1. Key Difference 1: Data Structure: Redis is a key-value store that offers various data structures such as strings, lists, sets, sorted sets, etc., making it highly flexible for different use cases. On the other hand, Scylla is a highly scalable NoSQL database that is built upon Apache Cassandra and focuses on providing wide column store functionality. It organizes data into tables with a defined schema, allowing for efficient querying and storage of large datasets.
2. Key Difference 2: Data Distribution: Redis utilizes a master-slave replication model, where multiple slave nodes can replicate data from a single master node. This provides high availability and read scalability but limits write scalability. In contrast, Scylla follows a masterless distributed architecture using consistent hashing and gossip protocols. It allows for automatic partitioning and distribution of data across multiple nodes, ensuring high write and read scalability.
3. Key Difference 3: Durability: Redis offers durability through a combination of techniques such as point-in-time snapshots, append-only files, and asynchronous replication. However, it is optimized for in-memory operations and may not be as suitable for scenarios with large datasets or high write frequencies. Scylla, on the other hand, is designed for durability, supporting both in-memory and on-disk storage. It integrates with distributed file systems and provides configurable levels of data persistence to meet various durability requirements.
4. Key Difference 4: Consistency Models: Redis supports multiple consistency models, including eventual consistency and strong consistency, depending on the data structure used. It allows applications to choose the level of consistency required for their use case. Scylla follows a tunable consistency model, giving developers the ability to define their desired consistency level on a per-operation basis. This flexibility allows for a trade-off between consistency, availability, and partition tolerance.
5. Key Difference 5: Query Language: Redis uses a simple query language that operates directly on its data structures. It supports a set of commands and pipelines, making it easy to manipulate and retrieve data. Scylla, being a wide column store, utilizes the CQL (Cassandra Query Language) which is similar to SQL but adapted for use in NoSQL databases. CQL provides powerful querying capabilities with support for filtering, data aggregation, and secondary indexes.
6. Key Difference 6: Performance and Scaling: Redis is known for its exceptional performance as an in-memory database. It can handle high volumes of operations with low latencies. However, when it comes to scaling beyond the capacity of a single node, Redis requires additional sharding and partitioning mechanisms. Scylla, being designed for horizontal scalability from the ground up, performs exceptionally well at massive scales. It efficiently utilizes hardware resources across a cluster of nodes, making it ideal for high-performance and high-throughput applications.

In Summary, Redis and Scylla differ in terms of their data structure flexibility, data distribution models, durability options, consistency models, query languages, and performance/scaling capabilities.