async

Async, short for asynchronous programming, refers to a style where operations that may take time, especially input/output, are started and then allowed to run without blocking the main execution thread. The results are obtained later, often via callbacks, futures, promises, or coroutines. The primary goal is to improve scalability and responsiveness by avoiding idle waiting on slow I/O.

In many environments, asynchronous code is driven by an event loop or scheduler. When an asynchronous operation is initiated, the program yields control; other tasks are executed while the operation completes. When the operation finishes, the event loop resumes the awaiting task, delivering the result or error.

Common abstractions include callbacks, promises (or futures), and coroutines, frequently combined with an async/await syntax that lets code appear synchronous while remaining non-blocking. Different languages implement these concepts with their own libraries and runtimes, but the underlying model is similar: start work, yield, and resume upon completion.

Use cases include network requests, file and database I/O, and streaming data, where the latency of external systems would otherwise stall a thread. Asynchronous approaches are particularly effective for I/O-bound workloads; CPU-bound tasks may require parallelism via threads or processes, often in combination with asynchronous design.

Caveats include added complexity in error handling, cancellation, and ordering; debugging and testing asynchronous code can be more challenging; improper use may lead to race conditions or resource leaks.