# Chapter 10 Extensions

“I try to know as many people as I can. You never know which one you’ll need.”

— Tyrion Lannister

In the previous chapter, Tuning, you learned how Spark processes data at large-scale by allowing users to configure the cluster resources, partition data implicitly or explicitly, execute commands across distributed compute nodes, shuffle data across them when needed, cache data to improve performance and serialize data efficiently over the network. You also learned how to configure the different Spark settings used while connecting, submitting a job, running and application and particular settings applicable only to R and R extensions that we will present in this chapter.

The Analysis, Modeling and Data chapters provided a foundation to read and understand most datasets. However, the functionality that was presented was scoped to Spark’s built-in features and tabular datasets. This chapter will go beyond tabular data and explore how to analyze and model networks of interconnected objects through graph processing, analyze genomics datasets, prepare data for deep learning, analyze geographic datasets and use advanced modeling libraries like H2O and XGBoost over large-scale datasets.

The combination of all the content presented in all the previous chapters should take care of most of your large-scale computing needs. However, for those few use cases where functionality is still lacking, the following chapters will provide tools to extend Spark yourself; either, through custom R transformation, custom Scala code or through recent new execution mode in Spark that enable analyzing real-time datasets. Although, before reinventing the wheel, we will present some of the extensions available in Spark.

## 10.1 Overview

In the Introduction chapter we presented the R community as a vibrant group of individuals collaborating with each other in many ways, one of them, by moving open science forward by creating R packages that can be installed from CRAN. In a similar way, but in a much smaller scale, the R community has contributed extensions that increase the functionality initially supported in Spark and R. Spark itself also provides support for creating Spark extensions and, in-fact, many R extensions make use of Spark extensions.

Extensions are constantly being created so this section will be outdated at any given point in time, in addition, we might not even be aware of many Spark and R extensions; however, at the very least we can track the extensions that are available in CRAN by looking at the “reverse imports” for sparklyr in CRAN. (“CRAN - Package Sparklyr” 2019) Extensions and R packages published in CRAN tend to be the most stable since when a package is published in CRAN, it will go through a review process which increases the overall quality of a contribution.

While we wish we could present all the extensions, we’ will’ve scoped this chapter to the extensions that should be the most interesting to most readers. You can find additional ones under the github.com/r-spark organization or by searching repos on GitHub with the sparklyr tag.

rsparkling
The rsparkling extensions allows you to use H2O and Spark from R. This extension is what we would consider advanced modeling in Spark. While Spark’s built-in modeling library, Spark MLlib, is quite useful in many cases; H2O’s modeling capabilities can compute additional statistical metrics and can provide performance and scalability improvements over Spark MLlib. We, ourselves, have not performed detailed comparisons nor benchmarks between MLlib and H2O; so this is something you will have to research on your own to create a complete picture of when to use H2O’s capabilities.
graphframes
The graphframes extensions adds support to process graphs in Spark. A graph is a structure that describes a set of objects in which some pairs of the objects are in some sense related. As you learned in the Introduction chapter, ranking web pages was an early motivation to develop precursors to Spark powered by MapReduce; web pages happen to form a graph if you consider a link between pages as the relationship between each pair of pages. Computing operations likes PageRank over graphs can be quite useful in web search and social networks to mention a few applications.
sparktf
The sparktf extension provides support to write TensorFlow records in Spark. TensorFlow is one of the leading deep learning frameworks and it is often used with large amounts of numerical data represented as TensorFlow records, a file format optimized for TensorFlow. Spark it is often used to process unstructured and large-scale datasets into smaller numerical datasets that can easily fit into a GPU. You can use this extension to save datasets in the TensorFLow record file format.
xgboost
The xgboost extension brings the well-known XGBoost modeling library to the world of large-scale computing. XGBoost is a scalable, portable and distributed library for gradient boosting. It became well known in the machine learning competition circles after its use in the winning solution of the Higgs Machine Learning Challenge (“Higgs Boson Machine Learning Challenge” 2019) and has remained popular in other Kaggle competitions since then.
variantspark
The variantspark extension provides an interface to use Variant Spark, a scalable toolkit for genome-wide association studies (GWAS). It currently provides functionality to build random forest models, estimating variable importance and reading Variant Call Format (VCF) files. While there are other random forest implementations in Spark, most of them are not optimized to deal with GWAS datasets, which usually come with thousands of samples and millions of variables.
geospark
The geospark extensions enables us to load and query large-scale geographic datasets. Usually datasets containing latitude and longitude points or complex areas defined in the Well-known Text (WKT) format, a text markup language for representing vector geometry objects on a map.

