tower/balance/p2c/
service.rs

1use super::super::error;
2use crate::discover::{Change, Discover};
3use crate::load::Load;
4use crate::ready_cache::{error::Failed, ReadyCache};
5use crate::util::rng::{sample_floyd2, HasherRng, Rng};
6use futures_util::future::{self, TryFutureExt};
7use std::hash::Hash;
8use std::marker::PhantomData;
9use std::{
10    fmt,
11    pin::Pin,
12    task::{ready, Context, Poll},
13};
14use tower_service::Service;
15use tracing::{debug, trace};
16
17/// Efficiently distributes requests across an arbitrary number of services.
18///
19/// See the [module-level documentation](..) for details.
20///
21/// Note that [`Balance`] requires that the [`Discover`] you use is [`Unpin`] in order to implement
22/// [`Service`]. This is because it needs to be accessed from [`Service::poll_ready`], which takes
23/// `&mut self`. You can achieve this easily by wrapping your [`Discover`] in [`Box::pin`] before you
24/// construct the [`Balance`] instance. For more details, see [#319].
25///
26/// [`Box::pin`]: std::boxed::Box::pin()
27/// [#319]: https://github.com/tower-rs/tower/issues/319
28pub struct Balance<D, Req>
29where
30    D: Discover,
31    D::Key: Hash,
32{
33    discover: D,
34
35    services: ReadyCache<D::Key, D::Service, Req>,
36    ready_index: Option<usize>,
37
38    rng: Box<dyn Rng + Send + Sync>,
39
40    _req: PhantomData<Req>,
41}
42
43impl<D: Discover, Req> fmt::Debug for Balance<D, Req>
44where
45    D: fmt::Debug,
46    D::Key: Hash + fmt::Debug,
47    D::Service: fmt::Debug,
48{
49    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
50        f.debug_struct("Balance")
51            .field("discover", &self.discover)
52            .field("services", &self.services)
53            .finish()
54    }
55}
56
57impl<D, Req> Balance<D, Req>
58where
59    D: Discover,
60    D::Key: Hash,
61    D::Service: Service<Req>,
62    <D::Service as Service<Req>>::Error: Into<crate::BoxError>,
63{
64    /// Constructs a load balancer that uses operating system entropy.
65    pub fn new(discover: D) -> Self {
66        Self::from_rng(discover, HasherRng::default())
67    }
68
69    /// Constructs a load balancer seeded with the provided random number generator.
70    pub fn from_rng<R: Rng + Send + Sync + 'static>(discover: D, rng: R) -> Self {
71        let rng = Box::new(rng);
72        Self {
73            rng,
74            discover,
75            services: ReadyCache::default(),
76            ready_index: None,
77
78            _req: PhantomData,
79        }
80    }
81
82    /// Returns the number of endpoints currently tracked by the balancer.
83    pub fn len(&self) -> usize {
84        self.services.len()
85    }
86
87    /// Returns whether or not the balancer is empty.
88    pub fn is_empty(&self) -> bool {
89        self.services.is_empty()
90    }
91}
92
93impl<D, Req> Balance<D, Req>
94where
95    D: Discover + Unpin,
96    D::Key: Hash + Clone,
97    D::Error: Into<crate::BoxError>,
98    D::Service: Service<Req> + Load,
99    <D::Service as Load>::Metric: std::fmt::Debug,
100    <D::Service as Service<Req>>::Error: Into<crate::BoxError>,
101{
102    /// Polls `discover` for updates, adding new items to `not_ready`.
103    ///
104    /// Removals may alter the order of either `ready` or `not_ready`.
105    fn update_pending_from_discover(
106        &mut self,
107        cx: &mut Context<'_>,
108    ) -> Poll<Option<Result<(), error::Discover>>> {
109        debug!("updating from discover");
110        loop {
111            match ready!(Pin::new(&mut self.discover).poll_discover(cx))
112                .transpose()
113                .map_err(|e| error::Discover(e.into()))?
114            {
115                None => return Poll::Ready(None),
116                Some(Change::Remove(key)) => {
117                    trace!("remove");
118                    self.services.evict(&key);
119                }
120                Some(Change::Insert(key, svc)) => {
121                    trace!("insert");
122                    // If this service already existed in the set, it will be
123                    // replaced as the new one becomes ready.
124                    self.services.push(key, svc);
125                }
126            }
127        }
128    }
129
130    fn promote_pending_to_ready(&mut self, cx: &mut Context<'_>) {
131        loop {
132            match self.services.poll_pending(cx) {
133                Poll::Ready(Ok(())) => {
134                    // There are no remaining pending services.
