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I think MongoDB 3.6 is when it became a "decent" DB.

Azure CosmosDB provides protocol level for MongoDB 3.6. This offers developers a neat way of using the power of distributed CosmosDB without sacrificing cloud portability.

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Cosmos' API is an emulation of MongoDB which differs in features, compatibility, and implementation from an actual MongoDB deployment. Cosmos' suggestion of API version support (eg 3.6) is referring to the MongoDB wire protocol rather than the full MongoDB server feature set for that version. There are also some inherent differences, such as Cosmos' Request Units (RUs) which need to be considered for capacity planning and costs: https://docs.microsoft.com/en-us/azure/cosmos-db/request-uni....

Those differences may be fine for some use cases, but definitely compromise portability if you want to run or test the same application with a database deployment on GCP, AWS, or your own infrastructure. The lowest common denominator is based on Cosmos DB's underlying limits and features (not a MongoDB server feature set): https://docs.microsoft.com/en-us/azure/cosmos-db/concepts-li....

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