Yaque: Yet Another QUEue

Yaque is yet another disk-backed persistent queue for Rust. It implements an SPSC channel using your OS' filesystem. Its main advantages over a simple VecDeque<T> are that

• You are not constrained by your RAM size, just by your disk size. This means you can store gigabytes of data without getting OOM killed.
• Your data is safe even if you program panics. All the queue state is written to the disk when the queue is dropped.
• Your data can persistence, that is, can exisit thrhough multiple executions of your program. Think of it as a very rudimentary kind of database.
• You can pass data between two processes.

Yaque is assynchronous and built directly on top of mio and notify. It is therefore completely agnostic to the runtime you are using for you application. It will work smoothly with tokio, with async-std or any other executor of your choice.

Sample usage

To create a new queue, just use the channel function, passing a directory path on which to mount the queue. If the directiory does not exist on creation, it (and possibly all its parent directories) will be created.

use yaque::channel;

let (mut sender, mut receiver) = channel("data/my-queue").await.unwrap();

You can also use Sender::open and Receiver::open to open only one half of the channel, if you need to.

The usage is similar to the MPSC channel in the standard library, except that the receiving method, Receiver::recv is assynchronous. Writing to the queue with the sender is basically lock-free and atomic.

sender.send(b"some data").unwrap();
let data = receiver.recv().await.unwrap();

assert_eq!(&*data, b"some data");

The returned value data is a kind of guard that implements Deref and DerefMut on the undelying type.

RecvGuard and transactional behavior

One important thing to notice is that reads from the queue are transactional. The Receiver::recv returns a RecvGuard that only commits the dequeing operation, that is, makes it official in the disk, when dropped. You can override this behavior using RecvGuard::rollback, although this will inccur in one more filesystem operation. If a thread panics while holding a RecvGuard, instead of commiting the dequeueing operation, it will try to rollback. If the rollback operation is unsuccessful, the operation will be commited (possibly with data loss), since to panic while panicking results in the process being aborted, which is Really Bad.

Batches

You can use the yaque queue to send and receive batches of data , too. The guarantees are the same as with single reads and writes, except that you may save on OS overhead when you send items, since only one disk operation is made. See Sender::send_batch, Receiver::recv_batch and Receiver::recv_while for more information on receiver batches.

Known issues and next steps

• This is a brand new project. Although I have tested it and it will certainly not implode your computer, don't trust your life on it yet.
• Wastes too much kernel time when the queue is small enough and the sender sends many frequent small messages non-atomically.
• I intend to make this an MPSC queue in the future.
• There are probably unknown bugs hidden in some corner case. If you find one, please fill an issue in GitHub. Pull requests and contributions are also greatly appreciated.

Licensing

This project is open source and licenced under the Apache 2.0 licence.