135                    debug_assert_eq!(self.services.pending_len(), 0);
136                    break;
137                }
138                Poll::Pending => {
139                    // None of the pending services are ready.
140                    debug_assert!(self.services.pending_len() > 0);
141                    break;
142                }
143                Poll::Ready(Err(error)) => {
144                    // An individual service was lost; continue processing
145                    // pending services.
146                    debug!(%error, "dropping failed endpoint");
147                }
148            }
149        }
150        trace!(
151            ready = %self.services.ready_len(),
152            pending = %self.services.pending_len(),
153            "poll_unready"
154        );
155    }
156
157    /// Performs P2C on inner services to find a suitable endpoint.
158    fn p2c_ready_index(&mut self) -> Option<usize> {
159        match self.services.ready_len() {
160            0 => None,
161            1 => Some(0),
162            len => {
163                // Get two distinct random indexes (in a random order) and
164                // compare the loads of the service at each index.
165                let [aidx, bidx] = sample_floyd2(&mut self.rng, len as u64);
166                debug_assert_ne!(aidx, bidx, "random indices must be distinct");
167
168                let aload = self.ready_index_load(aidx as usize);
169                let bload = self.ready_index_load(bidx as usize);
170                let chosen = if aload <= bload { aidx } else { bidx };
171
172                trace!(
173                    a.index = aidx,
174                    a.load = ?aload,
175                    b.index = bidx,
176                    b.load = ?bload,
177                    chosen = if chosen == aidx { "a" } else { "b" },
178                    "p2c",
179                );
180                Some(chosen as usize)
181            }
182        }
183    }
184
185    /// Accesses a ready endpoint by index and returns its current load.
186    fn ready_index_load(&self, index: usize) -> <D::Service as Load>::Metric {
187        let (_, svc) = self.services.get_ready_index(index).expect("invalid index");
188        svc.load()
189    }
190}
191
192impl<D, Req> Service<Req> for Balance<D, Req>
193where
194    D: Discover + Unpin,
195    D::Key: Hash + Clone,
196    D::Error: Into<crate::BoxError>,
197    D::Service: Service<Req> + Load,
198    <D::Service as Load>::Metric: std::fmt::Debug,
199    <D::Service as Service<Req>>::Error: Into<crate::BoxError>,
200{
201    type Response = <D::Service as Service<Req>>::Response;
202    type Error = crate::BoxError;
203    type Future = future::MapErr<
204        <D::Service as Service<Req>>::Future,
205        fn(<D::Service as Service<Req>>::Error) -> crate::BoxError,
206    >;
207
208    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
209        // `ready_index` may have already been set by a prior invocation. These
210        // updates cannot disturb the order of existing ready services.
211        let _ = self.update_pending_from_discover(cx)?;
212        self.promote_pending_to_ready(cx);
213
214        loop {
215            // If a service has already been selected, ensure that it is ready.
216            // This ensures that the underlying service is ready immediately
217            // before a request is dispatched to it (i.e. in the same task
218            // invocation). If, e.g., a failure detector has changed the state
219            // of the service, it may be evicted from the ready set so that
220            // another service can be selected.
221            if let Some(index) = self.ready_index.take() {
222                match self.services.check_ready_index(cx, index) {
223                    Ok(true) => {
224                        // The service remains ready.
225                        self.ready_index = Some(index);
226                        return Poll::Ready(Ok(()));
227                    }
228                    Ok(false) => {
229                        // The service is no longer ready. Try to find a new one.
230                        trace!("ready service became unavailable");
231                    }
232                    Err(Failed(_, error)) => {
233                        // The ready endpoint failed, so log the error and try
234                        // to find a new one.
235                        debug!(%error, "endpoint failed");
236                    }
237                }
238            }
239
240            // Select a new service by comparing two at random and using the
241            // lesser-loaded service.
242            self.ready_index = self.p2c_ready_index();
243            if self.ready_index.is_none() {
244                debug_assert_eq!(self.services.ready_len(), 0);
245                // We have previously registered interest in updates from
246                // discover and pending services.
247                return Poll::Pending;
248            }
249        }
250    }
251
252    fn call(&mut self, request: Req) -> Self::Future {
253        let index = self.ready_index.take().expect("called before ready");
254        self.services
255            .call_ready_index(index, request)
256            .map_err(Into::into)
257    }
258}
tower/balance/p2c/service.rs

tower/balance/p2c/
service.rs
