Not exactly. DocumentDB was primarily a document store. Cosmos allows you to store graphs and KV pairs as well.

I was unable to glean from a quick read of the consistency documentation, does CosmosDB support uniqueness and foreign key constraints?

Afraid not - there's no real concept of constraints with CosmosDB.

DocumentDB looks like a JSON document store?

It is.

Not really. It's a new database and is a superset of DocumentDB.

Azure Cosmos DB has been many years in the making. Azure Cosmos DB started as “Project Florence” in late 2010 to address developer the pain-points faced by large scale applications inside Microsoft. Observing that the challenges of building globally distributed apps are not a problem unique to Microsoft, in 2015 we made the first generation of this technology available to Azure developers in the form of DocumentDB. Since that time, we’ve been steadily adding new capabilities both in the database engine as well as, larger distributed system components. Azure Cosmos DB is the result. It is the next big leap in globally distributed, at scale, cloud databases. As a part of this release of Azure Cosmos DB, DocumentDB customers, with their data, are automatically Azure Cosmos DB customers. They now have access to the new system and capabilities offered by Azure Cosmos DB today as well as, as we keep evolving the service.

Does Cosmos DB support distributed transactions like Spanner? Would be nice to see a deeper dive into Transactions like how isolation works etc.

I'd also be interested in an answer. I'm guessing the answer is "no" based on my reading of the docs, but it'd be nice to confirm that.

Is GROUP BY support a possibility in the future? I was a little disappointed to see aggregates implemented without them after waiting for so long...

Here's a link for anyone who would like to vote for this feature. https://feedback.azure.com/forums/263030-documentdb/suggesti...

Thanks - this and other features are planned. Please stay tuned.