Before you learn how and when to use each extension, we should first briefly explain how extensions can be used with R and Spark.

First, a Spark extension is just and R package that happens to be aware of Spark. As any other R package, you will first have to install the R package. Once installed, it is important to know that you will need to reconnect to Spark before the extension can be used. So, in general, the pattern you should follow goes as follows:

library(sparkextension)
library(sparklyr)

sc <- spark_connect(master = "<master>")

Notice that sparklyr is loaded after the extensions to allow the extension to register properly. If you had to install and load a new extension you would simply have to disconnect first using spark_disconnect(sc), restart your R session, and repeat the steps above with the new extension.

It’s not hard to install and use Spark extensions from R; however, each extension can be a world on it’s own so most of the time you will have to spend time understanding what the extension is, when to use it and how to use it properly. The first extension you will learn about is the rsparkling extension which enables you to use H2O in Spark with R.

## 10.2 H2O

H2O is open-source software for large-scale modeling created by H2O.ai, which allows you to fit thousands of potential models as part of discovering patterns in data. You can consider using H2O to complement or replace Spark’s default modeling algorithms. It is common to Spark’s default modeling algorithms and transition to H2O when Spark’s algorithms fall short or when advanced functionality (like additional modeling metrics or automatic model selection) are desired

We can’t do justice to H2O’s great modeling capabilities in a single paragraph, explaining H2O properly will require a book in itself. Instead, we would like to recommend reading the “Practical machine learning with H2O” (Cook 2016) book to explore in-depth H2O’s modeling algorithms and features. In the meantime, you can use this section as a brief guide to get started using H2O in Spark with R.

In order to use H2O with Spark, it is important to know that there are four components involved: H2O, Sparkling Water, rsparkling and Spark. Sparkling Water allows users to combine the fast, scalable machine learning algorithms of H2O with the capabilities of Spark. You can think of Sparkling Water as a component bridging Spark with H2O and rsparkling as the R front-end for Sparkling Water, this is illustrated in Figure ??.

First, install rsparkling and h2o as specified in the rsparkling documentation site. (“RSparkling — H2o Sparkling Water 2.3.31 Documentation” 2019)

install.packages("h2o", type = "source",
repos = "http://h2o-release.s3.amazonaws.com/h2o/rel-yates/5/R")
install.packages("rsparkling", type = "source",
repos = "http://h2o-release.s3.amazonaws.com/sparkling-water/rel-2.3/31/R")

It is important to notice that you need to use compatible versions of Spark, Sparkling Water and H2O as specified in their docs; we present instructions for Spark 2.3, but using different Spark versions will require you to install different versions. So let’s start by checking the version of H2O by running,

packageVersion("h2o")
## [1] '3.24.0.5'
packageVersion("rsparkling")
## [1] '2.3.31'

We can then connect with the supported Spark versions as follows, you will have to adjust the master parameter for your particular cluster.

library(rsparkling)
library(sparklyr)
library(h2o)

sc <- spark_connect(master = "local", version = "2.3",
config = list(sparklyr.connect.timeout = 3 * 60))

cars <- copy_to(sc, mtcars)

H2O provides a web interface which can help you monitor training and access much of H2O’s functionality. The web interface can be accessed through h2o_flow(sc), it is referred to as H2O Flow and is shown in Figure ??.

When using H2O, you will have to convert your Spark DataFrame into and H2O DataFrame through as_h2o_frame:

cars_h2o <- as_h2o_frame(sc, cars)
cars_h2o
   mpg cyl disp  hp drat    wt  qsec vs am gear carb
1 21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
2 21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
3 22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
4 21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
5 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
6 18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

[32 rows x 11 columns] 

Then you can use many of the modeling functions available in the h2o package with ease. For instance, we can fit a generalized linear model with ease:

model <- h2o.glm(x = c("wt", "cyl"),
y = "mpg",
training_frame = cars_h2o,
lambda_search = TRUE)

H2O provides additional metrics not necessarily available in Spark’s modeling algorithms, the model that we just fit Residual Deviance is provided in the model while this would not be a standard metric when using Spark MLlib.

