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Type safe api reference

Jon Shea edited this page · 115 revisions

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There are two main concepts in the type-safe API: a TypedPipe[T] which is kind of a distributed list of objects of type T and a KeyedList[K,V] which represents some sharding of objects of key K and value V. There are a few KeyedList objects: Grouped[K,V], CoGrouped[K, V]. The former represents usual groupings, and the latter is used for cogroupings or joins.

Chapters:

Basics

Most of the Typed API is available simply by importing com.twitter.scalding._. Most sources, even the simple TextLine source, are typed (implement the TypedSource trait), which means it is easy to get a TypedPipe to begin performing operations on.

import com.twitter.scalding._

val lines : TypedPipe[String] = TypedPipe.from(TextLine("hello.txt"))

// do word count
lines.flatMap(_.split("\\s+"))
  .group
  .sum
  .write(TypedTsv[(String, Long)]("output"))

// reverse all lines in the file
val reversedLines : TypedPipe[String] = lines.map(_.reverse)
reversedLines.write(TypedTsv[String]("output.tsv"))

The above example generates the following error

<console>:18: error: Cannot prove that String <:< (K, V).
              lines.flatMap(_.split("\\s+")) .group .sum .write(TypedTsv[(String, Long)]("output"))
                                              ^

The problem appears to be that you are running the group function on a TypedPipe[String] when it expects a TypedPipe[(K,V)]. Essentially you need a pipe of tuples in order to group.

In the above example we show the preferred way to get a TypedPipe — using TypedPipe.from(), and then demonstrate running a map operation and writing out to a typed sink (TypedTsv).

Map-like functions

map

def map[U](f : T => U) : TypedPipe[U]

Converts a TypedPipe[T] to a TypedPipe[U] via f : T => U

case class Bird(name : String, winLb : Float, hinFt : Float, color : String)
val birds : TypedPipe[Bird] = getBirdPipe
val britishBirds : TypedPipe[(Float, Float)] =
  birds.map { bird =>
    val (weightInLbs, heightInFt) = (bird.winLb, bird.hinFt)
    (0.454 * weightInLbs, 0.305 * heightInFt)
  }

flatMap

def flatMap[U](f : T => Iterable[U]) : TypedPipe[U]

Converts a TypedPipe[T] to a TypedPipe[U] by applying f : T => Iterable[U] followed by flattening.

case class Book(title : String, author : String, text : String)
val books : TypedPipe[Book] = getBooks
val words : TypedPipe[String] = books.flatMap { _.text.split("\s+") }
//Example that uses Option. (either an animal name passes by in the pipe or nothing)
animals.flatMap { an =>
  if (an.kind == "bird") {Some(an.name)} else None
}

filter

def filter(f: T => Boolean): TypedPipe[T]

If you return true you keep the row, otherwise the row is ignored.

case class Animal(name : String, kind: String)
val animals : TypedPipe[Animal] = getAnimals
val birds = animals.filter { _.kind == "bird" }

filterNot

def filterNot(f : T => Boolean) : TypedPipe[T]

Acts like filter with a negated predicate - keeps the rows where the predicate function returns false, otherwise the row is ignored.

case class Animal(name : String, kind: String)
val animals : TypedPipe[Animal] = getAnimals
val notBirds = animals.filterNot { _.kind == "bird" }

collect

def collect(f: PartialFunction[T, U]): TypedPipe[U]

Filters and maps with Scala's partial function syntax (case):

case class Animal(name: String, kind: String)
val animals : TypedPipe[Animal] = getAnimals
val birds: TypedPipe[String] = animals.collect { case Animal(name, "bird") => name }
//This is the same as flatMapping an Option.

Creating Groups and Joining (CoGrouping)

Grouping

These are all methods on TypedPipe[T]. Notice that these methods do not return a TypedPipe[T] anymore; instead, they return Grouped[K,T].

groupBy

def groupBy[K](g: T => K)(implicit ord: Ordering[K]) : Grouped[K,T]

Call g : T => K on a TypedPipe[T] to create a Grouped[K,T]. Subsequent aggregation methods use K as the type of the grouping key. We can use any of the functions on Groups specified on the Fields Api to transform the Grouped[K, T] to a TypedPipe[U]. Notice that those functions act on T.

