Bun: Async Local Storage Issue With Fetch API

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Bun's Async Local Storage Issue with Fetch API: A Deep Dive

Hey guys! Today, we're diving deep into a fascinating issue encountered while using Bun, the blazing-fast JavaScript runtime. Specifically, we're going to explore why Async Local Storage (ALS) works seamlessly with Node.js's built-in HTTP module (node:http) but seems to stumble when used with the Fetch API within Bun.

Understanding the Problem

So, what's the buzz all about? Imagine you're building a cool web application, and you need to maintain context across asynchronous operations. This is where Async Local Storage comes to the rescue! It allows you to store data that's local to the current execution context, making it super handy for things like request tracing, authentication, and more.

The problem arises when you try to use ALS with Bun's fetch implementation. Some developers, like our friend Rossrobino, have noticed that code that works perfectly fine with node:http throws a ReferenceError when running with fetch. This error typically indicates that the context isn't being properly propagated, leaving you scratching your head.

The Technical Details

Let's break down the error message Rossrobino encountered:

ReferenceError: Context can only be obtained within a handler.
  at get (/Users/rossrobino/Projects/bun-async-repro/node_modules/ovr/dist/app/context.js:280:23)
  at Component (/Users/rossrobino/Projects/bun-async-repro/dist/server/app.js:5:21)
  at jsx (/Users/rossrobino/Projects/bun-async-repro/node_modules/ovr/dist/jsx/index.js:38:28)
  ...

This error essentially means that the code is trying to access the ALS context outside of a valid execution context. In Rossrobino's case, the Context.get() method, which is part of the ovr library, is failing because it can't find the expected context. The key takeaway here is that while node:http seems to play nicely with ALS, Bun's fetch might be a different ballgame.

To really get our hands dirty, let's delve into the provided reproduction repository. Rossrobino created a minimal example that showcases the issue perfectly. By running bun run:node, you can see the code working flawlessly with node:http. However, switch to bun run:fetch, and you'll encounter the dreaded ReferenceError.

The core of the problem lies in how Bun handles the execution context when using fetch. It appears that the context isn't being correctly propagated across asynchronous operations, leading to the ALS store being unavailable when needed.

Reproducing the Bug

To reproduce this bug, you can follow these steps:

  1. Clone the reproduction repository: git clone https://github.com/rossrobino/bun-async-repro
  2. Navigate to the project directory: cd bun-async-repro
  3. Install dependencies: bun install
  4. Run the Node.js HTTP server: bun run:node (This should work without issues)
  5. Run the Fetch API server: bun run:fetch (This will throw the error)

By following these steps, you'll be able to witness the discrepancy between node:http and fetch in Bun when it comes to Async Local Storage.

Why This Matters

So, why should you care about this? Well, ALS is a powerful tool for building robust and maintainable applications. If you're planning to use Bun's fetch API and rely on ALS for managing context, you might run into unexpected issues. This can lead to headaches during development and potentially impact the stability of your application.

Imagine you're building a microservices architecture, and you want to trace requests across different services. ALS could be your best friend for this, allowing you to easily propagate request IDs and other contextual information. However, if Bun's fetch doesn't play along, you might need to find alternative solutions or workarounds.

Furthermore, this issue highlights the importance of thorough testing and understanding the nuances of different runtime environments. Code that works perfectly in Node.js might not behave the same way in Bun, and vice versa. It's crucial to be aware of these differences and plan your development accordingly.

Diving Deeper: Potential Causes and Solutions

Now that we understand the problem, let's put on our detective hats and explore potential causes and solutions. Why is ALS behaving differently with fetch in Bun compared to node:http?

Potential Culprits

  1. Context Propagation in Bun's Fetch Implementation: The most likely culprit is how Bun's fetch implementation handles context propagation. It's possible that the context isn't being correctly passed along during asynchronous operations, leading to the ALS store becoming unavailable.
  2. Differences in Event Loop Handling: Bun and Node.js might have subtle differences in how they handle the event loop. These differences could potentially affect how ALS contexts are managed and accessed.
  3. Underlying Implementation of Async Local Storage: The underlying implementation of ALS in Bun might have subtle differences compared to Node.js. These differences could lead to compatibility issues in certain scenarios.

Possible Solutions and Workarounds

  1. Investigate Bun's Fetch Implementation: The Bun team needs to investigate how fetch handles context propagation and identify any potential bugs or areas for improvement. This might involve diving deep into the source code and understanding the intricacies of the implementation.
  2. Explore Alternative Context Management Techniques: If ALS isn't working reliably with Bun's fetch, developers might need to explore alternative context management techniques. This could involve manually passing context around or using other libraries or frameworks that provide similar functionality.
  3. Contribute to Bun's Development: The Bun community is known for its active involvement in the project's development. If you're passionate about solving this issue, consider contributing to Bun by submitting bug reports, suggesting solutions, or even contributing code.

A Real-World Analogy

Let's try to understand this issue with a real-world analogy. Imagine you're working in a busy restaurant. ALS is like a shared notepad that every waiter has access to. When a customer places an order (a request), the waiter writes down the order details on the notepad (ALS context). This notepad is then passed along to the kitchen staff, the bartender, and anyone else involved in fulfilling the order. This ensures that everyone has access to the same information throughout the process.

Now, imagine that in Bun's fetch scenario, the notepad sometimes gets lost or misplaced when it's passed between different people (asynchronous operations). This means that the kitchen staff might not have access to the order details, leading to confusion and errors. In contrast, node:http is like a well-oiled machine where the notepad is always passed along correctly, ensuring that everyone is on the same page.

The Importance of Community and Collaboration

This issue highlights the importance of community and collaboration in the open-source world. When developers encounter bugs or unexpected behavior, it's crucial to share their findings, create reproducible examples, and engage in discussions with the community. This helps to identify the root cause of the problem and work towards a solution.

Rossrobino's effort in creating a reproduction repository is a perfect example of this. By providing a clear and concise example, they've made it much easier for others to understand the issue and contribute to the solution. This collaborative approach is what makes the open-source ecosystem so powerful.

Wrapping Up

In conclusion, the Async Local Storage issue with Bun's fetch API is a fascinating problem that highlights the complexities of asynchronous programming and runtime environments. While it might seem like a niche issue, it has implications for developers who rely on ALS for managing context in their applications.

By understanding the problem, exploring potential causes, and collaborating with the community, we can work towards a solution that makes Bun an even more robust and reliable platform for building modern web applications. So, keep experimenting, keep exploring, and keep pushing the boundaries of what's possible!

Stay tuned for updates on this issue as the Bun team and the community continue to investigate and work towards a fix. And remember, if you encounter any similar issues, don't hesitate to share your findings and contribute to the collective knowledge of the community. Happy coding, guys!