model
...
MSE:  6.017684
RMSE:  2.453097
MAE:  1.940985
RMSLE:  0.1114801
Mean Residual Deviance :  6.017684
R^2 :  0.8289895
Null Deviance :1126.047
Null D.o.F. :31
Residual Deviance :192.5659
Residual D.o.F. :29
AIC :156.2425

Then you can run prediction over the generalized linear model, a similar approach would work for many other models available in H2O:

predictions <- as_h2o_frame(sc, copy_to(sc, data.frame(wt = 2, cyl = 6)))
h2o.predict(model, predictions)
   predict
1 24.05984

[1 row x 1 column]

H2O can also be used to perform automatic training and tuning of many models; meaning that, H2O can choose which model to use for you using AutoML. (“AutoML: Automatic Machine Learning” 2019)

automl <- h2o.automl(x = c("wt", "cyl"), y = "mpg",
training_frame = cars_h2o,
max_models = 20,
seed = 1)

For this particular dataset, H2O finds out that a Deep Learning model is a better fit than GLM7. Specifically, H2O’s AutoML explored using XGBoost, Deep Learning, GLM and a Stacked Ensemble models.

automl@leaderboard
model_id              mean_residual_dev…     rmse      mse      mae     rmsle
1 DeepLearning_…                6.541322 2.557601 6.541322 2.192295 0.1242028
2 XGBoost_grid_1…               6.958945 2.637981 6.958945 2.129421 0.1347795
3 XGBoost_grid_1_…              6.969577 2.639996 6.969577 2.178845 0.1336290
4 XGBoost_grid_1_…              7.266691 2.695680 7.266691 2.167930 0.1331849
5 StackedEnsemble…              7.304556 2.702694 7.304556 1.938982 0.1304792
6 XGBoost_3_…                   7.313948 2.704431 7.313948 2.088791 0.1348819

Rather than using the leaderboard, you can focus on the best model through automl@leader; for example, you can glance at the particular parameters from this Deep Learning model as follows:

tibble::tibble(parameter = names(automl@leader@parameters),
value = as.character(automl@leader@parameters))
# A tibble: 20 x 2
parameter                         values
<chr>                             <chr>
1 model_id                          DeepLearning_grid_1_AutoML…
2 training_frame                    automl_training_frame_rdd…
3 nfolds                            5
4 keep_cross_validation_models      FALSE
5 keep_cross_validation_predictions TRUE
6 fold_assignment                   Modulo
7 overwrite_with_best_model         FALSE
8 activation                        RectifierWithDropout
9 hidden                            200
10 epochs                            10003.6618461538
11 seed                              1
12 rho                               0.95
13 epsilon                           1e-06
14 input_dropout_ratio               0.2
15 hidden_dropout_ratios             0.4
16 stopping_rounds                   0
17 stopping_metric                   deviance
18 stopping_tolerance                0.05
19 x                                 c("cyl", "wt")
20 y                                 mpg 

You can then predict using the leader as follows,

h2o.predict(automl@leader, predictions)
   predict
1 30.74639

[1 row x 1 column] 

Many additional examples are available under spark.rstudio.com/guides/h2o, you can also request help from github.com/h2oai/sparkling-water/tree/master/r, the official GitHub repository for the rsparkling package.

The next extension, graphframes, will allow you to process large-scale relational datasets; however, before you start using it, make sure to disconnect with spark_disconnect(sc) and restart your R session since using a different extensions requires you to reconnect to Spark and reload sparklyr.

## 10.3 Graphs

The first paper in the history of graph theory was written by Leonhard Euler on the Seven Bridges of Königsberg in 1736. The problem was to devise a walk through the city that would cross each of those bridges once and only, the original diagram is shown in Figure ??.

Today, a graph is defined as an ordered pair $$G=(V,E)$$, with $$V$$ a set of vertices (nodes or points) and $$E \subseteq \{\{x, y\} | (x, y) ∈ \mathrm{V}^2 \land x \ne y\}$$ a set of edges (links or lines) which are either an unordered pair for undirected graphs or an ordered pair for directed graphs. The former describing links where the direction does not matter and the latter linked where it does.