Groups need an Ordering (i.e. a comparator) for the key K that we are grouping by. This is implemented for all the standard variable types that we use, in which case no explicit declaration is necessary.

case class Book(title: String, author: String, year: Int)
val books : TypedPipe[Book] = getBooks
val fields : TypedPipe[(String, String, Int)] = books.map { b => (b.title, b.author, b.year) }
//We want to group all the books based on their author
val byAuthor = fields.groupBy { case (title, author, year) => author }
//Now, we have Grouped[String*, (String, String, Int)**].
//*This String corresponds to the author.
//**This Tuple corresponds to the original fields found on the TypedPipe named fields.
byAuthor.size
//This creates a KeyedList[String, Int], where the String corresponds to the author and the Int corresponds to the number of books that the author wrote. KeyedList objects are automatically converted to TypedPipes as needed, or you can call .toTypedPipe if you prefer.

group (implicit grouping)

// uses scala's <:< to require that T is a subclass of (K, V).
def group[K, V](implicit ev: T <:< (K, V), ord : Ordering[K]): Grouped[K, V]

Special case of groupBy that can be called on TypedPipe[(K, V)]. Uses K as the grouping key.

groupAll (send everything to one reducer)

In scala there is a type that has one less value than Boolean, and that is Unit. There is only value in the type Unit. The value is written as ().

def groupAll: Grouped[Unit,T]

Uses Unit as the grouping key. Useful to send all tuples to 1 reducer.

Useful functions on Grouped[K,V].

val group: Grouped[K, V]
group.keys 
//Creates a TypedPipe[K] consisting of the keys in the (key, value) pairs of group.
group.values 
//Creates a TypedPipe[V] consisting of the values in the (key, value) pairs of group.
group.mapValues { values => mappingFunction(values) } 
//Creates a Grouped[K, V'], where the keys in the (key, value) pairs of group are unchanged, but the values are changed to V'.

Joining/CoGrouping

These are all methods on CoGroupable[K, V]. TypedPipe[K, V], Grouped[K, V] and even CoGrouped[K, V] are CoGroupable. If possible, put the CoGroupable with the fewest values per key on the right; this improves performance, but correctness is not impacted. In extreme cases failure to do so can lead to OutOfMemoryError's. First, we group the pipe by key of type K to get Grouped[K, V]. Then, we join with another group of the same key K, for example Grouped[K, W].

join (inner-join)

def join[W](smaller : CoGroupable[K, W]) : CoGrouped[K, (V, W)]

Note that CoGrouped extends KeyedListLike, so any reducing functions you are used to on Grouped will also work on a CoGrouped.

We already know K and V. The only type that could be specified in the join function is W, which is the value in the key-valued group of the smaller group.

//We have two libraries and we want to get a list of the books they have in common.
//The books of Library 2 have an additional field "copies."
case class Book(title: String, year: Int)
case class ExtendedBook(title: String, year: Int, copies: Long)
//Group the books of Library 1 by book title.
val library1 : TypedPipe[Book] = getBooks1
val L1 : TypedPipe[(String, Int)] = library1.map { b : Book => (b.title, b.year) }
val group1 = L1.groupBy { case (title, year) => title }
//Similarly, group the books of Library 2 by book title.
val library2 : TypedPipe[ExtendedBook] = getBooks2
val L2 : TypedPipe[(String, Int, Long)] = library2.map { b: ExtendedBook => (b.title, b.year, b.copies) }
val group2 = L2.groupBy { case (title, year, copies) => title }

//If Library 1 is larger than Library 2, then we do:
val theJoin: CoGrouped[String, ((String, Int), (String, Int, Long))] = group1.join(group2)

leftJoin

def leftJoin[W](smaller: CoGroupable[K, W]): CoGrouped[K, (V, Option[W])]

Using the definitions from the previous example, assume you are the general manager of Library 1 and you are interested in a complete list of all the books in your library. In addition, you would like to know, which of those books can also be found in Library 2, in case the ones in your library are being used:

val theLeftJoin : CoGrouped[String, ((String, Int), Option(String, Int, Long))] = group1.leftJoin(group2)

rightJoin

def rightJoin[W](smaller: CoGroupable[K, W]): CoGrouped[K, (Option[V], W)]

outerJoin

def outerJoin[W](smaller: CoGroupable[K, W]): CoGrouped[K, (Option[V], Option[W])]

