The MySQL server is built in a modular fashion:

The storage engines manage data storage and index management for MySQL. The MySQL server communicates with the storage engines through a defined API.

Each storage engine is a class with each instance of the class communicating with the MySQL server through a special handler interface.

Handlers are instanced on the basis of one handler for each thread that needs to work with a specific table. For example: If three connections all start working with the same table, three handler instances will need to be created.

Once a handler instance is created, the MySQL server issues commands to the handler to perform data storage and retrieval tasks such as opening a table, manipulating rows, and managing indexes.

Custom storage engines can be built in a progressive manner: Developers can start with a read-only storage engine and later add support for INSERT, UPDATE, and DELETE operations, and even later add support for indexing, transactions, and other advanced operations.