Module arc_swap::docs::limitations

Limitations and common pitfalls.

Sized types

This currently works only for Sized types. Unsized types have „fat pointers“, which are twice as large as the normal ones. The AtomicPtr doesn’t support them. One could use something like AtomicU128 for them. The catch is this doesn’t exist and the difference would make it really hard to implement the debt storage/stripped down hazard pointers.

A workaround is to use double indirection:

// This doesn't work:
// let data: ArcSwap<[u8]> = ArcSwap::new(Arc::from([1, 2, 3]));

// But this does:
let data: ArcSwap<Box<[u8]>> = ArcSwap::from_pointee(Box::new([1, 2, 3]));

It also may be possible to use ArcSwap with the triomphe::ThinArc (that crate needs enabling a feature flag to cooperate with ArcSwap).

Too many Guards

There’s only limited number of “fast” slots for borrowing from ArcSwap for each single thread (currently 8, but this might change in future versions). If these run out, the algorithm falls back to slower path.

If too many Guards are kept around, the performance might be poor. These are not intended to be stored in data structures or used across async yield points.

No Clone implementation

Previous version implemented Clone, but it turned out to be very confusing to people, since it created fully independent ArcSwap. Users expected the instances to be tied to each other, that store in one would change the result of future load of the other.

To emulate the original behaviour, one can do something like this:

let new = ArcSwap::new(old.load_full());