plugin.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<chapter xml:id="mysqlnd.plugin" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <title>MySQL Native Driver Plugin API</title>
 <para>
  The MySQL Native Driver Plugin API is a feature of MySQL Native
  Driver, or <literal>mysqlnd</literal>. <literal>Mysqlnd</literal>
  plugins operate in the layer between PHP applications and the MySQL
  server. This is comparable to MySQL Proxy. MySQL Proxy operates on a
  layer between any MySQL client application, for example, a PHP
  application and, the MySQL server. <literal>Mysqlnd</literal> plugins
  can undertake typical MySQL Proxy tasks such as load balancing,
  monitoring and performance optimizations. Due to the different
  architecture and location, <literal>mysqlnd</literal> plugins do not
  have some of MySQL Proxy's disadvantages. For example, with plugins,
  there is no single point of failure, no dedicated proxy server to
  deploy, and no new programming language to learn (Lua).
 </para>
 <para>
  A <literal>mysqlnd</literal> plugin can be thought of as an extension
  to <literal>mysqlnd</literal>. Plugins can intercept the majority of
  <literal>mysqlnd</literal> functions. The <literal>mysqlnd</literal>
  functions are called by the PHP MySQL extensions such as
  <literal>ext/mysql</literal>, <literal>ext/mysqli</literal>, and
  <literal>PDO_MYSQL</literal>. As a result, it is possible for a
  <literal>mysqlnd</literal> plugin to intercept all calls made to these
  extensions from the client application.
 </para>
 <para>
  Internal <literal>mysqlnd</literal> function calls can also be
  intercepted, or replaced. There are no restrictions on manipulating
  <literal>mysqlnd</literal> internal function tables. It is possible to
  set things up so that when certain <literal>mysqlnd</literal>
  functions are called by the extensions that use
  <literal>mysqlnd</literal>, the call is directed to the appropriate
  function in the <literal>mysqlnd</literal> plugin. The ability to
  manipulate <literal>mysqlnd</literal> internal function tables in this
  way allows maximum flexibility for plugins.
 </para>
 <para>
  <literal>Mysqlnd</literal> plugins are in fact PHP Extensions, written
  in C, that use the <literal>mysqlnd</literal> plugin API (which is
  built into MySQL Native Driver, <literal>mysqlnd</literal>). Plugins
  can be made 100% transparent to PHP applications. No application
  changes are needed because plugins operate on a different layer. The
  <literal>mysqlnd</literal> plugin can be thought of as operating in a
  layer below <literal>mysqlnd</literal>.
 </para>
 <para>
  The following list represents some possible applications of
  <literal>mysqlnd</literal> plugins.
 </para>
 <itemizedlist>
  <listitem>
   <para>
    Load Balancing
   </para>
   <itemizedlist>
    <listitem>
     <para>
      Read/Write Splitting. An example of this is the PECL/mysqlnd_ms
      (Master Slave) extension. This extension splits read/write queries
      for a replication setup.
     </para>
    </listitem>
    <listitem>
     <para>
      Failover
     </para>
    </listitem>
    <listitem>
     <para>
      Round-Robin, least loaded
     </para>
    </listitem>
   </itemizedlist>
  </listitem>
  <listitem>
   <para>
    Monitoring
   </para>
   <itemizedlist>
    <listitem>
     <para>
      Query Logging
     </para>
    </listitem>
    <listitem>
     <para>
      Query Analysis
     </para>
    </listitem>
    <listitem>
     <para>
      Query Auditing. An example of this is the PECL/mysqlnd_sip (SQL
      Injection Protection) extension. This extension inspects queries
      and executes only those that are allowed according to a ruleset.
     </para>
    </listitem>
   </itemizedlist>
  </listitem>
  <listitem>
   <para>
    Performance
   </para>
   <itemizedlist>
    <listitem>
     <para>
      Caching. An example of this is the PECL/mysqlnd_qc (Query Cache)
      extension.
