What does it mean that Azure Cosmos DB is multi-model?

Question

Looking at the new Azure cosmos database, I\'m a bit confused about the multi-model nature of it. Specifically, does it mean:

a) That the same underlying database/store

Answer 1

The accepted answer misses out on some points.

Cosmos DB is a NoSQL database, but it is highly distributed and we its storage format is Atom-Record-Sequence.

Why does that matter? We know that it accepts JSON as in- and output formats, that does not mean Cosmos stores its data as JSON, it could be any format actually. This helps us to reason about the multi-modelness of Cosmos: what you get when you execute a query according to a certain model is probably a projection or view of your data.

@JesseCarter already explained we can interchangeably use Document API and Graph API. Last week Table API got publicly announced and probably this API is not too different as well.

The guys over at Spectologic have written a nice blogpost about the Cross-API usage of Cosmos and have also pointed out that the multi-modelness is more cosmetics than internals, the only real exception seems Mongo. The interesting part gets pointed out in the chapter 'Switching the portal experience' here: https://blog.spectologic.com/2017/06/30/digging-into-cosmosdb-storage/

So maybe in the end it boils down to GlobalDocumentDb vs. MongoDb