# Yaque: Yet Another QUEue

Yaque is yet another disk-backed persistent queue (and mutex) 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 *persist*, that is, can exist through multiple executions
of your program. Think of it as a very rudimentary kind of database.
* You can pass data between two processes.

Yaque is _asynchronous_ 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 directory does not exist
on creation, it (and possibly all its parent directories) will be created.
```rust
use yaque::channel;

futures::executor::block_on(async {
    let (mut sender, mut receiver) = channel("data/my-queue").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 asynchronous. Writing to
the queue with the sender is basically lock-free and atomic.
```rust
use yaque::{channel, queue::try_clear};

futures::executor::block_on(async {
    // Open using the `channel` function or directly with the constructors.
    let (mut sender, mut receiver) = channel("data/my-queue").unwrap();
    
    // Send stuff with the sender...
    sender.send(b"some data").await.unwrap();

    // ... and receive it in the other side.
    let data = receiver.recv().await.unwrap();

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

    // Call this to make the changes to the queue permanent.
    // Not calling it will revert the state of the queue.
    data.commit();
});

// After everything is said and done, you may delete the queue.
// Use `clear` for awaiting for the queue to be released.
try_clear("data/my-queue").unwrap();
```
The returned value `data` is a kind of guard that implements `Deref` and
`DerefMut` on the underlying type.

## `queue::RecvGuard` and transactional behavior

One important thing to notice is that reads from the queue are
_transactional_. The `Receiver::recv` returns a `queue::RecvGuard` that acts as
a _dead man switch_. If dropped, it will revert the dequeue operation,
unless `queue::RecvGuard::commit` is explicitly called. This ensures that
the operation reverts on panics and early returns from errors (such as when
using the `?` notation). However, it is necessary to perform one more
filesystem operation while rolling back. During drop, this is done on a
"best effort" basis: if an error occurs, it is logged and ignored. This is done
because errors cannot propagate outside a drop and panics in drops risk the
program being aborted. If you _have_ any cleanup behavior for an error from
rolling back, you may call `queue::RecvGuard::rollback` which _will_ return the
underlying error.

## 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_until` for more information on receiver batches.

## Tired of `.await`ing? Timeouts are supported

If you need your application to not stall when nothing is being put on the
queue, you can use `Receiver::recv_timeout` and
`Receiver::recv_batch_timeout` to receive data, awaiting up to a
completion of a provided future, such as a delay or a channel. Here is an
example:
```rust
use yaque::channel;
use std::time::Duration;
use futures_timer::Delay;

futures::executor::block_on(async {
    let (mut sender, mut receiver) = channel("data/my-queue-2").unwrap();
    
    // receive some data up to a second
    let data = receiver
        .recv_timeout(Delay::new(Duration::from_secs(1)))
        .await
        .unwrap();

    // Nothing was sent, so no data...
    assert!(data.is_none());
    drop(data);
    
    // ... but if you do send something...
    sender.send(b"some data").await.unwrap();
 
    // ... now you receive something:
    let data = receiver
        .recv_timeout(Delay::new(Duration::from_secs(1)))
        .await
        .unwrap();

    assert_eq!(&*data.unwrap(), b"some data");  
});
```

## `Ctrl+C` and other unexpected events

First of all, "Don't panic©"! Writing to the queue is an atomic operation.
Therefore, unless there is something really wrong with your OS, you should be
fine in terms of data corruption most of the time.

In case of a panic (the program's, not the programmer's), the queue is
guaranteed to save all the most up-to-date metadata for the receiver. For
the reader it is even simpler: there is nothing to be saved in the first
place. The only exception to this guarantee is if the saving operation fails
due to an IO error. Remember that the program is not allowed to panic during
a panic. Therefore in this case, `yaque` will not attempt to recover from an
error.

The same thing cannot be said from OS signals. Signals from the OS are *not*
handled automatically by this library. It is understood that the application
programmer knows best how to handle them. If you chose to close queue on
`Ctrl+C` or other signals, you are in luck! Saving both sides of the queue
is [async-signal-safe](https://man7.org/linux/man-pages/man7/signal-safety.7.html)
so you may set up a bare signal hook directly using, for example,
`signal_hook`(https://docs.rs/signal-hook/), if you are the sort of person
that enjoys `unsafe` code. If not, there are a ton of completely safe
alternatives out there. Choose the one that suits you the best.

Unfortunately, there are also times when you get `aborted` or `killed`. These
signals cannot be handled by any library whatsoever. When this happens, not
everything is lost yet. We provied a whole module, `recovery`,
to aid you in automatic queue recovery. Please check the module for the
specific function names. From an architectural perspective, we offer two
different approaches to queue recovery, which may be suitable to different
use cases:

1. Recover with replay (the standard): we can reconstruct a _lower bound_
of the actual state of the queue during the crash, which consists of the
_maximum_ of the following two positions:
    * the bottom of the smallest segment still present in the directory.
    * the position indicated in the metadata file.

Since this is a lower bound, some elements may be replayed. If your
processing is _idempotent_, this will not be an issue and you lose no data
whatsoever.

2. Recover with loss: we can also reconstruct an _upper bound_ for the
actual state of the queue: the bottom of the second smallest segment in
the queue. In this case, the smallest segment is simply erased and the
receiver caries on as if nothing has happened. If replays are intollerable,
but some data loss is, this might be the right alternative for you. You can
limit data loss by constraining the segment size, configuring this option on
`SenderBuilder`.

If you really want to err on the safer side, you may use `Receiver::save`
to periodically back the receiver state up. Just choose you favorite timer
implementation and set a simple periodical task up every hundreds of milliseconds.
However, be warned that this is only a _mitigation_ of consistency problems, not
a solution.

## 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.~~ This code
is running in production for non-critical applications.
* Wastes too much kernel time when the queue is small enough and the sender
sends many frequent small messages non-atomically. You can mitigate that by
writing in batches to the queue.
* There are probably unknown bugs hidden in some corner case. If you find
one, please [fill an issue on GitHub](https://github.com/tokahuke/yaque/issues/new).
Pull requests and contributions are also greatly appreciated.