Like all KeyedListLike instances, CoGrouped has toTypedPipe to explicitly convert to TypedPipe. However, this is automatic (implicit from KeyedListLike[K, V, _] => TypedPipe[(K, V)] in object KeyedListLike).

val myJoin: CoGrouped[K, (V, W)]
val tpipe: TypedPipe[(K, (V, W))] = myJoin.toTypedPipe

Joining multiple streams

Since CoGrouped is CoGroupable it is perfectly legal to do a.join(b).join(c).leftJoin(d).outerJoin(e) and it will run in one map/reduce job, but the value type will be a bit ugly: (Option[(((A, B), C), Option[D])], Option[E]). To make this cleaner, in scalding 0.12 we introduce the MultiJoin object. MultiJoin(a, b, c, d) does an inner join with a value tuple of (A, B, C, D) as you might expect. You can also do MultiJoin.left or MultiJoin.outer.

Map-side (replicated) joins:

These methods do not require a reduce step, but should only be used on extremely small arguments since each mapper will read the entire argument to do the join.

cross

Suppose we want to send every value from one TypedPipe[U] to each value of a TypedPipe[T]. List(1,2,3) cross List(4,5) gives List((1,4),(1,5),(2,4),(2,5),(3,4),(3,5)). The final size is left.size * right.size.

// Implements a cross product.  The right side should be tiny.
def cross[U](tiny: TypedPipe[U]): TypedPipe[(T,U)]

hashJoin

A very efficient join, which works when the right side is tiny, is hashJoin. All the (key, value) pairs from the right side are stored in a hash table for quick retrieval. The hash table is replicated on every mapper and the hashJoin operation takes place entirely on the mappers (no reducers involved).

// Again, the right side should be tiny.
def hashJoin[W](tiny: HashJoinable[K, W]): TypedPipe[(K, (V, W))]

Tip: All groups and joins have .withReducers(n) to explicitly set the number of reducers for that step. For other options, please refer to: http://twitter.github.io/scalding/#com.twitter.scalding.typed.Grouped and http://twitter.github.io/scalding/#com.twitter.scalding.typed.CoGrouped

ValuePipe: working with values that will be computed

Sometimes we reduce everything down to one value:

val userFollowers: TypedPipe[(Long, Int)] = // function to get 

val topUsers: TypedPipe[Long] = allUsers
  .collect { case (uid, followers) if followers > 1000000 => uid }

// put it in a value:
val topUsers: ValuePipe[Set[Long]] = topUsers.map(Set(_)).sum

A value Pipe is a kind of future value: it is a value that will be computed by your job, but is not there yet. TypedPipe.sum returns a ValuePipe.

When you have this, you can then use it on another TypedPipe:

val allClickers: TypedPipe[Long] = //...

val topClickers = allClickers.filterWithValue(topUsers) { (clicker, optSet) =>
  optSet.get.contains(clicker) // keep the topUsers that are also clickers
}

You can also mapWithValue or flatMapWithValue. See ValuePipe.scala for more.

Records

Suppose you have many fields and you want to update just one or two. Did you know about the copy method on all case classes?

Consider this example:

scala> case class Record(name: String, weight: Double)
defined class Record

scala> List(Record("Bob", 180.3), Record("Lisa", 154.3))
res22: List[Record] = List(Record(Bob,180.3), Record(Lisa,154.3))

scala> List(Record("Bob", 180.3), Record("Lisa", 154.3)).map { r =>
  val w = r.weight + 10.0
  r.copy(weight = w)
}
res23: List[Record] = List(Record(Bob,190.3), Record(Lisa,164.3))

In exactly the same way, you can update just one or two fields in a case class on scalding with the typed API.

This is how we recommend making records, but WATCH OUT: you need to define case classes OUTSIDE of your job due to serialization reasons (otherwise they create circular references).

