A key-value engine serves as the bottommost layer in a key-value database, unless you are going to build your own file system or operating system. A key-value engine is crucial for a database because it manages all the persistent data directly.

Most key-value engines provide some common interfaces like Get, Put and Delete. Some engines also allow you to iterate the key-values in order efficiently, and most will provide special features for added efficiency.

Choosing a key value engine is the first step to build a database. Here are some important things we need to consider:

• The data structure. Different data structures are optimized for different workloads. Some are good for reads and some are good for writes, etc.
• Maturity. We don't need a storage engine to be fancy but we want it to be reliable. Buggy engines ruin everything you build on top of them. We recommend using a battle-tested storage engine which has been adopted by a lot of users.
• Performance. The performance of the storage engine limits the overall performance of the whole database. So make sure the storage engine meets your expectation and has the potential to improve along with the database.

In this chapter, we will do a comparison between two well-known data structures and guide you through the storage engine used in TiKV.