Spark's broadcast variables, used to broadcast immutable datasets to all nodes.
Broadcast[T] extends Serializable with Logging
A broadcast variable.
A broadcast variable. Broadcast variables allow the programmer to keep a read-only variable cached on each machine rather than shipping a copy of it with tasks. They can be used, for example, to give every node a copy of a large input dataset in an efficient manner. Spark also attempts to distribute broadcast variables using efficient broadcast algorithms to reduce communication cost.
Broadcast variables are created from a variable
vby calling org.apache.spark.SparkContext#broadcast. The broadcast variable is a wrapper around
v, and its value can be accessed by calling the
valuemethod. The interpreter session below shows this:
scala> val broadcastVar = sc.broadcast(Array(1, 2, 3)) broadcastVar: org.apache.spark.broadcast.Broadcast[Array[Int]] = Broadcast(0) scala> broadcastVar.value res0: Array[Int] = Array(1, 2, 3)
After the broadcast variable is created, it should be used instead of the value
vin any functions run on the cluster so that
vis not shipped to the nodes more than once. In addition, the object
vshould not be modified after it is broadcast in order to ensure that all nodes get the same value of the broadcast variable (e.g. if the variable is shipped to a new node later).
Type of the data contained in the broadcast variable.