     </para>
    </listitem>
    <listitem>
     <para>
      Throttling
     </para>
    </listitem>
    <listitem>
     <para>
      Sharding. An example of this is the PECL/mysqlnd_mc (Multi
      Connect) extension. This extension will attempt to split a SELECT
      statement into n-parts, using SELECT ... LIMIT part_1, SELECT
      LIMIT part_n. It sends the queries to distinct MySQL servers and
      merges the result at the client.
     </para>
    </listitem>
   </itemizedlist>
  </listitem>
 </itemizedlist>
 <para>
  <emphasis role="bold">MySQL Native Driver Plugins Available</emphasis>
 </para>
 <para>
  There are a number of mysqlnd plugins already available. These
  include:
 </para>
 <itemizedlist>
  <listitem>
   <para>
    <emphasis role="bold">PECL/mysqlnd_mc</emphasis> - Multi Connect
    plugin.
   </para>
  </listitem>
  <listitem>
   <para>
    <emphasis role="bold">PECL/mysqlnd_ms</emphasis> - Master Slave
    plugin.
   </para>
  </listitem>
  <listitem>
   <para>
    <emphasis role="bold">PECL/mysqlnd_qc</emphasis> - Query Cache
    plugin.
   </para>
  </listitem>
  <listitem>
   <para>
    <emphasis role="bold">PECL/mysqlnd_pscache</emphasis> - Prepared
    Statement Handle Cache plugin.
   </para>
  </listitem>
  <listitem>
   <para>
    <emphasis role="bold">PECL/mysqlnd_sip</emphasis> - SQL Injection
    Protection plugin.
   </para>
  </listitem>
  <listitem>
   <para>
    <emphasis role="bold">PECL/mysqlnd_uh</emphasis> - User Handler
    plugin.
   </para>
  </listitem>
 </itemizedlist>
 <section xml:id="mysqlnd.plugin.mysql-proxy">
  <title>A comparison of mysqlnd plugins with MySQL Proxy</title>
  <para>
   <literal>Mysqlnd</literal> plugins and MySQL Proxy are different
   technologies using different approaches. Both are valid tools for
   solving a variety of common tasks such as load balancing, monitoring,
   and performance enhancements. An important difference is that MySQL
   Proxy works with all MySQL clients, whereas
   <literal>mysqlnd</literal> plugins are specific to PHP applications.
  </para>
  <para>
   As a PHP Extension, a <literal>mysqlnd</literal> plugin gets
   installed on the PHP application server, along with the rest of PHP.
   MySQL Proxy can either be run on the PHP application server or can be
   installed on a dedicated machine to handle multiple PHP application
   servers.
  </para>
  <para>
   Deploying MySQL Proxy on the application server has two advantages:
  </para>
  <orderedlist>
   <listitem>
    <para>
     No single point of failure
    </para>
   </listitem>
   <listitem>
    <para>
     Easy to scale out (horizontal scale out, scale by client)
    </para>
   </listitem>
  </orderedlist>
  <para>
   MySQL Proxy (and <literal>mysqlnd</literal> plugins) can solve
   problems easily which otherwise would have required changes to
   existing applications.
  </para>
  <para>
   However, MySQL Proxy does have some disadvantages:
  </para>
  <itemizedlist>
   <listitem>
    <para>
     MySQL Proxy is a new component and technology to master and deploy.
    </para>
   </listitem>
   <listitem>
    <para>
     MySQL Proxy requires knowledge of the Lua scripting language.
    </para>
   </listitem>
  </itemizedlist>
  <para>
   MySQL Proxy can be customized with C and Lua programming. Lua is the
   preferred scripting language of MySQL Proxy. For most PHP experts Lua
   is a new language to learn. A <literal>mysqlnd</literal> plugin can
   be written in C. It is also possible to write plugins in PHP using
   <link xlink:href="&url.pecl.package;mysqlnd_uh">PECL/mysqlnd_uh</link>.
  </para>
  <para>
   MySQL Proxy runs as a daemon - a background process. MySQL Proxy can
   recall earlier decisions, as all state can be retained. However, a
   <literal>mysqlnd</literal> plugin is bound to the request-based
   lifecycle of PHP. MySQL Proxy can also share one-time computed
   results among multiple application servers. A
   <literal>mysqlnd</literal> plugin would need to store data in a
   persistent medium to be able to do this. Another daemon would need to
   be used for this purpose, such as Memcache. This gives MySQL Proxy an
   advantage in this case.
