tower/buffer/
service.rs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
use super::{
    future::ResponseFuture,
    message::Message,
    worker::{Handle, Worker},
};

use futures_core::ready;
use std::sync::Arc;
use std::task::{Context, Poll};
use tokio::sync::{mpsc, oneshot, OwnedSemaphorePermit, Semaphore};
use tokio_util::sync::PollSemaphore;
use tower_service::Service;

/// Adds an mpsc buffer in front of an inner service.
///
/// See the module documentation for more details.
#[derive(Debug)]
pub struct Buffer<T, Request>
where
    T: Service<Request>,
{
    // Note: this actually _is_ bounded, but rather than using Tokio's bounded
    // channel, we use Tokio's semaphore separately to implement the bound.
    tx: mpsc::UnboundedSender<Message<Request, T::Future>>,
    // When the buffer's channel is full, we want to exert backpressure in
    // `poll_ready`, so that callers such as load balancers could choose to call
    // another service rather than waiting for buffer capacity.
    //
    // Unfortunately, this can't be done easily using Tokio's bounded MPSC
    // channel, because it doesn't expose a polling-based interface, only an
    // `async fn ready`, which borrows the sender. Therefore, we implement our
    // own bounded MPSC on top of the unbounded channel, using a semaphore to
    // limit how many items are in the channel.
    semaphore: PollSemaphore,
    // The current semaphore permit, if one has been acquired.
    //
    // This is acquired in `poll_ready` and taken in `call`.
    permit: Option<OwnedSemaphorePermit>,
    handle: Handle,
}

impl<T, Request> Buffer<T, Request>
where
    T: Service<Request>,
    T::Error: Into<crate::BoxError>,
{
    /// Creates a new [`Buffer`] wrapping `service`.
    ///
    /// `bound` gives the maximal number of requests that can be queued for the service before
    /// backpressure is applied to callers.
    ///
    /// The default Tokio executor is used to run the given service, which means that this method
    /// must be called while on the Tokio runtime.
    ///
    /// # A note on choosing a `bound`
    ///
    /// When [`Buffer`]'s implementation of [`poll_ready`] returns [`Poll::Ready`], it reserves a
    /// slot in the channel for the forthcoming [`call`]. However, if this call doesn't arrive,
    /// this reserved slot may be held up for a long time. As a result, it's advisable to set
    /// `bound` to be at least the maximum number of concurrent requests the [`Buffer`] will see.
    /// If you do not, all the slots in the buffer may be held up by futures that have just called
    /// [`poll_ready`] but will not issue a [`call`], which prevents other senders from issuing new
    /// requests.
    ///
    /// [`Poll::Ready`]: std::task::Poll::Ready
    /// [`call`]: crate::Service::call
    /// [`poll_ready`]: crate::Service::poll_ready
    pub fn new(service: T, bound: usize) -> Self
    where
        T: Send + 'static,
        T::Future: Send,
        T::Error: Send + Sync,
        Request: Send + 'static,
    {
        let (service, worker) = Self::pair(service, bound);
        tokio::spawn(worker);
        service
    }

    /// Creates a new [`Buffer`] wrapping `service`, but returns the background worker.
    ///
    /// This is useful if you do not want to spawn directly onto the tokio runtime
    /// but instead want to use your own executor. This will return the [`Buffer`] and
    /// the background `Worker` that you can then spawn.
    pub fn pair(service: T, bound: usize) -> (Buffer<T, Request>, Worker<T, Request>)
    where
        T: Send + 'static,
        T::Error: Send + Sync,
        Request: Send + 'static,
    {
        let (tx, rx) = mpsc::unbounded_channel();
        let semaphore = Arc::new(Semaphore::new(bound));
        let (handle, worker) = Worker::new(service, rx, &semaphore);
        let buffer = Buffer {
            tx,
            handle,
            semaphore: PollSemaphore::new(semaphore),
            permit: None,
        };
        (buffer, worker)
    }

    fn get_worker_error(&self) -> crate::BoxError {
        self.handle.get_error_on_closed()
    }
}

impl<T, Request> Service<Request> for Buffer<T, Request>
where
    T: Service<Request>,
    T::Error: Into<crate::BoxError>,
{
    type Response = T::Response;
    type Error = crate::BoxError;
    type Future = ResponseFuture<T::Future>;

    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        // First, check if the worker is still alive.
        if self.tx.is_closed() {
            // If the inner service has errored, then we error here.
            return Poll::Ready(Err(self.get_worker_error()));
        }

        // Then, check if we've already acquired a permit.
        if self.permit.is_some() {
            // We've already reserved capacity to send a request. We're ready!
            return Poll::Ready(Ok(()));
        }

        // Finally, if we haven't already acquired a permit, poll the semaphore
        // to acquire one. If we acquire a permit, then there's enough buffer
        // capacity to send a new request. Otherwise, we need to wait for
        // capacity.
        let permit =
            ready!(self.semaphore.poll_acquire(cx)).ok_or_else(|| self.get_worker_error())?;
        self.permit = Some(permit);

        Poll::Ready(Ok(()))
    }

    fn call(&mut self, request: Request) -> Self::Future {
        tracing::trace!("sending request to buffer worker");
        let _permit = self
            .permit
            .take()
            .expect("buffer full; poll_ready must be called first");

        // get the current Span so that we can explicitly propagate it to the worker
        // if we didn't do this, events on the worker related to this span wouldn't be counted
        // towards that span since the worker would have no way of entering it.
        let span = tracing::Span::current();

        // If we've made it here, then a semaphore permit has already been
        // acquired, so we can freely allocate a oneshot.
        let (tx, rx) = oneshot::channel();

        match self.tx.send(Message {
            request,
            span,
            tx,
            _permit,
        }) {
            Err(_) => ResponseFuture::failed(self.get_worker_error()),
            Ok(_) => ResponseFuture::new(rx),
        }
    }
}

impl<T, Request> Clone for Buffer<T, Request>
where
    T: Service<Request>,
{
    fn clone(&self) -> Self {
        Self {
            tx: self.tx.clone(),
            handle: self.handle.clone(),
            semaphore: self.semaphore.clone(),
            // The new clone hasn't acquired a permit yet. It will when it's
            // next polled ready.
            permit: None,
        }
    }
}