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The newer code tends to fit into the pattern of modules better. So in our definition, a module is a piece of code that logically belongs together in some way, and performs a certain critical function in the server. It parses the configuration file and the command-line arguments, allocates glo-bal memory buffers, initializes global variables and structures, loads the access control tables, and performs a number of other initialization tasks.

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Once the initial-ization job is complete, the Initialization Module passes control to the Connection Manager, which starts listening for connections from clients in a loop. When a client connects to the database server, the Connection Manager performs a number of low-level network protocol tasks and then passes control to the Thread Manager, which in turn supplies a thread to handle the connection which from now on will be referred to as the Connection Thread. The Connection Thread might be created anew, or retrieved from the thread cache and called to active duty.

The credentials of the connecting user are verified, and the client may now issue requests.

Understanding Mysql Internals

The Connection Thread passes the request data to the Command Dispatcher. Some requests, known in the MySQL code terminology as commands, can be accommo-dated by the Command Dispatcher directly, while more complex ones need to be redirected to another module.

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A typical command may request the server to run a query, change the active database, report the status, send a continuous dump of the replication updates, close the connection, or perform some other operation. In MySQL server terminology, there are two types of client requests: a query and a command. A query is anything that has to go through the parser. A command is a request that can be executed without the need to invoke the parser.


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We will use the term query in the context of MySQL internals. What we would call a query is sometimes called an SQL statement. MySQL Architecture 5 If full query logging is enabled, the Command Dispatcher will ask the Logging Mod-ule to log the query or the command to the plain-text log prior to the dispatch.

Thus in the full logging configuration all queries will be logged, even the ones that are not syntactically correct and will never be executed, immediately returning an error. The Query Cache Module checks whether the query is of the type that can be cached, and if there exists a previously computed cached result that is still valid. In the case of a hit, the execution is short-circuited at this point, the cached result is returned to the user, and the Connection Thread receives control and is now ready to process another command.

If the Query Cache Module reports a miss, the query goes to the Parser, which will make a decision on how to transfer control based on the query type. Select queries are forwarded to the Optimizer; updates, inserts, deletes, and table-creation and schema-altering queries go to the respective Table Modification Modules; queries that check, repair, update key statistics, or defragment the table go to the Table Maintenance module; queries related to replication go to the Replication Module; and status requests go to the Status Reporting Module.

At this point, each of the modules that will receive control from the Parser passes the list of tables involved in the query to the Access Control Module and then, upon suc-cess, to the Table Manager, which opens the tables and acquires the necessary locks.

Now the table operation module is ready to proceed with its specific task and will issue a number of requests to the Abstracted Storage Engine Module for low-level operations such as inserting or updating a record, retrieving the records based on a key value, or performing an operation on the table level, such as repairing it or updating the index statistics. The Abstracted Storage Engine Module will automatically translate the calls to the corresponding methods of the specific Storage Engine Module via object polymor-phism.

In other words, when dealing with a Storage Engine object, the caller thinks it is dealing with an abstracted one, when in fact the object is of a more specific type: it is the Storage Engine object corresponding to the given table type. The interface methods are virtual, which creates the effect of transparency. The correct method will be called, and the caller does not need to be aware of the exact object type of the Storage Engine object.

As the query or command is being processed, the corresponding module may send parts of the result set to the client as they become available.

MySQL DBA #9 MySQL InnoDB Storage Engine

It may also send warn-ings or an error message. If an error message is issued, both the client and the server will understand that the query or command has failed and take the appropriate mea-sures. The client will not accept any more result set, warning, or error message data for the given query, while the server will always transfer control to the Connection Thread after issuing an error. Note that since MySQL does not use exceptions for reasons of implementation stability and portability, all calls on all levels must be checked for errors with the appropriate transfer of control in the case of failure.

If the low-level module has made a modification to the data in some way and if the binary update logging is enabled, the module will be responsible for asking the Log-ging Module to log the update event to the binary update log, sometimes known as the replication log, or, among MySQL developers and power users, the binlog. Once the task is completed, the execution flow returns to the Connection Thread, which performs the necessary clean-up and waits for another query or command from the client.

The session continues until the client issues the Quit command. In addition to interacting with regular clients, a server may receive a command from a replication slave to continuously read its binary update log.


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This command will be handled by the Replication Master Module. They take care of propagating updates that happened on the master to the slave. It is possible for the same server to be configured as both a master and a slave. It is also responsible for encrypting the data using the OpenSSL library calls if the connection options are set appropriately. As they perform their respective tasks, the core components of the server heavily rely on the Core API. MySQL developers are encouraged to avoid direct libc calls, and use the Core API to facilitate ports to new platforms and code optimization in the future.

Figure illustrates the core modules and their interaction. MySQL Architecture 7 Client makes request Authenticates user upon first request Figure One purpose of the dis-cussion is to connect the conceptual language used earlier with the actual source. In addition, we will cover the some of the history of each component and try to esti-mate its future development path. Frequent references to the source will be made, and you may find it helpful to open the mentioned files in a text editor and locate the function references.

This can also be done in a debugger, as shown in Chapter 3. That chapter will also tell you how to get the source code.

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Understanding Mysql Internals: Discovering And Improving A Great Database

Server Initialization Module The Server Initialization Module is responsible for the server initialization on star-tup. Some other functions of interest follow. One big chunk of initialization code that used to be under main got reorganized gradually into a number of helper functions over the lifetime of the code. In version 5. Overall, this area of the code is fairly stable. Based on the past history, we should anticipate possible incremental additions in the future as new features that require special initialization on startup are added.

However, a rewrite of this code is unlikely. Connection Manager The Connection Manager listens for incoming connections from clients, and dis-patches the requests to the Thread Manager. However, it deserves to be classified as a separate module due to its critical role in the operation of the server. The abun-dance of ifdef directives speaks to the challenge of porting networking code to a variety of operating systems. Over time, the code evolved somewhat to accommodate quirks in the network sys-tem calls of different operating systems. Further changes might be necessary in the future as new ports are attempted, or as the different operating system vendors intro-duce MySQL Architecture 9 new quirks into new versions of their products.

Thread Manager The Thread Manager is responsible for keeping track of threads and for making sure a thread is allocated to handle the connection from a client. This is another very small module. Objects of the THD type are thread descrip-tors, and are critical in the operation of most of the server modules. Many functions take a THD pointer as their first argument.

The thread management code was significantly reworked in version 3.

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Since then it has not been changed significantly. It is rea-sonable to expect that it will not receive any significant changes in the future. However, if we, in our abstraction, consider the THD class itself as part of this mod-ule, we have a different story as far as changes are concerned. The addition of new features such as prepared statements, server-side cursors, and stored procedures led to a significant rework of THD in versions 4. Connection Thread The Connection Thread is the heart of the work of processing client requests on an established connection.

This module is also very small. However, despite its size, it deserves to be classified as a module due to its role in the server.