MySQL Native Driver Plugin Architecture

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This section provides an overview of the mysqlnd plugin architecture.

MySQL Native Driver Overview

Before developing mysqlnd plugins, it is useful to know a little of how mysqlnd itself is organized. Mysqlnd consists of the following modules:

**The mysqlnd organization chart, per module**
Modules Statistics	mysqlnd_statistics.c
Connection	mysqlnd.c
Resultset	mysqlnd_result.c
Resultset Metadata	mysqlnd_result_meta.c
Statement	mysqlnd_ps.c
Network	mysqlnd_net.c
Wire protocol	mysqlnd_wireprotocol.c

C Object Oriented Paradigm

At the code level, mysqlnd uses a C pattern for implementing object orientation.

In C you use a struct to represent an object. Members of the struct represent object properties. Struct members pointing to functions represent methods.

Unlike with other languages such as C++ or Java, there are no fixed rules on inheritance in the C object oriented paradigm. However, there are some conventions that need to be followed that will be discussed later.

The PHP Life Cycle

When considering the PHP life cycle there are two basic cycles:

PHP engine startup and shutdown cycle
Request cycle

When the PHP engine starts up it will call the module initialization (MINIT) function of each registered extension. This allows each module to setup variables and allocate resources that will exist for the lifetime of the PHP engine process. When the PHP engine shuts down it will call the module shutdown (MSHUTDOWN) function of each extension.

During the lifetime of the PHP engine it will receive a number of requests. Each request constitutes another life cycle. On each request the PHP engine will call the request initialization function of each extension. The extension can perform any variable setup and resource allocation required for request processing. As the request cycle ends the engine calls the request shutdown (RSHUTDOWN) function of each extension so the extension can perform any cleanup required.

How a plugin works

A mysqlnd plugin works by intercepting calls made to mysqlnd by extensions that use mysqlnd. This is achieved by obtaining the mysqlnd function table, backing it up, and replacing it by a custom function table, which calls the functions of the plugin as required.

The following code shows how the mysqlnd function table is replaced:

struct st_mysqlnd_conn_methods org_methods;void minit_register_hooks(TSRMLS_D) {
  
  struct st_mysqlnd_conn_methods * current_methods
    = mysqlnd_conn_get_methods();  
  memcpy(&org_methods, current_methods,
    sizeof(struct st_mysqlnd_conn_methods);  
  current_methods->query = MYSQLND_METHOD(my_conn_class, query);
}

Connection function table manipulations must be done during Module Initialization (MINIT). The function table is a global shared resource. In an multi-threaded environment, with a TSRM build, the manipulation of a global shared resource during the request processing will almost certainly result in conflicts.

Note:

Do not use any fixed-size logic when manipulating the mysqlnd function table: new methods may be added at the end of the function table. The function table may change at any time in the future.

Calling parent methods

If the original function table entries are backed up, it is still possible to call the original function table entries - the parent methods.

In some cases, such as for Connection::stmt_init(), it is vital to call the parent method prior to any other activity in the derived method.

MYSQLND_METHOD(my_conn_class, query)(MYSQLND *conn,
  const char *query, unsigned int query_len TSRMLS_DC) {  php_printf("my_conn_class::query(query = %s)\n", query);  query = "SELECT 'query rewritten' FROM DUAL";
  query_len = strlen(query);  return org_methods.query(conn, query, query_len); 
}

Extending properties

A mysqlnd object is represented by a C struct. It is not possible to add a member to a C struct at run time. Users of mysqlnd objects cannot simply add properties to the objects.

Arbitrary data (properties) can be added to a mysqlnd objects using an appropriate function of the mysqlnd_plugin_get_plugin_<object>_data() family. When allocating an object mysqlnd reserves space at the end of the object to hold a void * pointer to arbitrary data. mysqlnd reserves space for one void * pointer per plugin.

The following table shows how to calculate the position of the pointer for a specific plugin:

**Pointer calculations for mysqlnd**
Memory address	Contents
0	Beginning of the mysqlnd object C struct
n	End of the mysqlnd object C struct
n + (m x sizeof(void*))	void* to object data of the m-th plugin

If you plan to subclass any of the mysqlnd object constructors, which is allowed, you must keep this in mind!

