#queue #access #limit #async #value #number


limit the number of simultaneous accesses to a value

2 stable releases

1.1.0 May 25, 2020
1.0.0 May 25, 2020

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Used in 2 crates


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An access queue allows at most N concurrent accesses to guarded type. It is an async concurrency primitive intended to support certain backpressure patterns.

// This queue limits the number of simultaneous references to `inner` to 4
let queue = AccessQueue::new(inner, 4);

// get an inner reference
let inner1 = queue.access().await;

// get more (from other concurrent tasks)
let inner2 = queue.access().await;
let inner3 = queue.access().await;
let inner4 = queue.access().await;

// this access has to wait, because there are already 4 accesses ongoing
// (note: you should not call access multiple times from within the same
//  scope. this example is meant to simulate accessing from multiple
//  tasks concurrently)
let inner5 = queue.access().await;

When constructing an access queue, users set how many accesses are allowed to occur at the same time. Then, using the AccessQueue::access API, they get a future which resolves to a type that dereferences to the guarded value. This limits how many accesses can occur simultaneously, causing accesses beyond the limit to wait until previous accesses have completed.

Accesses are always processed in a first-in, first-out order. Accesses which are awaited first get access before accesses which are awaited later.

The normal AccessQueue::access API manages accesses in simple patterns, but more complex patterns are supported as well:

  • With AccessQueue::block and AccessQueue::release, you can augment the number of accesses without actually accessing the value, blocking some accesses or releasing more.
  • With AccessQueue::skip_queue you can access the inner value without waiting in the queue at all.
  • With AccessGuard::hold_indefinitely you can keep an access even after it drops from scope, never releasing it back to the queue.


AccessQueue is a concurrency primitive, but it does not allow mutable access to any value because it never guarantees that any access is exclusive. For this reason, AccessQueue is 100% safe coe, because its correctness properties do not have anything to do with memory safety.