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VHT: Vertical hoeffding tree

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IoT big data requires new machine learning methods able to scale to large size of data arriving at high speed. Decision trees are popular machine learning models since they are very effective, yet easy to interpret and visualize. In the literature, we can find distributed algorithms for learning decision trees, and also streaming algorithms, but not algorithms that combine both features. In this paper we present the Vertical Hoeffding Tree (VHT), the first distributed streaming algorithm for learning decision trees. It features a novel way of distributing decision trees via vertical parallelism. The algorithm is implemented on top of Apache SAMOA, a platform for mining big data streams, and thus able to run on real-world clusters. Our experiments to study the accuracy and throughput of VHT prove its ability to scale while attaining superior performance compared to sequential decision trees.
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VHT: Vertical Hoeffding Tree
Nicolas Kourtellis Gianmarco De Francisci Morales Albert Bifet Arinto Murdopo
Telefonica Research Qatar Computing Research Institute Telecom ParisTech LARC-SMU
nicolas.kourtellis@telefonica.com gdfm@acm.org albert@albertbifet.com arintom@smu.edu.sg
ABSTRACT
IoT Big Data requires new machine learning methods able
to scale to large size of data arriving at high speed. Deci-
sion trees are popular machine learning models since they
are very effective, yet easy to interpret and visualize. In
the literature, we can find distributed algorithms for learn-
ing decision trees, and also streaming algorithms, but not
algorithms that combine both features. In this paper we
present the Vertical Hoeffding Tree (VHT), the first dis-
tributed streaming algorithm for learning decision trees. It
features a novel way of distributing decision trees via ver-
tical parallelism. The algorithm is implemented on top of
Apache SAMOA, a platform for mining distributed data
streams, and thus able to run on real-world clusters. We
run several experiments to study the accuracy and through-
put performance of our new VHT algorithm, as well as its
ability to scale while keeping its superior performance with
respect to non-distributed decision trees.
1. INTRODUCTION
Nowadays, we generate data from many of our daily ac-
tivities as we interact with software systems continuously.
The posts in a social network like Twitter or Facebook, the
purchases with a credit card, the clicks in a website, or the
access to the GPS, can all potentially produce useful in-
formation for interested parties. The recent advancements
in mobile devices and wearable technology have further in-
creased the rate and amount of data being generated. People
now generate data anywhere, anytime, by using a multitude
of gadgets and technologies. In the limit, the Internet of
Things (IoT) will continuously produce data without any
human intervention, thus leading to a dramatic increase vol-
ume and velocity of data. It is estimated that the IoT will
consist of almost 50 billion objects by 2020.1
There is a common pattern to most modern data sources:
data is generated continuously, as a stream. Extracting
1http://www.cisco.com/c/dam/en_us/about/ac79/docs/
innov/IoT_IBSG_0411FINAL.pdf
knowledge from these massive streams of data to create
models, and using them, e.g., to choose a suitable business
strategy, or to improve healthcare services, can generate sub-
stantial competitive advantages. Many applications need to
process incoming data and react on-the-fly by using compre-
hensible prediction mechanisms. For example, when a bank
monitors the transactions of its clients to detect frauds, it
needs to identify and verify a fraud as soon as the transac-
tion is performed, and immediately either block it, or adjust
the prediction mechanism.
Streaming data analytic systems need to process and man-
age data streams in a fast and efficient way, due to the strin-
gent restrictions in terms of time and memory imposed by
the streaming setting. The input to the system is an un-
bounded stream arriving at high speed. Therefore, we need
to use simple models that scale gracefully with the amount
of data. Additionally, we need to let the model take the
right decision online. But how can we trust that the model
is right? A way to create trust is to enhance understand-
ing of the model and its interpretability, for instance via
visualization. There are several models which satisfy both
requirements, however, for reasons we discuss further, in this
work we focus on decision trees.
A decision tree is a classic decision support tool that uses
a tree-like model. In machine learning, it can be used for
both classification and regression [4]. At its core, a decision
tree is a model where internal nodes are tests on attributes,
branches are possible outcomes of these tests, and leafs are
decisions, e.g., a class assignment.
Decision trees, and in general tree-based classifiers, are
widely popular, for several reasons. First, the model is very
easy to interpret. It is easy to understand how the model
reaches a classification decision, and the relative importance
of features. Trees are also easy to visualize, and to modify
according to domain knowledge. Second, prediction is very
fast. Once the model is trained, classifying a new instance
requires just a logarithmic number of very fast checks (in the
size of the model). For this reason, they are commonly used
in one of the most time-sensitive domains nowadays – Web
search [5, 16]. Third, trees are powerful classifiers that can
model non-linear relationships. Indeed, their performance,
especially when used in ensemble methods such as boosting,
bagging, and random forests, is outstanding [9].
Learning the optimal decision tree for a given labeled
dataset is NP-complete even for very simple settings [13].
Practical methods for building tree models usually employ
a greedy heuristic that optimizes decisions locally at each
node [4]. In a nutshell, the greedy heuristic starts with
1
arXiv:1607.08325v1 [cs.DC] 28 Jul 2016
an empty node (the root) as the initial model, and works
by recursively sorting the whole dataset through the cur-
rent model. Each leaf of the tree collects statistics on the
distribution of attribute-class co-occurrences in the part of
dataset that reaches the leaf. When all the dataset has been
analyzed, each leaf picks the best attribute according to a
splitting criterion (e.g., entropy or information gain). Then,
it becomes an internal node that branches on that attribute,
splits the dataset into newly created children leaves, and
calls the procedure recursively for these leaves. The proce-
dure usually stops when the leaf is pure (i.e., only one class
reaches the leaf), or when the number of instances reaching
the leaf is small enough. This recursive greedy heuristic is
inherently a batch process, as it needs to process the whole
dataset before taking a split decision. However, streaming
variants of tree learners also exist.
The Hoeffding tree [8] (a.k.a. VFDT) is a streaming de-
cision tree learner with statistical guarantees. In particular,
by leveraging the Chernoff-Hoeffding bound [12], it guaran-
tees that the learned model is asymptotically close to the
model learned by the batch greedy heuristic, under mild as-
sumptions.
The learning algorithm is very simple. Each leaf keeps
track of the statistics for the portion of the stream it is
reached by, and computes the best two attributes according
to the splitting criterion. Let ∆Gbe the difference between
the value of the functions that represent the splitting cri-
terion of these two attributes. Let be a quantity that
depends on a user-defined confidence parameter δ, and that
decreases with the number of instances processed. When
G > , then the currently best attribute is selected to
split the leaf. The Hoeffding bound guarantees that this
choice is the correct one with probability larger than 1 δ.