Aggregation and Stream Processing

Both Grouped[K, R] and CoGrouped[K, R] extend KeyedListLike[K, R, _], which is the class that represents sublists of R sharded by K. The following methods are the main aggregations or stream processes you can run.

sum: Generalized reduction

def sum[U >: V](implicit s: Semigroup[U]): KeyedListLike[K, U]

Scalding uses a type from Algebird called a Semigroup for sums. A semigroup is just a reduce function that has the property that plus(plus(a, b), c) == plus(a, plus(b, c)). The default Semigroup is what you probably expect: addition for numbers, union for sets, concatenation for lists, maps do an outer join on their keys and then do the semigroup for their value types.

If there is no sorting on the values, scalding assumes that order does not matter and it will partially apply the sum on the mappers. This can dramatically reduce the communication cost of the job depending on how many keys there are in your data set.

reduce: an ad-hoc Semigroup

def reduce(fn: (V, V) => V): KeyedListLike[K, V]

This defines the plus function for a Semigroup, and then calls sum with that Semigroup. See the documentation there.

aggregate: Using Aggregators for reusablity

def aggregate[B,C](a: Aggregator[V, B, C]): KeyedListLike[K, C]

check the aggregator tutorial for more explanation and examples.

foldLeft and fold

def foldLeft[U](init: U)(fn: (U, V) => U): KeyedListLike[K, U]

foldLeft is used where you might make a loop in some language. U is the some state you are updating every time you see a new value V. An example might be training a model on some data. U is your model. V are you data points. Your fn looks like: foldLeft(defaultModel) { (model, data) => updateModel(model, data) }.

def fold[U](f: Fold[V, U]): KeyedListLike[K, U]
def foldWithKey[U](fn: K => Fold[V, U]): KeyedListLike[K, U]

A com.twitter.algebird.Fold is an instance that encapsulates a fold function. The value of this is two fold:

  1. Logic can be packaged in a Fold and shared across many jobs, for instance Fold.size
  2. Folds can be combined together so many functions can be applied in one pass over the data.
val myWork: Fold[Int, (Long, Boolean, Int)] = Fold.size
               .join(Fold.forall { i: Int => i > 0 })
               .join(Fold.sum[Int])
               .map { case ((size, pos), sum) => (size, pos, sum) }

Folds are similar to Aggregators, with the exception that they MUST be run only on the reducers. If you can express an aggregation in terms of Aggregators, it is worthwhile to do so in that it can give you map-side reduction before going to the reducers.

mapGroup and mapValueStream: totally general reducer functions

def mapGroup[U](fn: (K, Iterator[V]) => Iterator[U]): KeyedListLike[K, U]
def mapValueStream[U](fn: Iterator[V] => Iterator[U]): KeyedListLike[K, U]

It is pretty rare that you need a reduction that is not a sum, aggregate or fold, but it might occasionally come up. If you find yourself reaching for this very often, it might be a sign that you have not quite grokked how to use Aggregators or Folds.

These functions give you an Iterator over the values on your reducer, and in the case of mapGroup the key, and you can transform just the values, not the key. If you need to change the key, output the new key and value in the U type, and then discard the keys using the .values method.

Using mapGroup/mapValueStream always forces all the data to reducers. Realizing the entire stream of values at once (i.e. manually reversing or rescanning the data) can explode the memory, so prefer to operate one at time on the Iterators you are given.

Example: Datacubing

A common pattern is called data-cubing. This is where you have some commutative sum that you want to materialize sums of all possible binary queries where part of the key is present or absent (making each point of the key space into a hyper-cube). Here is an example of how to do this with the typed-API:

The Fields-based API Reference has a builder-pattern object called GroupBuilder which allows you to easily create a tuple of several parallel aggregations, e.g. counting, summing, and taking the max, all in one pass through the data. The type-safe API has a way to do this, but it involves implementing a type-class for your object and using KeyedList.sum on the tuple. Below we give an example.

case class MyObject(name : String, ridesFixie : Boolean, rimmedGlasses : Boolean, income : Double) {
  def hipsterScore = { List(ridesFixie, rimmedGlasses).map { if(_) 1.0 else 0.0 }.sum + 1.0/income }
}

