# Memo

## Transformation Operations

* **Functions**
  * filter(boolean) - keep elements when boolean is true
  * flatMap - similar to Map, but FlatMap allows returning 0, 1 or more elements. It applies to each element of RDD and it returns the result as new RDD.
  * flatMapValues - similar to flatMap, operates on the value only.
  * map- operates on the entire record
  * mapValues - operates on the value only
  * reduceByKey -  sum up value(s) by key
    * rdd.reduceByKey((x,y) => x + y)  // x is one of the two values, y is the other.
    * rdd.reduceByKey(lambda x, y: x + y)  // with lambda style
* **Remark**
  * With key/value data, use mapValues() / flatMapValues() instead of map() / flatMap() if the transformation doesn't affect the keys for efficiency.
  * About partitioning: if you applied any custom partitioning to your RDD (e.g. using partitionBy), using map would "forget" that partitioner (the result will revert to default partitioning) as the keys might have changed; mapValues, however, preserves any partitioner set on the RDD.

Creating RDD with collection

* parallelize
  * The 2nd parameter is to set the number of partitions which is optional.
* makeRDD

## Shared Variables

* Broadcast Variables

  * A read-only variable cached on each machine.

  ```python
  conf = SparkConf().setMaster("local").setAppName("PopularMovies")
  sc = SparkContext(conf = conf)

  def loadMovieNames():
    movieNames = {}
    with open("ml-100k/u.ITEM") as f:
        for line in f:
            fields = line.split('|')
            movieNames[int(fields[0])] = fields[1]
    return movieNames

  nameDict = sc.broadcast(loadMovieNames())
  ```

  * For example, to give every node a copy of a large input dataset in an efficient manner. 
* Accumulators
  * Only “added” to through an associative and commutative operation and can therefore be efficiently supported in parallel.
  * Methods supported: .add() and .value


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