Retries

README.md

@@ -315,7 +315,6 @@ There are some helper methods which might be useful when writing forked code:
 
 * `forever { ... }` repeatedly evaluates the given code block forever
 * `repeatWhile { ... }` repeatedly evaluates the given code block, as long as it returns `true`
-* `retry(times, sleep) { ... }` retries the given block up to the given number of times
 * `uninterruptible { ... }` evaluates the given code block making sure it can't be interrupted
 
 ## Syntax
@@ -634,6 +633,10 @@ Please see [the respective ADR](doc/adr/0001-error-propagation-in-channels.md) f
 Channels are back-pressured, as the `.send` operation is blocking until there's a receiver thread available, or if 
 there's enough space in the buffer. The processing space is bound by the total size of channel buffers.
 
+## Retries
+
+The retries mechanism allows to retry a failing operation according to a given policy (e.g. retry 3 times with a 100ms delay between attempts). See the [full docs](doc/retries.md) for details.
+
 ## Kafka sources & drains
 
 Dependency:

core/src/main/scala/ox/control.scala

@@ -1,7 +1,5 @@
 package ox
 
-import scala.concurrent.duration.FiniteDuration
-
 def forever(f: => Unit): Nothing =
   while true do f
   throw new RuntimeException("can't get here")
@@ -16,25 +14,6 @@ def repeatUntil(f: => Boolean): Unit =
   var loop = true
   while loop do loop = !f
 
-// TODO: retry schedules
-def retry[T](times: Int, sleep: FiniteDuration)(f: => T): T =
-  try f
-  catch
-    case e: Throwable =>
-      if times > 0
-      then
-        Thread.sleep(sleep.toMillis)
-        retry(times - 1, sleep)(f)
-      else throw e
-
-def retryEither[E, T](times: Int, sleep: FiniteDuration)(f: => Either[E, T]): Either[E, T] =
-  f match
-    case r: Right[E, T] => r
-    case Left(_) if times > 0 =>
-      Thread.sleep(sleep.toMillis)
-      retry(times - 1, sleep)(f)
-    case l: Left[E, T] => l
-
 def uninterruptible[T](f: => T): T =
   scoped {
     val t = fork(f)

core/src/main/scala/ox/retry/Jitter.scala

@@ -0,0 +1,25 @@
+package ox.retry
+
+/** A random factor used for calculating the delay between subsequent retries when a backoff strategy is used for calculating the delay.
+  *
+  * The purpose of jitter is to avoid clustering of subsequent retries, i.e. to reduce the number of clients calling a service exactly at
+  * the same time - which can result in subsequent failures, contrary to what you would expect from retrying. By introducing randomness to
+  * the delays, the retries become more evenly distributed over time.
+  *
+  * See the <a href="https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/">AWS Architecture Blog article on backoff and
+  * jitter</a> for a more in-depth explanation.
+  *
+  * Depending on the algorithm, the jitter can affect the delay in different ways - see the concrete variants for more details.
+  */
+enum Jitter:
+  /** No jitter, i.e. the delay just uses an exponential backoff with no adjustments. */
+  case None
+
+  /** Full jitter, i.e. the delay is a random value between 0 and the calculated backoff delay. */
+  case Full
+
+  /** Equal jitter, i.e. the delay is half of the calculated backoff delay plus a random value between 0 and the other half. */
+  case Equal
+
+  /** Decorrelated jitter, i.e. the delay is a random value between the initial delay and the last delay multiplied by 3. */
+  case Decorrelated

