Why Look Beyond Hadoop Map-Reduce?
Perhaps you are a video service provider and would like to optimize the end user experience by choosing the appropriate content distribution network based on dynamic network conditions. Or you are a government regulatory body that needs to classify Internet pages into porn or non-porn in order to filter porn pages—which has to be achieved at high throughput and in real-time. Or you are a telecom/mobile service provider, or you work for one, and you are worried about customer churn (churn refers to a customer leaving the provider and choosing a competitor, or new customers joining in leaving competitors). How you wish you had known that the last customer who was on the phone with your call center had tweeted with negative sentiments about you a day before. Or you are a retail storeowner and you would love to have predictions about the customers’ buying patterns after they enter the store so that you can run promotions on your products and expect an increase in sales. Or you are a healthcare insurance provider for whom it is imperative to compute the probability that a customer is likely to be hospitalized in the next year so that you can fix appropriate premiums. Or you are a Chief Technology Officer (CTO) of a financial product company who wishes that you could have real-time trading/predictive algorithms that can help your bottom line. Or you work for an electronic manufacturing company and you would like to predict failures and identify root causes during test runs so that the subsequent real-runs are effective. Welcome to the world of possibilities, thanks to big data analytics.
Analytics has been around for a long time now—North Carolina State University ran a project called “Statistical Analysis System (SAS)” for agricultural research in the late 1960s that led to the formation of the SAS Company. The only difference between the terms analysis and analytics is that analytics is about analyzing data and converting it into actionable insights. The term Business Intelligence (BI) is also used often to refer to analysis in a business environment, possibly originating in a 1958 article by Peter Luhn (Luhn 1958). Lots of BI applications were run over data warehouses, even quite recently. The evolution of “big data” in contrast to the “analytics” term has been quite recent, as explained next.
The term big data seems to have been used first by John R. Mashey, then chief scientist of Silicon Graphics Inc. (SGI), in a Usenix conference invited talk titled “Big Data and the Next Big Wave of InfraStress,” the transcript of which is available at http://static.usenix.org/event/usenix99/invited_talks/mashey.pdf. The term was also used in a paper (Bryson et al. 1999) published in the Communications of the Association for Computing Machinery (ACM). The report (Laney 2001) from the META group (now Gartner) was the first to identify the 3 Vs (volume, variety, and velocity) perspective of big data. Google’s seminal paper on Map-Reduce (MR; Dean and Ghemawat 2004) was the trigger that led to lots of developments in the big data space. Though the MR paradigm was known in the functional programming literature, the paper provided scalable implementations of the paradigm on a cluster of nodes. The paper, along with Apache Hadoop, the open source implementation of the MR paradigm, enabled end users to process large data sets on a cluster of nodes—a usability paradigm shift. Hadoop, which comprises the MR implementation, along with the Hadoop Distributed File System (HDFS), has now become the de facto standard for data processing, with a lot of industrial game changers such as Disney, Sears, Walmart, and AT&T having their own Hadoop cluster installations.
Hadoop is good for a number of use cases, including those in which the data can be partitioned into independent chunks—the embarrassingly parallel applications, as is widely known. Hindrances to widespread adoption of Hadoop across Enterprises include the following:
- Lack of Object Database Connectivity (ODBC)—A lot of BI tools are forced to build separate Hadoop connectors.
Hadoop’s lack of suitability for all types of applications:
- If data splits are interrelated or computation needs to access data across splits, this might involve joins and might not run efficiently over Hadoop. For example, imagine that you have a set of stocks and the set of values of those stocks at various time points. It is required to compute correlations across stocks—can you check when Apple falls? What is the probability of Samsung too falling the next day? The computation cannot be split into independent chunks—you may have to compute correlation between stocks in different chunks, if the chunks carry different stocks. If the data is split along the time line, you would still need to compute correlation between stock prices at different points of time, which may be in different chunks.
- For iterative computations, Hadoop MR is not well-suited for two reasons. One is the overhead of fetching data from HDFS for each iteration (which can be amortized by a distributed caching layer), and the other is the lack of long-lived MR jobs in Hadoop. Typically, there is a termination condition check that must be executed outside of the MR job, so as to determine whether the computation is complete. This implies that new MR jobs need to be initialized for each iteration in Hadoop—the overhead of initialization could overwhelm computation for the iteration and could cause significant performance hits.