  </para>
  <para>
   MySQL Proxy works on top of the wire protocol. With MySQL Proxy you
   have to parse and reverse engineer the MySQL Client Server Protocol.
   Actions are limited to those that can be achieved by manipulating the
   communication protocol. If the wire protocol changes (which happens
   very rarely) MySQL Proxy scripts would need to be changed as well.
  </para>
  <para>
   <literal>Mysqlnd</literal> plugins work on top of the C API, which
   mirrors the <literal>libmysqlclient</literal> client.
   This C API is basically a wrapper around the MySQL Client Server
   protocol, or wire protocol, as it is sometimes called. You can
   intercept all C API calls. PHP makes use of the C API, therefore you
   can hook all PHP calls, without the need to program at the level of
   the wire protocol.
  </para>
  <para>
   <literal>Mysqlnd</literal> implements the wire protocol. Plugins can
   therefore parse, reverse engineer, manipulate and even replace the
   communication protocol. However, this is usually not required.
  </para>
  <para>
   As plugins allow you to create implementations that use two levels (C
   API and wire protocol), they have greater flexibility than MySQL
   Proxy. If a <literal>mysqlnd</literal> plugin is implemented using
   the C API, any subsequent changes to the wire protocol do not require
   changes to the plugin itself.
  </para>
 </section>
 <section xml:id="mysqlnd.plugin.obtaining">
  <title>Obtaining the mysqlnd plugin API</title>
  <para>
   The <literal>mysqlnd</literal> plugin API is simply part of the MySQL
   Native Driver PHP extension, <literal>ext/mysqlnd</literal>.
   Development started on the <literal>mysqlnd</literal> plugin API in
   December 2009. It is developed as part of the PHP source repository,
   and as such is available to the public either via Git, or through
   source snapshot downloads.
  </para>
  <para>
   Plugin developers can determine the <literal>mysqlnd</literal>
   version through accessing <literal>MYSQLND_VERSION</literal>, which
   is a string of the format <quote>mysqlnd 8.3.17</quote>, or through
   <literal>MYSQLND_VERSION_ID</literal>, which is an integer such as
   50007. Developers can calculate the version number as follows:
  </para>
  <table xml:id="mysqlnd.plugin.version-id">
   <title>MYSQLND_VERSION_ID calculation table</title>
   <tgroup cols="2">
    <thead>
     <row>
      <entry>Version (part)</entry>
      <entry>Example</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry>Major*10000</entry>
      <entry>5*10000 = 50000</entry>
     </row>
     <row>
      <entry>Minor*100</entry>
      <entry>0*100 = 0</entry>
     </row>
     <row>
      <entry>Patch</entry>
      <entry>7 = 7</entry>
     </row>
     <row>
      <entry>MYSQLND_VERSION_ID</entry>
      <entry>50007</entry>
     </row>
    </tbody>
   </tgroup>
  </table>
  <para>
   During development, developers should refer to the
   <literal>mysqlnd</literal> version number for compatibility and
   version tests, as several iterations of <literal>mysqlnd</literal>
   could occur during the lifetime of a PHP development branch with a
   single PHP version number.
  </para>
 </section>
 <section xml:id="mysqlnd.plugin.architecture">
  <title>MySQL Native Driver Plugin Architecture</title>
  <para>
   This section provides an overview of the <literal>mysqlnd</literal>
   plugin architecture.
  </para>
  <para>
   <emphasis role="bold">MySQL Native Driver Overview</emphasis>
  </para>
  <para>
   Before developing <literal>mysqlnd</literal> plugins, it is useful to
   know a little of how <literal>mysqlnd</literal> itself is organized.