Streaming algorithms are only one of the two main ways
to deal with massive datasets, the other being distributed
algorithms [6]. However, even though streaming algorithms
are very efficient, they are still bounded by the limits of a
single machine. As argued by Agarwal et al. [1], “there are
natural reasons for studying distributed machine learning
on a cluster.” Nowadays, the data itself is usually already
distributed, and the cost of moving it to a single machine is
too high. Furthermore, cluster computing with commodity
servers is economically more viable than using powerful sin-
gle machines, as testified by innumerable web companies [2].
Finally, “the largest problem solvable by a single machine
will always be constrained by the rate at which the hard-
ware improves, which has been steadily dwarfed by the rate
at which our data sizes have been increasing over the past
decade” [1].
For all the aforementioned reasons, the goal of the cur-
rent work is to propose a tree learning algorithm for the
streaming setting that runs in a distributed environment.
By combining the efficiency of streaming algorithms with
the scalability of distributed processing we aim at provid-
ing a practical tool to tackle the complexities of “big data”,
namely its velocity and volume. Specifically, we develop our
algorithm in the context of Apache SAMOA [7], an open-
source platform for mining big data streams.2
We name our algorithm the Vertical Hoeffding Tree (VHT).
The vertical part stands for the type of parallelism we em-
ploy, namely, vertical data parallelism. Similarly to the orig-
2http://samoa.incubator.apache.org
inal Hoeffding tree, the VHT features anytime prediction
and continuous learning.
Naturally, the combination of streaming and distributed
algorithms presents its own unique challenges. Other ap-
proaches have been proposed for parallel algorithms, which
however do not take into account the characteristics of mod-
ern, shared-nothing cluster computing environments [3].
Concisely, we make the following contributions:
we propose VHT, the first distributed streaming algo-
rithms for learning decision trees;
in doing so, we explore a novel way of parallelizing deci-
sion trees via vertical parallelism;
we deploy our algorithm on top of SAMOA, and run
it on a real-world Storm cluster to test scalability and
accuracy;
we experiment with large datasets of tens of thousands
of attributes and obtain high accuracy (up to 80%) and
high throughput (offering up to 20xspeedup over serial
streaming solutions).
The outline of the paper is as follows. We discuss related
work in Section 2, and some preliminary concepts in Sec-
tion 3. We present the new VHT algorithm in Section 4, var-
ious optimization and implementation details in Section 5,
and an empirical evaluation in Section 6, with several ex-
perimental setups on real and synthetic datasets. Finally,
with Section 7 we conclude this work.
2. RELATED WORK
The literature abounds with streaming and distributed
machine learning algorithms, though none of these features
both characteristics simultaneously. Reviewing all these al-
gorithms is out of the scope of this paper, so we focus our
attention on decision trees. We also review the few attempts
at creating distributed streaming learning algorithms that
have been proposed so far.
Algorithms. One of the pioneer works in decision tree
induction for the streaming setting is the Very Fast Decision
Tree algorithm (VFDT) [8]. This work focuses on alleviating
the bottleneck of machine learning application in terms of
time and memory, i.e. the conventional algorithm is not able
to process it due to limited processing time and memory. Its
main contribution is the usage of the Hoeffding Bound to
decide the number of data required to achieve certain level of
confidence. This work has been the basis for a large number
of improvements, such as dealing with concept drift [11] and
handling continuous numeric attributes [10].
PLANET [15] is a framework for learning tree models
on massive datasets. This framework utilizes MapReduce
to provide scalability. The authors propose a PLANET
scheduler to transform steps in decision tree induction into
MapReduce jobs that can be executed in parallel. PLANET
uses task-parallelism where each task (node splitting) is ex-
ecuted by one MapReduce jobs that runs independently.
Clearly, MapReduce is a batch programming paradigm which
is not suited to deal with streams of data.
Ye et al. [20] show how to distribute and parallelize Gra-
dient Boosted Decision Trees (GBDT). The authors first
implement MapReduce-based GBDT that employs horizon-
tal data partitioning. Converting GBDT to MapReduce
model is fairly straightforward. However, due to high over-
head from HDFS as communication medium when splitting
nodes, the authors conclude that MapReduce is not suitable
2
for this kind of algorithm. The authors then implement
GBDT by using MPI. This implementation uses vertical
data partitioning by splitting the data based on their at-
tributes. This partitioning technique minimizes inter-machine
communication cost. Vertical parallelism is also the data
partitioning strategy we choose for the VHT algorithm.
While technically not a tree, Vu et al. [19] propose the first
distributed streaming rule-based regression algorithm. The
algorithm is in spirit similar to the VHT, as it uses vertical
parallelism and runs on top of distributed SPEs. However,
it creates a different kind of model and deals with regression
rather than classification.
Frameworks. We identify two frameworks that belong
to the category of distributed streaming machine learning:
Jubatus and StormMOA. Jubatus3is an example of dis-
tributed streaming machine learning framework. It includes
a library for streaming machine learning such as regres-
sion, classification, recommendation, anomaly detection and
graph mining. It introduces the local ML model concept
which means there can be multiple models running at the
same time and they process different sets of data. Using this
technique, Jubatus achieves horizontal scalability via hori-
zontal parallelism in partitioning data. We test horizontal
parallelism in our experiments, by implementing a horizon-
tally scaled version of the hoeffding tree.
Jubatus establishes tight coupling between the machine
learning library implementation and the underlying distributed
stream processing engine (SPE). The reason is Jubatus builds
and implements its own custom distributed SPE. In addi-
tion, Jubatus does not offer any tree learning algorithm, as
all of its models need to be linear by construction.
StormMOA4is a project to combine MOA with Storm to
satisfy the need of scalable implementation of streaming ML
frameworks. It uses Storm’s Trident abstraction and MOA
library to implement OzaBag and OzaBoost[14].
Similarly to Jubatus, StormMOA also establishes tight
coupling between MOA (the machine learning library) and
Storm (the underlying distributed SPE). This coupling pre-
vents StormMOA’s extension by using other SPEs to exe-
cute the machine learning library.
StormMOA only allows to run a single model in each
Storm bolt (processor). This characteristic restricts the kind
of models that can be run in parallel to ensembles. The
sharding algorithm we use in the experimental section can
be seen as an instance of this type of framework.