The other perspective of Hadoop suitability can be understood by looking at the characterization of the computation paradigms required for analytics on massive data sets, from the National Academies Press (NRC 2013). They term the seven categories as seven “giants” in contrast with the “dwarf” terminology that was used to characterize fundamental computational tasks in the super-computing literature (Asanovic et al. 2006). These are the seven “giants”:
- Basic statistics: This category involves basic statistical operations such as computing the mean, median, and variance, as well as things like order statistics and counting. The operations are typically O(N) for N points and are typically embarrassingly parallel, so perfect for Hadoop.
- Linear algebraic computations: These computations involve linear systems, eigenvalue problems, inverses from problems such as linear regression, and Principal Component Analysis (PCA). Linear regression is doable over Hadoop (Mahout has the implementation), whereas PCA is not easy. Moreover, a formulation of multivariate statistics in matrix form is difficult to realize over Hadoop. Examples of this type include kernel PCA and kernel regression.
- Generalized N-body problems: These are problems that involve distances, kernels, or other kinds of similarity between points or sets of points (tuples). Computational complexity is typically O(N2) or even O(N3). The typical problems include range searches, nearest neighbor search problems, and nonlinear dimension reduction methods. The simpler solutions of N-body problems such as k-means clustering are solvable over Hadoop, but not the complex ones such as kernel PCA, kernel Support Vector Machines (SVM), and kernel discriminant analysis.
- Graph theoretic computations: Problems that involve graph as the data or that can be modeled graphically fall into this category. The computations on graph data include centrality, commute distances, and ranking. When the statistical model is a graph, graph search is important, as are computing probabilities which are operations known as inference. Some graph theoretic computations that can be posed as linear algebra problems can be solved over Hadoop, within the limitations specified under giant 2. Euclidean graph problems are hard to realize over Hadoop as they become generalized N-body problems. Moreover, major computational challenges arise when you are dealing with large sparse graphs; partitioning them across a cluster is hard.
- Optimizations: Optimization problems involve minimizing (convex) or maximizing (concave) a function that can be referred to as an objective, a loss, a cost, or an energy function. These problems can be solved in various ways. Stochastic approaches are amenable to be implemented in Hadoop. (Mahout has an implementation of stochastic gradient descent.) Linear or quadratic programming approaches are harder to realize over Hadoop, because they involve complex iterations and operations on large matrices, especially at high dimensions. One approach to solve optimization problems has been shown to be solvable on Hadoop, but by realizing a construct known as All-Reduce (Agarwal et al. 2011). However, this approach might not be fault-tolerant and might not be generalizable. Conjugate gradient descent (CGD), due to its iterative nature, is also hard to realize over Hadoop. The work of Stephen Boyd and his colleagues from Stanford has precisely addressed this giant. Their paper (Boyd et al. 2011) provides insights on how to combine dual decomposition and augmented Lagrangian into an optimization algorithm known as Alternating Direction Method of Multipliers (ADMM). The ADMM has been realized efficiently over Message Passing Interface (MPI), whereas the Hadoop implementation would require several iterations and might not be so efficient.
- Integrations: The mathematical operation of integration of functions is important in big data analytics. They arise in Bayesian inference as well as in random effects models. Quadrature approaches that are sufficient for low-dimensional integrals might be realizable on Hadoop, but not those for high-dimensional integration which arise in Bayesian inference approach for big data analytical problems. (Most recent applications of big data deal with high-dimensional data—this is corroborated among others by Boyd et al. 2011.) For example, one common approach for solving high-dimensional integrals is the Markov Chain Monte Carlo (MCMC) (Andrieu 2003), which is hard to realize over Hadoop. MCMC is iterative in nature because the chain must converge to a stationary distribution, which might happen after several iterations only.
