<p>Do you need <em>durable</em> data?</p> <p>The biggest issue here is what should happen in a machine crash situation. If someone pulls out the power from your server should the system resume in exactly the same state that it was in when the power went out?</p> <p>It is tempting to think that all data needs to be durable but don't make ephemeral information like catalogue browsing history or an 'I like this' style button-click equivalent to, say, the placing of an order or the creation of a user account. Durable data comes at the cost of speed, as it means coordinating the persistence of the data.</p>