core/src/main/scala/ox/retry/ResultPolicy.scala

@@ -0,0 +1,37 @@
+package ox.retry
+
+/** A policy that allows to customize when a non-erroneous result is considered successful and when an error is worth retrying (which allows
+  * for failing fast on certain errors).
+  *
+  * @param isSuccess
+  *   A function that determines whether a non-erroneous result is considered successful. By default, every non-erroneous result is
+  *   considered successful.
+  * @param isWorthRetrying
+  *   A function that determines whether an error is worth retrying. By default, all errors are retried.
+  * @tparam E
+  *   The error type of the operation. For operations returning a `T` or a `Try[T]`, this is fixed to `Throwable`. For operations returning
+  *   an `Either[E, T]`, this can be any `E`.
+  * @tparam T
+  *   The successful result type for the operation.
+  */
+case class ResultPolicy[E, T](isSuccess: T => Boolean = (_: T) => true, isWorthRetrying: E => Boolean = (_: E) => true)
+
+object ResultPolicy:
+  /** A policy that considers every non-erroneous result successful and retries on any error. */
+  def default[E, T]: ResultPolicy[E, T] = ResultPolicy()
+
+  /** A policy that customizes when a non-erroneous result is considered successful, and retries all errors
+    *
+    * @param isSuccess
+    *   A predicate that indicates whether a non-erroneous result is considered successful.
+    */
+  def successfulWhen[E, T](isSuccess: T => Boolean): ResultPolicy[E, T] = ResultPolicy(isSuccess = isSuccess)
+
+  /** A policy that customizes which errors are retried, and considers every non-erroneous result successful
+    * @param isWorthRetrying
+    *   A predicate that indicates whether an erroneous result should be retried..
+    */
+  def retryWhen[E, T](isWorthRetrying: E => Boolean): ResultPolicy[E, T] = ResultPolicy(isWorthRetrying = isWorthRetrying)
+
+  /** A policy that considers every non-erroneous result successful and never retries any error, i.e. fails fast */
+  def neverRetry[E, T]: ResultPolicy[E, T] = ResultPolicy(isWorthRetrying = _ => false)

core/src/main/scala/ox/retry/RetryPolicy.scala

@@ -0,0 +1,106 @@
+package ox.retry
+
+import scala.concurrent.duration.*
+
+/** A policy that defines how to retry a failed operation.
+  *
+  * @param schedule
+  *   The retry schedule which determines the maximum number of retries and the delay between subsequent attempts to execute the operation.
+  *   See [[Schedule]] for more details.
+  * @param resultPolicy
+  *   A policy that allows to customize when a non-erroneous result is considered successful and when an error is worth retrying (which
+  *   allows for failing fast on certain errors). See [[ResultPolicy]] for more details.
+  * @tparam E
+  *   The error type of the operation. For operations returning a `T` or a `Try[T]`, this is fixed to `Throwable`. For operations returning
+  *   an `Either[E, T]`, this can be any `E`.
+  * @tparam T
+  *   The successful result type for the operation.
+  */
+case class RetryPolicy[E, T](schedule: Schedule, resultPolicy: ResultPolicy[E, T] = ResultPolicy.default[E, T])
+
+object RetryPolicy:
+  /** Creates a policy that retries up to a given number of times, with no delay between subsequent attempts, using a default
+    * [[ResultPolicy]].
+    *
+    * This is a shorthand for {{{RetryPolicy(Schedule.Immediate(maxRetries))}}}
+    *
+    * @param maxRetries
+    *   The maximum number of retries.
+    */
+  def immediate[E, T](maxRetries: Int): RetryPolicy[E, T] = RetryPolicy(Schedule.Immediate(maxRetries))
+
