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Apache Spark Tutorial: Get Started With Serving ML Models With Spark

An estimated 463 exabytes of data will be produced each day by the year 2025. Data scientists will need to make sense out of this data. Obviously, you can’t process, nor store big data on any single computer. Big data needs to be stored in a cluster of computers. 

This makes the machine learning process more complex. Luckily, there are tools that were built specifically for dealing with big data. Apache Spark is one of those tools. 

Let’s take a look at how you can use Apache Spark to process big data. 

What is Apache Spark? 

Apache Spark is an open-source engine for analyzing and processing big data. A Spark application has a driver program, which runs the user’s main function. It’s also responsible for executing parallel operations in a cluster. 

A cluster in this context refers to a group of nodes. Each node is a single machine or server. A unit of work sent to the executor is known as a task, and a parallel computation involving multiple tasks is known as a job. 

The SparkContext

Applications in Spark are controlled by the SparkContext. It connects to the cluster manager, as you can see in the diagram above. There are several cluster managers, namely:

  • Spark’s own standalone cluster manager
  • Mesos
  • YARN

The cluster manager allocates resources to Spark applications. 

Spark executors

Executors live on the worker node, and they run computations and store your application’s data. Worker nodes are responsible for running application code in the cluster. 

Every application in Spark will have its own executor. SparkContext sends tasks to the executor. Applications are isolated from each other, because every application has its own executor program. 

Why use Apache Spark

There are other tools, like Hadoop, that can be used for distributed computing. Why would you use Apache Spark?

  • it’s 100 times faster than Hadoop thanks to in-memory computation 
  • Apache Spark can be used in Java, Scala, Python, R, or SQL
  • you can run Apache Spark on Hadoop, Apache Mesos, Kubernetes, or in the cloud 

Apache Spark installation

It’s expected that you’ll be running Spark in a cluster of computers, for example a cloud environment. However, if you’re a beginner with Spark, there are quicker alternatives to get started. Let’s take a look at some of them. 

Setting up Spark on Google Colab

To run Spark on Google Colab, you need two things: `openjdk` and `findspark`. 

`findspark` is the package that makes Spark importable on Google Colab. You will also need to download Spark and extract it.

!apt-get install openjdk-8-jdk-headless -qq > /dev/null
!wget -q https://www-us.apache.org/dist/spark/spark-3.0.1/spark-3.0.1-bin-hadoop2.7.tgz
!tar xf spark-3.0.1-bin-hadoop2.7.tgz
!pip install findspark

Next step, set some environment variables based on the location where Spark and Java were installed. 

import os
os.environ["JAVA_HOME"] = "/usr/lib/jvm/java-8-openjdk-amd64"
os.environ["SPARK_HOME"] = "/content/spark-3.0.1-bin-hadoop2.7"

Next, use `findspark` to make Spark importable. 

import findspark
findspark.init()

You need to create the `SparkContext` any time you want to use Spark. Create an instance of the `SparkContext` to confirm that Spark was installed successfully. 

import pyspark
sc = pyspark.SparkContext()

Installing Apache Spark on your local machine

How can you install Spark on your local machine? This process can get very complicated, very quickly. It might not be worth the effort, since you won’t actually run a production-like cluster on your local machine. 

It’s just easier and quicker to use a container to run Spark’s installation on your local machine. You will need to have Docker installed in order to install Spark using a container. 

The next step is to pull the Spark image from Docker Hub. Once this is done, you will be able to access a ready notebook at localhost:8888. 

$ docker run -p 8888:8888 jupyter/pyspark-notebook

When you navigate to the Notebook, you can start a new SparkContext to confirm the installation. 

import pyspark
sc = pyspark.SparkContext()
Apache spark context

Running Apache Spark on Databricks

The most hassle-free alternative is to use the Databricks community edition. The only thing you need to do is set up a free account. This article will assume that you’re running Spark on Databricks. However, all the other alternatives we mentioned will suffice. 

