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copied!<p>First, don't confuse CAP "Availability" with "High Availability". They have nothing to do with each other. The A in CAP simply means "All DB nodes can answer queries". To get High Availability, you must be in multiple data centers, you must have robust documented procedures for maintenance, expansion, etc. None of that depends on your CAP choice.</p> <p>Second, be realistic about your requirements. A stock-trading application might have a requirement for 100% uptime, because every second of downtime could loose millions of dollars. On the other hand, I'm guessing your pizza joint might loose tens of dollars for every minute it's down. So it doesn't make sense to spend millions trying to keep it up. Try to compute your actual costs.</p> <p>Third, always evaluate your choice vs mainstream. You could just go CA (MySQL) and quickly fail-over to the slaves when problems happen. Be realistic about the costs (and risks) of building on new technology. If you <em>really</em> expect your system to run for 5 years without downtime, ask for proof that someone else has run that database for 5 years without downtime.</p> <p>If you go "AP" and have remote people (drivers, etc.) then you'll need to write an app that stores their data on their phone and sends it in the background (with retries). Of course, you could do this regardless of weather your database was CA or AP.</p> <p>If you want high uptimes, you can either:</p> <ul> <li><p>Increase MTBF (Mean Time Between Failures) - Buy redundant power supplies, buy dual ethernet cards, etc..</p></li> <li><p>Decrease MTTR (Mean Time To Recovery) - Just make sure when failure happens you can recover quickly. (Fail over to slave)</p></li> </ul> <p>I've seen people spend tens of thousands of dollars on MTBF, only to be down for 8 hours while they restore their backup. It makes more sense to ensure MTTR is low before attacking MTBF.</p>

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