   <literal>Mysqlnd</literal> consists of the following modules:
  </para>
  <table xml:id="mysqlnd.plugin.orgchart">
   <title>The mysqlnd organization chart, per module</title>
   <tgroup cols="2">
    <thead>
     <row>
      <entry>Modules Statistics</entry>
      <entry>mysqlnd_statistics.c</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry>Connection</entry>
      <entry>mysqlnd.c</entry>
     </row>
     <row>
      <entry>Resultset</entry>
      <entry>mysqlnd_result.c</entry>
     </row>
     <row>
      <entry>Resultset Metadata</entry>
      <entry>mysqlnd_result_meta.c</entry>
     </row>
     <row>
      <entry>Statement</entry>
      <entry>mysqlnd_ps.c</entry>
     </row>
     <row>
      <entry>Network</entry>
      <entry>mysqlnd_net.c</entry>
     </row>
     <row>
      <entry>Wire protocol</entry>
      <entry>mysqlnd_wireprotocol.c</entry>
     </row>
    </tbody>
   </tgroup>
  </table>
  <para>
   <emphasis role="bold">C Object-Oriented Paradigm</emphasis>
  </para>
  <para>
   At the code level, <literal>mysqlnd</literal> uses a C pattern for
   implementing object orientation.
  </para>
  <para>
   In C you use a <literal>struct</literal> to represent an object.
   Members of the struct represent object properties. Struct members
   pointing to functions represent methods.
  </para>
  <para>
   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.
  </para>
  <para>
   <emphasis role="bold">The PHP Life Cycle</emphasis>
  </para>
  <para>
   When considering the PHP life cycle there are two basic cycles:
  </para>
  <itemizedlist>
   <listitem>
    <para>
     PHP engine startup and shutdown cycle
    </para>
   </listitem>
   <listitem>
    <para>
     Request cycle
    </para>
   </listitem>
  </itemizedlist>
  <para>
   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.
  </para>
  <para>
   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.
  </para>
  <para>
   <emphasis role="bold">How a plugin works</emphasis>
  </para>
  <para>
   A <literal>mysqlnd</literal> plugin works by intercepting calls made
   to <literal>mysqlnd</literal> by extensions that use
   <literal>mysqlnd</literal>. This is achieved by obtaining the
   <literal>mysqlnd</literal> function table, backing it up, and
   replacing it by a custom function table, which calls the functions of
   the plugin as required.
  </para>
  <para>
   The following code shows how the <literal>mysqlnd</literal> function
   table is replaced:
  </para>
<programlisting>
<![CDATA[
/* a place to store original function table */
struct st_mysqlnd_conn_methods org_methods;

void minit_register_hooks(TSRMLS_D) {
  /* active function table */
  struct st_mysqlnd_conn_methods * current_methods
    = mysqlnd_conn_get_methods();

  /* backup original function table */
  memcpy(&org_methods, current_methods,
    sizeof(struct st_mysqlnd_conn_methods);

  /* install new methods */
  current_methods->query = MYSQLND_METHOD(my_conn_class, query);
}
]]>
</programlisting>
  <para>
   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.
  </para>
  <note>
   <para>
    Do not use any fixed-size logic when manipulating the
    <literal>mysqlnd</literal> 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.
   </para>
  </note>
  <para>
   <emphasis role="bold">Calling parent methods</emphasis>
  </para>
  <para>
   If the original function table entries are backed up, it is still
   possible to call the original function table entries - the parent
   methods.
  </para>
  <para>
   In some cases, such as for
   <literal>Connection::stmt_init()</literal>, it is vital to call the
   parent method prior to any other activity in the derived method.
  </para>
<programlisting>
<![CDATA[
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); /* return with call to parent */
}
]]>
</programlisting>
  <para>
   <emphasis role="bold">Extending properties</emphasis>
  </para>
  <para>
   A <literal>mysqlnd</literal> object is represented by a C struct. It
   is not possible to add a member to a C struct at run time. Users of
   <literal>mysqlnd</literal> objects cannot simply add properties to
   the objects.
  </para>
  <para>
   Arbitrary data (properties) can be added to a
   <literal>mysqlnd</literal> objects using an appropriate function of
   the
   <literal>mysqlnd_plugin_get_plugin_&lt;object&gt;_data()</literal>
   family. When allocating an object <literal>mysqlnd</literal> reserves
   space at the end of the object to hold a <literal>void *</literal>
   pointer to arbitrary data. <literal>mysqlnd</literal> reserves space
   for one <literal>void *</literal> pointer per plugin.
