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Set a cap on the number of services in pool (#291) 

This is very useful to avoid spinning up a million database connections for example (like I just did).
This commit is contained in:
Jon Gjengset 2019-05-31 15:12:01 -04:00 committed by GitHub
parent 82e7b8a27b
commit 313530c875
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1 changed files with 27 additions and 2 deletions
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29
tower-balance/src/pool.rs
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@ -40,6 +40,7 @@ where
target: Target,
load: Load,
services: usize,
limit: Option<usize>,
}
impl<MS, Target, Request> Discover for PoolDiscoverer<MS, Target, Request>
@ -61,6 +62,14 @@ where
if let Load::High = self.load {
if self.making.is_none() {
if self
.limit
.map(|limit| self.services >= limit)
.unwrap_or(false)
{
return Ok(Async::NotReady);
}
try_ready!(self.maker.poll_ready());
// TODO: it'd be great if we could avoid the clone here and use, say, &Target
self.making = Some(self.maker.make_service(self.target.clone()));
@ -105,6 +114,7 @@ pub struct Builder {
high: f64,
init: f64,
alpha: f64,
limit: Option<usize>,
}
impl Default for Builder {
@ -114,6 +124,7 @@ impl Default for Builder {
low: 0.00001,
high: 0.2,
alpha: 0.03,
limit: None,
}
}
}
@ -176,6 +187,17 @@ impl Builder {
self
}
/// The maximum number of backing `Service` instances to maintain.
///
/// When the limit is reached, the load estimate is clamped to the high load threshhold, and no
/// new service is spawned.
///
/// No maximum limit is imposed by default.
pub fn max_services(&mut self, limit: Option<usize>) -> &mut Self {
self.limit = limit;
self
}
/// See [`Pool::new`].
pub fn build<C, MS, Target, Request>(
&self,
@ -196,6 +218,7 @@ impl Builder {
target,
load: Load::Normal,
services: 0,
limit: self.limit,
};
Pool {
@ -276,8 +299,10 @@ where
if self.ewma > self.options.high {
self.balance.discover.load = Load::High;
// don't reset the EWMA -- in theory, poll_ready should now start returning
// `Ready`, so we won't try to launch another service immediately.
// don't reset the EWMA -- in theory, poll_ready should now start returning
// `Ready`, so we won't try to launch another service immediately.
// we clamp it to high though in case the # of services is limited.
self.ewma = self.options.high;
} else {
self.balance.discover.load = Load::Normal;
}