1 files changed, 69 insertions, 0 deletions
diff --git a/core-util/src/main/scala/xyz/driver/core/Refresh.scala b/core-util/src/main/scala/xyz/driver/core/Refresh.scala
new file mode 100644
index 0000000..6db9c26
--- /dev/null
+++ b/core-util/src/main/scala/xyz/driver/core/Refresh.scala
@@ -0,0 +1,69 @@
+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 _
+  }
+
+}