the-algorithm/tweetypie/server/src/main/scala/com/twitter/tweetypie/store/TweetStoreEvent.scala

package com.twitter.tweetypie
package store

import com.twitter.finagle.tracing.Trace
import com.twitter.tweetypie.store.TweetStoreEvent.RetryStrategy
import com.twitter.tweetypie.thriftscala._

object TweetStoreEvent {

  /**
   * Parent trait for indicating what type of retry strategy to apply to event handlers
   * for the corresponding event type.  Different classes of events use different strategies.
   */
  sealed trait RetryStrategy

  /**
   * Indicates that the event type doesn't support retries.
   */
  case object NoRetry extends RetryStrategy

  /**
   * Indicates that if an event handler encounters a failure, it should enqueue a
   * retry to be performed asynchronously.
   */
  case class EnqueueAsyncRetry(enqueueRetry: (ThriftTweetService, AsyncWriteAction) => Future[Unit])
      extends RetryStrategy

  /**
   * Indicates that if an event handler encounters a failure, it should retry
   * the event locally some number of times, before eventually given up and scribing
   * the failure.
   */
  case class LocalRetryThenScribeFailure(toFailedAsyncWrite: AsyncWriteAction => FailedAsyncWrite)
      extends RetryStrategy

  /**
   * Indicates that if an event handler encounters a failure, it should retry
   * the event locally some number of times.
   */
  case object ReplicatedEventLocalRetry extends RetryStrategy
}

/**
 * The abstract parent class for all TweetStoreEvent types.
 */
sealed trait TweetStoreEvent {
  val name: String

  val traceId: Long = Trace.id.traceId.toLong

  /**
   * Indicates a particular retry behavior that should be applied to event handlers for
   * the corresponding event type.  The specifics of the strategy might depend upon the
   * specific TweetStore implementation.
   */
  def retryStrategy: RetryStrategy
}

abstract class SyncTweetStoreEvent(val name: String) extends TweetStoreEvent {
  override def retryStrategy: RetryStrategy = TweetStoreEvent.NoRetry
}

abstract class AsyncTweetStoreEvent(val name: String) extends TweetStoreEvent {
  def enqueueRetry(service: ThriftTweetService, action: AsyncWriteAction): Future[Unit]

  override def retryStrategy: RetryStrategy = TweetStoreEvent.EnqueueAsyncRetry(enqueueRetry)
}

abstract class ReplicatedTweetStoreEvent(val name: String) extends TweetStoreEvent {
  override def retryStrategy: RetryStrategy = TweetStoreEvent.ReplicatedEventLocalRetry
}

/**
 * A trait for all TweetStoreEvents that become TweetEvents.
 */
trait TweetStoreTweetEvent {
  val timestamp: Time

  val optUser: Option[User]

  /**
   * Most TweetStoreTweetEvents map to a single TweetEvent, but some
   * optionally map to an event and others map to multiple events, so
   * this method needs to return a Seq of TweetEventData.
   */
  def toTweetEventData: Seq[TweetEventData]
}

/**
 * The abstract parent class for an event that indicates a particular action
 * for a particular event that needs to be retried via the async-write-retrying mechanism.
 */
abstract class TweetStoreRetryEvent[E <: AsyncTweetStoreEvent] extends TweetStoreEvent {
  override val name = "async_write_retry"

  def action: AsyncWriteAction
  def event: E

  def eventType: AsyncWriteEventType

  def scribedTweetOnFailure: Option[Tweet]

  override def retryStrategy: RetryStrategy =
    TweetStoreEvent.LocalRetryThenScribeFailure(action =>
      FailedAsyncWrite(eventType, action, scribedTweetOnFailure))
}

/**
 * Functions as a disjunction between an event type E and it's corresonding
 * retry event type TweetStoreRetryEvent[E]
 */
case class TweetStoreEventOrRetry[E <: AsyncTweetStoreEvent](
  event: E,
  toRetry: Option[TweetStoreRetryEvent[E]]) {
  def toInitial: Option[E] = if (retryAction.isDefined) None else Some(event)
  def retryAction: Option[RetryStrategy] = toRetry.map(_.retryStrategy)
  def hydrate(f: E => Future[E]): Future[TweetStoreEventOrRetry[E]] =
    f(event).map(e => copy(event = e))
}

object TweetStoreEventOrRetry {
  def apply[E <: AsyncTweetStoreEvent, R <: TweetStoreRetryEvent[E]](
    event: E,
    retryAction: Option[AsyncWriteAction],
    toRetryEvent: (AsyncWriteAction, E) => R
  ): TweetStoreEventOrRetry[E] =
    TweetStoreEventOrRetry(event, retryAction.map(action => toRetryEvent(action, event)))

  object First {

    /** matches against TweetStoreEventOrRetry instances for an initial event */
    def unapply[E <: AsyncTweetStoreEvent](it: TweetStoreEventOrRetry[E]): Option[E] =
      it.toInitial
  }

  object Retry {

    /** matches against TweetStoreEventOrRetry instances for a retry event */
    def unapply[E <: AsyncTweetStoreEvent](
      it: TweetStoreEventOrRetry[E]
    ): Option[TweetStoreRetryEvent[E]] =
      it.toRetry
  }
}