Recent content on Russell Cohen
Given the level of distrust in election systems in recent years, I became curious about verifiable voting systems – systems in which you can ensure that your vote was really counted, and counted correctly. Systems in which everyone (or at least interested parties) can verify that election results precisely reflect the votes cast. The verifiable voting system I’ll describe is pretty…
This document is adapted from an internal document I wrote for one of my clients, LogicHub. They’ve graciously allowed me to adapt it and share it as the blog post you’re reading now. As a reviewer, your primary goal is to find bugs before code is merged. Your secondary goal is to keep the code quality as high as possible, within reason and the time constraints of the moment. Depending…
There’s a common misconception that neural networks’ recent success on a slew of problems is due to the increasing speed and decreasing cost of GPUs. In reality, that’s not the case. Modern processing power plays a critical role, but only when combined with a series of innovations in architecture and training. You can’t process million-image datasets like ImageNet without a…
Many seasoned engineers working in JVM based languages have seen errors like this: [error] (run-main-0) java.lang.OutOfMemoryError: unable to create native thread: [error] java.lang.OutOfMemoryError: unable to create native thread: [error] at java.base/java.lang.Thread.start0(Native Method) [error] at java.base/java.lang.Thread.start(Thread.java:813) ... [error] at…
To say my first foray into Rust was a frustrating struggle would be an understatement. I picked a terrible first project that left me neck deep in Rust’s trickiest areas right off the bat. I was excited to try again. A few years ago I wrote Sumoshell, a CLI App for log analysis. I’d wanted to improve it for a while, so porting it to Rust seemed like a nice way to kill two birds with…
Written in Rust, angle-grinder enables sophisticated log file analysis from the terminal. Angle grinder is a distributed as a small, statically linked binary. That means it will run anywhere without dependencies so it’s easy to install on whatever server your logs are on.
Somehow I made it this far without actually understanding how sudo works. For years, I’ve just typed sudo, typed my password, and revelled in my new, magical, root super powers. The other day and I finally looked into it – to be honest, the mechanism is not at all what I expected. After going through the basics, we’ll walk through creating our own version of sudo. How Sudo Works…
Two years ago in February 2016, frustrated and not feeling motivated by the 9 to 5, I quit my job. But, living in the Bay area is expensive, so I started freelancing. It’s been a life changing experience – I have control of my schedule, I have way less stress, and coding is fun again. It’s definitely not for everyone. If you don’t mind a little uncertainty and less choice…
Before I start this post, let me preface it by saying that I’m not an experienced Rustacean by any means. The excellent foks at /r/rust gave a lot of helpful feedback and cleared up some misconceptions I had. Futher errata and corrections are appreciated. This post is aimed at helping other fledgling rust-learners avoid my mistake. First, by helping Rust learners pick good introductory…
Someone recently lamented to me that try as they might, they can’t seem to instill a culture of non-manual testing in their team. This problem pervades startups, especially in those with a lot of newer developers. My theory: To foster a culture where software engineers are internally motivated to write good tests, make it harder to run your app locally and easier to write and run tests. To…
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If you read my recent post about Postgres you may have noted that Postgres operates primarily with fixed-size blocks of memory called “pages.” By default, Postgres pages are 8KB. This number is tuned to match operating system page sizes which are tuned to match hardware cache sizes. If you were to run Postgres on hardware with different cache sizes than Postgres was tuned for, you may…
Many software engineers use database indexes every day, but few of us really understand how they work. In this post I’ll explain: How indexing works in Postgres using B-Trees What B-Trees are Why they are a good fit for this problem Indexes in Postgres Postgres actually offers 4 different kinds of indexes for different use cases. In this post I’ll be focusing on the…
Python 2 and Python 3 provide arbitrary precision integers. This means that you don’t need to worry about overflowing or underflowing the integer datatypes, ever. 2**10000? No problem. It makes it pretty convenient to write code. Floats, however, are not arbitrary precision: >>> a = 2 ** 10000 >>> a * .1 Traceback (most recent call last): File "<stdin>", line 1, in <module> OverflowError:…
