2-second API call
        weather_data = f"Sunny in {city}"
        print(f"WeatherFetcherNode: Got weather: {weather_data}")
        return weather_data

    async def post_async(self, shared, prep_res, exec_res):
        shared["weather_report"] = exec_res
        print(f"WeatherFetcherNode: Weather report stored in shared.")
        return "done" # Action to signify completion

In this WeatherFetcherNode:

  • All methods are async def.
  • exec_async uses await asyncio.sleep(2) to pause for 2 seconds. If this were a real application, it might be await http_client.get(...). While this await is active, other asynchronous tasks in your program could run.

Orchestrating with AsyncFlow

To run AsyncNodes, you need an AsyncFlow. As we saw in Chapter 4: Flow (Flow, AsyncFlow), an AsyncFlow can manage both AsyncNodes and regular Nodes. When it encounters an AsyncNode, it will correctly await its asynchronous methods.

Let’s set up an AsyncFlow to use our WeatherFetcherNode.

1. Instantiate your Node(s):

weather_node = WeatherFetcherNode()
# You could have other nodes here, sync or async

2. (Optional) Define Transitions: If you have multiple nodes, you define transitions as usual. Since we only have one node, its returned action "done" will simply end this branch of the flow.

# Example: weather_node - "done" >> some_other_node
# For this example, we'll let it end.

3. Create the AsyncFlow:

from pocketflow import AsyncFlow

weather_flow = AsyncFlow(start=weather_node)

4. Run the AsyncFlow: Running an AsyncFlow requires await because the flow itself is an asynchronous operation. You’ll typically do this inside an async function.

# main.py
import asyncio

# Assume WeatherFetcherNode is defined as above
# Assume weather_flow is created as above

async def main():
    shared_data = {"city_name": "London"}
    print("Starting weather flow...")
    await weather_flow.run_async(shared_data) # Use run_async()
    print("Weather flow finished.")
    print(f"Final shared data: {shared_data}")

if __name__ == "__main__":
    asyncio.run(main()) # Standard way to run an async main function

Expected Output/Behavior:

When you run main.py:

  1. “Starting weather flow…” is printed.
  2. WeatherFetcherNode: Preparing to fetch weather for London. is printed.
  3. WeatherFetcherNode: Calling weather API for London... is printed.
  4. The program will pause here for about 2 seconds (due to await asyncio.sleep(2)). If other async tasks were scheduled, Python’s event loop could run them during this time.
  5. After 2 seconds:
    • WeatherFetcherNode: Got weather: Sunny in London is printed.
    • WeatherFetcherNode: Weather report stored in shared. is printed.
  6. “Weather flow finished.” is printed.
  7. Final shared data: {'city_name': 'London', 'weather_report': 'Sunny in London'} is printed.

The key is that during the 2-second “API call,” a well-structured asynchronous application wouldn’t be frozen. It could be handling other user requests, updating a UI, or performing other background tasks.

What Happens Under the Hood?

When an AsyncFlow runs an AsyncNode, it leverages Python’s asyncio event loop.

  1. AsyncFlow starts: You call await my_async_flow.run_async(shared).
  2. Node Execution: The AsyncFlow’s orchestrator (_orch_async) identifies the current node.
  3. Calling _run_async: If the current node is an AsyncNode (like our WeatherFetcherNode), the AsyncFlow calls await current_node._run_async(shared).
  4. Inside AsyncNode:
    • _run_async calls await self.prep_async(shared).
    • Then, await self._exec(prep_result) (which internally calls await self.exec_async(prep_result)).
    • Finally, await self.post_async(shared, prep_result, exec_result).
  5. The await Keyword: When an AsyncNode’s method encounters an await statement (e.g., await asyncio.sleep(2) or await some_api_call()), execution of that specific node’s task pauses. Control is yielded back to the asyncio event loop.
  6. Event Loop Magic: The event loop can then run other pending asynchronous tasks. It keeps track of the paused task.
  7. Task Resumes: When the awaited operation completes (e.g., asyncio.sleep(2) finishes, or the API responds), the event loop resumes the paused AsyncNode task from where it left off.
  8. Action and Next Node: The AsyncNode’s post_async eventually returns an action, and the AsyncFlow determines the next node, continuing the process.

Here’s a sequence diagram to visualize it:

Diving into PocketFlow’s Code (Simplified):

  • AsyncNode’s Execution (pocketflow/__init__.py): The AsyncNode has an _run_async method: 
    # Inside AsyncNode class
async def _run_async(self, shared):
    p = await self.prep_async(shared)
    e = await self._exec(p) # _exec calls exec_async with retries
    return await self.post_async(shared, p, e)

    And its _exec method handles calling exec_async (and retries, similar to Node but async):

    # Inside AsyncNode class (simplified _exec)
async def _exec(self, prep_res): 
    # ... (retry loop) ...
    try:
        return await self.exec_async(prep_res) # Key: await exec_async
    except Exception as e:
        # ... (fallback logic) ...
  • AsyncFlow’s Orchestration (pocketflow/__init__.py): The AsyncFlow has an _orch_async method that handles running nodes: 
    # Inside AsyncFlow class
async def _orch_async(self, shared, params=None):
    curr, p, last_action = self.start_node, (params or {}), None
    while curr:
        # ... (set params for current node) ...
        if isinstance(curr, AsyncNode):
            last_action = await curr._run_async(shared) # AWAIT AsyncNode
        else:
            last_action = curr._run(shared) # Run sync Node normally
        curr = self.get_next_node(curr, last_action)
    return last_action

    Notice how it checks if curr is an AsyncNode and uses await curr._run_async(shared) if it is. Otherwise, for regular synchronous Nodes (BaseNode, Node, AsyncNode), it calls curr._run(shared) directly.

Benefits of Asynchronous Processing

  1. Responsiveness: Your application doesn’t freeze while waiting for I/O tasks. This is vital for user interfaces or servers handling multiple requests.
  2. Improved Throughput: For applications with many I/O-bound tasks (e.g., making multiple API calls), asynchronous processing allows these tasks to overlap their waiting periods, leading to faster overall completion. Imagine our chef preparing multiple simmering dishes at once!
  3. Efficient Resource Usage: Threads can be resource-intensive. asyncio often uses a single thread more efficiently by switching between tasks during their I/O wait times.

Use AsyncNode and AsyncFlow when your workflow involves tasks that spend significant time waiting for external operations.

Conclusion

You’ve now unlocked the power of asynchronous processing in PocketFlow with AsyncNode and AsyncFlow!

  • Asynchronous operations prevent your application from freezing during I/O-bound tasks like API calls.
  • AsyncNode defines its logic with async def methods (prep_async, exec_async, post_async) and uses await for non-blocking waits.
  • AsyncFlow orchestrates these AsyncNodes (and regular Nodes) using await flow.run_async().
  • This approach leads to more responsive and efficient applications, especially when dealing with network requests or user interactions.

This “asynchronous chef” model is incredibly useful. What if you have many similar items to process, perhaps even asynchronously and in parallel? That’s where batch processing comes in.

Next up: Chapter 6: Batch Processing (BatchNode, BatchFlow, AsyncParallelBatchNode)

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