3. PRELIMINARIES
This section introduces the background needed to under-
stand the VHT algorithm. First, we review the literature on
inducing decision trees on a stream. Then, we present the
programming paradigm offered by Apache SAMOA.
3.1 Hoeffding Tree
A decision tree consists of a tree structure, where each
internal node corresponds to a test on an attribute. The
node splits into a branch for each attribute value (for dis-
crete attributes), or a set of branches according to ranges of
the value (for continuous attributes). Leaves contain clas-
sification predictors, usually majority class classifiers, i.e.,
3http://jubat.us/en
4http://github.com/vpa1977/stormmoa
Algorithm 1 HoeffdingTreeInduction(X,HT ,δ)
Require: X, a labeled training instance.
Require: HT , the current decision tree.
1: Use HT to sort Xinto a leaf l
2: Update sufficient statistic in l
3: Increment nl, the number of instances seen at l
4: if nlmod nmin = 0 and not all instances seen at l
belong to the same class then
5: For each attribute, compute Gl(Xi)
6: Let Xabe the attribute with highest Gl
7: Let Xbbe the attribute with second highest Gl
8: Compute the Hoeffding bound =qR2ln(1)
2nl
9: if Xa6=Xand (Gl(Xa)Gl(Xb)>  or  < τ )
then
10: Replace lwith an internal node branching on Xa
11: for all branches of the split do
12: Add a new leaf with derived sufficient statistic
from the split node
13: end for
14: end if
15: end if
each leaf predicts the class belonging to the majority of the
instances that reach the leaf.
Decision tree models are very easy to interpret and vi-
sualize. The class predicted by a tree can be explained in
terms of a sequence of tests on its attributes. Each attribute
contributes to the final decision, and it’s easy to understand
the importance of each attribute.5
The Hoeffding tree or VFDT is a very fast decision tree for
streaming data. Its main characteristic is that rather than
reusing instances recursively down the tree, it uses them
only once. Algorithm 1 shows a high-level description of the
Hoeffding tree.
At the beginning of the learning phase, it creates a tree
with only a single node. The Hoeffding tree induction Algo-
rithm 1 is invoked for each training instance Xthat arrives.
First, the algorithm sorts the instance into a leaf l(line 1).
This leaf is a learning leaf, therefore the algorithm updates
the sufficient statistic in l(line 2). In this case, the sufficient
statistic is the class distribution for each value of each at-
tribute. In practice, the algorithm increases a counter nijk ,
for attribute i, value j, and class k. The algorithm also in-
crements the number of instances (nl) seen at leaf lbased
on X’s weight (line 3).
A single instance usually does not change the distribution
significantly enough, therefore the algorithm tries to grow
the tree only after a certain number of instances nmin has
been sorted to the leaf. In addition, the algorithm does not
grow the tree if all the instances that reached lbelong to
the same class (line 4).
To grow the tree, the algorithm attempts to find a good
attribute to split the leaf on. The algorithm iterates through
each attribute and calculates the corresponding splitting cri-
terion Gl(Xi) (line 5). This criterion is an information-
theoretic function, such as entropy or information gain, which
is computed by making use of the counters nijk. The algo-
rithm also computes the criterion for the scenario where no
5http://blog.datadive.net/interpreting-random-
forests
3
split takes places (X). Domingos and Hulten [8] refer to this
inclusion of a no-split scenario with the term pre-pruning.
The algorithm then chooses the best (Xa) and the second
best (Xb) attributes based on the criterion (lines 6 and 7).
By using these chosen attributes, it computes the difference
of their splitting criterion values ∆Gl=Gl(Xa)Gl(Xb).
To determine whether the leaf needs to be split, it compares
the difference ∆Glto the Hoeffding bound for the current
confidence parameter δ(where Ris the range of possible
values of the criterion). If the difference is larger than the
bound (∆Gl> ), then Xais the best attribute with high
confidence 1 δ, and can therefore be used to split the leaf.
Line 9 shows the complete condition to split the leaf.
If the best attribute is the no-split scenario (X), the al-
gorithm does not perform any split. The algorithm also
uses a tie-breaking τmechanism to handle the case where
the difference in splitting criterion between Xaand Xbis
very small. If the Hoeffding bound becomes smaller than τ
(∆Gl<  < τ), then the current best attribute is chosen
regardless of the values of ∆Gl.
If the algorithm splits the node, it replaces the leaf lwith
an internal node. It also creates branches based on the best
attribute that lead to newly created leaves and initializes
these leaves using the class distribution observed at the best
attribute the branches are starting from (lines 10 to 13).
3.2 SAMOA
Apache SAMOA6is an open-source distributed stream
mining platform initially developed at Yahoo Labs [6]. It
allows easy implementation and deployment of distributed
streaming machine learning algorithms on supported dis-
tributed stream processing engines (DSPEs). Besides, it
provides the ability to integrate new DSPEs into the frame-
work and leverage their scalability for performing big data
mining.
SAMOA is both a framework and a library. As a frame-
work, it allows the algorithm developer to abstract from the
underlying execution engine, and therefore reuse their code
on different engines. It features a pluggable architecture
that allows it to run on several distributed stream process-
ing engines such as Storm,7Samza,8and Flink.9This capa-
bility is achieved by designing a minimal API that captures
the essence of modern DSPEs. This API also allows to eas-
ily write new bindings to port SAMOA to new execution
engines. SAMOA takes care of hiding the differences of the
underlying DSPEs in terms of API and deployment.
An algorithm in SAMOA is represented by a directed
graph of operators that communicate via messages along
streams which connect pairs of nodes. Borrowing the ter-
minology from Storm, this graph is called a Topology. Each
node in a Topology is a Processor that sends messages through
aStream. A Processor is a container for the code that imple-
ments the algorithm. At runtime, several parallel replicas
of a Processor are instantiated by the framework. Repli-
cas work in parallel, with each receiving and processing a
portion of the input stream. These replicas can be instanti-
ated on the same or different physical computing resources,
according to the DSPE used. A Stream can have a single
source but several destinations (akin to a pub-sub system).
6https://samoa.incubator.apache.org
7https://storm.apache.org
8https://samza.apache.org
9https://flink.apache.org
Split
Source Model Aggregator
Instance
Shuffle Grouping
Key Grouping
All Grouping
Local Statistics
Attributes
Figure 1: High level diagram of the VHT topology.
A Topology is built by using a Topology Builder, which con-
nects the various pieces of user code to the platform code
and performs the necessary bookkeeping in the background.