- Alignment problems: The alignment problems are those that involve matching between data objects or sets of objects. They occur in various domains—image de-duplication, matching catalogs from different instruments in astronomy, multiple sequence alignments used in computational biology, and so on. The simpler approaches in which the alignment problem can be posed as a linear algebra problem can be realized over Hadoop. But the other forms might be hard to realize over Hadoop—when either dynamic programming is used or Hidden Markov Models (HMMs) are used. It must be noted that dynamic programming needs iterations/recursions. The catalog cross-matching problem can be posed as a generalized N-body problem, and the discussion outlined earlier in point 3 applies.
To summarize, giant 1 is perfect for Hadoop, and in all other giants, simpler problems or smaller versions of the giants are doable in Hadoop—in fact, we can call them dwarfs, Hadoopable problems/algorithms! The limitations of Hadoop and its lack of suitability for certain classes of applications have motivated some researchers to come up with alternatives. Researchers at the University of Berkeley have proposed “Spark” as one such alternative—in other words, Spark could be seen as the next-generation data processing alternative to Hadoop in the big data space. In the previous seven giants categorization, Spark would be efficient for
- Complex linear algebraic problems (giant 2)
- Generalized N-body problems (giant 3), such as kernel SVMs and kernel PCA
- Certain optimization problems (giant 4), for example, approaches involving CGD
An effort has been made to apply Spark for another giant, namely, graph theoretic computations in GraphX (Xin et al. 2013). It would be an interesting area of further research to estimate the efficiency of Spark for other classes of problems or other giants such as integrations and alignment problems.
The key idea distinguishing Spark is its in-memory computation, allowing data to be cached in memory across iterations/interactions. Initial performance studies have shown that Spark can be 100 times faster than Hadoop for certain applications. This book explores Spark as well as the other components of the Berkeley Data Analytics Stack (BDAS), a data processing alternative to Hadoop, especially in the realm of big data analytics that involves realizing machine learning (ML) algorithms. When using the term big data analytics, I refer to the capability to ask questions on large data sets and answer them appropriately, possibly by using ML techniques as the foundation. I will also discuss the alternatives to Spark in this space—systems such as HaLoop and Twister.
The other dimension for which the beyond-Hadoop thinking is required is for real-time analytics. It can be inferred that Hadoop is basically a batch processing system and is not well suited for real-time computations. Consequently, if analytical algorithms are required to be run in real time or near real time, Storm from Twitter has emerged as an interesting alternative in this space, although there are other promising contenders, including S4 from Yahoo and Akka from Typesafe. Storm has matured faster and has more production use cases than the others. Thus, I will discuss Storm in more detail in the later chapters of this book—though I will also attempt a comparison with the other alternatives for real-time analytics.
The third dimension where beyond-Hadoop thinking is required is when there are specific complex data structures that need specialized processing—a graph is one such example. Twitter, Facebook, and LinkedIn, as well as a host of other social networking sites, have such graphs. They need to perform operations on the graphs, for example, searching for people you might know on LinkedIn or a graph search in Facebook (Perry 2013). There have been some efforts to use Hadoop for graph processing, such as Intel’s GraphBuilder. However, as outlined in the GraphBuilder paper (Jain et al. 2013), it is targeted at construction and transformation and is useful for building the initial graph from structured or unstructured data. GraphLab (Low et al. 2012) has emerged as an important alternative for processing graphs efficiently. By processing, I mean running page ranking or other ML algorithms on the graph. GraphBuilder can be used for constructing the graph, which can then be fed into GraphLab for processing. GraphLab is focused on giant 4, graph theoretic computations. The use of GraphLab for any of the other giants is an interesting topic of further research.
The emerging focus of big data analytics is to make traditional techniques, such as market basket analysis, scale, and work on large data sets. This is reflected in the approach of SAS and other traditional vendors to build Hadoop connectors. The other emerging approach for analytics focuses on new algorithms or techniques from ML and data mining for solving complex analytical problems, including those in video and real-time analytics. My perspective is that Hadoop is just one such paradigm, with a whole new set of others that are emerging, including Bulk Synchronous Parallel (BSP)-based paradigms and graph processing paradigms, which are more suited to realize iterative ML algorithms. The following discussion should help clarify the big data analytics spectrum, especially from an ML realization perspective. This should help put in perspective some of the key aspects of the book and establish the beyond-Hadoop thinking along the three dimensions of real-time analytics, graph computations, and batch analytics that involve complex problems (giants 2 through 7).