32-bit MongoDB processes are limited to about 2 gb of data. This has come as a surprise to a lot of people who are used to not having to worry about that. The reason for this is that the MongoDB storage engine uses memory-mapped files for performance.
By not supporting more than 2gb on 32-bit, we’ve been able to keep our code much simpler and cleaner. This greatly reduces the number of bugs, and reduces the time that we need to release a 1.0 product. The world is moving toward all 64-bit very quickly. Right now there aren’t too many people for whom 64-bit is a problem, and in the long term, we think this will be a non-issue.
Reaching into Objects
MongoDB is a JSON-style store. Just like JSON, we can nest objects within other objects, and also arrays of data within objects. This then suggests the question or issue: how does one perform a query on nested objects? Index keys in nested objects? This is very important of course. The following doc page explains the method. http://www.mongodb.org/display/DOCS/Dot+Notation
Safe Software Deployment: The 180 Rule
In my last post , I talked about the anxiety developers feel when they deploy software, and the negative impact that fear has on innovation. Today, I’m offering the first of four methods I’ve used to help teams overcome that fear: The 180 Rule. Developers need to be able to get software into production, and if it doesn’t work, back it out of production as quickly as possible and return the system to its prior working state. If they have confidence that they can detect problems and fix them, they can feel more confident about deploying. All deployments have the same overall stages: Deployment: You roll the software from staging to production, either in pieces -- by directing more and more transactions to it -- or by flipping a switch. This involves getting binaries or configuration files reliably to production and having the system start using them. Monitoring: How does the system behave under live load? Do we have signals that the software is behaving correctly and performantly? It’s essential that this monitoring focuses more on the existing functionality than just the “Happy Path” of the new functionality. In other words, did we damage the system through the rollout? Rollback: If there is any hint that the system is not working correctly, the change needs to be quickly rolled back from production. In a sense, a rollback is a kind of deployment, because you’re making another change to the live system: returning it to a prior state. The “180” in the name of the rule has a double meaning. Of course, we’re referring here to the “180 degree” about-face of a rollback. But it’s also a reference to an achievable goal of any deployment. I believe that any environment should be able to deploy software to production and roll it back if it doesn’t work in three minutes, or 180 seconds. This gives 60 seconds to roll binaries to the fleet and point your customers to them, 60 seconds to see if the transaction loads or your canaries see problems, and then 60 seconds to roll back the binaries or configurations if needed. Of course, in your industry or for your product, you might need this to be shorter. But the bottom line is that a failed software deployment should not live in production for more than three minutes. Developers follow these three stages all the time, and they often do it manually. I know what you’re thinking: “How can any human being deploy, monitor, and roll back software that fast?” And that is the hidden beauty of the 180 Rule. The only way to meet this requirement is by automating the process. Instead of making the decisions, we must teach the computers how to gather the information and make the decisions themselves. Sadly, this is a fundamental change for many companies. But it’s a necessary change. Because the alternative is hoping things will work while fearing that they will not. And that makes developers loath to deploy software. Sure, there are a lot of tools out there that help with deployments. But this is not an off-the-shelf, set-it-and-forget-it scenario. You, as the developer, must provide those tools with the right metrics to monitor and the right scripts to both deploy the software and possibly roll it back. The 180 Rule does not specify which tools to use. Instead it forces developers to create rigorous scripts and metrics, and ensure they can reliably detect and fix problems quickly. There’s a gotcha that many of you are thinking of: The 180 Rule is not applicable if the deployment is not reversible. For example, deploying a refactored relational schema can be a big problem, because a new schema might introduce information loss that prevents a roll-back. Or the deployment might delete some old config files that aren’t used by the new software. I’ll talk more about how to avoid wicked problems like these in my subsequent posts. But for now, I’m interested to hear what you think of The 180 Rule, and whether you’re using any similar heuristics in your approach to safe deployment.