A processor receives Content Events via a Stream. Al-
gorithm developers instantiate a Stream by associating it
with exactly one source Processor. When the destination
Processor wants to connect to a Stream, it needs to specify
the grouping mechanism which determines how the Stream
partitions and routes the transported Content Events. Cur-
rently there are three grouping mechanisms in SAMOA:
Shuffle grouping, which routes the Content Events in a
round-robin way among the corresponding Processor in-
stances. This grouping ensures that each Processor in-
stance receives the same number of Content Events from
the stream.
Key grouping, which routes the Content Events based
on their key, i.e., the Content Events with the same key
are always routed by the Stream to the same Processor
instance.
All grouping, which replicates the Content Events and
broadcasts them to all downstream Processor instances.
4. ALGORITHM
In this section, we explain the details of our proposed algo-
rithm, the Vertical Hoeffding Tree, which is a data-parallel,
distributed version of the Hoeffding tree described in Sec-
tion 3. First, we describe the parallelization and the ideas
behind our design choice. Then, we present the engineering
details and optimizations we employed to obtain the best
performance.
4.1 Vertical Parallelism
Data parallelism is a way of distributing work across dif-
ferent nodes in a parallel computing environment such as
a cluster. In this setting, each node executes the same
operation on different parts of the dataset. Contrast this
definition with task parallelism (aka pipelined parallelism),
where each node executes a different operator and the whole
dataset flows through each node at different stages. When
applicable, data parallelism is able to scale to much larger
4
deployments, for two reasons: (i) data has usually much
higher intrinsic parallelism that can be leveraged compared
to tasks, and (ii) it is easier to balance the load of a data-
parallel application compared to a task-parallel one. These
attributes have led to the high popularity of the currently
available DSPEs. For these reasons, we employ data paral-
lelism in the design of VHT.
In machine learning, it is common to think about data
in matrix form. A typical linear classification formulation
requires to find a vector xsuch that A·xb, where Ais the
data matrix and bis a class label vector. The matrix Ais n×
m-dimensional, with nbeing the number of data instances
and mbeing the number of attributes of the dataset.
Clearly, there are two ways to slice this data matrix to
obtain data parallelism: by row or by column. The for-
mer is called horizontal paral lelism, the latter vertical paral-
lelism. With horizontal parallelism, data instances are inde-
pendent from each other, and can be processed in isolation
while considering all available attributes. With vertical par-
allelism, instead, attributes are considered independent from
each other.
The fundamental operation of the algorithm is to accu-
mulate statistics nijk (i.e., counters) for triplets of attribute
i, value j, and class k, for each leaf of the tree. The counters
for each leaf are independent, so let us consider the case for
a single leaf. These counters, together with the learned tree
structure, form the state of the VHT algorithm.
Different kinds of parallelism distribute the counters across
computing nodes in different ways. When using horizontal
parallelism, the instances are distributed randomly, there-
fore multiple instances of the same counter need to exist
on several nodes. On the other hand, when using vertical
parallelism, the counters for one attribute are grouped on a
single node.
This latter design has several advantages. First, by hav-
ing a single copy of the counter, the memory requirements
for the model are the same as in the sequential version. In
contrast, with horizontal parallelism a single attribute may
be kept track on every node, thus the memory requirements
grow linearly with the parallelism level. Second, by having
each attribute being tracked independently, the computa-
tion of the split criterion can be performed in parallel by
several nodes. Conversely, with horizontal partitioning the
algorithm needs to (centrally) aggregate the partial counters
before being able to compute the splitting criterion.
Of course, the vertically-parallel design has also its draw-
backs. In particular, horizontal parallelism achieves a good
load balance much more easily, even though solutions for
these problems have recently been proposed [17, 18]. In ad-
dition, if the instance stream arrives in row-format, it needs
to be transformed in column-format, and this transforma-
tion generates additional CPU overhead at the source. In-
deed, each attribute that constitutes an instance needs to
be sent independently, and needs to carry the class label of
its instance. Therefore, both the number of messages and
the size of the data transferred increase.
Nevertheless, as shown in Section 6, the advantages of
vertical parallelism outweigh its disadvantages for several
real-world settings.
4.2 Algorithm Structure
We are now ready to explain the structure of the VHT
algorithm. Recall from Section 3 that there are two main
Algorithm 2 Model Aggregator: VerticalHoeffding-
TreeInduction(E,V HT tree)
Require: Eis a training instance from source PI, wrapped
in instance content event
Require: V HT tree is the current state of the decision tree
in model-aggregator PI
1: Use V HT tree to sort Einto a leaf l
2: Send attribute content events to local-statistic PIs
3: Increment the number of instances seen at l(which is
nl)
4: if nlmod nmin = 0 and not all instances seen at l
belong to the same class then
5: Add linto the list of splitting leaves
6: Send compute content event with the id of leaf lto all
local-statistic PIs
7: end if
Algorithm 3 Local Statistic: UpdateLocal-
Statistic(attribute,local statistic)
Require: attribute is an attribute content event
Require: local statistic is the local statistic, could be im-
plemented as Table < leaf id, attribute id >
1: Update local statistic with data in attribute: attribute
value, class value and instance weights
parts to the Hoeffding tree algorithm: sorting the instances
through the current model, and accumulating statistics of
the stream at each leaf node. This separation offers a neat
cut point to modularize the algorithm in two separate com-
ponents. We call the first component model aggregator, and
the second component local statistics. Figure 1 presents a
visual depiction of the algorithm, specifically, of its compo-
nents and of how the data flow among them.
The model aggregator holds the current model (the tree)
produced so far. Its main duty is to receive the incoming
instances and sort them to the correct leaf. If the instance
is unlabeled, the model predicts the label at the leaf and
sends it downstream (e.g., for evaluation). Otherwise, if the
instance is labeled it is used as training data. The VHT
decomposes the instance into its constituent attributes, at-
taches the class label to each, and sends them independently
to the following stage, the local statistics. Algorithm 2 is a
pseudocode for the model aggregator.
The local statistics contain the sufficient statistics nijk
for a set of attribute-value-class triples. Conceptually, the
local statistics Processor can be viewed as a large distributed
table, indexed by leaf id (row), and attribute id (column).
The value of the cell represents a set of counters, one for
each pair of attribute value and class. The local statistics
simply accumulate statistics on the data sent to it by the
model aggregator. Pseudocode for the update function is
shown in Algorithm 3.
In SAMOA, we implement vertical parallelism by connect-
ing the model to the statistics via key grouping. We use a
composite key made by the leaf id and the attribute id. Hor-
izontal parallelism can similarly be implemented via shuffle
grouping on the instances themselves.
