&gt; <i>you can fit 500 billion of them in memory</i><p>You will also need space for fulltext indexes, indexes per hashtag (a set which grows constantly and never shrinks, but has a power distribution, meaning that some hot tags will overwhelm any single machine and it constantly changes), the ability to call up any tweet from any point in history (growing by one beefy Dell server per day), the ability to fan out to 1 or 10 or 10 million followers and so on. The fanout problem turns sharding into a lumpy problem because again, there is a power law distribution that can overwhelm any single machine.<p>Then there&#x27;s the fun part that Twitter is so reliable these days that when something else on the internet breaks, we check Twitter for updates. Robustness at that scale is more than buying a pair of SSDs and setting up RAID.<p>One thing I&#x27;ve learnt from being a consulting engineer is to stop assuming other engineers are morons. Sometimes we do things out of ignorance, or out of fear, or out of necessity. But often, when you inquire, there is a solid non-obvious reason for non-trivial diversions from the simplest thing that could possibly work.<p>I always find it useful to give the benefit of the doubt.