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CHAPTER 7 BUILDING YOUR OWN STORAGE ENGINE
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WHAT IS A SINGLETON
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There are situations in object-oriented programming when you may need to limit object creation such that only one object instantiation is made for a given class. One reason for this may be that the class protects a shared set of operations or data. For example, if you had a manager class designed to be a gatekeeper for access to a specific resource or data, you might be tempted to create a static or global reference to this object and therefore permit only one instance in the entire application. However, the use of global instances and constant structures or access functions flies in the face of the object-oriented mantra. Instead of doing that, you can create a specialized form of the object that restricts creation to only one instance so that it can be shared by all areas (objects) in the application. These special one-time-creation objects are called singletons. (For more information on singletons, see the article Creating Singleton Objects Using Visual C++ by T. Kulathu Sarma at www.codeproject.com/gen/design/singleton.asp.) There are a variety of ways to create singletons: Static variables Heap-registration Runtime type information (RTTI) Self-registering Smart singletons (like smart pointers) Now that you know what a singleton is, you re probably thinking that you ve been creating these your entire career but didn t know it!
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Note The pluggable storage engine isn t the only pluggable mechanism in MySQL. MySQL permits you to
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use pluggable text parsers and even user-defined functions. Future releases on MySQL may include pluggable stored procedure language processors.
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Basic Process
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The basic process for adding a new storage engine can be described as a series of stages. After all, a storage engine does not merely consist of a few lines of code; therefore the most natural way to develop something of this size and complexity is through an iterative process, where a small part of the system is developed and tested prior to moving on to another more complicated portion. In the tutorial that follows, I start with the most basic of functions and gradually add functionality until a fully functional storage engine emerges. The first few stages create and add the basic data read and write mechanisms. Later stages add indexing and transaction support. Depending on what features you want to add to your own storage engine, you may not need to complete all of the stages. A functional storage engine should support, at a minimum, the functions defined in the first four stages.2 The following list describes each of the stages:
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2. Some special storage engines may not need to write data at all. For example, the BLACKHOLE storage engine does not actually implement any write functions. Hey, it s a blackhole!
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CHAPTER 7 BUILDING YOUR OWN STORAGE ENGINE
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1. Stubbing the engine The first step in the process is creating the basic storage engine that can be plugged into the server. The basic source code files are created, the storage engine is established as a derivative of the handler base class, and the storage engine itself is plugged into the server source code. 2. Working with tables A storage engine would not be very interesting if it didn t have a means of creating, opening, closing, and deleting files. This stage is where you set up the basic file-handling routines and establish that the engine is working with the files correctly. 3. Reading and writing data To complete the most basic of storage engines, you must implement the read and write methods to read and write data from and to the storage medium.3 This stage is where you add those methods to read data in the storage medium format and translate them to the MySQL internal data format. Likewise, you write out the data from the MySQL internal data format to the storage medium. 4. Updating and deleting data To make the storage engine something that can be used in applications, you must also implement those methods that allow for altering data in the storage engine. This stage is where the resolution of updates and deletion of data is implemented. 5. Indexing the data A fully functional storage engine should also include the ability to permit fast random reads and range queries. This stage is where you implement the second-most complex operation of file access methods indexing. I have provided an index class that should make this step easier for you to explore on your own. 6. Adding transaction support The last stage of the process involves adding transaction support to the storage engine. It is at this stage that the storage engine becomes a truly relational database storage mechanism suitable for use in transactional environments. This is the most complex operation of file-access methods. Throughout this process, you should be conducting testing and debugging at every stage. In the sections that follow, I ll show you examples of debugging a storage engine and writing tests to test the various stages. All of the normal debugging and trace mechanisms can be used in the storage engine. You can also use the interactive debuggers and get in to see the code in action!
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