Azure CosmosDB is a truly global, highly available geo-replicated NoSQL data store. CosmosDB is available in almost all Azure regions, and it is also possible to configure geo-replication between all of these regions. CosmosDB allows for creating collections that are replicated across multiple regions asynchronously. It also provides flexibility in determining the consistency level, while configuring the replication strategy for high availability. These consistency levels help an architect to determine the criticality of the availability of data in other regions. These consistency levels are as follows:
- Strong: This ensures that every replicated region gets its data before returning to the user.
- Bounded staleness: This ensures that data is not stale in read regions beyond a point—either a fixed number of writes or a time span.
- Sessions: This ensures that data is consistent in a session.
- Ordered prefixes: This is when the writes will come to replicated regions in a similar order as they were written in the write region.
- Eventual: It is possible to have dirty reads here, and there is no SLA for determining the availability of data. It follows the principle of eventual consistency.