What does it mean that Azure Cosmos DB is multi-model?
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 can be queried multiple ways concurrently so that I can use both gremlin graph queries and mongodb api against the same collections.
- or -
b) Does it mean that you can choose a different model (graph, key value, column, document) at the time of provisioning your Cosmos DB and that is how the data will be stored from then on.
The brochure makes it sound like a), but using the Azure dashboard to create a cosmos instance it makes it seem like b) since you have to choose a model type at creation.
Additionally, the literature makes reference to columnar data, but I don't see the option for it at create time.
Answer
Cosmos DB is a single NoSQL data engine, an evolution of Document DB. When you create a container ("database instance") you choose the most relevant API for your use case which optimises the way you interact with the underling data store and how the data is persisted in to that store.
So, depending on the API chosen, it projects the desired model (graph, column, key value or document) on to the underlying store.
You can only use one API against a container, multiple are not possible due to the way the data is stored and retrieved. The API dictates the storage model - graph, key value, column etc, but they all map back on to the same technology under the hood.
Thanks to @Jesse Carter's comment below it appears you are however able to mix and match the graph and DocumentSQL APIs.
Multi-model, multi-API support
Azure Cosmos DB natively supports multiple data models including documents, key-value, graph, and column-family. The core content-model of Cosmos DB’s database engine is based on atom-record-sequence (ARS). Atoms consist of a small set of primitive types like string, bool, and number. Records are structs composed of these types. Sequences are arrays consisting of atoms, records, or sequences. The database engine can efficiently translate and project different data models onto the ARS-based data model. The core data model of Cosmos DB is natively accessible from dynamically typed programming languages and can be exposed as-is as JSON.
The service also supports popular database APIs for data access and querying. Cosmos DB’s database engine currently supports DocumentDB SQL, MongoDB, Azure Tables (preview), and Gremlin (preview). You can continue to build applications using popular OSS APIs and get all the benefits of a battle-tested and fully managed, globally distributed database service.