  </para>
  <para>
   The following table shows how to calculate the position of the
   pointer for a specific plugin:
  </para>
  <table xml:id="mysqlnd.plugin.pointercalc">
   <title>Pointer calculations for mysqlnd</title>
   <tgroup cols="2">
    <thead>
     <row>
      <entry>Memory address</entry>
      <entry>Contents</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry>0</entry>
      <entry>Beginning of the mysqlnd object C struct</entry>
     </row>
     <row>
      <entry>n</entry>
      <entry>End of the mysqlnd object C struct</entry>
     </row>
     <row>
      <entry>n + (m x sizeof(void*))</entry>
      <entry>void* to object data of the m-th plugin</entry>
     </row>
    </tbody>
   </tgroup>
  </table>
  <para>
   If you plan to subclass any of the <literal>mysqlnd</literal> object
   constructors, which is allowed, you must keep this in mind!
  </para>
  <para>
   The following code shows extending properties:
  </para>
<programlisting>
<![CDATA[
/* any data we want to associate */
typedef struct my_conn_properties {
  unsigned long query_counter;
} MY_CONN_PROPERTIES;

/* plugin id */
unsigned int my_plugin_id;

void minit_register_hooks(TSRMLS_D) {
  /* obtain unique plugin ID */
  my_plugin_id = mysqlnd_plugin_register();
  /* snip - see Extending Connection: methods */
}

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;
}
]]>
</programlisting>
  <para>
   The plugin developer is responsible for the management of plugin data
   memory.
  </para>
  <para>
   Use of the <literal>mysqlnd</literal> memory allocator is recommended
   for plugin data. These functions are named using the convention:
   <literal>mnd_*loc()</literal>. The <literal>mysqlnd</literal>
   allocator has some useful features, such as the ability to use a
   debug allocator in a non-debug build.
  </para>
  <table xml:id="mysqlnd.plugin.subclass">
   <title>When and how to subclass</title>
   <tgroup cols="4">
    <thead>
     <row>
      <entry></entry>
      <entry>When to subclass?</entry>
      <entry>Each instance has its own private function table?</entry>
      <entry>How to subclass?</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry>Connection (MYSQLND)</entry>
      <entry>MINIT</entry>
      <entry>No</entry>
      <entry>mysqlnd_conn_get_methods()</entry>
     </row>
     <row>
      <entry>Resultset (MYSQLND_RES)</entry>
      <entry>MINIT or later</entry>
      <entry>Yes</entry>
      <entry>mysqlnd_result_get_methods() or object method function table
       manipulation</entry>
     </row>
     <row>
      <entry>Resultset Meta (MYSQLND_RES_METADATA)</entry>
      <entry>MINIT</entry>
      <entry>No</entry>
      <entry>mysqlnd_result_metadata_get_methods()</entry>
     </row>
     <row>
      <entry>Statement (MYSQLND_STMT)</entry>
      <entry>MINIT</entry>
      <entry>No</entry>
      <entry>mysqlnd_stmt_get_methods()</entry>
     </row>
     <row>
      <entry>Network (MYSQLND_NET)</entry>
      <entry>MINIT or later</entry>
      <entry>Yes</entry>
      <entry>mysqlnd_net_get_methods() or object method function table manipulation</entry>
     </row>
     <row>
      <entry>Wire protocol (MYSQLND_PROTOCOL)</entry>
      <entry>MINIT or later</entry>
      <entry>Yes</entry>
      <entry>mysqlnd_protocol_get_methods() or object method function table
       manipulation</entry>
     </row>
    </tbody>
   </tgroup>
  </table>
  <para>
   You must not manipulate function tables at any time later than MINIT
   if it is not allowed according to the above table.
  </para>
  <para>
   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.
  </para>
  <para>
   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.
  </para>
  <para>
   The advantage of the shared function table approach is performance.
   There is no need to copy a function table for each and every object.
