Collaborative Filtering - RDD-based API

2020-01-21
  • Table of contents {:toc}

Collaborative filtering

Collaborative filtering is commonly used for recommender systems. These techniques aim to fill in the missing entries of a user-item association matrix. spark.mllib currently supports model-based collaborative filtering, in which users and products are described by a small set of latent factors that can be used to predict missing entries. spark.mllib uses the alternating least squares (ALS) algorithm to learn these latent factors. The implementation in spark.mllib has the following parameters:

  • numBlocks is the number of blocks used to parallelize computation (set to -1 to auto-configure).
  • rank is the number of features to use (also referred to as the number of latent factors).
  • iterations is the number of iterations of ALS to run. ALS typically converges to a reasonable solution in 20 iterations or less.
  • lambda specifies the regularization parameter in ALS.
  • implicitPrefs specifies whether to use the explicit feedback ALS variant or one adapted for implicit feedback data.
  • alpha is a parameter applicable to the implicit feedback variant of ALS that governs the baseline confidence in preference observations.

Explicit vs. implicit feedback

The standard approach to matrix factorization-based collaborative filtering treats the entries in the user-item matrix as explicit preferences given by the user to the item, for example, users giving ratings to movies.

It is common in many real-world use cases to only have access to implicit feedback (e.g. views, clicks, purchases, likes, shares etc.). The approach used in spark.mllib to deal with such data is taken from Collaborative Filtering for Implicit Feedback Datasets. Essentially, instead of trying to model the matrix of ratings directly, this approach treats the data as numbers representing the strength in observations of user actions (such as the number of clicks, or the cumulative duration someone spent viewing a movie). Those numbers are then related to the level of confidence in observed user preferences, rather than explicit ratings given to items. The model then tries to find latent factors that can be used to predict the expected preference of a user for an item.

Scaling of the regularization parameter

Since v1.1, we scale the regularization parameter lambda in solving each least squares problem by the number of ratings the user generated in updating user factors, or the number of ratings the product received in updating product factors. This approach is named "ALS-WR" and discussed in the paper "Large-Scale Parallel Collaborative Filtering for the Netflix Prize". It makes lambda less dependent on the scale of the dataset, so we can apply the best parameter learned from a sampled subset to the full dataset and expect similar performance.

Examples

In the following example, we load rating data. Each row consists of a user, a product and a rating. We use the default [ALS.train()](api/scala/index.html#org.apache.spark.mllib.recommendation.ALS$) method which assumes ratings are explicit. We evaluate the recommendation model by measuring the Mean Squared Error of rating prediction. Refer to the [`ALS` Scala docs](api/scala/index.html#org.apache.spark.mllib.recommendation.ALS) for more details on the API. {% include_example scala/org/apache/spark/examples/mllib/RecommendationExample.scala %} If the rating matrix is derived from another source of information (i.e. it is inferred from other signals), you can use the `trainImplicit` method to get better results. {% highlight scala %} val alpha = 0.01 val lambda = 0.01 val model = ALS.trainImplicit(ratings, rank, numIterations, lambda, alpha) {% endhighlight %}
All of MLlib's methods use Java-friendly types, so you can import and call them there the same way you do in Scala. The only caveat is that the methods take Scala RDD objects, while the Spark Java API uses a separate `JavaRDD` class. You can convert a Java RDD to a Scala one by calling `.rdd()` on your `JavaRDD` object. A self-contained application example that is equivalent to the provided example in Scala is given below: Refer to the [`ALS` Java docs](api/java/org/apache/spark/mllib/recommendation/ALS.html) for more details on the API. {% include_example java/org/apache/spark/examples/mllib/JavaRecommendationExample.java %}
In the following example we load rating data. Each row consists of a user, a product and a rating. We use the default ALS.train() method which assumes ratings are explicit. We evaluate the recommendation by measuring the Mean Squared Error of rating prediction. Refer to the [`ALS` Python docs](api/python/pyspark.mllib.html#pyspark.mllib.recommendation.ALS) for more details on the API. {% include_example python/mllib/recommendation_example.py %} If the rating matrix is derived from other source of information (i.e. it is inferred from other signals), you can use the trainImplicit method to get better results. {% highlight python %} # Build the recommendation model using Alternating Least Squares based on implicit ratings model = ALS.trainImplicit(ratings, rank, numIterations, alpha=0.01) {% endhighlight %}

In order to run the above application, follow the instructions provided in the Self-Contained Applications section of the Spark Quick Start guide. Be sure to also include spark-mllib to your build file as a dependency.

Tutorial

The training exercises from the Spark Summit 2014 include a hands-on tutorial for personalized movie recommendation with spark.mllib.