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  • D3 Round Two: How to Blend HTML5 Canvas with SVG to Speed Up Rendering

    Soon after the publication of “Digging Into D3 Internals to Eliminate Jank,” I was pleased to see that it had sparked a discussion on Twitter, with D3 community members, notably Noah Veltman and Mike Bostock, sharing suggestions for improving our rendering solution:

    A suggestion we received both in this discussion and on lobste.rs was to use canvas to render the data points. We had originally avoided canvas because of time constraints, lack of team familiarity with canvas, and the complications it introduced with regards to mouse interactions. However, Noah proposed a combination of SVG and canvas that strikes a balance between canvas’ performance and SVG’s convenience, complete with a demo:

    It piqued my interest, and so I decided to explore it in some more detail here.

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  • The Saga of Concurrent DNS in Python, and the Defeat of the Wicked Mutex Troll

    creaky old wooden chest with medieval-style engravings and a scroll depicting the history of getaddrinfo on macOS

    Tell us about the time you made DNS resolution concurrent in Python on Mac and BSD.

    No, no, you do not want to hear that story, my friends. It is nothing but old lore and #ifdefs.

    But you made Python more scalable. The saga of Steve Jobs was sung to you by a mysterious wizard with a fanciful nickname! Tell us!

    Gather round, then. I will tell you how I unearthed a lost secret, unbound Python from old shackles, and banished an ancient and horrible Mutex Troll.

    Let us begin at the beginning.


    A long time ago, in the 1980s, a coven of Berkeley sorcerers crafted an operating system. They named it after themselves: the Berkeley Software Distribution, or BSD. For generations they nurtured it, growing it and adding features. One night, they conjured a powerful function that could resolve hostnames to IPv4 or IPv6 addresses. It was called getaddrinfo. The function was mighty, but in years to come it would grow dangerous, for the sorcerers had not made getaddrinfo thread-safe.

    As ages passed, BSD spawned many offspring. There were FreeBSD, OpenBSD, NetBSD, and in time, Mac OS X. Each made its copy of getaddrinfo thread safe, at different times and different ways. Some operating systems retained scribes who recorded these events in the annals. Some did not.

    Because getaddrinfo is ringed round with mystery, the artisans who make cross-platform network libraries have mistrusted it. Is it thread safe or not? Often, they hired a Mutex Troll to stand guard and prevent more than one thread from using getaddrinfo concurrently. The most widespread such library is Python’s own socket module, distributed with Python’s standard library. On Mac and other BSDs, the Python interpreter hires a Mutex Troll, who demands that each Python thread hold a special lock while calling getaddrinfo.

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  • MongoDB’s JavaScript Fuzzer: Harnessing the Havoc (2/2)

    Fuzz testing is a method for subjecting a codebase to a tide of hostile input to supplement the test cases engineers create on their own. In part one of this pair, we looked at the hybrid nature of our fuzzer – how it combines “smart” and “dumb” fuzzing to produce input random enough to provoke bugs, but structured enough to pass input validation and test meaningful codepaths. To wrap up, I’ll discuss how we isolate signal from the noise a fuzzer intrinsically produces, and the tooling that augments the root cause analyses we do when the fuzzer finds an issue.

    An unbridled fuzzer creates too much noise

    Fuzz testing is a game of random numbers. That randomness makes the fuzzer powerful… too powerful. Without some careful harnessing, it would just blow itself up all the time by creating errors within the testing code itself. Take the following block of code, which is something you would see in one of MongoDB’s JavaScript tests:

    while(coll.count() < 654321)
        assert(coll.update({a:1}, {$set: {...}}))
    

    This code does a large number of updates to a document stored in MongoDB. If we were to put it through the fuzzer, a possible test-case that the fuzzer could produce is this:

    while(true) 
        assert(coll.update({}, {$set: {"a.654321" : 1}}))
    

    The new code now tests something completely different. It tries to set the 654321th element in an array stored in all documents in some MongoDB collection.

    Now, this is an interesting test-case. Using the $set operator with such a large array may not be something we thought of testing explicitly and could trigger a bug (in fact it does). But the interaction between the fuzzed true condition and the residual while loop is going to hang the test! Unless, that is, the assert call in the while loop fails, which could happen if the line defining coll in the original test (not shown here) is mutated or deleted by the fuzzer, leaving coll undefined. If the assert call fails, it would be caught by the Mongo shell and cause it to terminate.

    But neither the hang nor the assertion failure are caused by bugs in MongoDB. They are just byproducts of a randomly generated test-case, and they represent the two classes of noise we have to filter out of our fuzz testing: branch logic and assertion failures.

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  • MongoDB’s JavaScript Fuzzer: Creating Chaos (1/2)

    As MongoDB becomes more feature rich and complex with time, our need for more sophisticated bug-finding methods grows as well. We recently added a homegrown JavaScript fuzzer to our toolkit, and it is now our most prolific bug finding tool, responsible for finding almost 200 bugs over the course of two release cycles. These bugs span a range of MongoDB components from sharding to the storage engine, with symptoms ranging from deadlocks to data inconsistency. We run the fuzzer as part of our continuous integration system, Evergreen, where it frequently catches bugs in newly committed code.