  </para>
  <table xml:id="mysqlnd.plugin.constatus">
   <title>Constructor status</title>
   <tgroup cols="4">
    <thead>
     <row>
      <entry>Type</entry>
      <entry>Allocation, construction, reset</entry>
      <entry>Can be modified?</entry>
      <entry>Caller</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry>Connection (MYSQLND)</entry>
      <entry>mysqlnd_init()</entry>
      <entry>No</entry>
      <entry>mysqlnd_connect()</entry>
     </row>
     <row>
      <entry>Resultset(MYSQLND_RES)</entry>
      <entry><para>
        Allocation:
       </para>
       <itemizedlist>
        <listitem>
         <para>
          Connection::result_init()
         </para>
        </listitem>
       </itemizedlist>
       <para>
        Reset and re-initialized during:
       </para>
       <itemizedlist>
        <listitem>
         <para>
          Result::use_result()
         </para>
        </listitem>
        <listitem>
         <para>
          Result::store_result
         </para>
        </listitem>
       </itemizedlist></entry>
      <entry>Yes, but call parent!</entry>
      <entry><itemizedlist>
        <listitem>
         <para>
          Connection::list_fields()
         </para>
        </listitem>
        <listitem>
         <para>
          Statement::get_result()
         </para>
        </listitem>
        <listitem>
         <para>
          Statement::prepare() (Metadata only)
         </para>
        </listitem>
        <listitem>
         <para>
          Statement::resultMetaData()
         </para>
        </listitem>
       </itemizedlist></entry>
     </row>
     <row>
      <entry>Resultset Meta (MYSQLND_RES_METADATA)</entry>
      <entry>Connection::result_meta_init()</entry>
      <entry>Yes, but call parent!</entry>
      <entry>Result::read_result_metadata()</entry>
     </row>
     <row>
      <entry>Statement (MYSQLND_STMT)</entry>
      <entry>Connection::stmt_init()</entry>
      <entry>Yes, but call parent!</entry>
      <entry>Connection::stmt_init()</entry>
     </row>
     <row>
      <entry>Network (MYSQLND_NET)</entry>
      <entry>mysqlnd_net_init()</entry>
      <entry>No</entry>
      <entry>Connection::init()</entry>
     </row>
     <row>
      <entry>Wire protocol (MYSQLND_PROTOCOL)</entry>
      <entry>mysqlnd_protocol_init()</entry>
      <entry>No</entry>
      <entry>Connection::init()</entry>
     </row>
    </tbody>
   </tgroup>
  </table>
  <para>
   It is strongly recommended that you do not entirely replace a
   constructor. The constructors perform memory allocations. The memory
   allocations are vital for the <literal>mysqlnd</literal> plugin API
   and the object logic of <literal>mysqlnd</literal>. 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.
  </para>
  <para>
   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.
  </para>
  <table xml:id="mysqlnd.plugin.deststatus">
   <title>Destruction status</title>
   <tgroup cols="3">
    <thead>
     <row>
      <entry>Type</entry>
      <entry>Derived method must call parent?</entry>
      <entry>Destructor</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry>Connection</entry>
      <entry>yes, after method execution</entry>
      <entry>free_contents(), end_psession()</entry>
     </row>
     <row>
      <entry>Resultset</entry>
      <entry>yes, after method execution</entry>
      <entry>free_result()</entry>
     </row>
     <row>
      <entry>Resultset Meta</entry>
      <entry>yes, after method execution</entry>
      <entry>free()</entry>
     </row>
     <row>
      <entry>Statement</entry>
      <entry>yes, after method execution</entry>
      <entry>dtor(), free_stmt_content()</entry>
     </row>
     <row>
      <entry>Network</entry>
      <entry>yes, after method execution</entry>
      <entry>free()</entry>
     </row>
     <row>
      <entry>Wire protocol</entry>
      <entry>yes, after method execution</entry>
      <entry>free()</entry>
     </row>
    </tbody>
   </tgroup>
  </table>
  <para>
   The destructors are the appropriate place to free properties,
   <literal>mysqlnd_plugin_get_plugin_<replaceable>&lt;object&gt;</replaceable>_data()</literal>.