Messages. During the execution of the VHT, the type of
events that are being sent and received from the different
parts of the algorithm are summarized in Table 1.
5
Table 1: Different type of content events used during the execution of the VHT algorithm.
Name Parameters From To
instance <attribute 1, . . . , attribute m, class C > Source PI Model-Aggregator PI
attribute <attribute id, attribute value, class C > Model Aggregator PI Local Statistic PI id =<leaf id + attribute id >
compute <leaf id > Model Aggregator PI All Local Statistic PIs
local-result < Xlocal
a, Xlocal
b>Local Statistic PI id Model Aggregator PI
drop <leaf id > Model Aggregator PI All Local Statistic PIs
Algorithm 4 Local Statistic: ReceiveCompute-
Message(compute,local statistic)
Require: compute is an compute content event
Require: local statistic is the local statistic, could be im-
plemented as Table < leaf id, attribute id >
1: Get leaf lID from compute content event
2: For each attribute ithat belongs to leaf lin local statis-
tic, compute Gl(Xi)
3: Find Xlocal
a, which is the attribute with highest Glbased
on the local statistic
4: Find Xlocal
b, which is the attribute with second highest
Glbased on the local statistic
5: Send Xlocal
aand Xlocal
busing local-result content
event to model-aggregator PI via computation-result
stream
Leaf splitting. Periodically, the model aggregator will try
to see if the model needs to evolve by splitting a leaf. When
a sufficient number of instances have been sorted through a
leaf, it will send a broadcast message to the statistics, ask-
ing to compute the split criterion for the given leaf id. The
statistics will get the table corresponding to the leaf, and
for each attribute compute the splitting criterion in paral-
lel (e.g., information gain or entropy). Each local statis-
tic Processor will then send back to the model the top two
attributes according to the chosen criterion, together with
their scores. The model aggregator simply needs to compute
the overall top two attributes, apply the Hoeffding bound,
and see whether the leaf needs to be split. Refer to Algo-
rithm 4 for a pseudocode.
Two cases can arise: the leaf needs splitting, or it doesn’t.
In the latter case, the algorithm simply continues without
taking any action. In the former case instead, the model
modifies the tree by splitting the leaf on the selected at-
tribute, and generating one new leaf for each possible value
of the branch. Then, it broadcasts a drop message contain-
ing the former leaf id to the local statistics. This message
is needed to release the resources held by the leaf and make
space for the newly created leaves. Subsequently, the tree
can resume sorting instances to the new leaves. The local
statistics will create a new table for the new leaves lazily,
whenever they first receive a previously unseen leaf id. Al-
gorithm 5 shows this part of the process. In its simplest
version, while the tree adjustment is performed, the algo-
rithm drops the new incoming instances. We show in the
next section an optimized version that buffers them to im-
prove accuracy.
5. VHT OPTIMIZATIONS
We now introduce three optimizations that improve the
performance of the VHT: model replication,optimistic split
Algorithm 5 Model Aggregator: Receive(local result,
V HT tree)
Require: local result is an local-result content event
Require: V HT tree is the current state of the decision tree
in model-aggregator PI
1: Get correct leaf lfrom the list of splitting leaves
2: Update Xaand Xbin the splitting leaf lwith Xlocal
aand
Xlocal
bfrom local result
3: if local results from all local-statistic PIs received or
time out reached then
4: Compute Hoeffding bound =qR2ln(1)
2nl
5: if Xa6=Xand (Gl(Xa)Gl(Xb)>  or  < τ )
then
6: Replace lwith a split-node on Xa
7: for all branches of the split do
8: Add a new leaf with derived sufficient statistic
from the split node
9: end for
10: Send drop content event with id of leaf lto all local-
statistic PIs
11: end if
12: end if
Control
Split
Result
Source (n) Model (n) Stats (n) Evaluator (1)
AttributeInstance
Shuffle Grouping
Key Grouping
All Grouping
Figure 2: Deployment diagram for VHT.
execution, and instance buffering. The first deals with the
throughput and I/O capability of the algorithm, by remov-
ing its single bottleneck at the model aggregator. The latter
two instead deal with the problem computing the split cri-
terion in a distributed environment.
Model replication. If the model is maintained in a single
Processor, it can easily become a bottleneck in the construc-
tion and maintenance of the tree, especially under high in-
stance arrival rate. Instead, a parallel replication and main-
tenance of the model on multiple Processors allows for higher
throughput, but indeed comes with increased management
complexity and possible accuracy drop.
In order to materialize the model replication of the VHT,
two issues must be resolved: how to distribute incoming in-
stances to models, and how to perform consistent leaf split-
ting across all models, thus guaranteeing consistent main-
6
tenance of the tree in all models. The first issue can be
easily solved via shuffle grouping from the source Proces-
sor to all parallel model aggregator Processors. Assuming p
parallel models, shuffle grouping routes incoming instances
in a round-robin fashion among them, guaranteeing equal
split of instances among the models.
The second issue, however, requires a more elaborate so-
lution, because of two reasons. First, in a fully distributed
mode, each model can decide to send a control message to
the statistics at any time. To escape the problem of having
inconsistent models, one model (e.g., the first to be created
in the topology construction) is appointed the role of the pri-
mary model and is responsible for broadcasting the control
message to the statistics. The frequency of this broadcast is
adjusted to take into account the level of model parallelism
p, and that each model receives 1/p-th of the total instances.
Second, the exact number of instances nlseen at each
leaf lis not available at any central point. Instead, each
model handles a portion of the stream and thus only a partial
number of instances for a leaf l(n0
l) is available to each model
for the computation of the Hoeffding bound . To remedy
this problem, a naive approach would be to estimate, at the
models, that nln0
l×p. However, this approach can over-
or under-estimate the true number of instances seen per leaf,
if the instances are not dense, i.e., they don’t contain values
for all attributes.
A better approach is to have the local statistics broadcast
back to all models their estimation of n0
l, along with their
top two attributes Xlocal
aand Xlocal
b. Note that, for sparse
instances, the value of n0
lis still an estimate and not an exact
value due to the way attributes are distributed to statistics
via key grouping on the composite key (leaf id, attribute id).
Indeed, for dense instances, each instance gets decomposed
into all its attributes, so the model sends a message per
instance to the statistics. However, for sparse instances,
each instance gets decomposed into a subset of its attributes
(those with non-zero values). Therefore, each local statistic
may receive a different number of instances per leaf, and thus
its counter for n0
lunderestimates the real value nl. However,
the models can now independently compute n00
l= max n0
l,
the maximum over all received estimates n0
l. This value
is a good estimate of the true value of nl, especially for
real-world skewed datasets, where one of the attributes is
extremely frequent. Finally, the models can compute the
Hoeffding bound for the particular leaf, and decide in a
consistent manner across all models whether to split, thus
allowing the maintenance of the same tree in all model pro-
cessors.