+  /** Creates a policy that retries indefinitely, with no delay between subsequent attempts, using a default [[ResultPolicy]].
+    *
+    * This is a shorthand for {{{RetryPolicy(Schedule.Immediate.forever)}}}
+    */
+  def immediateForever[E, T]: RetryPolicy[E, T] = RetryPolicy(Schedule.Immediate.forever)
+
+  /** Creates a policy that retries up to a given number of times, with a fixed delay between subsequent attempts, using a default
+    * [[ResultPolicy]].
+    *
+    * This is a shorthand for {{{RetryPolicy(Schedule.Delay(maxRetries, delay))}}}
+    *
+    * @param maxRetries
+    *   The maximum number of retries.
+    * @param delay
+    *   The delay between subsequent attempts.
+    */
+  def delay[E, T](maxRetries: Int, delay: FiniteDuration): RetryPolicy[E, T] = RetryPolicy(Schedule.Delay(maxRetries, delay))
+
+  /** Creates a policy that retries indefinitely, with a fixed delay between subsequent attempts, using a default [[ResultPolicy]].
+    *
+    * This is a shorthand for {{{RetryPolicy(Schedule.Delay.forever(delay))}}}
+    *
+    * @param delay
+    *   The delay between subsequent attempts.
+    */
+  def delayForever[E, T](delay: FiniteDuration): RetryPolicy[E, T] = RetryPolicy(Schedule.Delay.forever(delay))
+
+  /** Creates a policy that retries up to a given number of times, with an increasing delay (backoff) between subsequent attempts, using a
+    * default [[ResultPolicy]].
+    *
+    * The backoff is exponential with base 2 (i.e. the next delay is twice as long as the previous one), starting at the given initial delay
+    * and capped at the given maximum delay.
+    *
+    * This is a shorthand for {{{RetryPolicy(Schedule.Backoff(maxRetries, initialDelay, maxDelay, jitter))}}}
+    *
+    * @param maxRetries
+    *   The maximum number of retries.
+    * @param initialDelay
+    *   The delay before the first retry.
+    * @param maxDelay
+    *   The maximum delay between subsequent retries. Defaults to 1 minute.
+    * @param jitter
+    *   A random factor used for calculating the delay between subsequent retries. See [[Jitter]] for more details. Defaults to no jitter,
+    * i.e. an exponential backoff with no adjustments.
+    */
+  def backoff[E, T](
+      maxRetries: Int,
+      initialDelay: FiniteDuration,
+      maxDelay: FiniteDuration = 1.minute,
+      jitter: Jitter = Jitter.None
+  ): RetryPolicy[E, T] =
+    RetryPolicy(Schedule.Backoff(maxRetries, initialDelay, maxDelay, jitter))
+
+  /** Creates a policy that retries indefinitely, with an increasing delay (backoff) between subsequent attempts, using a default
+    * [[ResultPolicy]].
+    *
+    * The backoff is exponential with base 2 (i.e. the next delay is twice as long as the previous one), starting at the given initial delay
+    * and capped at the given maximum delay.
+    *
+    * This is a shorthand for {{{RetryPolicy(Schedule.Backoff.forever(initialDelay, maxDelay, jitter))}}}
+    *
+    * @param initialDelay
+    *   The delay before the first retry.
+    * @param maxDelay
+    *   The maximum delay between subsequent retries. Defaults to 1 minute.
+    * @param jitter
+    *   A random factor used for calculating the delay between subsequent retries. See [[Jitter]] for more details. Defaults to no jitter,
+    * i.e. an exponential backoff with no adjustments.
+    */
+  def backoffForever[E, T](
+      initialDelay: FiniteDuration,
+      maxDelay: FiniteDuration = 1.minute,
+      jitter: Jitter = Jitter.None
+  ): RetryPolicy[E, T] =
+    RetryPolicy(Schedule.Backoff.forever(initialDelay, maxDelay, jitter))

