How Apache Spark works internally

5 minute read

Apache Spark Q&A’s

Apache Spark - Execution Model

1. How does the spark context read the user code and convert it to tasks?

The driver code generates job, stages and tasks. The entire driver code can be called as one application and each action constitutes a job. When a job is submitted is to the driver, the job is divided into logical plan and physical plan.

During logical plan the transformations() build the computation, chaning a series of RDD. Since each action() triggers a job, during the physical plan the complete dependency graph of transformations is divided in to stages. Unlike hadoop where the execution process is fixed map-shuffle-sort-aggregate, spark doesn’t have fixed execution process. The data is computed when they are actually needed in a flow fashion. It starts from the final result of RDD and check backwards the RDD chain to find what RDDs and parititons are needed for computing the final result. During the backtracking if it encounters the ShuffleDependency it cuts the data flow and forms a new stage leaving the chainings of RDD by NarrowDepedency. So it the ShuffleDependency that break out for a new stage.

Within each stage the tasks are exectued and data is pipelined through the transformations. The number of tasks is equivalent to number of partitions number in the RDDs of each stage. All the tasks are packaged in TaskSet and sent to the TaskScheduler.The Driver actor sends the serialized tasks to CoarseGrainedExecutorBackend Actor on worker node. Upon receiving , the executor deserializes the it to a normal task and runs to get the result. TaskScheduler will be notified that the task is finished, and its result will be processed

If the received task on driver is the last task in the stage, then next stage will be submitted. If the stage is already the last one, dagScheduler will be informed that the job is finished.

From spark 1.4 release, new visualization is added in the Spark UI. Where we can see the DAG visualization of different stages.

How spark driver serializes the task that is sent to executors? RDD goes through series of transformations with user defined functions / method in object. And these functions are passed to the executors in the form of tasks. These tasks are instances of a Scala class defined in spark-core.

How do the executors know what is the method that needs to be executed which is wrapped in the task class?

The executor receive a RPC msg with a task description

How exactly is the serialization helpful here?

Yes, the task contains code that is serialized by a closureSerializer

How does the spark context read the user code and convert it to tasks?

In a REPL envirenment, spark compile the user code to class files and put on a file server, the executor implements a custom classloader which load the class from the file server on the driver side; the class is actually a function to run against a iterator of records

Spark function passing fundamentally is based on Java Serialization. In Java you can pass any arbitrary code to other machine over network, in can be simple case class or any class with any behavior.

Only one requirement — serialized class needs to be in class path of target JVM.

On startup when you use spark-submit it distributes your jar file to all Spark worker node, it allows driver to pass serialized functions to worker node, and because serialized class is in class path any function that sent from driver can be deserialized.

Spark doesn’t define any specific Task class for RDD transformation. If you use it from Scala for map operations you are sending serialized versions of scala Function1.

If you use aggregate/reduce by key etc, it can be Function2. Anyway, it’s not anything Spark specific, it’s just plain Scala (Java) class.

Apache Spark - repartition() vs coalesce()

Keep in mind that repartitioning your data is a fairly expensive operation. Spark also has an optimized version of repartition() called coalesce() that allows avoiding data movement, but only if you are decreasing the number of RDD partitions.

It avoids a full shuffle. If it’s known that the number is decreasing then the executor can safely keep data on the minimum number of partitions, only moving the data off the extra nodes, onto the nodes that we kept.

So, it would go something like this:

Node 1 = 1,2,3 Node 2 = 4,5,6 Node 3 = 7,8,9 Node 4 = 10,11,12 Then coalesce down to 2 partitions:

Node 1 = 1,2,3 + (10,11,12) Node 3 = 7,8,9 + (4,5,6) Notice that Node 1 and Node 3 did not require its original data to move.

2. What’s the differences between Apache Spark coalesce and repartition?

The repartition algorithm does a full shuffle of the data and creates equal sized partitions of data. Coalesce combines existing partitions to avoid a full shuffle.

Apache Spark - Shuffle hash join vs Broadcast hash join

The default implementation of a join in Spark is a shuffled hash join. The shuffled hash join ensures that data on each partition will contain the same keys by partitioning the second dataset with the same default partitioner as the first, so that the keys with the same hash value from both datasets are in the same partition. While this approach always works, it can be more expensive than necessary because it requires a shuffle. The shuffle can be avoided if:

Both RDDs have a known partitioner.

One of the datasets is small enough to fit in memory, in which case we can do a broadcast hash join

The easiest optimization is that if one of the datasets is small enough to fit in memory, it should be broadcasted to every compute node. This use case is very common as data needs to be combined with side data, such as a dictionary, all the time.

Mostly joins are slow due to too much data being shuffled over the network. With the Broadcast join, the smaller dataset is copied to all the worker nodes so the original parallelism of the larger DataFrame is maintained.

Example :
  import org.apache.spark.sql.functions.broadcast  
  val employeesDF = employeesRDD.toDF
  val departmentsDF = departmentsRDD.toDF

  // materializing the department data
  val tmpDepartments = broadcast(departmentsDF.as("departments"))

  import context.implicits._
  employeesDF.join(broadcast(tmpDepartments), 
  $"depId" === $"id",  // join by employees.depID == departments.id 
  "inner").show()