As a simple example, we can use the highschool dataset from the ggraph package which tracks friendship among high school boys. In this dataset, the vertices are the students and the edges describe pairs of students who happen to be friends in a particular year.

install.packages("ggraph")
install.packages("igraph")
ggraph::highschool
# A tibble: 506 x 3
from    to  year
<dbl> <dbl> <dbl>
1     1    14  1957
2     1    15  1957
3     1    21  1957
4     1    54  1957
5     1    55  1957
6     2    21  1957
7     2    22  1957
8     3     9  1957
9     3    15  1957
10     4     5  1957
# … with 496 more rows

While the highschool dataset can be easily processed in R, even medium size graph datasets can be hard to process without distributing this work across a cluster of machines which Spark is suited for. Spark supports processing graphs through the graphframes extension which in turn uses the GraphX Spark component. GraphX is Apache Spark’s API for graphs and graph-parallel computation and allows you to seamlessly work with both graphs and collection, it’s comparable in performance to the fastest specialized graph processing systems and provides a growing library of graph algorithms.

A graph in Spark is also represented as a dataframe of edges and vertices; however, the format is slightly different since we will need to construct a dataframe for vertices. Lets first install the GraphFrames extension,

install.packages("graphframes")

Followed by connecting, copying the highschool dataset and transforming the graph to the format that this extension expects, we will scope this dataset to the friendships of year 1957.

library(graphframes)
library(sparklyr)
library(dplyr)

sc <- spark_connect(master = "local", version = "2.3")
highschool_tbl <- copy_to(sc, ggraph::highschool, "highschool") %>%
filter(year == 1957) %>%
transmute(from = as.character(as.integer(from)),
to = as.character(as.integer(to)))

from_tbl <- highschool_tbl %>% distinct(from) %>% transmute(id = from)
to_tbl <- highschool_tbl %>% distinct(to) %>% transmute(id = to)

vertices_tbl <- distinct(sdf_bind_rows(from_tbl, to_tbl))
edges_tbl <- highschool_tbl %>% transmute(src = from, dst = to)

The vertices_tbl table is expected to have a single id column:

vertices_tbl
# Source: spark<?> [?? x 1]
id
<chr>
1 1
2 34
3 37
4 43
5 44
6 45
7 56
8 57
9 65
10 71
# … with more rows

While the edges_tbl is expected to have a src and dst columns:

edges_tbl
# Source: spark<?> [?? x 2]
src   dst
<chr> <chr>
1 1     14
2 1     15
3 1     21
4 1     54
5 1     55
6 2     21
7 2     22
8 3     9
9 3     15
10 4     5
# … with more rows

You can now create a GraphFrame,

graph <- gf_graphframe(vertices_tbl, edges_tbl)

We can now use this graph to start analyzing this dataset. For instance, by finding out how many friends on average every one has, this is referred as the degree or valency of a vertex:

gf_degrees(graph) %>% summarise(friends = mean(degree))
# Source: spark<?> [?? x 1]
friends
<dbl>
1    6.94

We can then find what the shortest path to some specific vertex (person for this dataset). Since the data is anonymized, we can just pick the person identified as 33 and find how many degrees of separation exist between them:

gf_shortest_paths(graph, 33) %>%
filter(size(distances) > 0) %>%
mutate(distance = explode(map_values(distances))) %>%
select(id, distance)
# Source: spark<?> [?? x 2]
id    distance
<chr>    <int>
1 19           5
2 5            4
3 27           6
4 4            4
5 11           6
6 23           4
7 36           1
8 26           2
9 33           0
10 18           5
# … with more rows

Finally, we can also compute PageRank over this graph, which was presented in the Introduction chapter as Google’s web page ranking algorithm:

gf_graphframe(vertices_tbl, edges_tbl) %>%
gf_pagerank(reset_prob = 0.15, max_iter = 10L)
GraphFrame
Vertices:
Database: spark_connection
$id <dbl> 12, 12, 14, 14, 27, 27, 55, 55, 64, 64, 41, 41, 47, 47, 6…$ pagerank <dbl> 0.3573460, 0.3573460, 0.3893665, 0.3893665, 0.2362396, 0.…
Edges:
Database: spark_connection
$src <dbl> 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 12, 12, 12,…$ dst    <dbl> 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17,…
$weight <dbl> 0.25000000, 0.25000000, 0.25000000, 0.25000000, 0.25000000,… To give you some insights into this dataset, Figure ?? plots this chart using the ggraph and highlights the highest PageRank scores for this dataset, highschool_tbl %>% igraph::graph_from_data_frame(directed = FALSE) %>% ggraph(layout = 'kk') + geom_edge_link(alpha = 0.2, arrow = arrow(length = unit(2, 'mm')), end_cap = circle(2, 'mm'), start_cap = circle(2, 'mm')) + geom_node_point(size = 2, alpha = 0.4) There are many more graph algorithms provided in graphframes, to mention some: bread depth search, connected components, label propagation for detecting communities, strongly connected components and triangle count. For questions on this extension refer to the official GitHub repo, github.com/rstudio/graphframes. We will now present a popular gradient boosting framework, make sure to disconnect, restart before trying the next extension. ## 10.4 XGBoost A decision tree is a flowchart-like structure in which each internal node represents a test on an attribute, each branch represents the outcome of the test, and each leaf node represents a class label. For example, the diagram in Figure ?? shows an a decision tree that could help classify if an employee is likely to leave given a set of factors, like job satisfaction and overtime. When a decision tree is used to predict continuous variables instead of discrete outcomes, say, how likely someone is to leave a company, decision trees are referred to as regression trees. While a decision tree representation is quite easy to understand and to interpret, finding out the decisions in the tree requires mathematical techniques like gradient descent to find a local minimum. Gradient descent takes steps proportional to the negative of the gradient of the function at the current point. The gradient is represented by $$\nabla$$, the learning rate by $$\gamma$$ and one simply starts from a given state $$a_n$$ and compute the next iteration $$a_{n+1}$$ by simply following the direction of the gradient: $$a_{n+1} = a_n - \gamma \nabla F(a_n)$$ XGBoost is an open-source software library which provides a gradient boosting framework. It aims to provide a scalable, portable and distributed gradient boosting for training gradient-boosted decision trees (GBDT) and gradient-boosted regression trees (GBDT). Gradient-boosted means xgboost uses gradient descent and boostrin, which is a technique that chooses each predictor sequentially. sparkxgb is an extension that you can use to train XGBoost models in Spark; however, please beware that currently Windows is unsupported. To use this extension, first install it from CRAN: install.packages("sparkxgb") Then you would need to import the sparkxgb extension followed by your usual Spark connection code, adjusting master as needed: library(sparkxgb) library(sparklyr) library(dplyr) sc <- spark_connect(master = "local", version = "2.3") For this example, we will use the attrition dataset from the rsample package which you would need to install with install.packages("rsample"). This dataset is a fictional dataset created by IBM data scientists to uncover the factors that lead to employee attrition. attrition <- copy_to(sc, rsample::attrition) attrition # Source: spark<?> [?? x 31] Age Attrition BusinessTravel DailyRate Department DistanceFromHome <int> <chr> <chr> <int> <chr> <int> 1 41 Yes Travel_Rarely 1102 Sales 1 2 49 No Travel_Freque… 279 Research_… 8 3 37 Yes Travel_Rarely 1373 Research_… 2 4 33 No Travel_Freque… 1392 Research_… 3 5 27 No Travel_Rarely 591 Research_… 2 6 32 No Travel_Freque… 1005 Research_… 2 7 59 No Travel_Rarely 1324 Research_… 3 8 30 No Travel_Rarely 1358 Research_… 24 9 38 No Travel_Freque… 216 Research_… 23 10 36 No Travel_Rarely 1299 Research_… 27 # … with more rows, and 25 more variables: Education <chr>, # EducationField <chr>, EnvironmentSatisfaction <chr>, Gender <chr>, # HourlyRate <int>, JobInvolvement <chr>, JobLevel <int>, JobRole <chr>, # JobSatisfaction <chr>, MaritalStatus <chr>, MonthlyIncome <int>, # MonthlyRate <int>, NumCompaniesWorked <int>, OverTime <chr>, # PercentSalaryHike <int>, PerformanceRating <chr>, # RelationshipSatisfaction <chr>, StockOptionLevel <int>, # TotalWorkingYears <int>, TrainingTimesLastYear <int>, # WorkLifeBalance <chr>, YearsAtCompany <int>, YearsInCurrentRole <int>, # YearsSinceLastPromotion <int>, YearsWithCurrManager <int> To build an XGBoost model in Spark use xgboost_classifier(), we will compute attrition against all other features by using the Attrition ~ . formula and specify two for the number of classes since the attrition attribute tracks only whether an employee leaves or not. Then you can use ml_predict() to predict over large-scale datasets: xgb_model <- xgboost_classifier(attrition, Attrition ~ ., num_class = 2, num_round = 50, max_depth = 4) xgb_model %>% ml_predict(attrition) %>% select(Attrition, predicted_label, starts_with("probability_")) %>% glimpse() Observations: ?? Variables: 4 Database: spark_connection$ Attrition       <chr> "Yes", "No", "Yes", "No", "No", "No", "No", "No", "No", …