Optimistic split execution. In the simplest version of
the VHT algorithm, whenever the decision on splitting is
being taken, labeled instances are simply thrown away. This
behavior clearly wastes useful data, and is thus not desirable.
Note that there are two possible outcomes when a split
decision is taken. If the algorithm decides to split the cur-
rent leaf, all the statistics accumulated so far for the leaf are
dropped. Otherwise, the leaf keeps accumulating statistics.
In either case, the algorithm is better served by using the in-
stances that arrive during the split. If the split is taken, the
in-transit instances do not have any effect in any case. How-
ever, if the split is not taken, the instances can be correctly
used to accumulate statistics.
Given these observations, we modify the VHT algorithm
to keep sending instances that arrive during splits to the
local statistics. We call this variant of the algorithm wk(0).
Instance buffering. The feedback for a split from the local
statistics to the model aggregator comes with a delay that
can affect the performance of the model. While the model
is waiting to receive this feedback from the local statistics
to decide whether a split should be taken, the information
from the instances that arrive can be lost if the node splits.
To avoid this waste, we add a buffer to store instances in
the model during a split decision. The algorithm can re-
play these instances if the model decides to split. That is,
instances that arrive during a split decision are sent down-
stream and are accounted for in the current local statistics.
If a split occurs, these statistics are dropped, and the in-
stances are replayed from the buffer before resuming with
normal operations. Conversely, if no split occurs, the buffer
is simply dropped.
To avoid increasing the memory pressure of the algorithm,
the buffer resides on disk. The access to the buffer is se-
quential both while writing and when reading, so it does
not represent a bottleneck for the algorithm. We also limit
the maximum size of the buffer, to avoid delaying newly ar-
riving instances excessively. The optimal size of the buffer
depends on the number of attributes of the instances, the
arrival rate, the delay of the feedback from the local statis-
tics, and the specific hardware configuration. Therefore, we
let the user customize its size with a parameter z, and we
refer to this version of the algorithm as wk(z).
Timeout. Each model waits for a timeout to receive all re-
sponses back from the statistics, before computing the new
splits. This timeout is primarily a system check to avoid
the model waiting indefinitely. This timeout parameter may
impact the performance of the tree: if it’s too large, many
instances will not be stored in the available buffer and there-
fore lost. Thus, the size of the buffer is closely related to this
timeout parameter, allowing it to have enough instances to
be replayed if the model has a leaf that decides to split.
6. EXPERIMENTS
In our experimental evaluation of the VHT method, we
aim to study the following questions:
Q1: How does a centralized VHT compare to a centralized
hoeffding tree (available in MOA) with respect to accu-
racy and throughput?
Q2: How does the vertical parallelism used by VHT com-
pare to horizontal parallelism?
Q3: What is the effect of number and density of attributes?
Q4: How does discarding or buffering instances affect the
performance of VHT?
6.1 Experimental setup
In order to study these questions, we experiment with five
datasets (two synthetic generators and three real datasets),
five different versions of the hoeffding tree algorithm, and up
to four levels of computing parallelism. We measure classi-
fication accuracy during the execution and at the end, and
throughput (number of classified instances per second). We
execute each experimental configuration ten times, and re-
port the average of these measures.
Synthetic datasets. We use synthetic data streams pro-
duced by two random generators: one for dense and one for
sparse attributes.
7
Dense attributes are extracted from a random decision
tree. We test different number of attributes, and include
both categorical and numerical types. The label for each
configuration is the number of categorical-numerical used
(e.g, 100-100 means the configuration has 100 categorical
and 100 numerical attributes). We produce 10 differently
seeded streams with 1M instances for each tree, with
one of two balanced classes in each instance, and take
measurements every 100k instances.
Sparse attributes are extracted from a random tweet
generator. We test different dimensionalities for the at-
tribute space: 100, 1k, 10k. These attributes represent
the appearance of words from a predefined bag-of-words.
On average, the generator produces 15 words per tweet
(size of a tweet is Gaussian), and uses a Zipf distribu-
tion with skew z= 1.5 to select words from the bag. We
produce 10 differently seeded streams with 1M tweets in
each stream. Each tweet has a binary class chosen uni-
formly at random, which conditions the Zipf distribution
used to generate the words.
Real datasets. We also test VHT on three real data streams
to assess its performance on benchmark data.10
(elec) Electricity. This dataset has 45312 instances, 8
numerical attributes and 2 classes.
(phy) Particle Physics. This dataset has 50000 instances
for training, 78 numerical attributes and 2 classes.
(covtype) CovertypeNorm. This dataset has 581012 in-
stances, 54 numerical attributes and 7 classes.
Algorithms. We compare the following versions of the ho-
effding tree algorithm.
MOA: This is the standard Hoeffding tree in MOA.
local: This algorithm executes VHT in a local, sequen-
tial execution engine. All split decisions are made in
a sequential manner in the same process, with no com-
munication and feedback delays between statistics and
model.
wok: This algorithm discards instances that arrive dur-
ing a split decision. This version is the vanilla VHT.
wk(z): This algorithm sends instances that arrive dur-
ing a split decision downstream. In also adds instances
to a buffer of size zuntil full. If the split decision is
taken, it replays the instances in the buffer through the
new tree model. Otherwise, it discards the buffer, as the
instances have already been incorporated in the statistics
downstream.
sharding: Splits the incoming stream horizontally among
an ensemble of Hoeffding trees. The final prediction
is computed by majority voting. This method is an
instance of horizontal parallelism applied to Hoeffding
trees.
Experimental configuration. All experiments are per-
formed on a Linux server with 24 cores (Intel Xeon X5650),
clocked at 2.67GHz, L1d cache: 32kB, L1i cache: 32kB, L2
cache: 256kB, L3 cache: 12288kB, and 65GB of main mem-
ory. On this server, we run a Storm cluster (v0.9.3) and
zookeeper (v3.4.6). We use SAMOA v0.4.0 (development
version) and MOA v2016.04 available from the respective
project websites.
