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Did I miss it, or did they not say why they picked CosmoDB? Postgres has also sharding, so instead of moving to a different DB they could have added a new postgres instance with sharding for the new requests.

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At a guess CosmosDb NoSql was a good choice for dumping user contexts sharded rather than use Postgres schema or JSONB. Citus is the obvious choice for this with Postgres but Azure had poor support until recently 2024 they have preview Azure Database for Postgres with Elastic Clusters, which is basically Azure Database for Postgres Flexible Server with Citus extension installed. In the end they aren’t paying Microsoft what usual customer are, so even though CosmosDb NoSql is expensive (RU based) and the SDK is horrible, it probably served as a good stopgap until elastic clusters is fully out of preview. That’s my guess anyway.

Might've had something to do with explaining behavior to their app teams. "Use this new DB product where it's sharded" might be easier than "here's a new postgres endpoint like the old one but now if you join on user ID it's inconsistent".

(not that that's an excuse, but i've seen similar things before)

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