  </para>
  <para>
   The listed destructors may not be equivalent to the actual
   <literal>mysqlnd</literal> 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
   <literal>mysqlnd</literal>.
  </para>
  <para>
   The recommended method for plugins is to simply hook the methods,
   free your memory and call the parent implementation immediately
   following this.
  </para>
 </section>
 <section xml:id="mysqlnd.plugin.api">
  <title>The mysqlnd plugin API</title>
  <para>
   The following is a list of functions provided in the
   <literal>mysqlnd</literal> plugin API:
  </para>
  <itemizedlist>
   <listitem>
    <para>
     mysqlnd_plugin_register()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_plugin_count()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_plugin_get_plugin_connection_data()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_plugin_get_plugin_result_data()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_plugin_get_plugin_stmt_data()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_plugin_get_plugin_net_data()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_plugin_get_plugin_protocol_data()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_conn_get_methods()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_result_get_methods()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_result_meta_get_methods()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_stmt_get_methods()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_net_get_methods()
    </para>
   </listitem>
   <listitem>
    <para>
     mysqlnd_protocol_get_methods()
    </para>
   </listitem>
  </itemizedlist>
  <para>
   There is no formal definition of what a plugin is and how a plugin
   mechanism works.
  </para>
  <para>
   Components often found in plugins mechanisms are:
  </para>
  <itemizedlist>
   <listitem>
    <para>
     A plugin manager
    </para>
   </listitem>
   <listitem>
    <para>
     A plugin API
    </para>
   </listitem>
   <listitem>
    <para>
     Application services (or modules)
    </para>
   </listitem>
   <listitem>
    <para>
     Application service APIs (or module APIs)
    </para>
   </listitem>
  </itemizedlist>
  <para>
   The <literal>mysqlnd</literal> plugin concept employs these features,
   and additionally enjoys an open architecture.
  </para>
  <para>
   <emphasis role="bold">No Restrictions</emphasis>
  </para>
  <para>
   A plugin has full access to the inner workings of
   <literal>mysqlnd</literal>. There are no security limits or
   restrictions. Everything can be overwritten to implement friendly or
   hostile algorithms. It is recommended you only deploy plugins from a
   trusted source.
  </para>
  <para>
   As discussed previously, plugins can use pointers freely. These
   pointers are not restricted in any way, and can point into another
   plugin's data. Simple offset arithmetic can be used to read another
   plugin's data.
  </para>
  <para>
   It is recommended that you write cooperative plugins, and that you
   always call the parent method. The plugins should always cooperate
   with <literal>mysqlnd</literal> itself.
  </para>
  <table xml:id="mysqlnd.plugin.chaining">
   <title>Issues: an example of chaining and cooperation</title>
   <tgroup cols="3">
    <thead>
     <row>
      <entry>Extension</entry>
      <entry>mysqlnd.query() pointer</entry>
      <entry>call stack if calling parent</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry>ext/mysqlnd</entry>
      <entry>mysqlnd.query()</entry>
      <entry>mysqlnd.query</entry>
     </row>
     <row>
      <entry>ext/mysqlnd_cache</entry>
      <entry>mysqlnd_cache.query()</entry>
      <entry><orderedlist>
        <listitem>
         <para>
          mysqlnd_cache.query()
         </para>
        </listitem>
        <listitem>
         <para>
          mysqlnd.query
         </para>
        </listitem>
       </orderedlist></entry>
     </row>
     <row>
      <entry>ext/mysqlnd_monitor</entry>
      <entry>mysqlnd_monitor.query()</entry>
      <entry><orderedlist>
        <listitem>
         <para>
          mysqlnd_monitor.query()
         </para>
        </listitem>
        <listitem>
         <para>
          mysqlnd_cache.query()
         </para>
        </listitem>
        <listitem>
         <para>
          mysqlnd.query
         </para>
        </listitem>
       </orderedlist></entry>
     </row>
    </tbody>
   </tgroup>
  </table>
  <para>
   In this scenario, a cache (<literal>ext/mysqlnd_cache</literal>) and
   a monitor (<literal>ext/mysqlnd_monitor</literal>) plugin are loaded.
