Azure Cosmos DB vs Redis: What are the differences?

Azure Cosmos DB and Redis are both widely used distributed databases. Let's discuss the key differences between Azure Cosmos DB and Redis:

1. Database Model: Azure Cosmos DB is a document-oriented database that provides a schema-less data model. It allows you to store and query JSON documents natively. On the other hand, Redis is an in-memory data structure store that supports various data structures such as strings, lists, sets, hashes, and more. It is primarily used as a key-value store.

2. Scalability and Global Distribution: Azure Cosmos DB offers global distribution and scalable performance out of the box. It ensures low latency access to data from any region around the world and allows you to scale both read and write throughput independently. Redis also provides scalability, but it requires additional configuration and setup to achieve global distribution.

3. Data Persistence: Azure Cosmos DB offers multiple levels of data consistency, including strong consistency, bounded staleness, session consistency, and eventual consistency. It persists data durably across multiple Azure regions. In contrast, Redis is an in-memory database by default, and you need to configure it for persistence by periodically saving the data to disk or using replication.

4. Querying Capabilities: Azure Cosmos DB provides a SQL-like query language called SQL API, which allows complex queries on JSON documents. It also supports other APIs like Gremlin (graph database), MongoDB, Cassandra, and Azure Table Storage API. Redis, on the other hand, primarily supports simple key-based operations and does not have built-in support for complex querying.

5. TTL (Time-to-Live) and Expiry: Azure Cosmos DB allows you to set a time-to-live (TTL) for individual documents, so they automatically expire and get deleted after a specific duration. Redis also supports TTL, but it is more commonly used for setting expiry on individual keys rather than entire documents.

6. Data Replication and High Availability: Azure Cosmos DB provides automatic multi-region replication to ensure high availability and fault tolerance. It replicates data across multiple regions and provides automatic failover in case of region-wide failures. Redis supports replication as well but requires manual configuration and set up for high availability scenarios.

In summary, choose Azure Cosmos DB if you need a globally distributed, document-oriented database with strong querying capabilities and automatic replication. Choose Redis if you need an in-memory data store with support for various data structures and manual replication setup.