core/src/main/scala/ox/retry/Schedule.scala

@@ -0,0 +1,119 @@
+package ox.retry
+
+import scala.concurrent.duration.*
+import scala.util.Random
+
+private[retry] sealed trait Schedule:
+  def nextDelay(attempt: Int, lastDelay: Option[FiniteDuration]): FiniteDuration
+
+object Schedule:
+
+  private[retry] sealed trait Finite extends Schedule:
+    def maxRetries: Int
+
+  private[retry] sealed trait Infinite extends Schedule
+
+  /** A schedule that retries up to a given number of times, with no delay between subsequent attempts.
+    *
+    * @param maxRetries
+    *   The maximum number of retries.
+    */
+  case class Immediate(maxRetries: Int) extends Finite:
+    override def nextDelay(attempt: Int, lastDelay: Option[FiniteDuration]): FiniteDuration = Duration.Zero
+
+  object Immediate:
+    /** A schedule that retries indefinitely, with no delay between subsequent attempts. */
+    def forever: Infinite = ImmediateForever
+
+  private case object ImmediateForever extends Infinite:
+    override def nextDelay(attempt: Int, lastDelay: Option[FiniteDuration]): FiniteDuration = Duration.Zero
+
+  /** A schedule that retries up to a given number of times, with a fixed delay between subsequent attempts.
+    *
+    * @param maxRetries
+    *   The maximum number of retries.
+    * @param delay
+    *   The delay between subsequent attempts.
+    */
+  case class Delay(maxRetries: Int, delay: FiniteDuration) extends Finite:
+    override def nextDelay(attempt: Int, lastDelay: Option[FiniteDuration]): FiniteDuration = delay
+
+  object Delay:
+    /** A schedule that retries indefinitely, with a fixed delay between subsequent attempts.
+      *
+      * @param delay
+      *   The delay between subsequent attempts.
+      */
+    def forever(delay: FiniteDuration): Infinite = DelayForever(delay)
+
+  case class DelayForever private[retry] (delay: FiniteDuration) extends Infinite:
+    override def nextDelay(attempt: Int, lastDelay: Option[FiniteDuration]): FiniteDuration = delay
+
+  /** A schedule that retries up to a given number of times, with an increasing delay (backoff) between subsequent attempts.
+    *
+    * The backoff is exponential with base 2 (i.e. the next delay is twice as long as the previous one), starting at the given initial delay
+    * and capped at the given maximum delay.
+    *
+    * @param maxRetries
+    *   The maximum number of retries.
+    * @param initialDelay
+    *   The delay before the first retry.
+    * @param maxDelay
+    *   The maximum delay between subsequent retries.
+    * @param jitter
+    *   A random factor used for calculating the delay between subsequent retries. See [[Jitter]] for more details. Defaults to no jitter,
+    * i.e. an exponential backoff with no adjustments.
+    */
+  case class Backoff(
+      maxRetries: Int,
+      initialDelay: FiniteDuration,
+      maxDelay: FiniteDuration = 1.minute,
+      jitter: Jitter = Jitter.None
+  ) extends Finite:
+    override def nextDelay(attempt: Int, lastDelay: Option[FiniteDuration]): FiniteDuration =
+      Backoff.nextDelay(attempt, initialDelay, maxDelay, jitter, lastDelay)
+
+  object Backoff:
+    private[retry] def delay(attempt: Int, initialDelay: FiniteDuration, maxDelay: FiniteDuration): FiniteDuration =
+      // converting Duration <-> Long back and forth to avoid exceeding maximum duration
+      (initialDelay.toMillis * Math.pow(2, attempt)).toLong.min(maxDelay.toMillis).millis
+
+    private[retry] def nextDelay(
+        attempt: Int,
+        initialDelay: FiniteDuration,
+        maxDelay: FiniteDuration,
+        jitter: Jitter,
+        lastDelay: Option[FiniteDuration]
+    ): FiniteDuration =
+      def backoffDelay = Backoff.delay(attempt, initialDelay, maxDelay)
+
+      jitter match
+        case Jitter.None => backoffDelay
+        case Jitter.Full => Random.between(0, backoffDelay.toMillis).millis
+        case Jitter.Equal =>
+          val backoff = backoffDelay.toMillis
+          (backoff / 2 + Random.between(0, backoff / 2)).millis
+        case Jitter.Decorrelated =>
+          val last = lastDelay.getOrElse(initialDelay).toMillis
+          Random.between(initialDelay.toMillis, last * 3).millis
+
+    /** A schedule that retries indefinitely, with an increasing delay (backoff) between subsequent attempts.
+      *
+      * The backoff is exponential with base 2 (i.e. the next delay is twice as long as the previous one), starting at the given initial
+      * delay and capped at the given maximum delay.
+      *
+      * @param initialDelay
+      *   The delay before the first retry.
+      * @param maxDelay
+      *   The maximum delay between subsequent retries.
+      * @param jitter
+      *   A random factor used for calculating the delay between subsequent retries. See [[Jitter]] for more details. Defaults to no jitter,
+      * i.e. an exponential backoff with no adjustments.
+      */
+    def forever(initialDelay: FiniteDuration, maxDelay: FiniteDuration = 1.minute, jitter: Jitter = Jitter.None): Infinite =
+      BackoffForever(initialDelay, maxDelay, jitter)
+
+  case class BackoffForever private[retry] (initialDelay: FiniteDuration, maxDelay: FiniteDuration = 1.minute, jitter: Jitter = Jitter.None)
+      extends Infinite:
+    override def nextDelay(attempt: Int, lastDelay: Option[FiniteDuration]): FiniteDuration =
+      Backoff.nextDelay(attempt, initialDelay, maxDelay, jitter, lastDelay)