Understanding Async Context - James Snell, Node Congress 2024

Learn how to seamlessly store and retrieve values across promises in your Node.js applications with AsyncLocalStorage, a centralized storage mechanism built on top ofExecutionContext and async_hooks APIs.

Key takeaways

AsyncLocalStorage is a centralized storage mechanism for Node.js, allowing to store and retrieve values across promises.
It is built on top of the ExecutionContext API and uses the async_hooks module.
When a promise is created, a new frame is created, which is a copy of the current frame.
The frame is immutable and is reset to a new instance every time ALS run is called.
The runtime storage frame is used to store the current frame, which is used to retrieve the current value of an async local storage instance.
AsyncLocalStorage uses a copy-on-write strategy to avoid modifying the original frame.
The async local storage instance itself is a map that stores key-value pairs.
When a new value is set, a new frame is created with a copy of the current frame and the new value.
Each call to ALS run sets the current frame to the new frame.
When a task is run, its captured frame is temporarily set as the current frame.
The async local storage instance can be used to store and retrieve values across promises.
AsyncLocalStorage is a drop-in replacement for existing code, and will work with most promise chains.
The async local storage instance uses a map to store key-value pairs.
The initial frame is empty, and each call to ALS run creates a new frame and sets it as the current frame.
The async local storage instance uses a copy-on-write strategy to avoid modifying the original frame.
When a promise is created, a new task is created that captures a reference to the current frame.

Understanding Async Context - James Snell, Node Congress 2024

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