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package xyz.driver.core
import java.time.Instant
import java.util.concurrent.atomic.AtomicReference
import scala.concurrent.{ExecutionContext, Future, Promise}
import scala.concurrent.duration.Duration
/** A single-value asynchronous cache with TTL.
*
* Slightly adapted from
* [[https://github.com/twitter/util/blob/ae0ab09134414438af9dfaa88a4613cecbff4741/util-cache/src/main/scala/com/twitter/cache/Refresh.scala
* Twitter's "util" library]]
*
* Released under the Apache License 2.0.
*/
object Refresh {
/** Creates a function that will provide a cached value for a given time-to-live (TTL).
*
* It avoids the "thundering herd" problem if multiple requests arrive
* simultanously and the cached value has expired or is unset.
*
* Usage example:
* {{{
* def freshToken(): Future[String] = // expensive network call to get an access token
* val getToken: () => Future[String] = Refresh.every(1.hour)(freshToken())
*
* getToken() // new token is issued
* getToken() // subsequent calls use the cached token
* // wait 1 hour
* getToken() // new token is issued
* }}}
*
* @param ttl Time-To-Live duration to cache a computed value.
* @param compute Call-by-name operation that eventually computes a value to
* be cached. Note that if the computation (i.e. the future) fails, the value
* is not cached.
* @param ec The execution context in which valeu computations will be run.
* @return A zero-arg function that returns the cached value.
*/
def every[A](ttl: Duration)(compute: => Future[A])(implicit ec: ExecutionContext): () => Future[A] = {
val ref = new AtomicReference[(Future[A], Instant)](
(Future.failed(new NoSuchElementException("Cached value was never computed")), Instant.MIN)
)
def refresh(): Future[A] = {
val tuple = ref.get
val (cachedValue, lastRetrieved) = tuple
val now = Instant.now
if (now.getEpochSecond < lastRetrieved.getEpochSecond + ttl.toSeconds) {
cachedValue
} else {
val p = Promise[A]
val nextTuple = (p.future, now)
if (ref.compareAndSet(tuple, nextTuple)) {
compute.onComplete { done =>
if (done.isFailure) {
ref.set((p.future, lastRetrieved)) // don't update retrieval time in case of failure
}
p.complete(done)
}
}
refresh()
}
}
refresh _
}
}
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