In particular, we ran a beta test on all our engineers’ machines, comparing Watchman’s answers for real user queries with the actual file system results and logging any differences. Through heavy stress testing and internal dogfooding, we identified and fixed many of the issues and race conditions that are common in file system monitoring. Mercurial’s design made integrating with Watchman straightforward, but we expected Watchman to have bugs, so we developed a strategy to address them safely. We decided to query our build system’s file monitor, Watchman, to see which files have changed. This has been tried before, even for Mercurial, but making it work reliably is surprisingly challenging. We solved this by monitoring the file system for changes. The Git approach doesn’t scale, and the Perforce approach isn’t friendly. Git examines every file and naturally becomes slower and slower as the number of files increases, while Perforce “cheats” by forcing users to tell it which files they are going to edit. These helped, but we also wanted to make more fundamental changes to address the problem of scale.įor a repository as large as ours, a major bottleneck is simply finding out what files have changed. These range from new graph algorithms to rewrites of tight loops in native code. To narrow this performance gap, we’ve contributed over 500 patches to Mercurial over the last year and a half. When we first started working on Mercurial, we found that it was slower than Git in several notable areas. Just as importantly, the Mercurial developer community is actively helping us address our scaling problems by reviewing our patches and keeping our scale in mind when designing new features.
FACEBOOK SPADES PLUS HACK CODE
Importantly, it’s written mostly in clean, modular Python (with some native code for hot paths), making it deeply extensible.
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Mercurial is a distributed source control system similar to Git, with many equivalent features. After much deliberation, we concluded that Git’s internals would be difficult to work with for an ambitious scaling project. Our engineers were comfortable with Git and we preferred to stay with a familiar tool, so we took a long, hard look at improving it to work at scale. But instead of building a new system from scratch, we decided to take an existing one and make it scale. We realized that we’d have to solve this ourselves.
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On top of that, the idea that the scaling constraints of our source control system should dictate our code structure just doesn’t sit well with us.
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Splitting it up would make large, atomic refactorings more difficult. Even at our current scale, we often make large changes throughout our code base, and having a single repository is useful for continuous modernization. We could have spent a lot of time making it more modular in a way that would be friendly to a source control tool, but there are a number of benefits to using a single repository. Our code base has grown organically and its internal dependencies are very complex. We looked at the available options and found none that were both fast and easy to use at scale. Based on those projections, it appeared likely that our then-current technology, a Subversion server with a Git mirror, would become a productivity bottleneck very soon. Two years ago, as we saw our repository continue to grow at a staggering rate, we sat down and extrapolated our growth forward a few years. Given our size and complexity-and Facebook’s practice of shipping code twice a day–improving our source control is one way we help our engineers move fast. With thousands of commits a week across hundreds of thousands of files, Facebook’s main source repository is enormous–many times larger than even the Linux kernel, which checked in at 17 million lines of code and 44,000 files in 2013.