$predicted_label <chr> "No", "Yes", "No", "Yes", "Yes", "Yes", "Yes", "Yes", "Y…$ probability_No  <dbl> 0.753938094, 0.024780750, 0.915146366, 0.143568754, 0.07…
$probability_Yes <dbl> 0.24606191, 0.97521925, 0.08485363, 0.85643125, 0.927375… XGBoost became well known in the competition circles after its use in the winning solution of the Higgs Machine Learning Challenge which uses the ATLAS experiment to identify the Higgs boson. Since then, it has become a popular model and used for a large number of Kaggle competitions. However, decision trees could prove limiting especially in datasets with non tabular data like images, audio and text which you can tackle with deep learning models, should we remind you to disconnect and restart? ## 10.5 Deep Learning A perceptron is a mathematical model introduced by Rosenblatt (Rosenblatt 1958) who developed it as a theory for a hypothetical nervous system. The perceptron maps stimuli to numeric inputs that are weighted into a threshold function that activates only when enough stimuli is present, mathematically: $$f(x) = \begin{cases} 1 & \sum_{i=1}^m w_i x_i + b > 0\\ 0 & \text{otherwise} \end{cases}$$ Minsky found out that a single perceptron can only classify datasets that are linearly separable; however, he also presented in his perceptrons book (Minsky and Papert 2017) that layering perceptrons would bring additional classification capabilities, the original diagram showcasing a multi-layered perceptron is presented in Figure ??. Before we start, let’s first install all the packages we are about to use, install.packages("sparktf") install.packages("tfdatasets") Using Spark we can create a multi-layer perceptron classifier with ml_multilayer_perceptron_classifier() and gradient descent to classify and predict over large datasets. Gradient descent was introduced to layered perceptrons by Geoff Hinton (Ackley, Hinton, and Sejnowski 1985) and like XGBoost, it also makes use of gradient descent. library(sparktf) library(sparklyr) sc <- spark_connect(master = "local", version = "2.3") attrition <- copy_to(sc, rsample::attrition) nn_model <- ml_multilayer_perceptron_classifier( attrition, Attrition ~ Age + DailyRate + DistanceFromHome + MonthlyIncome, layers = c(4, 3, 2), solver = "gd") nn_model %>% ml_predict(attrition) %>% select(Attrition, predicted_label, starts_with("probability_")) %>% glimpse() Observations: ?? Variables: 4 Database: spark_connection$ Attrition       <chr> "Yes", "No", "Yes", "No", "No", "No", "No", "No", "No"…
$predicted_label <chr> "No", "No", "No", "No", "No", "No", "No", "No", "No", …$ probability_No  <dbl> 0.8439275, 0.8439275, 0.8439275, 0.8439275, 0.8439275,…
$probability_Yes <dbl> 0.1560725, 0.1560725, 0.1560725, 0.1560725, 0.1560725,… Notice that the columns must be numeric so you will have to manually convert them with feature transforming techniques presented in the Modeling chapter. It is natural to try to add more layers to classify more complex datasets; however, adding too many layers will cause the gradient to vanish and other techniques will have to use these deep layered networks also known as, deep learning models. Deep learning models solve the vanishing gradient problem by making use of special activation functions, dropout, data augmentation and graphic processing units (GPUs). You can use Spark to retrieve and preprocess large datasets into numerical-only datasets that can fit in a GPU for deep learning training. Tensorflow is one of the most popular deep learning frameworks and supports a binary format known as TensorFlow Records. You can write TensorFlow records using the sparktf in Spark which you can prepare to process in GPU instances with libraries like Keras or TensorFlow. You can then preprocess large datasets in Spark and write it as TensorFlow records using spark_write_tf(): copy_to(sc, iris) %>% ft_string_indexer_model( "Species", "label", labels = c("setosa", "versicolor", "virginica") ) %>% spark_write_tfrecord(path = "tfrecord") Once trained, you can use the tfdatasets package to load the dataset followed by training with keras or tensorflow. You will also need to install the TensorFlow runtime with install_tensorflow() and install Python on your own. To learn more about training deep learning models with Keras we recommend reading “Deep Learning with R”. (Chollet and Allaire 2018) tensorflow::install_tensorflow() tfdatasets::tfrecord_dataset("tfrecord/part-r-00000") <DatasetV1Adapter shapes: (), types: tf.string> Training deep learning models in a single local node with one or more GPUs is often enough for most applications; however, recent state-of-the-art deep learning models train using distributed computing frameworks like Apache Spark. Distributed computing frameworks are used to achieve higher petaflops each day the systems spends training these models. OpenAI analyzed trends in the field of Artificial Intelligence and cluster computing (“AI and Compute” 2019) and illustrated in Figure ??. It should be obvious from the figure that there is a trend in recent years to use distributed computing frameworks. Training large-scale deep learning models is possible in Spark and TensorFlow through frameworks like Horovod. Today, it’s possible to use Horovod with Spark