Spark Session
In Spark 2.0, SparkSession has been introduced. It provides a single point of entry for interaction with Spark functionality. It allows user accessing DataFrame and Dataset APIs.
In older Spark versions, the user had to create Spark configuration (sparkConf), SparkContext (sc) and then sqlContext. In Spark 2.0 all is done with SparkSession (spark), which encapsulates the above mentioned trio, HiveContext and StreamingContext.
In order to use DataFrame API in Spark 1.6, SQLContext needs to be used. When running Spark, new Spark application is started by creating SparkContext object (represents a connection to computing cluster). From SparkContext, SQLContext can be created (the main entry point for Spark DataFrame and SQL functionality). A SQLContext can be used to create DataFrames, which allows you to direct operations on your data.
It was confusing when to use SparkContext and when to use SQLContext in Spark 1.6. All this is hidden under a layer called SparkSession. See the following object types from PySpark driver:
SparkSession
>>> type(spark)
<class 'pyspark.sql.session.SparkSession'>
SparkContext
>>> type(sc)
<class 'pyspark.context.SparkContext'>
SqlContext
>>> type(sqlContext)
<class 'pyspark.sql.context.SQLContext'>
The Spark session has to be created when using spark-submit command. An example from the documentation on how to do that:
>>> spark = SparkSession.builder \
| ... .master("local") \
| ... .appName("Word Count") \
| ... .config("spark.some.config.option", "some-value") \
| ... .getOrCreate()
The spark handle is created and first DataFrames can be created.
When using local driver programs (pyspark, spark-sql, spark-shell or sparkR), the SparkSession is automatically initialized. Example with sparkR:
>>> sparkR
SQL
SQL in Spark 2.0 supports SQL:2003 (latest revision is SQL:2011). Spark SQL can run all 99 TPC-DS queries. Subquery support has been improved. More on the SQL improvements here.
Spark SQL
Spark SQL can be accessed through SparkSession. A table can be created and SQL queries can be executed against it. Example:
myDF.createOrReplaceTempView("my_table")
resultDF = spark.sql("SELECT col1, col2 from my_table")
Running the following command
>>> spark.catalog.listTables()
Returns the tables available to SparkSession.
Driver program
In Spark, communication occurs between driver and executors. The driver has Spark jobs to run, it splits them into tasks and submits them to executors for completion. The results are delivered back to the driver.
Every Spark application has a driver program which launches various parallel operations on executor JVMs. The JVMs are running either in a cluster or locally on the same machine. Pyspark is an example of a local driver program. Example:
pyspark --master yarn --deploy-mode cluster
Error: Cluster deploy mode is not applicable to Spark shells.
If you run in cluster mode that means that the client that submitted the job is detached from the Spark application and its further behavior does not influence the application. If you shut down the computer that submitted the application to the cluster, the job will continue to run. The driver is on one of the nodes in Spark cluster. Command spark-submit with property –deploy-mode cluster does this.
If you run in client mode, that means that the client you are running the application from is the client. In Spark HistoryServer, under tab Executors, in table Executors, you can read that the address of the driver matches the address of the computer the command has been sent from.
The driver program creates distributed datasets on the cluster and applies operations to those datasets. Driver programs access Spark through a SparkContext object.
Listing attributes
Pythons dir() lists all attributes accessible through the parameter
Example for spark:
>>> dir(spark)
['Builder', '__class__', '__delattr__', '__dict__', '__doc__', '__enter__', '__exit__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', '_conf', '_createFromLocal', '_createFromRDD', '_inferSchema', '_inferSchemaFromList', '_instantiatedContext', '_jsc', '_jsparkSession', '_jvm', '_jwrapped', '_sc', '_wrapped', 'builder', 'catalog', 'conf', 'createDataFrame', 'newSession', 'range', 'read', 'readStream', 'sparkContext', 'sql', 'stop', 'streams', 'table', 'udf', 'version']
Help
Python’s help() lists attributes and examples. Example:
>>> help(spark)
Opens extensive help and examples.
RDD
SQLContext is created from lower level SparkContext. SparkContext is used to create Resilient Distributed Datasets (RDDs). RDD is Spark’s way of representing data internally. DataFrames are implemented in terms of RDDs.
It is possible to interact directly with RDDs, but DataFrames are preferred. They are faster and perform no matter of the language you use (Python, R, Scala, Java). Whether you express your computations in Spark SQL or Python, Java, Scala, or R, the underlying code generated is identical because all execution planning undergoes the same Catalyst optimizer.
DFs are made of partitions – converting a DF to an RDD to check number of partitions:
>>> tweetDF.rdd.getNumPartitions()
Data is split into partitions.
How to optimize:
If there are 3 slots, perfect is to have partitions = x*3
Repartition DF:
>>> tweetDF.repartition(6)
DataFrames
In Spark, base DataFrame is first created. Either by generating a Dataset using spark.range method (for learning purposes) or by reading file(s) or tables and returning a DataFrame. Operations can then be applied to it. DataFrame is immutable, once created, it cannot be changed. Each transformation creates a new DataFrame. In the end, one or more actions can be applied to the DataFrame.
DataFrame consists of series of Row objects. Each of them has a set of named columns.
DataFrame must have a schema, each of which has a name and a type. Some datasources have schemas built into them, although it is possible to define a schema and introduce it as a parameter when creating a new DataFrame.
Running:
help(spark.createDataFrame)
Returns:
createDataFrame(self, data, schema=None, samplingRatio=None) method of pyspark.sql.session.SparkSession instance
Dataset
DataFrame and DataSet are unified in Scala and Java. In Python and R, DataFrame is the main interface.
Test in Scala:
scala> import org.apache.spark.sql._
scala> classOf[DataFrame] == classOf[Dataset[_]]
res5: Boolean = true
Checking SQL package in Spark 2.0
package object sql {
/**
* Converts a logical plan into zero or more SparkPlans. This API is exposed for experimenting
* with the query planner and is not designed to be stable across spark releases. Developers
* writing libraries should instead consider using the stable APIs provided in
* [[org.apache.spark.sql.sources]]
*/
@DeveloperApi
type Strategy = SparkStrategy
type DataFrame = Dataset[Row]
}
Pandas DataFrame
Spark DF can be converted to Pandas DF
>>> import pandas as pd
Then you can use .toPandas() at the end of the Spark DF to convert to Pandas DF.
Cache
Putting DataFrame in memory: Spark uses Tungsten binary format to columnar compress data in memory. The number of partitions in memory is equal to the number of partitions defined on the RDD under the DataFrame. The data in memory is equally divided per partition (for example, 600MB in memory, 6 partitions -> 100MB per partition).
The size of data shrinks when cached. Example: 1,6GB file on disk is cached in memory with size 618,6 MB.
When an action is executed, and if the DataFrame is cached, the stages BEFORE the cache was executed are skipped, because the data is already partitioned in memory.
Ideal partition size is between 100MB to 200MB, so number of partitions should be adjusted to that, not the other way around.
Cache is not an action, which means it will be executed when the next action is executed. However, if you cache a table, it WILL be cached right away.
Sources
How to use SparkSession in Apache Spark 2.0
Using Apache Spark 2.0 to Analyze the City of San Francisco’s Open Data
Modern Spark DataFrame and Dataset (Intermediate Tutorial)
Spark SQL, DataFrames and Datasets Guide
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