    In part one of two, we examine how our fuzzer hybridizes the two main types of fuzzing to achieve greater coverage than either method alone could accomplish. Part two will focus on the pragmatics of running the fuzzer in a production setting and distilling a root cause from the complex output fuzz tests often produce.

    What’s a fuzzer?

    Fuzzing, or fuzz testing, is a technique of generating randomized, unexpected, and invalid inputs to a program to trigger untested code paths. Fuzzing was originally developed in the 1980s and has since proven to be effective at ensuring the stability of a wide range of systems, from filesystems to distributed clusters to browsers. As people attempt to make fuzzing more effective, two philosophies have emerged: smart, and dumb fuzzing. And as the state of the art evolves, the techniques that are used to implement fuzzers are being partitioned into categories, chief among them being “generational” and “mutational.” In many popular fuzzing tools, smart fuzzing corresponds to generational techniques, and dumb fuzzing to mutational techniques, but as we will see, this is not an intrinsic relationship. Indeed, in our case, the situation is precisely reversed.

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  • Investing In CS4All: Training Teachers and Helping Them Build Curricula

    Until last year, Jeremy Mellema was a history teacher. Now, he’s teaching computer programming. When I visited his class in the Bronx this month, he had 30 students with 30 MacBooks, completing exercises in Python. They had just finished a lesson on data types, and now they were tackling variables. In Jeremy’s class, the first variable assignment is:

    tupac = "Greatest of All Time!!"
    

    Computer Science for All

    A year ago, New York City mayor Bill de Blasio announced Computer Science for All, an $80 million public-private partnership. The goal is to teach computer science to every student at every public school. But first, the schools need curricula and 5000 teachers need training.

    Here at MongoDB, our VP of Education Shannon Bradshaw oversees MongoDB University, which trains IT professionals to use MongoDB. When he heard about CS4All, he wanted us to contribute. He proposed that we set aside budget for two paid fellowships, and recruit public school teachers to spend the summer with us. We would develop them as teachers, and help build curricula they could take back into schools this fall. MongoDB staff would share our expertise, our office space, our equipment, and the MongoDB software itself.

    Shannon pitched his proposal to the company like this: “As many of us know, it’s still unusual for students to encounter computer science, let alone databases, in their classrooms before entering college. I believe this absence directly contributes to the gender and racial disparity we see today across our industry.” The CS4All project improves access to these subjects for many more students in our city, and MongoDB could be part of it from the beginning.

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  • Succeeding With ClangFormat, Part 3: Persisting The Change

    Cartoon of coders arguing about formatting and not seeing a bug

    If you’ve been following our series on succeeding with ClangFormat, you already know all about why we did it and the steps we took to ensure the migration went well. In this concluding post, we’ll talk about how to succeed after the integration and reformat are complete. We learned some valuable lessons about what happens in the immediate aftermath of bringing ClangFormat into our system and have been refining our workflows ever since. Here’s a look at our occasionally bumpy road and how you might have a smoother one.

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  • Evergreen Continuous Integration: Why We Reinvented The Wheel

    We’ve all been there: you’re pitching a solution when one of your team members interjects, “let’s not reinvent the wheel, here.” Whether it’s based on fear or wisdom, the charge of reinventing the wheel is a death sentence for ideas. It typically isn’t worth the time and resources to implement a new version of an old, ubiquitous idea—though you’d never know that with all the different kinds of actual, literal wheels you use every day.

    All different kinds of wheels

    For most developers, continuous integration (CI)—the automated building and testing of new code pushed into your repository—is one of those never-reinvented wheels. You set up one of a few long-standing solutions like Travis or Jenkins, rejigger your test code to fit into that solution’s organizational model, and then avoid messing with it too much. Here at MongoDB, challenging this approach rewarded us incredibly.

    Instead of working around an off-the-shelf solution that didn’t fit our needs, we wound up reinventing the wheel and built our own continuous integration system called Evergreen. It gives us a powerful, efficient infrastructure that lets us test changes quickly – and keeps our engineers happy as well. Our journey to creating Evergreen was born of necessity and stalked by uncertainty, but we don’t regret it. Reinventing the wheel allowed us to build a near-perfect CI tool for our use case, seriously evaluate powerful new technologies, and have a lot of fun doing it.

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  • Succeeding With ClangFormat, Part 2: The Big Reformat

    Cartoon of coders arguing about formatting and not seeing a bug

    When properly integrated into a toolchain, ClangFormat can entirely do away with time wasted on discussion and enforcement of code formatting. In part 1 of this series, I laid out the case for doing so, the factors that doomed our prior attempt, and the approach we took to get it right the next time. In this part I’ll walk through all the details that have to be considered before drawing up a functional specification and reaching the next milestone: codebase conversion.

    Setting the format

    Landing on a format was surprisingly easy, considering how contentious formatting choices can be. In this area, MongoDB has the benefit of being towards the larger end of team size. In a large shop, developers seem more understanding that “there is a way of doing things” that might not be their personal preference. But regardless of your team’s size, everyone has to agree on the fundamental principle that a standard is more important that which standard. Start your initiative with obtaining buy-in, and demonstrate your commitment to solving disputes fairly, and you will find this step is not as fraught as you might expect.

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