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PredictionIO's DASE architecture brings the separation-of-concerns design principle to predictive engine development. DASE stands for the following components of an engine:

  • Data - includes Data Source and Data Preparator
  • Algorithm(s)
  • Serving
  • Evaluator

Let's look at the code and see how you can customize the engine you built from the Product Ranking Template.

Evaluator will not be covered in this tutorial.

The Engine Design

As you can see from the Quick Start, MyProductRanking takes a JSON prediction query, e.g. { "user": "u2", "items": ["i1", "i3", "i10", "i2", "i5", "i31", "i9"] }, and return a JSON predicted result. In MyProductRanking/src/main/scala/Engine.scala, the Query case class defines the format of such query:

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case class Query(
  user: String,
  items: List[String]
) extends Serializable

The PredictedResult case class defines the format of predicted result, such as

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{"itemScores":[
  {"item":"i5","score":1.0038217983580324},
  {"item":"i3","score":0.00598658734782459},
  {"item":"i2","score":0.004048103059012265},
  {"item":"i9","score":-1.966935819737517E-4},
  {"item":"i1","score":-0.0016841195307744916},
  {"item":"i31","score":-0.0019770986240634503},
  {"item":"i10","score":-0.0031498317618844918}],
  "isOriginal":false}

with:

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case class PredictedResult(
  itemScores: Array[ItemScore],
  isOriginal: Boolean // set to true if the items are not ranked at all.
) extends Serializable

case class ItemScore(
  item: String,
  score: Double
) extends Serializable

Finally, ProductRankingEngine is the Engine Factory that defines the components this engine will use: Data Source, Data Preparator, Algorithm(s) and Serving components.

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object ProductRankingEngine extends IEngineFactory {
  def apply() = {
    new Engine(
      classOf[DataSource],
      classOf[Preparator],
      Map("als" -> classOf[ALSAlgorithm]),
      classOf[Serving])
  }
}

Spark MLlib

The PredictionIO Product Ranking Engine Template integrates Spark's MLlib ALS algorithm under the DASE architecture. We will take a closer look at the DASE code below.

The MLlib ALS algorithm takes training data of RDD type, i.e. RDD[Rating] and train a model, which is a MatrixFactorizationModel object.

You can visit here to learn more about MLlib's ALS collaborative filtering algorithm.

Data

In the DASE architecture, data is prepared by 2 components sequentially: DataSource and DataPreparator. They take data from the data store and prepare them for Algorithm.

Data Source

In MyProductRanking/src/main/scala/DataSource.scala, the readTraining method of class DataSource reads and selects data from the Event Store (data store of the Event Server). It returns TrainingData.

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case class DataSourceParams(appName: String) extends Params

class DataSource(val dsp: DataSourceParams)
  extends PDataSource[TrainingData,
      EmptyEvaluationInfo, Query, EmptyActualResult] {

  @transient lazy val logger = Logger[this.type]

  override
  def readTraining(sc: SparkContext): TrainingData = {

    // create a RDD of (entityID, User)
    val usersRDD: RDD[(String, User)] = PEventStore.aggregateProperties(...) ...

    // create a RDD of (entityID, Item)
    val itemsRDD: RDD[(String, Item)] = PEventStore.aggregateProperties(...) ...

    // get all "user" "view" "item" events
    val viewEventsRDD: RDD[ViewEvent] = PEventStore.find(...) ...

    new TrainingData(
      users = usersRDD,
      items = itemsRDD,
      viewEvents = viewEventsRDD
    )
  }
}

PredictionIO automatically loads the parameters of datasource specified in MyProductRanking/engine.json, including appName, to dsp.

In engine.json:

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{
  ...
  "datasource": {
    "params" : {
      "appName": "MyApp1"
    }
  },
  ...
}

In readTraining(), PEventStore is an object which provides function to access data that is collected by PredictionIO Event Server.

This Product Ranking Engine Template requires "user" and "item" entities that are set by events.

PEventStore.aggregateProperties(...) aggregates properties of the user and item that are set, unset, or delete by special events $set, $unset and $delete. Please refer to Event API for more details of using these events.

The following code aggregates the properties of user and then map each result to a User() object.

