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Most likely? Time.

Using off the shelf software means you mostly design how to plumb things together and how to make them correct , safe and scalable.

The things you mention, on the other hand, carry the same requirements but are also much complex to develop AND to maintain.

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The social network VK internally uses highly specialized database engines per business domain. They don't use stock DBs. They have a DB engine for posts, a DB engine for likes, etc. They have a team of DB engineers. Their DB load was around 250 mln RPS 3 years ago. Stock DBs were harder to scale for them. I guess if you have immense highload, having a team of DB engineers can be cheaper because you can save a lot on servers. I reviewed their code. A DB engine's source code is pretty compact and simple (relatively speaking) because they deal with very specific domain entities, so they don't have to account for all the possible user query combinations that a general-purpose DB would have to support. It was mostly shards+binlog+snapshots+views in RAM. Considering that Telegram was founded by former VK engineers, I suspect they have something similar.

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