from R using the reticulate package since Horovod requires Python and Open MPI which goes beyond the scope of this book. Instead, we will introduce a different Spark extension in the domain of genomics. ## 10.6 Genomics The human genome consists of two copies of about three billion base pairs of DNA within the 23 chromosome pairs, Figure ?? shows the organization of the genome into chromosomes. DNA strands are composed of nucleotides, each composed of one of four nitrogen-containing nucleobases: cytosine (C), guanine (G), adenine (A) or thymine (T). (“Human Genome” 2019) Since the DNA of all humans is nearly identical, we only need to store the differences from the reference genome in the form of a Variant Call Format (VCF) file. VariantSpark is a framework based on scala and spark to analyze genome datasets. It is being developed by CSIRO Bioinformatics team in Australia. VariantSpark was tested on datasets with 3000 samples each one containing 80 million features in either unsupervised clustering approaches and supervised applications, like classification and regression. VariantSpark can read VCF files and run analyses while using familiar Spark DataFrames. To get started, install variantspark from CRAN, connect to Spark and retrieve a vsc connection to VariantSpark: library(variantspark) library(sparklyr) sc <- spark_connect(master = "local", version = "2.3", config = list(sparklyr.connect.timeout = 3 * 60)) vsc <- vs_connect(sc) We can start by loading a VCF file, vsc_data <- system.file("extdata/", package = "variantspark") hipster_vcf <- vs_read_vcf(vsc, file.path(vsc_data, "hipster.vcf.bz2")) hipster_labels <- vs_read_csv(vsc, file.path(vsc_data, "hipster_labels.txt")) labels <- vs_read_labels(vsc, file.path(vsc_data, "hipster_labels.txt")) VariantSpark uses Random Forest to assign an Importance score to each tested variant reflecting its association to the interest phenotype. A variant with higher Importance score implies it is more strongly associated with the phenotype of interest. You can compute the Importance and transform it into a Spark table as follows, importance_tbl <- vs_importance_analysis(vsc, hipster_vcf, labels, n_trees = 100) %>% importance_tbl() importance_tbl # Source: spark<?> [?? x 2] variable importance <chr> <dbl> 1 2_109511398 0 2 2_109511454 0 3 2_109511463 0.00000164 4 2_109511467 0.00000309 5 2_109511478 0 6 2_109511497 0 7 2_109511525 0 8 2_109511527 0 9 2_109511532 0 10 2_109511579 0 # … with more rows You can then use dplyr and ggplot2 to transform the output and visualize it, library(dplyr) library(ggplot2) importance_df <- importance_tbl %>% arrange(-importance) %>% head(20) %>% collect() ggplot(importance_df) + aes(x = variable, y = importance) + geom_bar(stat = 'identity') + scale_x_discrete(limits = importance_df[order(importance_df$importance), 1]$variable) + coord_flip() This concludes a brief introduction to genomic analysis in Spark using the variantspark extensions. Next, we will move away from microscopic genes, to macroscopic datasets that contain geographic locations across the world. ## 10.7 Spatial geospark enables distributed geospatial computing using a grammar compatible with dplyr and sf package which provides a set of tools for working with geospatial vectors. You can install geospark from GitHub as follows: install.packages("remotes") remotes::install_github("r-spark/geospark") Then we will initialize the geospark extension and connect to Spark: library(geospark) library(sparklyr) sc <- spark_connect(master = "local", version = "2.3") Next we will load a spatial dataset containing polygons and points. polygons <- system.file("examples/polygons.txt", package="geospark") %>% read.table(sep="|", col.names = c("area", "geom")) points <- system.file("examples/points.txt", package="geospark") %>% read.table(sep = "|", col.names = c("city", "state", "geom")) polygons_wkt <- copy_to(sc, polygons) points_wkt <- copy_to(sc, points) There are various spatial operations defined in geospark, which Figure ?? describes. These operations allow you to control how geospatial data should be queried based on overlap, intersection, disjoint sets, etc. For instance, we can use these operations to find the polygons that contain a given set of points using st_contains(), library(dplyr) polygons_wkt <- mutate(polygons_wkt, y = st_geomfromwkt(geom)) points_wkt <- mutate(points_wkt, x = st_geomfromwkt(geom)) inner_join(polygons_wkt, points_wkt, sql_on = sql("st_contains(y,x)")) %>% group_by(area, state) %>% summarise(cnt = n())  # Source: spark<?> [?? x 3] # Groups: area area state cnt <chr> <chr> <dbl> 1 california area CA 10 2 new york area NY 9 3 dakota area ND 10 4 texas area TX 10 5 dakota area SD 1 You can also plot these datasets by collecting a subset of the entire dataset or aggregating the geometries in spark before collecting them, one package you should look into is the sf package. We will now start closing this chapter by presenting a couple troubleshooting techniques applicable to all extensions. ## 10.8 Troubleshooting When using a new extension for the first time, we recommend increasing the connection timeout since Spark will usually have to download extension dependencies and changing logging to verbose to help troubleshoot when the download process does not complete: config <- spark_config() config$sparklyr.connect.timeout <- 3 * 60
config\$sparklyr.log.console = TRUE

