Category: General

Identifying Bot Commenters on Reddit using Benford’s Law

michael

How can you identify non-human actors on social media? There are many ways to do this, and each method has its strengths and weaknesses. In this post, I discuss how to use Benford’s Law to identify non-human actors in user interaction logs. Application of Benford’s Law Benford’s Law is an observation that a collection of numbers that measure naturally occurring events of items tend to have a logarithm frequency distribution for the first digit of these numbers. The are several characteristics of a naturally occurring set of numbers that Benford’s Law takes advantage of: The order of magnitude of the number in the set varies uniformly The numbers vary with multiplicative fluctuations The distribution of numbers is scale invariant The exact distribution of first digits that Benford’s Law predicts is: This results in a distribution that looks like this: For a collection of numbers, if the frequency of the numbers’ first digits does not align well with the distribution shown Read More …

Benchmarking Software for PySpark on Apache Spark Clusters

michael

Now that my Personal Compute Cluster is uninhibited by CPU overheating, I want to turn my configuration to work as efficiently as possible for the type of workloads I place on it. I searched around for Apache Spark benchmarking software, however most of what I found was either too older (circa Spark 1.x) or too arcane. I was able to get Ewan Higgs’s implementation of TeraSort working on my cluster, but it was written in Scala and not necessarily representative of the type of operations I would use in PySpark. So I set out to write my own. The primary goal of my benchmarking approach is to have a standard set of data and operations that I can compare the performance of before and after some change I make to my Spark deployment and be confident that any change in performance was due to the change in the Spark deployment and not due to variability in the benchmark. This attribute Read More …

Airline Flight Data Analysis – Airport “PageRank”

michael

Now that we have the Airline On-Time Performance data set loaded into parquet files on the Personal Compute Cluster, we can take a look to see what the data set tells us about the state of air transportation in the United States. The first thing I will look at is determining which are the most important airports in the United States. A simple way to determine the most important airport is to count number of flights that handled. However, given that most airline routes leverage a hub-and-spoke system, simply counting flights in and out does not convey how important the airport is for routing airline traffic in general. This because certain hub airports might be critical junctures for the flights in other airports, and as a result, those hub airports might be considered more important even if they have identical or even less flight counts. So how can we identify the most important airports in the United States considering how Read More …

Airline Flight Data Analysis – Part 1 – Data Preparation (Reprise)

michael

As some of you know, I previously explored building a Spark cluster using ODROID XU-4 single board computers. I was able to demonstrate some utility with this cluster, but it was limited. One analysis I attempted looking at was the Airline On-Time Performance data set collected by the United States Department of Transportation. The XU-4 cluster allowed me to make summarization graphs of the data, but not much more. The primary constraint was the RAM pool the cluster had, which was 8 GB total RAM across the four nodes, and the 10 year data set was greater than 30 GB uncompressed. Now that I have built the Personal Compute Cluster, I decided to revise this data set to see if I could do more sophisticated analysis. The short answer is I can. But before I do that we need to prepare the raw data that we download from the Department of Transportation’s website into a format. Specifically we need to Read More …

ARM7 CPUs, Double Alignment, and Apache Spark

michael

I haven’t posted an update to my data analysis projects in a while. Partly because my day job has been a bit busy lately, and partly because what time I do have for my recreational coding has been taken up by a problem I was experiencing with Apache Spark. I started have stability problems on my ODROID XU4 cluster. I didn’t fully understand the cause at first, thinking for the longest time it was my own code. In the end, it proved to be a bug in spark, or more specifically, an incompatibility between Spark’s memory management and the ARM71 platform of my ODROID XU4 cluster. The issue has to do with how some CPUs operate on double floating point values. These CPUs, including the 32-bit ARM71 CPU found in the ODROID XU4, requires that when the CPU operates on a double floating point value the 8 bytes of memory used to contain the value should be aligned to 8 byte Read More …

Using Custom Hive UDFs With PySpark

michael

Using Python to develop on Apache Spark is easy and familiar for many developers. However, due to the fact that Spark runs in a JVM, when your Python code interacts with the underlying Spark system, there can be an expensive process of data serialization and deserialization between the JVM and the Python interpreter. If you do most of your data manipulation using data frames in PySpark, you generally avoid this serialization cost because the Python code ends up being more of a high-level coordinator of the data frame operations rather than doing low-level operations on the data itself. This changes if you ever write a UDF in Python. To avoid the JVM-to-Python data serialization costs, you can use a Hive UDF written in Java. Creating a Hive UDF and then using it within PySpark can be a bit circuitous, but it does speed up your PySpark data frame flows if they are using Python UDFs. To illustrate this, I will rework the Read More …