// Monoid which chooses the highest hipster score
implicit val hipsterMonoid = new Monoid[MyObject] {
  def zero = MyObject("zeroHipster", false, false, Double.NegativeInfinity)
  def plus(left : MyObject, right : MyObject) = { List(left, right).maxBy { _.hipsterScore } }
}

// Now let's count our fixie riders find the biggest hipster
val everybody : TypedPipe[MyObject] = getPeople
// Now we want to know how many total people, fixie riders, and how many rimmed-glasses wearers,
// as well as the biggest hipster:
everybody.map { person => (1L, if(person.ridesFixie) 1L else 0L, if(person.rimmedGlasses) 1L else 0L, person) }
  .groupAll
  .sum
  //Throw away the unit key created by groupAll
  .values
  // annoying, but mapping a single part in the tuple is a pain (here we want .name)
  .map { results => (results._1, results._2, results._3, results._4.name)
  .write(TypedTsv[(Long, Long, Long, String)]("maxHipster"))

Scalding automatically knows how to sum tuples (it does so element-wise, see GeneratedAbstractAlgebra.scala.

Powerful Aggregation with Algebird

See this example on Locality Sensitive Hashing via @argyris.

Interoperating between Fields API and Type-safe API

If you can avoid the Fields API, we recommend it. But if you have legacy code that you want to keep while you are migrating to the Type-safe API, there are methods to help you.

Generally, all the methods from the Fields-based API Reference are present with the following exceptions:

  1. The mapping functions always replace the input with the output. map and flatMap in the Type safe API are similar to the mapTo and flatMapTo functions (respectively) in the Fields-based API.
  2. Due to the previous statement, there is no need to name fields.

If you import TDsl._ you get an enrichment on cascading Pipe objects to jump into a Typed block:

  pipe.typed(('in0, 'in1) -> 'out) { tpipe : TypedPipe[(Int,Int)] =>
     tpipe.groupBy { x => 1 } //groups on all the input tuples (equivalent to groupAll)
     .mapValues { tup => tup._1 + tup._2 } //sum the two values in each tuple
     .sum //sum all the tuple sums (i.e. sum everything)
     .values // discard the key which is 1
  }

In this example, we start off with a cascading Pipe (pipe), which has the 'in0 and 'in1 fields. We use the method typed in order to create a new TypedPipe (tpipe). Then, we apply all of our functions on the TypedPipe[(Int, Int)] to obtain a TypedPipe[Int] which has the total sum. Finally, this is converted back into the cascading Pipe (pipe) with the single field 'out, which contains a single Tuple holding the total sum.

Converting pipes

  • To go from a pipe to a TypedPipe[T]: mypipe.toTypedPipe[T](Fields_Kept). Fields_Kept specifies the fields in mypipe that we want to keep in the Typed Pipe.
  • To go from a TypedPipe[T] to a pipe: myTypedPipe.toPipe(f: Fields) method. Since we go from a Typed to a cascading pipe, we actually need to give names to the fields.

Example:

import TDsl._

case class Bird(name : String, winLb : Float, color : String)
val birds : TypedPipe[Bird] = getBirdPipe
birds.toPipe('name, 'winLb, 'color) //Cascading Pipe with the 3 specified fields.
birds.toTypedPipe[(String, String)]('name, 'color) //Typed Pipe (keeping only some fields)

Advanced examples:

import TDsl._

case class Bird(name : String, winLb : Float, hinFt : Float, color : String)
val birds : TypedPipe[Bird] = getBirdPipe
birds.toPipe('name, 'color)

val p : TypedPipe[(Double, Double)] = TypedTsv[(Double,Double)](input, ('a, 'b)).toTypedPipe[(Double, Double)]('a, 'b)

TypedPipe[MyClass] is slightly more involved, but you can get it in several ways. One straightforward way is:

object Bird {
  def fromTuple(t : (Double, Double)) : Bird = Bird(t._1, t._2)
}

case class Bird(weight : Double, height : Double) {
  def toTuple : (Double, Double) = { (weight, height) }
}

import TDsl._
val birds : TypedPipe[Bird] = TypedTsv[(Double, Double)](path, ('weight, 'height)).map{ Bird.fromTuple(_) }
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