sc <- spark_connect(master = "local", config = config)

In addition, you should know that Apache IVY is a popular dependency manager focusing on flexibility, simplicity, and is used by Apache Spark while installing extensions. When the connection fails while using an extension, consider clearing your IVY Cache by running:

unlink("~/.ivy2", recursive = TRUE)

In addition, you can also consider opening GitHub issues from the following extensions repos to get help from the extension authors:

## 10.9 Recap

This chapter provided a brief overview on using some of the Spark extensions available in R, which happens to be as easy as installing a package. You then learned how to use the rsparkling extension which provides access to H2O in Spark to which provides additional modeling functionality like enhanced metrics and ability to automatically select models. We then jumped to graphframes, an extension to help you process relational datasets which are formally referred as graphs; you learned how to compute simple connection metrics or run complex algorithms like pagerank.

THe XGBoost and Deep Learning sections provided alternate modeling techniques that use gradient descent, the former over decision trees and the latter over deep multi-layered perceptrons where Spark can be used to preprocess datasets into records that can then be later consument by TensorFlow and Keras using the sparktf extension. The last two sections introduced extensions to process genomic and spatial datasets through the variantspark and geospark extensions.

These extensions and many more, provide a comprehensive library of advanced functionality that in combination with analysis and modeling techniques presented, should cover most tasks required to run in computing clusters. However, when functionality is lacking, you can consider writing your own extension as we will present in the Contributing chapter or you can apply custom transformations over each partition using R code as we will describe in the next chapter, Distributed R.

### References

“CRAN - Package Sparklyr.” 2019. https://cran.r-project.org/web/packages/sparklyr/index.html.

“Higgs Boson Machine Learning Challenge.” 2019. https://www.kaggle.com/c/higgs-boson.

Cook, Darren. 2016. Practical Machine Learning with H2o: Powerful, Scalable Techniques for Deep Learning and Ai. O’Reilly Media, Inc.

“RSparkling — H2o Sparkling Water 2.3.31 Documentation.” 2019. http://docs.h2o.ai/sparkling-water/2.3/latest-stable/doc/rsparkling.html.

“AutoML: Automatic Machine Learning.” 2019. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/automl.html.

Rosenblatt, Frank. 1958. “The Perceptron: A Probabilistic Model for Information Storage and Organization in the Brain.” Psychological Review.

Minsky, Marvin, and Seymour A Papert. 2017. Perceptrons: An Introduction to Computational Geometry. MIT press.

Ackley, David H, Geoffrey E Hinton, and Terrence J Sejnowski. 1985. “A Learning Algorithm for Boltzmann Machines.” Cognitive Science.

Chollet, Francois, and J.J. Allaire. 2018. Deep Learning with R. Manning Publications.

“AI and Compute.” 2019. https://openai.com/blog/ai-and-compute.

“Human Genome.” 2019. https://en.wikipedia.org/wiki/Human_genome.

1. Notice that AutoML uses cross-validation which we did not use in GLM.