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    // create a RDD of (entityID, User)
    val usersRDD: RDD[(String, User)] = PEventStore.aggregateProperties(
      appName = dsp.appName,
      entityType = "user"
    )(sc).map { case (entityId, properties) =>
      val user = try {
        // placeholder for expanding user properties
        User()
      } catch {
        case e: Exception => {
          logger.error(s"Failed to get properties ${properties} of" +
            s" user ${entityId}. Exception: ${e}.")
          throw e
        }
      }
      (entityId, user)
    }.cache()

In the template, User() object is a simple dummy as a placeholder for you to customize and expand.

Similarly, the following code aggregates the properties of item and then map each result to a Item() object.

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    // create a RDD of (entityID, Item)
    val itemsRDD: RDD[(String, Item)] = PEventStore.aggregateProperties(
      appName = dsp.appName,
      entityType = "item"
    )(sc).map { case (entityId, properties) =>
      val item = try {
        // placeholder for expanding item properties
        Item()
      } catch {
        case e: Exception => {
          logger.error(s"Failed to get properties ${properties} of" +
            s" item ${entityId}. Exception: ${e}.")
          throw e
        }
      }
      (entityId, item)
    }.cache()

In the template, Item() object is a simple dummy as a placeholder for you to customize and expand.

PEventStore.find(...) specifies the events that you want to read. In this case, "user view item" events are read and then each is mapped to a ViewEvent() object.

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    // get all "user" "view" "item" events
    val viewEventsRDD: RDD[ViewEvent] = PEventStore.find(
      appName = dsp.appName,
      entityType = Some("user"),
      eventNames = Some(List("view")),
      // targetEntityType is optional field of an event.
      targetEntityType = Some(Some("item")))(sc)
      // PEventStore.find() returns RDD[Event]
      .map { event =>
        val viewEvent = try {
          event.event match {
            case "view" => ViewEvent(
              user = event.entityId,
              item = event.targetEntityId.get,
              t = event.eventTime.getMillis)
            case _ => throw new Exception(s"Unexpected event ${event} is read.")
          }
        } catch {
          case e: Exception => {
            logger.error(s"Cannot convert ${event} to ViewEvent." +
              s" Exception: ${e}.")
            throw e
          }
        }
        viewEvent
      }.cache()

ViewEvent case class is defined as:

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case class ViewEvent(user: String, item: String, t: Long)

For flexibility, this template is designed to support user ID and item ID in String.

TrainingData contains an RDD of User, Item and ViewEvent objects. The class definition of TrainingData is:

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class TrainingData(
  val users: RDD[(String, User)],
  val items: RDD[(String, Item)],
  val viewEvents: RDD[ViewEvent]
) extends Serializable { ... }

PredictionIO then passes the returned TrainingData object to Data Preparator.

Data Preparator

In MyProductRanking/src/main/scala/Preparator.scala, the prepare method of class Preparator takes TrainingData as its input and performs any necessary feature selection and data processing tasks. At the end, it returns PreparedData which should contain the data Algorithm needs.

By default, prepare simply copies the unprocessed TrainingData data to PreparedData:

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class Preparator
  extends PPreparator[TrainingData, PreparedData] {

  def prepare(sc: SparkContext, trainingData: TrainingData): PreparedData = {
    new PreparedData(
      users = trainingData.users,
      items = trainingData.items,
      viewEvents = trainingData.viewEvents)
  }
}

class PreparedData(
  val users: RDD[(String, User)],
  val items: RDD[(String, Item)],
  val viewEvents: RDD[ViewEvent]
) extends Serializable

PredictionIO passes the returned PreparedData object to Algorithm's train function.

Algorithm

In MyProductRanking/src/main/scala/ALSAlgorithm.scala, the two methods of the algorithm class are train and predict. train is responsible for training the predictive model;predict is responsible for using this model to make prediction.

train(...)

train is called when you run pio train. This is where MLlib ALS algorithm, i.e. ALS.trainImplicit(), is used to train a predictive model.

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  def train(sc: SparkContext, data: PreparedData): ALSModel = {

    ...

    // create User and item's String ID to integer index BiMap
    val userStringIntMap = BiMap.stringInt(data.users.keys)
    val itemStringIntMap = BiMap.stringInt(data.items.keys)

    val mllibRatings = data.viewEvents
      .map { r =>
        // Convert user and item String IDs to Int index for MLlib
        val uindex = userStringIntMap.getOrElse(r.user, -1)
        val iindex = itemStringIntMap.getOrElse(r.item, -1)

        if (uindex == -1)
          logger.info(s"Couldn't convert nonexistent user ID ${r.user}"
            + " to Int index.")

        if (iindex == -1)
          logger.info(s"Couldn't convert nonexistent item ID ${r.item}"
            + " to Int index.")

