[DF] Add first tutorial for distributed rdf

tutorials/CMakeLists.txt

@@ -575,6 +575,16 @@ if(ROOT_pyroot_FOUND)
     list(APPEND pyveto pyroot/pyroot004_NumbaDeclare.py)
   endif()
 
+  # Rules specific to distributed RDataFrame with Pyspark 
+  # Disable distributed RDF tutorials if we didn't check dependencies in the environment first
+  if(NOT test_distrdf_pyspark)
+    list(APPEND pyveto dataframe/distrdf*)
+  endif()
+  # Use main Python executable to run in PySpark driver and executors
+  if(test_distrdf_pyspark)
+    list(APPEND ROOT_environ PYSPARK_PYTHON=${PYTHON_EXECUTABLE_Development_Main})
+  endif()
+
   find_python_module(xgboost QUIET)
   if(NOT PY_XGBOOST_FOUND OR NOT dataframe)
     file(GLOB tmva_veto_py RELATIVE ${CMAKE_CURRENT_SOURCE_DIR} tmva/tmva10*.py)

tutorials/dataframe/distrdf001_spark_connection.py

@@ -0,0 +1,77 @@
+## \file
+## \ingroup tutorial_dataframe
+## \notebook -draw
+## Configure a Spark connection and fill two histograms distributedly.
+##
+## This tutorial shows the ingredients needed to setup the connection to a Spark
+## cluster, namely a SparkConf object holding configuration parameters and a
+## SparkContext object created with the desired options. After this initial
+## setup, an RDataFrame with distributed capabilities is created and connected
+## to the SparkContext instance. Finally, a couple of histograms are drawn from
+## the created columns in the dataset.
+##
+## \macro_code
+## \macro_image
+##
+## \date March 2021
+## \author Vincenzo Eduardo Padulano
+import pyspark
+import ROOT
+
+# Point RDataFrame calls to Spark RDataFrame object
+RDataFrame = ROOT.RDF.Experimental.Distributed.Spark.RDataFrame
+
+# Setup the connection to Spark
+# First create a dictionary with keys representing Spark specific configuration
+# parameters. In this tutorial we use the following configuration parameters:
+#
+#     1. spark.app.name: The name of the Spark application
+#     2. spark.master: The Spark endpoint responsible for running the
+#         application. With the syntax "local[4]" we signal Spark we want to run
+#         locally on the same machine with 4 cores, each running a separate
+#         process. The default behaviour of a Spark application would run
+#         locally on the same machine with as many concurrent processes as
+#         available cores, that could be also written as "local[*]".
+#
+# If you have access to a remote cluster you should substitute the endpoint URL
+# of your Spark master in the form "spark://HOST:PORT" in the value of
+# `spark.master`. Depending on the availability of your cluster you may request
+# more computing nodes or cores per node with a similar configuration:
+#
+#     sparkconf = pyspark.SparkConf().setAll(
+#                     {"spark.master": "spark://HOST:PORT",
+#                      "spark.executor.instances": <number_of_nodes>,
+#                      "spark.executor.cores" <cores_per_node>,}.items())
+#
+# You can find all configuration options and more details in the official Spark
+# documentation at https://spark.apache.org/docs/latest/configuration.html .
+
+# Create a SparkConf object with all the desired Spark configuration parameters
+sparkconf = pyspark.SparkConf().setAll(
+    {"spark.app.name": "distrdf001_spark_connection",
+     "spark.master": "local[4]", }.items())
+# Create a SparkContext with the configuration stored in `sparkconf`
+sparkcontext = pyspark.SparkContext(conf=sparkconf)
+
+# Create an RDataFrame that will use Spark as a backend for computations
+df = RDataFrame(1000, sparkcontext=sparkcontext)
+
+# Set the random seed and define two columns of the dataset with random numbers.
+ROOT.gRandom.SetSeed(1)
+df_1 = df.Define("gaus", "gRandom->Gaus(10, 1)").Define("exponential", "gRandom->Exp(10)")
+
+# Book an histogram for each column
+h_gaus = df_1.Histo1D(("gaus", "Normal distribution", 50, 0, 30), "gaus")
+h_exp = df_1.Histo1D(("exponential", "Exponential distribution", 50, 0, 30), "exponential")
+
+# Plot the histograms side by side on a canvas
+c = ROOT.TCanvas("distrdf001", "distrdf001", 1280, 800)
+c.Divide(2, 1)
+c.cd(1)
+h_gaus.DrawCopy()
+c.cd(2)
+h_exp.DrawCopy()
+
+# Save the canvas
+c.SaveAs("distrdf001_spark_connection.png")
+print("Saved figure to distrdf001_spark_connection.png")