Sync vs. Async: When to Use Each

Async programming sounds great, but that doesn’t mean everything should be asynchronous.

The distinction becomes clearer when we think about I/O-bound and CPU-bound work.

I/O-bound operations spend a significant amount of their time waiting for something outside the CPU: network requests, database calls, file operations and similar tasks. These are often excellent candidates for async programming because there is useful time to reclaim while the application waits.

CPU-bound operations are different. If you’re performing a heavy calculation, for example, the CPU is actively doing the work rather than waiting for something else to happen. Making that code asynchronous doesn’t suddenly make the computation faster.

CPU-bound work may instead benefit from parallelism, where computation is divided so that multiple pieces of work can execute at the same time, potentially across multiple CPU cores.

Async programming can also introduce additional complexity and some runtime overhead, so there’s little benefit in making code asynchronous simply for the sake of it. It is most useful when your application would otherwise spend meaningful time blocked waiting for I/O.