The following code shows extending properties:

typedef struct my_conn_properties {
  unsigned long query_counter;
} MY_CONN_PROPERTIES;
unsigned int my_plugin_id;void minit_register_hooks(TSRMLS_D) {
  
  my_plugin_id = mysqlnd_plugin_register();
  
}static MY_CONN_PROPERTIES** get_conn_properties(const MYSQLND *conn TSRMLS_DC) {
  MY_CONN_PROPERTIES** props;
  props = (MY_CONN_PROPERTIES**)mysqlnd_plugin_get_plugin_connection_data(
    conn, my_plugin_id);
  if (!props || !(*props)) {
    *props = mnd_pecalloc(1, sizeof(MY_CONN_PROPERTIES), conn->persistent);
    (*props)->query_counter = 0;
  }
  return props;
}

The plugin developer is responsible for the management of plugin data memory.

Use of the mysqlnd memory allocator is recommended for plugin data. These functions are named using the convention: mnd_*loc(). The mysqlnd allocator has some useful features, such as the ability to use a debug allocator in a non-debug build.

**When and how to subclass**
	When to subclass?	Each instance has its own private function table?	How to subclass?
Connection (MYSQLND)	MINIT	No	mysqlnd_conn_get_methods()
Resultset (MYSQLND_RES)	MINIT or later	Yes	mysqlnd_result_get_methods() or object method function table manipulation
Resultset Meta (MYSQLND_RES_METADATA)	MINIT	No	mysqlnd_result_metadata_get_methods()
Statement (MYSQLND_STMT)	MINIT	No	mysqlnd_stmt_get_methods()
Network (MYSQLND_NET)	MINIT or later	Yes	mysqlnd_net_get_methods() or object method function table manipulation
Wire protocol (MYSQLND_PROTOCOL)	MINIT or later	Yes	mysqlnd_protocol_get_methods() or object method function table manipulation

You must not manipulate function tables at any time later than MINIT if it is not allowed according to the above table.

Some classes contain a pointer to the method function table. All instances of such a class will share the same function table. To avoid chaos, in particular in threaded environments, such function tables must only be manipulated during MINIT.

Other classes use copies of a globally shared function table. The class function table copy is created together with the object. Each object uses its own function table. This gives you two options: you can manipulate the default function table of an object at MINIT, and you can additionally refine methods of an object without impacting other instances of the same class.

The advantage of the shared function table approach is performance. There is no need to copy a function table for each and every object.

**Constructor status**
	Allocation, construction, reset	Can be modified?	Caller
Connection (MYSQLND)	mysqlnd_init()	No	mysqlnd_connect()
Resultset(MYSQLND_RES)	Allocation: Connection::result_init() Reset and re-initialized during: Result::use_result() Result::store_result	Yes, but call parent!	Connection::list_fields() Statement::get_result() Statement::prepare() (Metadata only) Statement::resultMetaData()
Resultset Meta (MYSQLND_RES_METADATA)	Connection::result_meta_init()	Yes, but call parent!	Result::read_result_metadata()
Statement (MYSQLND_STMT)	Connection::stmt_init()	Yes, but call parent!	Connection::stmt_init()
Network (MYSQLND_NET)	mysqlnd_net_init()	No	Connection::init()
Wire protocol (MYSQLND_PROTOCOL)	mysqlnd_protocol_init()	No	Connection::init()

It is strongly recommended that you do not entirely replace a constructor. The constructors perform memory allocations. The memory allocations are vital for the mysqlnd plugin API and the object logic of mysqlnd. If you do not care about warnings and insist on hooking the constructors, you should at least call the parent constructor before doing anything in your constructor.

Regardless of all warnings, it can be useful to subclass constructors. Constructors are the perfect place for modifying the function tables of objects with non-shared object tables, such as Resultset, Network, Wire Protocol.

**Destruction status**
	Derived method must call parent?	Destructor
Connection	yes, after method execution	free_contents(), end_psession()
Resultset	yes, after method execution	free_result()
Resultset Meta	yes, after method execution	free()
Statement	yes, after method execution	dtor(), free_stmt_content()
Network	yes, after method execution	free()
Wire protocol	yes, after method execution	free()

The destructors are the appropriate place to free properties, mysqlnd_plugin_get_plugin_<object>_data().

The listed destructors may not be equivalent to the actual mysqlnd method freeing the object itself. However, they are the best possible place for you to hook in and free your plugin data. As with constructors you may replace the methods entirely but this is not recommended. If multiple methods are listed in the above table you will need to hook all of the listed methods and free your plugin data in whichever method is called first by mysqlnd.

The recommended method for plugins is to simply hook the methods, free your memory and call the parent implementation immediately following this.

Caution

Due to a bug in PHP versions 5.3.0 to 5.3.3, plugins do not associate plugin data with a persistent connection. This is because ext/mysql and ext/mysqli do not trigger all the necessary mysqlnd end_psession() method calls and the plugin may therefore leak memory. This has been fixed in PHP 5.3.4.