        ((uindex, iindex), 1)
      }.filter { case ((u, i), v) =>
        // keep events with valid user and item index
        (u != -1) && (i != -1)
      }.reduceByKey(_ + _) // aggregate all view events of same user-item pair
      .map { case ((u, i), v) =>
        // MLlibRating requires integer index for user and item
        MLlibRating(u, i, v)
      }

    // MLLib ALS cannot handle empty training data.
    require(!mllibRatings.take(1).isEmpty,
      s"mllibRatings cannot be empty." +
      " Please check if your events contain valid user and item ID.")

    // seed for MLlib ALS
    val seed = ap.seed.getOrElse(System.nanoTime)

    val m = ALS.trainImplicit(
      ratings = mllibRatings,
      rank = ap.rank,
      iterations = ap.numIterations,
      lambda = ap.lambda,
      blocks = -1,
      alpha = 1.0,
      seed = seed)

    new ALSModel(
      rank = m.rank,
      userFeatures = m.userFeatures.collectAsMap.toMap,
      productFeatures = m.productFeatures.collectAsMap.toMap,
      userStringIntMap = userStringIntMap,
      itemStringIntMap = itemStringIntMap
    )
  }

Working with Spark MLlib's ALS.trainImplicit(....)

MLlib ALS does not support String user ID and item ID. ALS.trainImplicit thus also assumes int-only Rating object. First, you can rename MLlib's Integer-only Rating to MLlibRating for clarity:

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import org.apache.spark.mllib.recommendation.{Rating => MLlibRating}

In order to use MLlib's ALS algorithm, we need to convert the viewEvents into MLlibRating. There are two things we need to handle:

  1. Map user and item String ID of the ViewEvent into Integer ID, as required by MLlibRating.
  2. ViewEvent object is an implicit event that does not have an explicit rating value. ALS.trainImplicit() supports implicit preference. If the MLlibRating has higher rating value, it means higher confidence that the user prefers the item. Hence we can aggregate how many times the user has viewed the item to indicate the confidence level that the user may prefer the item.

You create a bi-directional map with BiMap.stringInt which maps each String record to an Integer index.

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val userStringIntMap = BiMap.stringInt(data.users.keys)
val itemStringIntMap = BiMap.stringInt(data.items.keys)

Then convert the user and item String ID in each ViewEvent to Int with these BiMaps. We use default -1 if the user or item String ID couldn't be found in the BiMap and filter out these events with invalid user and item ID later. After filtering, we use reduceByKey() to add up all values for the same key (uindex, iindex) and then finally map to MLlibRating object.

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val mllibRatings = data.viewEvents
  .map { r =>
    // Convert user and item String IDs to Int index for MLlib
    val uindex = userStringIntMap.getOrElse(r.user, -1)
    val iindex = itemStringIntMap.getOrElse(r.item, -1)

    if (uindex == -1)
      logger.info(s"Couldn't convert nonexistent user ID ${r.user}"
        + " to Int index.")

    if (iindex == -1)
      logger.info(s"Couldn't convert nonexistent item ID ${r.item}"
        + " to Int index.")

    ((uindex, iindex), 1)
  }.filter { case ((u, i), v) =>
    // keep events with valid user and item index
    (u != -1) && (i != -1)
  }.reduceByKey(_ + _) // aggregate all view events of same user-item pair
  .map { case ((u, i), v) =>
    // MLlibRating requires integer index for user and item
    MLlibRating(u, i, v)
  }

In addition to RDD[MLlibRating], ALS.trainImplicit takes the following parameters: rank, iterations, lambda and seed.

The values of these parameters are specified in algorithms of MyProductRanking/engine.json:

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{
  ...
  "algorithms": [
    {
      "name": "als",
      "params": {
        "rank": 10,
        "numIterations": 20,
        "lambda": 0.01,
        "seed": 3
      }
    }
  ]
  ...
}

PredictionIO will automatically loads these values into the constructor ap, which has a corresponding case class ALSAlgorithmParams:

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case class ALSAlgorithmParams(
  rank: Int,
  numIterations: Int,
  lambda: Double,
  seed: Option[Long]) extends Params

The seed parameter is an optional parameter, which is used by MLlib ALS algorithm internally to generate random values. If the seed is not specified, current system time would be used and hence each train may produce different results. Specify a fixed value for the seed if you want to have deterministic result (For example, when you are testing).

