A stretch... Data is organized by schemas, data is stored on disk (or memory), but nothing like a good old school disk. to illustrate data
A stretch… Data is organized by schemas, data is stored on disk (or memory), but nothing like a good old school disk. to illustrate data

In this fifth episode of DataFriday, I will dig into the schema linked to each dataframe in Apache Spark. I will rename columns, create columns, and analyze the result.

I am also briefly talking about partitions in Apache Spark.

This post is strictly following the first lab (lab #200) of Spark in Action, 2nd edition. The dataset being used is the Wake County (North Carolina, USA) list of restaurants. As the book is now published, I will go a bit over the book as well.

The code is available on GitHub.

For convenience, the Java code is added here. You will see the main steps of this small application:

  1. Open an Apache Spark session.
  2. Load a CSV file in a dataframe.
  3. Manipulate the dataframe’s schema: rename some columns, drop others.
package net.jgp.books.spark.ch03.lab200_ingestion_schema_manipulation;

import static org.apache.spark.sql.functions.concat;
import static org.apache.spark.sql.functions.lit;

import org.apache.spark.Partition;
import org.apache.spark.sql.Dataset;
import org.apache.spark.sql.Row;
import org.apache.spark.sql.SparkSession;

/**
 * CSV ingestion in a dataframe and manipulation.
 * 
 * @author jgp
 */
public class IngestionSchemaManipulationApp {

  /**
   * main() is your entry point to the application.
   * 
   * @param args
   */
  public static void main(String[] args) {
    IngestionSchemaManipulationApp app =
        new IngestionSchemaManipulationApp();
    app.start();
  }

  /**
   * The processing code.
   */
  private void start() {
    // Creates a session on a local master
    SparkSession spark = SparkSession.builder()
        .appName("Restaurants in Wake County, NC")
        .master("local")
        .getOrCreate();

    // Reads a CSV file with header, called
    // Restaurants_in_Wake_County_NC.csv,
    // stores it in a dataframe
    Dataset<Row> df = spark.read().format("csv")
        .option("header", "true")
        .load("data/Restaurants_in_Wake_County_NC.csv");
    System.out.println("*** Right after ingestion");
    df.show(5);
    df.printSchema();
    System.out.println("We have " + df.count() + " records.");

    // Let's transform our dataframe
    df = df.withColumn("county", lit("Wake"))
        .withColumnRenamed("HSISID", "datasetId")
        .withColumnRenamed("NAME", "name")
        .withColumnRenamed("ADDRESS1", "address1")
        .withColumnRenamed("ADDRESS2", "address2")
        .withColumnRenamed("CITY", "city")
        .withColumnRenamed("STATE", "state")
        .withColumnRenamed("POSTALCODE", "zip")
        .withColumnRenamed("PHONENUMBER", "tel")
        .withColumnRenamed("RESTAURANTOPENDATE", "dateStart")
        .withColumnRenamed("FACILITYTYPE", "type")
        .withColumnRenamed("X", "geoX")
        .withColumnRenamed("Y", "geoY")
        .drop("OBJECTID")
        .drop("PERMITID")
        .drop("GEOCODESTATUS");
    df = df.withColumn("id", concat(
        df.col("state"),
        lit("_"),
        df.col("county"), lit("_"),
        df.col("datasetId")));

    // Shows at most 5 rows from the dataframe
    System.out.println("*** Dataframe transformed");
    df.show(5);

    // for book only
    Dataset<Row> dfUsedForBook = df.drop("address2")
        .drop("zip")
        .drop("tel")
        .drop("dateStart")
        .drop("geoX")
        .drop("geoY")
        .drop("address1")
        .drop("datasetId");
    dfUsedForBook.show(5, 15);
    // end

    df.printSchema();

    System.out.println("*** Looking at partitions");
    Partition[] partitions = df.rdd().partitions();
    int partitionCount = partitions.length;
    System.out.println("Partition count before repartition: " +
        partitionCount);

    df = df.repartition(4);
    System.out.println("Partition count after repartition: " +
        df.rdd().partitions().length);
  }
}

This is the first lab of chapter 3, more labs are coming… More details are available in the book as well.

More resources:

The YouTube channel for DataFriday lists all episodes. You can attend the live show every Friday morning at 8 AM EST, check out the DataFriday page.