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2.1 MySQL Architecture
It will greatly aid your thinking about storage engines and the capabilities they bring to MySQL if you have a good mental picture of where they fit. Figure 2-1 provides a logical view of MySQL. It doesn't necessarily reflect the low-level implementation, which is bound to be more complicated and less clear cut. However, it does serve as a guide that will help you understand how storage engines fit in to MySQL. (The NDB storage engine was added to MySQL just before this book was printed. Watch for it in the second edition.)
The topmost layer is composed of the services that aren't unique to MySQL. They're services most network-based client/server tools or servers need: connection handling, authentication, security, etc.
The second layer is where things get interesting. Much of the brains inside MySQL live here, including query parsing, analysis, optimization, caching, and all the built-in functions (dates, times, math, encryption, etc.). Any functionality provided across storage engines lives at this level. Stored procedures, which will arrive in MySQL 5.0, also reside in this layer.
The third layer is made up of storage engines. They're responsible for the storage and retrieval of all data stored "in" MySQL. Like the various filesystems available for Linux, each storage engine has its own benefits and drawbacks. The good news is that many of the differences are transparent at the query layer.
The interface between the second and third layers is a single API not specific to any given storage engine. This API is made up of roughly 20 low-level functions that perform operations such as "begin a transaction" or "fetch the row that has this primary key" and so on. The storage engines don't deal with SQL or communicate with each other; they simply respond to requests from the higher levels within MySQL.
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