Airline Flight Data Analysis – Part 2 – Analyzing On-Time Performance

michael

In my last post on this topic, we loaded the Airline On-Time Performance data set collected by the United States Department of Transportation into a Parquet file to greatly improve the speed at which the data can be analyzed. Now, let’s take a first look at the data by graphing the average airline-caused flight delay by airline. This is a rather straightforward analysis, but is a good one to get started with the data set. Open a Jupyter python notebook on the cluster in the first cell indicate that we will be using MatPlotLib to do graphing: %matplotlib inline Then, in the next cell load data frames for the airline on time activity and airline meta data based on the parquet files built in the last post. Note that I am using QFS as my distributed file system. If you are using HDFS, simply update the file URLs as needed. air_data = spark.read.parquet(‘qfs://master:20000/user/michael/data/airline_data’) airlines = spark.read.parquet(‘qfs://master:20000/user/michael/data/airline_id_table’) Now we are ready to process Read More …

Airline Flight Data Analysis – Part 1 – Data Preparation

michael

UPDATE – I have a more modern version of this post with larger data sets available here. This data analysis project is to explore what insights can be derived from the Airline On-Time Performance data set collected by the United States Department of Transportation. The data can be downloaded in month chunks from the Bureau of Transportation Statistics website. The data gets downloaded as a raw CSV file, which is something that Spark can easily load. However, if you download 10+ years of data from the Bureau of Transportation Statistics (meaning you downloaded 120+ one month CSV files from the site), that would collectively represent 30+ GB of data. For commercial scale Spark clusters, 30 GB of text data is a trivial task. However, if you are running Spark on the ODROID XU4 cluster or in local mode on your Mac laptop, 30+ GB of text data is substantial. So, before we can do any analysis of the dataset, we need to Read More …

Coverting Apache Access Logs to Parquet Backed Data Frames on Spark

michael

One of the analysis I am looking to do with my ODROID XU4 cluster is to take a look at various access patterns on my website by analyzing the Apache http access logs. Analyzing Apache access logs directly in Spark can be slow due to them being unstructured text logs. Converting to the logs to a data frame backed by partitioned parquet files can make subsequent analysis much faster. The first task is to create a mapper that can be used in Spark convert a row int eh access log to a Spark Row object. A Python 3 mapper would look like: # Parse an Apache access log. Assumes Python 3 import re from pyspark.sql import Row from datetime import datetime APACHE_ACCESS_LOG_PATTERN = ‘^(\S+) (\S+) (\S+) \[([\w:/]+\s[+\-]\d{4})\] “(\S+) (\S+) (\S+)” (\d{3}) (\d+) “((?:[^”]|”)+)” “((?:[^”]|”)+)”$’ DATETIME_PARSE_PATTERN = ‘%d/%b/%Y:%H:%M:%S %z’ # Returns a Row containing the Apache Access Log info def parse_apache_log_line(logline): match = re.search(APACHE_ACCESS_LOG_PATTERN, logline) if match is None: return None Read More …

What Every Data Scientist Should Know About Floating-Point Arithmetic

michael

In 1991 David Goldberg at Xerox PARC published the seminal paper on floating point arithmetic titled “What every computer scientist should know about floating-point arithmetic.” This paper was especially influential in the 1990’s and early 2000’s when limitations in computer hardware drove people to operate in a regime that most exposes them to the limitations of floating point arithmetic, specifically using 32 bit floats for storing and calculating floating point numbers. With modern 64 bit architecture and low storage and RAM costs, the incentive to use 32 bit floats is no longer there and computer scientist generally use 64 bit double precision floats, which greatly reduces them to the issues of floating point arithmetic. However, 64 bit floats do not remove the issues, it just proverbially kicks the can down the road waiting for usage patterns to evolve to once again cause floating point issues to become prominent again. With Big Data and extremely large data sets, both data scientists and computer scientists need to Read More …