Databricks

Apache Spark basics

The most fundamental thing to understand in Apache Spark is how it represents data. The main representations are:

  • Resilient Distributed Datasets (RDDs)
  • Dataframes

Resilient Distributed Datasets (RDDs)

A Resilient Distributed Datasets (RDD) is a fault-tolerant collection of elements that can be operated on in parallel. RDDs automatically recover from faults. This format is a great choice when:

  • you’re working with unstructured data, such as media or text streams
  • you need to perform low-level transformations on a dataset

RDD creation

You can create an RDD by parallelizing a collection from the driver program, or by using a dataset in the file system. Spark will determine the optimal number of partitions for splitting the dataset. However, you can pass this as the second argument to the `parallelize` function:

my_list = [1, 2, 3, 4, 5,6]
my_list_distributed = sc.parallelize(my_list,3)

RDD operations

In Spark there are two main type of operations:

  • transformations
  • actions 

Spark transformations

Transformations will create a new dataset from one that already exists. An example of this operation is `map`. Transformations are lazy, meaning that they don’t return the result of their computation right away. Computation only happens when an action requests for the result. 

my_list = [2,3,4,5,6]
def power_two(x):
  return x**2
list(map(power_two,my_list))

Other transformations in Spark include:

  • `filter` – returns a dataset after selecting items that return true on a certain condition
  • `union` – returns the union of two datasets

Spark actions

An action will return a value after a certain computation on the dataset. For example, the `reduce` function.

results = map(power_two,my_list)
for result in results:
  print(result)
Apache guide code 1

There are a couple of other actions that you can perform with Spark:

  • `collect` to return the dataset as an array
  • `count` to return the number of items in a dataset
  • `take(n)` to return the first n elements of a dataset
  • `first` to return the first element in a dataset

RDD persistence

Spark allows you to improve the performance of your application by caching your dataset. Computations can be saved in memory and retrieved from the cache the next time they’re needed. Caching in Spark is fault-tolerant, so any partitions that are lost will be recomputed using the transformations that created them.

df.cache()

Dataframes

Spark Dataframes are immutable, and very similar to Pandas DataFrames. As you’ll see later, Spark DataFrames can be queried using SQL. You will see the usage of DataFrames and SQL in Spark as you go through the machine learning example below. 


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Machine Learning in Spark

You can do machine learning in Spark using `pyspark.ml`. This module ships with Spark, so you don’t need to look for it or install it. Once you log in to your Databricks account, create a cluster. The notebook that’s needed for this exercise will run in that cluster. 

Apache guide - new cluster

When your cluster is ready, create a notebook. Next, you will need to define the source of your data. Databricks lets you upload the data or link from one of their partner providers. When uploading the data, it needs to be less than 2GB. 

Apache guide - new table

For the sake of illustration, let’s use this heart disease dataset from UCI Machine Learning. 

Data exploration with Spark 

Note the path to your dataset once you upload it to Databricks so that you can use it to load the data. Also, make sure your notebook is connected to the cluster. 

Apache guide code
df = spark.read.csv('/FileStore/tables/heart.csv',sep=',',inferSchema='true',header='true')

Notice that you didn’t have to create a SparkContext. This is because Databricks does this for you by default. You can type `sc` on a cell to confirm that. 

Apache guide code

You can also click the link to the Spark UI to see more information about your cluster. 

Spark UI

The quickest way to run data exploration on Databricks is to use the `display(df)` function. This function is unique to Databricks so it will not work if you’re on Google Colab or your local machine. On those platforms you can use:

df.display()
Apache guide plot

Clicking on the plot options button will give you more charts and options that you can try. You can download the plots as well. 

