With more than 25 photos and 90 likes every second, we store a lot of data here at Instagram. To make sure all of our important data fits into memory and is available quickly for our users, we’ve begun to shard our data — in other words, place the data in many smaller buckets, each holding a part of the data.

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https://engineering.instagram.com/sharding-ids-at-instagram-1cf5a71e5a5c