Twitter is very read-heavy. Think about the average timeline — Tweets don't change (they recently introduced "editing", but it's not an in-place edit). That mea

Twitter is very read-heavy. Think about the average timeline — Tweets don't change (they recently introduced "editing", but it's not an in-place edit). That means many things can be heavily cached, and most people on Twitter aren't writing Tweets, so the write volume is relatively low compared to the read volume. Now think about the average chat app like Slack or Teams. Those have a very different read-write ratio. Some things can be cached for long periods of time, but many other entities become invalidated quickly, and are subject to intricate permissions policies. As for sharding, Slack uses Vitess: https://slack.engineering/scaling-datastores-at-slack-with-vitess/ https://slack.engineering/scaling-datastores-at-slack-with-v...