As mentioned before, you can run filter operations.

df.filter((df.age>20) & (df.target=='1')).show()
Apache guide code

Spark also lets you run `group by` operations similarly to how you would do them in Pandas. Let’s see how that can be done.

from pyspark.sql import functions as F
df.groupBy(["sex"]).agg(
    F.mean("age").alias("Mean Age")
    ).show()
Apache guide code

If you’re coming from the world of SQL, you might be interested in querying the data frame as if it was an SQL table. You can do that by using the `SQLContext` to register a temporary SQL table. After that, you can run SQL queries normally.

from pyspark.sql import SQLContext
sqlContext = SQLContext(sc)
df.registerTempTable('df_table')
df_sql = sqlContext.sql('select age,target,sex,slope,cp from df_table where age>30 ORDER BY age DESC')
df_sql.show()
Apache guide code

Data preprocessing with Spark

Once you’re done with data exploration, the next step is to convert the data into a format that would be accepted by Spark’s MLlib. In this case, the features need to be transformed into a single vector that will be passed to the machine learning model. 

This can be done using the `VectorAssembler`. Let’s import it and instantiate it using the features in the dataset.

from pyspark.ml.feature import VectorAssembler
feat_cols = ['age',
 'sex',
 'cp',
 'trestbps',
 'chol',
 'fbs',
 'restecg',
 'thalach',
 'exang',
 'oldpeak',
 'slope',
 'ca',
 'thal']
vec_assember = VectorAssembler(inputCols = feat_cols, outputCol='features' )

The next step is to use it to transform the data frame. 

final_data = vec_assember.transform(df)

The `take` function can be used to view a part of the dataset. You will notice a vector known as `features` that contains all the features. 

final_data.take(2)
Apache guide code

Next, split the dataset into a training and testing set.

training,testing = final_data.randomSplit([0.7,0.3],seed=42)
Apache guide code

Building the Machine Learning model with PySpark MLlib

The next step is to fit this dataset to the algorithm. Let’s use Logistic Regression here. 
Start by importing it. You can access more algorithms here.

from pyspark.ml.classification import LogisticRegression

Let’s create an instance of the algorithm while passing the label column name and the name of the features. 

lr = LogisticRegression(labelCol='target',featuresCol='features')

Now fit the model to the training set. 

lrModel = lr.fit(training)

Evaluating the model

Prior to evaluating the model you have to run the predictions. Use the `transform` function. 

predictions = lrModel.transform(testing)

You can view these predictions by selecting some columns. 

predictions.select('target','prediction','probability','age','sex').show()
Apache guide code

Since this is binary classification, the `BinaryClassificationEvaluator` function can be used to evaluate the model.

from pyspark.ml.evaluation import BinaryClassificationEvaluator
evaluator.evaluate(predictions)
Apache guide code

Serving Apache Spark Machine Learning models 

Databricks lets you serve your machine learning models with MLflow. However, the easiest way to serve your model is to send it to a model registry like Neptune.

lrModel.save("model.pkl")
neptune.log_artifact(“model.pkl”)

Final thoughts  

In this article, you’ve seen that Apache Spark can be used for analyzing and building machine learning models for handling big data. We also covered the core concepts of using Apache Spark: 

  • what Apache Spark is
  • data representations in Apache Spark
  • various methods for installing Apache Spark
  • running Spark on Databricks, your local machine, and on Google Colab
  • building and saving machine learning models using Apache Spark

You’re now armed with the skills you need to explore and build machine learning models using big data. Thanks for reading!


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ML Experiment Tracking: What It Is, Why It Matters, and How to Implement It

10 mins read | Jakub Czakon | Posted November 26, 2020

Let me share a story that I’ve heard too many times.

”… We were developing an ML model with my team, we ran a lot of experiments and got promising results…

…unfortunately, we couldn’t tell exactly what performed best because we forgot to save some model parameters and dataset versions…

…after a few weeks, we weren’t even sure what we have actually tried and we needed to re-run pretty much everything”

– unfortunate ML researcher.

And the truth is, when you develop ML models you will run a lot of experiments.

Those experiments may:

  • use different models and model hyperparameters
  • use different training or evaluation data, 
  • run different code (including this small change that you wanted to test quickly)
  • run the same code in a different environment (not knowing which PyTorch or Tensorflow version was installed)

And as a result, they can produce completely different evaluation metrics. 

Keeping track of all that information can very quickly become really hard. Especially if you want to organize and compare those experiments and feel confident that you know which setup produced the best result.  

This is where ML experiment tracking comes in. 

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