Sync or async?

You can use Pavex's attributes on all functions and methods, no matter if they are synchronous or asynchronous.

Be careful, though! Synchronous components will block the current thread until they execute to completion.

That's not a concern if you are performing an operation that's guaranteed to be fast (e.g. converting an error into a response). It becomes an issue if you're doing work that's potentially slow. In the wild, there are two main categories of "potentially slow" operations:

• Input/output (I/O) operations (e.g. reading from a file, querying a database, etc.)
• CPU-intensive computations (e.g. hashing a password, parsing a large file, etc.)

As a rule of thumb:

I/O	CPU-intensive	Function type	Notes
Yes	No	Async	Use async libraries for the I/O portion. If the I/O interface is synchronous, use tokio::task::spawn_blocking.
No	Yes	Async	Use tokio::task::spawn_blocking for the CPU-intensive portion.
Yes	Yes	Async	See above.
No	No	Sync	You can also make it asynchronous, it doesn't matter.

Check out Alice Rhyl's excellent article to learn more about what "blocking" means in the world of asynchronous Rust.