10http://moa.cms.waikato.ac.nz/datasets/,
http://osmot.cs.cornell.edu/kddcup/datasets.html
80
85
90
95
100
10-10 100-100 1k-1k 10k-10k
% accuracy
nominal attributes - numerical attributes
Dense attributes
local
moa
100 1k 10k
attributes
Sparse attributes
Figure 3: Accuracy of VHT executed in local mode on
SAMOA compared to MOA, for dense and sparse datasets.
100
101
102
103
104
105
10-10 100-100 1k-1k 10k-10k
Execution time (s)
nominal attributes - numerical attributes
Dense attributes
local
moa
100 1k 10k
attributes
Sparse attributes
Figure 4: Execution time of VHT in local mode on
SAMOA compared to MOA, for dense and sparse datasets.
We use several parallelism levels in the range of p=
2,...,16, depending on the experimental configuration. For
dense instances, we stop at p= 8 due to memory constraints,
while for sparse instances we scale up to p= 16. We disable
model replication (i.e., use a single model aggregator), as in
our setup the model is not the bottleneck.
6.2 Accuracy and time of VHT local vs. MOA
In this first set of experiments, we test if VHT is perform-
ing as well as its counterpart hoeffding tree in MOA. This is
mostly a sanity check to confirm that the algorithm used to
build the VHT does not affect the performance of the tree
when all instances are processed sequentially by the model.
To verify this fact, we execute VHT local and MOA with
both dense and sparse instances. Figure 3 shows that VHT
local achieves the same accuracy as MOA, even besting it at
times. However, VHT local always takes longer than MOA
to execute, as shown by Figure 4. Indeed, the local execu-
tion engine of SAMOA is optimized for simplicity rather
than speed. Therefore, the additional overhead required to
interface VHT to DSPEs is not amortized by scaling the
algorithm out. Future optimized versions of VHT and the
local execution engine should be able to close this gap.
6.3 Accuracy of VHT local vs. distributed
Next, we compare the performance of VHT local with
VHT built in a distributed fashion over multiple processors
for scalability. We use up to p= 8 parallel statistics, due to
memory restrictions, as our setup runs on a single machine.
In this set of experiments we compare the different versions
of VHT, wok and wk(z), to understand what is the impact
of keeping instances for training after a model’s split. Accu-
racy of the model might be affected, compared to the local
execution, due to delays in the feedback loop between statis-
tics and model. That is, instances arriving during a split will
8
be classified using an older version of the model compared
to the sequential execution. As our target is a distributed
system where independent processes run without coordina-
tion, this delay is a characteristic of the algorithm as much
as of the distributed SPE we employ.
We expect that buffering instances and replaying them
when a split is decided would improve the accuracy of the
model. In fact, this is the case for dense instances with a
small number of attributes (i.e., around 200), as shown in
Figure 5. However, when the number of available attributes
increases significantly, the load imposed on the model seems
to outweigh the benefits of keeping the instances for replay-
ing. We conjecture that the increased load in computing the
splitting criterion in the statistics further delays the feed-
back to compute the split. Therefore, a larger number of
instances are classified with an older model, thus negatively
affecting the accuracy of the tree. In this case, the addi-
tional load imposed by replaying the buffer further delays
the split decision. For this reason, the accuracy for VHT
wk(z) drops by about 30% compared to VHT local. Con-
versely, the accuracy of VHT wok drops more gracefully,
and is always within 18% of the local version.
VHT always performs approximatively 10% better than
sharding. For dense instances with a large number of at-
tributes (20k), sharding fails to complete due to its mem-
ory requirements exceeding the available memory. Indeed,
sharding builds a full model for each shard, on a subset of
the stream. Therefore, its memory requirements are ptimes
higher than a standard hoeffding tree.
When using sparse instances, the number of attributes
per instance is constant, while the dimensionality of the at-
tribute space increases. In this scenario, increasing the num-
ber of attributes does not put additional load on the system.
Indeed, Figure 6 shows that the accuracy of all versions is
quite similar, and close to the local one. This observation is
in line with our conjecture that the overload on the system
is the cause for the drop in accuracy on dense instances.
We also study how the accuracy evolves over time. In gen-
eral, the accuracy of all algorithms is rather stable, as shown
in Figures 7 and 8. For instances with 10 to 100 attributes,
all algorithms perform similarly. For dense instances, the
versions of VHT with buffering, wk(z), outperform wok,
which in turn outperforms sharding. This result confirms
that buffering is beneficial for small number of attributes.
When the number of attributes increases to a few thousand
per instance, the performance of these more elaborate al-
gorithms drops considerably. However, the VHT wok con-
tinues to perform relatively well and better than sharding.
This performance, coupled with good speedup over MOA
(as shown next) makes it a viable option for streams with a
large number of attributes and a large number of instances.
6.4 Speedup of VHT distributed vs. MOA
Since the accuracy of VHT wk(z) is not satisfactory for
both types of instances, next we focus our investigation on
VHT wok.
Figure 9 shows the speedup of VHT for dense instances.
VHT wok is about 2-10 times faster than VHT local and up
to 4 times faster than MOA. Clearly, the algorithm achieves
a higher speedup when more attributes are present in each
instance, as (i) there is more opportunity for paralleliza-
tion, and (ii) the implicit load shedding caused by discard-
ing instances during splits has a larger effect. Even though
0
20
40
60
80
100
2105 4105 6105 8105 1106
% accuracy
instances
p=2, u=10, o=10
local
moa
wk(0)
wk(1k)
wk(10k)
wok
sharding
0
20
40
60
80
100
2105 4105 6105 8105 1106
instances
p=2, u=1000, o=1000
local
moa
wk(0)
wk(1k)
wk(10k)
wok
sharding
Figure 7: Evolution of accuracy with respect to instances
arriving, for several versions of VHT (local, wok,wk(z))
and sharding, for dense datasets.
0
20
40
60
80
100
2105 4105 6105 8105 1106
% accuracy
instances
p=2, a=100
local
moa
wk(0)
wk(1k)
wk(10k)
wok
sharding
0
20
40
60
80
100
2105 4105 6105 8105 1106
instances
p=2, a=10000
local
moa
wk(0)
wk(1k)
wk(10k)
wok
sharding
Figure 8: Evolution of accuracy with respect to instances
arriving, for several versions of VHT (local, wok,wk(z))
and sharding, for sparse datasets.
sharding performs well in speedup with respect to MOA
on small number of attributes, it fails to build a model for
large number of attributes due to running out of memory.
In addition, even for a small number of attributes, VHT
wok outperforms sharding with a parallelism of 8. Thus, it
is clear from the results that the vertical parallelism used
by VHT offers better scaling behavior than the horizontal
parallelism used by sharding.