ALS.trainImplicit() then returns a MatrixFactorizationModel model which contains two RDDs: userFeatures and productFeatures. They correspond to the user X latent features matrix and item X latent features matrix, respectively. In this case, we will make use of both userFeatures and productFeatures matrix to rank the items for the user. These matrixes are stored as local model. You could see the ALSModel class is defined as:

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class ALSModel(
  val rank: Int,
  val userFeatures: Map[Int, Array[Double]],
  val productFeatures: Map[Int, Array[Double]],
  val userStringIntMap: BiMap[String, Int],
  val itemStringIntMap: BiMap[String, Int]
) extends Serializable { ... }

PredictionIO will automatically store the returned model, i.e. ALSModel in this example.

predict(...)

predict is called when you send a JSON query to http://localhost:8000/queries.json. PredictionIO converts the query, such as { "user": "u2", "items": ["i1", "i3", "i10", "i2", "i5", "i31", "i9"] } to the Query class you defined previously.

To rank the calculated the ranked scores of the items, we first look up the feature vector of this user (if the user exists). Then we look up the feature vectors of the items in query (if the items exist). The score is the dot product of the user and item feature vectors. The items are then sorted by the score.

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  def predict(model: ALSModel, query: Query): PredictedResult = {

    val itemStringIntMap = model.itemStringIntMap
    val productFeatures = model.productFeatures

    // default itemScores array if items are not ranked at all
    lazy val notRankedItemScores =
      query.items.map(i => ItemScore(i, 0)).toArray

    model.userStringIntMap.get(query.user).map { userIndex =>
      // lookup userFeature for the user
      model.userFeatures.get(userIndex)
    }.flatten // flatten Option[Option[Array[Double]]] to Option[Array[Double]]
    .map { userFeature =>
      val scores: Vector[Option[Double]] = query.items.toVector
        .par // convert to parallel collection for parallel lookup
        .map { iid =>
          // convert query item id to index
          val featureOpt: Option[Array[Double]] = itemStringIntMap.get(iid)
            // productFeatures may not contain the item
            .map (index => productFeatures.get(index))
            // flatten Option[Option[Array[Double]]] to Option[Array[Double]]
            .flatten

          featureOpt.map(f => dotProduct(f, userFeature))
        }.seq // convert back to sequential collection

      // check if all scores is None (get rid of all None and see if empty)
      val isAllNone = scores.flatten.isEmpty

      if (isAllNone) {
        logger.info(s"No productFeature for all items ${query.items}.")
        PredictedResult(
          itemScores = notRankedItemScores,
          isOriginal = true
        )
      } else {
        // sort the score
        val ord = Ordering.by[ItemScore, Double](_.score).reverse
        val sorted = query.items.zip(scores).map{ case (iid, scoreOpt) =>
          ItemScore(
            item = iid,
            score = scoreOpt.getOrElse[Double](0)
          )
        }.sorted(ord).toArray

        PredictedResult(
          itemScores = sorted,
          isOriginal = false
        )
      }
    }.getOrElse {
      logger.info(s"No userFeature found for user ${query.user}.")
      PredictedResult(
        itemScores = notRankedItemScores,
        isOriginal = true
      )
    }

  }

PredictionIO passes the returned PredictedResult object to Serving.

Serving

The serve method of class Serving processes predicted result. It is also responsible for combining multiple predicted results into one if you have more than one predictive model. Serving then returns the final predicted result. PredictionIO will convert it to a JSON response automatically.

In MyProductRanking/src/main/scala/Serving.scala,

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class Serving
  extends LServing[Query, PredictedResult] {

  override
  def serve(query: Query,
    predictedResults: Seq[PredictedResult]): PredictedResult = {
    predictedResults.head
  }
}

When you send a JSON query to http://localhost:8000/queries.json, PredictedResult from all models will be passed to serve as a sequence, i.e. Seq[PredictedResult].

An engine can train multiple models if you specify more than one Algorithm component in object RecommendationEngine inside Engine.scala. Since only one ALSAlgorithm is implemented by default, this Seq contains one element.