Finding the median in a list seems like a trivial problem, but doing so in linear time turns out to be tricky. In this post I’m going to walk through one of my favorite algorithms, the median-of-medians approach to find the median of a list in deterministic linear time. Although proving that this algorithm runs in linear time is a bit tricky, this post is targeted at readers with only a…
In the last 2 months, at two separate clients, I ran into the same type of deadlock, so I figured it was worth writing about to save someone else from the same fate. In short: When you have a pool of resources, the same thread must not acquire a second resource while already holding a resource. Otherwise, multiple threads can deadlock when they have all acquired their first resource but none can…
I always wondered how Google Authenticator style 2-factor codes worked. The process of going from QR code to rotating 6-digit pin seemed a bit magical. A few days ago, my curiosity found itself coupled with some free time. Here’s what I found: What’s in the QR Code I scanned the QR code from Github with a barcode scanning app. Here’s what’s inside:…
Few data-structures are more ubiquitous in real-world development than the hash table. Nearly every major programming features an implementation in its standard library or built into the runtime. Yet, there is no conclusive best strategy to implement one and the major programming languages diverge widely in their implementations! I did a survey of the Hash map implementations in Go, Python, Ruby,…
Even if you go to bed at 9, getting up at 5AM is hard. It’s always darker than you expect and it’s usually freezing. In any case, Calley and I crawled out of our respective vans at 5AM in the Tuolumne Meadows campground, shoveled down some cold cereal, and rolled out for the day’s objective, the West Ridge of Mount Conness. TM Herbert, legendary Tuolumne badass, described the…
If you click on IPA (international phonetic alphabet) text on wikipedia, it links to a generic IPA page, which I always found frustrating. I created a chrome extension which adds a play button next to IPA text to read it aloud. Done in an afternoon, it’s almost certainly the highest users:time ratio of any project I’ve done. It simply extracts the IPA text from wikipedia and then uses…
A recent outage lead me to investigate Postgres unique constraints more deeply. Postgres implements unique constraints by creating a unique index – an index that can only contain unique values.1 It turns out that unique indices and concurrent transactions can interact in nasty and surprising ways. Before I get into the “why”, here are the implications: When two transactions…
As I was learning to program, Python lists seemed totally magical to me. I imagined them as being implemented by some sort of magical datastructure that was part linked-list, part array that was perfect for everything. As I grew as an engineer, it occurred that this was unlikely. I guessed (correctly) that rather than some sort of magical implementation, it was just backed by a resizable array. I…
An elasticsearch client library for Scala. It focuses on being easy to extend and modify while targeting the REST API for backwards compatibility. I was the primary author of this library while I was working at Sumo Logic and I currently maintain it.
re:search is a search engine to find professors that share your research interests. Leah and I originally built this in PHP for 6.470 Junior year (and won the MIT Utility award). A few years later we redid it with React and Scala.
Written in Go, Sumoshell enables sophisticated log file analysis including parsing, aggregating and graphing on the command line.
The code for this post, as well as the post itself, are on github. This post is part 3 of a 3 part series. Part 1: Parsing Part 2: Generate an NFA Part 3: Evaluate an NFA Evaluating the NFA NFAs, DFAs and Regular Expressions Recall from part 2 that there are two types of finite automata: deterministic and non-deterministic. They have one key difference: A non-deterministic finite automata can have…
The code for this post, as well as the post itself, are on github. This post is part 2 of a 3 part series. Part 1: Parsing Part 2: Generate an NFA Part 3: Evaluate an NFA Converting the Parse Tree to an NFA In the last post, we transformed the flat string representation of a regular expression to the hierarchical parse tree form. In this post we’ll transform the parse tree to a state…
The code for this post, as well as the post itself, are on github. This post is part 1 of a 3 part series. Part 1: Parsing Part 2: Generate an NFA Part 3: Evaluate an NFA Until recently, regular expressions seemed magical to me. I never understood how you could determine if a string matched a given regular expression. Now I know! Here’s how I implemented a basic regular expression engine in…
After 4 days climbing long moderates in Tuolumne Meadows, Eben and I were stoked to get out of the park for the weekend and escape the crowds on some eastside alpine rock. After a few minutes of flipping through the guidebook, we settled on the North Arete on Bear Creek Spire. The granite prow of Bear Creek Spire rises to 13713’, roughly 1500’ above the scree at its base. The North…