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How MongoDB's Journaling Works
This was originally posted to Kristina Chodorow’s blog, Snail in a Turtleneck I was working on a section on the gooey innards of journaling for The Definitive Guide , but then I realized it’s an implementation detail that most people won’t care about. However, I had all of these nice diagrams just laying around. Good idea, Patrick! So, how does journaling work? Your disk has your data files and your journal files, which we’ll represent like this: When you start up mongod , it maps your data files to a shared view . Basically, the operating system says: “Okay, your data file is 2,000 bytes on disk. I’ll map that to memory address 1,000,000-1,002,000. So, if you read the memory at memory address 1,000,042, you’ll be getting the 42nd byte of the file.“ (Also, the data won’t necessary be loaded until you actually access that memory.) This memory is still backed by the file: if you make changes in memory, the operating system will flush these changes to the underlying file. This is basically how mongod works without journaling: it asks the operating system to flush in-memory changes every 60 seconds. However, with journaling, mongod makes a second mapping, this one to a private view . Incidentally, this is why enabling journalling doubles the amount of virtual memory mongod uses. Note that the private view is not connected to the data file, so the operating system cannot flush any changes from the private view to disk. Now, when you do a write, mongod writes this to the private view. mongod will then write this change to the journal file, creating a little description of which bytes in which file changed. The journal appends each change description it gets. At this point, the write is safe. If mongod crashes, the journal can replay the change, even though it hasn’t made it to the data file yet. The journal will then replay this change on the shared view. Finally, at a glacial speed compared to everything else, the shared view will be flushed to disk. By default, mongod requests that the OS do this every 60 seconds. The last step is that mongod remaps the shared view to the private view. This prevents the private view from getting too “dirty” (having too many changes from the shared view it was mapped from). And that’s how journaling works. Thanks to Richard, who gave the best explanation of this I’ve heard (Richard is going to be teaching an online course on MongoDB this fall, if you’re interested in more wisdom from the source).
Australian Start-Up Ynomia Is Building an IoT Platform to Transform the Construction Industry and its Hostile Environments
The trillion dollar construction industry has not yet experienced the same revolution in technology you might have expected. Low levels of R&D and difficult working environments have led to a lack of innovation and fundamental improvements have been slow. But one Australian start-up is changing that by building an Internet of Things (IoT) platform to harness construction and jobsite data in real time. “Productivity in construction is down there with hunting and fishing as one of the least productive industries per capita in the entire world. It's a space that's ripe for people to come in and really help,” explains Rob Postill , CTO at Ynomia. Ynomia has already been closely involved with many prestigious construction projects, including the residential N06 development in London’s famous 2012 Olympic Village. It was also integral to the construction of the Victoria University Tower in Australia. Link to Podcast Episode Here “These projects involve massive outflow of money: think about glass facades on modern buildings, which can represent 20-30 percent of the overall project cost. They are largely produced in China and can take 12 weeks to get here,” says Postill. “Meanwhile, the plasterer, the plumber, the electrician are all waiting for those glass facades to be put on so it is safe for them to work. If you get it wrong, you can go in the deep red very quickly.” To tackle these longstanding challenges, Ynomia aims to address the lack of connectivity, transparency and data management on construction sites, which has traditionally resulted in the inefficient use of critical personnel, equipment and materials; compressed timelines; and unpredictable cash flows. To optimize productivity, Ynomia offers a simple end-to-end technology solution that creates a Connected Jobsite. Helping teams manage materials, tools, and people across the worksite in real time. IOT in a Hostile Environment The deployment of technology in construction is often fraught with risk. As a result, construction sites are still largely run on paper, such as blueprints, diagrams and models as well as the more traditional invoices and filing. At the same time, there is a constant need to track progress and monitor massive volumes of information across the entire supply chain. Engineers, builders, electricians, plumbers, and all the other associated professionals need to know what they need to do, where they need to be, and when they need to start. “The environment is hostile to technology like GPS, computers, and mobile phone reception because you have a lot of Faraday cages and lots of water and dust,” explains Postill. “You can't have somebody wandering around a construction site with a laptop; it'll get trashed pretty quickly." Enter MongoDB Atlas “On a site, you might be talking about materials, then if you add to that plant & equipment, or bins, or tools etc, you're rapidly getting into thousands and thousands of tags, talking all the time, every day,” said Postill. That means thousands of tags now send millions of readings on Ynomia building sites around the world. All these IoT data packets must be stored efficiently and accurately so Ynomia can reassemble the history of what has happened and track tagged inventory, personnel, and vehicles around the site. Many of the tag events are also safety critical so accuracy is a vital component and packets can't be missed. To address these needs Ynomia was looking for a database that was scalable, flexible, resilient and could easily handle a wide variety of fast-changing sensor data captured from multiple devices. The final component Postill was looking for in a database layer was freedom: a database that didn't lock them into a single cloud platform as they were still in the early stages of assessing cloud partners. The Commonwealth Scientific and Industrial Research Organisation , which Postill had worked with in the past, suggested MongoDB , a general purpose, document-based database built for modern applications. “The most important factor was that the database is event-driven, which I knew would be difficult in the traditional relational model. We deal with millions of tag readings a day, which is a massive wall of data,” said Postill. A Cloud Database Ynomia is using MongoDB Atlas , the global cloud database service, now hosted on Microsoft Azure. Atlas offers best-in-class automation and proven practices that combine availability, scalability, and compliance with the most demanding data security and privacy standards. “When we started we didn't know enough about the problem and we didn't want to be constrained," explained Postill. "MongoDB Atlas gives us a cloud environment that moves with us. It allows us to understand what is happening and make changes to the architecture as we go." Postill says this combination of flexibility and management tooling also allows his developers to focus on business value not undifferentiated code. One example Postill gave was cluster administration: "Cluster administration for a start-up like us is wasted work," he said. "We’re not solving the customer's problem. We're not moving anything on. We’re focusing on the wrong thing. For us to be able to just make that problem go away is huge. Why wouldn’t you?" Atlas also gives Ynomia the option to spin out new clusters seamlessly anywhere in the world. This allows customers to keep data local to their construction site, improving latency and helping solve for regional data regulations. Real Time Analytics The company has also deployed MongoDB Charts, which takes this live data and automatically provides a real time view. Charts is the fastest and easiest way to visualize event data directly from MongoDB in order to act instantly and decisively based on the real-time insights generated by event-driven architecture. It allows Ynomia to share dashboards so all the right people can see what they need to and can collaborate accordingly. “Charts enables us to quickly visualize information without having to build more expensive tools, both internally and externally, to examine our data,” comments Postill. “As a startup, we go through this journey of: what are we doing and how are we doing it? There's a lot of stuff we are finding out along the way on how we slice and re-slice our data using Charts.” A Platform for Future Growth Ynomia is targeting a huge market and is set for ambitious growth in the coming years. How the platform, and its underlying architecture, can continue to scale and evolve will be crucial to enabling that business growth. “We do anything we can to keep things simple,” concluded Postill. “We pick technology partners that save us from spending time we shouldn't spend so we can solve real problems. We pick technologies that roll with the punches and that's MongoDB.” When we started we didn't know enough about the problem and we didn't want to be constrained," explained Postill. "MongoDB Atlas gives us a cloud environment that moves with us. It allows us to understand what is happening and make changes to the architecture as we go. Rob Postill, CTO, Ynomia