When testing the algorithms on sparse instances, as shown
in Figure 10, we notice that VHT wok can reach up to 60
times the throughput of VHT local and 20 times the one
of MOA (for clarity we only show the results with respect
to MOA). Similarly to what observed for dense instances,
a higher speedup is observed when a larger number of at-
tributes are present for the model to process. This very large
superlinear speedup (20×with p= 2), is due to the aggres-
sive load shedding implicit in the wok version of VHT. The
algorithm actually performs consistently less work than the
local version and MOA.
However, note that for sparse instances the algorithm pro-
cesses a constant number of attributes, albeit from an in-
creasingly larger space. Therefore, in this setup, wok has a
constant overhead for processing each sparse instance, dif-
ferently from the dense case. VHT wok outperforms shard-
ing in most scenarios and especially for larger numbers of
attributes and larger parallelism.
Increased parallelism does not impact accuracy of the
model (see Figure 5 and Figure 6), but its throughput is
improved. Boosting the parallelism from 2 to 4 makes VHT
wok up to 2 times faster. However, adding more proces-
sors does not improve speedup, and in some cases there
is a slowdown due to additional communication overhead
(for dense instances). Particularly for sparse instances, par-
9
0
20
40
60
80
100
10-10 100-100 1k-1k 10k-10k
% accuracy
parallelism = 2
sharding wok wk(0) wk(1k) wk(10k) local
0
20
40
60
80
100
10-10 100-100 1k-1k 10k-10k
nominal attributes - numerical attributes
parallelism = 4
0
20
40
60
80
100
10-10 100-100 1k-1k 10k-10k
parallelism = 8
Figure 5: Accuracy of several versions of VHT (local, wok,wk(z)) and sharding, for dense datasets.
0
20
40
60
80
100
100 1k 10k
% accuracy
attributes
parallelism = 2
sharding wok wk(0) wk(1k) wk(10k) local
0
20
40
60
80
100
100 1k 10k
attributes
parallelism = 4
0
20
40
60
80
100
100 1k 10k
attributes
parallelism = 8
0
20
40
60
80
100
100 1k 10k
attributes
parallelism = 16
Figure 6: Accuracy of several versions of VHT (local, wok,wk(z)) and sharding, for sparse datasets.
1
2
3
4
5
10-10 100-100 1k-1k 10k-10k
Speedup over moa
parallelism = 2
sharding
wok
1
2
3
4
5
10-10 100-100 1k-1k 10k-10k
nominal attributes - numerical attributes
parallelism = 4
1
2
3
4
5
10-10 100-100 1k-1k 10k-10k
parallelism = 8
Figure 9: Speedup of VHT wok executed on SAMOA compared to MOA for dense datasets.
10
allelism does not impact accuracy which enables handling
large sparse data streams while achieving high speedup over
MOA.
6.5 Performance on real-world datasets
Tables 2 and 3 show the performance of VHT, either run-
ning in a local mode or in a distributed fashion over a storm
cluster of a few processors. We also test two different ver-
sions of VHT: wok and wk(0). In the same tables we com-
pare VHT’s performance with MOA and sharding.
The results from these real datasets demonstrate that
VHT can perform similarly to MOA with respect to ac-
curacy and at the same time process the instances faster.
In fact, for the larger dataset, covtypeNorm, VHT wok
exhibits 1.8 speedup with respect to MOA, even though
the number of attributes is not very large (54 numeric at-
tributes). VHT wok also performs better than sharding,
even though the latter is faster in some cases. However, the
speedup offered by sharding decreases when the parallelism
level is increased from 2 to 4 shards.
Table 2: Average accuracy (%) for different algorithms,
with parallelism level (p), on the real-world datasets.
dataset MOA VHT Sharding
local wok wok wk(0) wk(0)
p=2 p=4 p=2 p=4 p=2 p=4
elec 75.4 75.4 75.0 75.2 75.4 75.6 74.7 74.3
phy 63.3 63.8 62.6 62.7 63.8 63.7 62.4 61.4
covtype 67.9 68.4 68.0 68.8 67.5 68.0 67.9 60.0
Table 3: Average execution time (seconds) for different
algorithms, with parallelism level (p), on the real-world
datasets.
Dataset MOA VHT Sharding
local wok wok wk(0) wk(0)
p=2 p=4 p=2 p=4 p=2 p=4
elec 1.09 1 2 2 2 2 2 2.33
phy 5.41 4 3.25 4 3 3.75 3 4
covtype 21.77 16 12 12 13 12 9 11
6.6 Summary
In conclusion, our VHT algorithm has the following per-
formance traits. We learned that for a small number of
attributes, it helps to buffer incoming instances that can
be used in future decisions of split. For larger number of
attributes, the load in the model can be high and larger de-
lays can be observed in the integration of the feedback from
the local statistics into the model. In this case, buffered in-
stances may not be used on the most up-to-date model and
this can penalize the overall accuracy of the model.
With respect to a centralized sequential tree model (MOA),
it processes dense instances with thousands of attributes up
to 4×faster with only 10 20% drop in accuracy. It can
also process sparse instances with thousands of attributes up
to 20×faster with only 5 10% drop in accuracy. Also, its
ability to build the tree in a distributed fashion using tens of
processors allows it to scale and accommodate thousands of
attributes and parse millions of instances. Competing meth-
ods cannot handle these data sizes due to increased memory
and computational complexity.
7. CONCLUSION
The rapid increase observed in the number of users of so-
cial media or of IoT devices has lead to a multifold increase
in the data available for analysis. For these data to be an-
alyzed and business or other learnings to be extracted, new
machine learning methods are required, able to scale to a
large size of fast data arriving at high speeds.
In this paper we presented the Vertical Hoeffding Tree
(VHT), the first distributed streaming algorithm for learn-
ing decision trees that can be used for performing classifica-
tion tasks on such large data streams arriving at high rates.
VHT features a novel way of distributing decision trees via
vertical parallelism. The algorithm is implemented on top
of Apache SAMOA, a platform for mining big data streams,
and is thus able to run on real-world clusters.
Through exhaustive experimentation, and in comparison
to a centralized sequential tree model, we show that VHT
can process dense and sparse instances with thousands of
attributes up to 4×and 20×faster, respectively, and with
small degradation in accuracy. Also, VHT’s ability to build
the decision tree in a distributed fashion using tens of pro-
cessors allows it to scale and accommodate thousands of at-
tributes and parse millions of instances. We also show that
competing distributed methods cannot handle the same data
sizes due to memory and computational complexity.
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