   Both subclass <literal>Connection::query()</literal>. Plugin
   registration happens at <literal>MINIT</literal> using the logic
   shown previously. PHP calls extensions in alphabetical order by
   default. Plugins are not aware of each other and do not set extension
   dependencies.
  </para>
  <para>
   By default the plugins call the parent implementation of the query
   method in their derived version of the method.
  </para>
  <para>
   <emphasis role="bold">PHP Extension Recap</emphasis>
  </para>
  <para>
   This is a recap of what happens when using an example plugin,
   <literal>ext/mysqlnd_plugin</literal>, which exposes the
   <literal>mysqlnd</literal> C plugin API to PHP:
  </para>
  <itemizedlist>
   <listitem>
    <para>
     Any PHP MySQL application tries to establish a connection to
     192.168.2.29
    </para>
   </listitem>
   <listitem>
    <para>
     The PHP application will either use <literal>ext/mysql</literal>,
     <literal>ext/mysqli</literal> or <literal>PDO_MYSQL</literal>. All
     three PHP MySQL extensions use <literal>mysqlnd</literal> to
     establish the connection to 192.168.2.29.
    </para>
   </listitem>
   <listitem>
    <para>
     <literal>Mysqlnd</literal> calls its connect method, which has been
     subclassed by <literal>ext/mysqlnd_plugin</literal>.
    </para>
   </listitem>
   <listitem>
    <para>
     <literal>ext/mysqlnd_plugin</literal> calls the userspace hook
     <literal>proxy::connect()</literal> registered by the user.
    </para>
   </listitem>
   <listitem>
    <para>
     The userspace hook changes the connection host IP from 192.168.2.29
     to 127.0.0.1 and returns the connection established by
     <literal>parent::connect()</literal>.
    </para>
   </listitem>
   <listitem>
    <para>
     <literal>ext/mysqlnd_plugin</literal> performs the equivalent of
     <literal>parent::connect(127.0.0.1)</literal> by calling the
     original <literal>mysqlnd</literal> method for establishing a
     connection.
    </para>
   </listitem>
   <listitem>
    <para>
     <literal>ext/mysqlnd</literal> establishes a connection and returns
     to <literal>ext/mysqlnd_plugin</literal>.
     <literal>ext/mysqlnd_plugin</literal> returns as well.
    </para>
   </listitem>
   <listitem>
    <para>
     Whatever PHP MySQL extension had been used by the application, it
     receives a connection to 127.0.0.1. The PHP MySQL extension itself
     returns to the PHP application. The circle is closed.
    </para>
   </listitem>
  </itemizedlist>
 </section>
 <section xml:id="mysqlnd.plugin.developing">
  <title>Getting started building a mysqlnd plugin</title>
  <para>
   It is important to remember that a <literal>mysqlnd</literal> plugin
   is itself a PHP extension.
  </para>
  <para>
   The following code shows the basic structure of the MINIT function
   that will be used in the typical <literal>mysqlnd</literal> plugin:
  </para>
<programlisting>
<![CDATA[
/* my_php_mysqlnd_plugin.c */

 static PHP_MINIT_FUNCTION(mysqlnd_plugin) {
  /* globals, ini entries, resources, classes */

  /* register mysqlnd plugin */
  mysqlnd_plugin_id = mysqlnd_plugin_register();

  conn_m = mysqlnd_get_conn_methods();
  memcpy(org_conn_m, conn_m,
    sizeof(struct st_mysqlnd_conn_methods));

  conn_m->query = MYSQLND_METHOD(mysqlnd_plugin_conn, query);
  conn_m->connect = MYSQLND_METHOD(mysqlnd_plugin_conn, connect);
}
]]>
</programlisting>
<programlisting>
<![CDATA[
/* my_mysqlnd_plugin.c */

 enum_func_status MYSQLND_METHOD(mysqlnd_plugin_conn, query)(/* ... */) {
  /* ... */
}
enum_func_status MYSQLND_METHOD(mysqlnd_plugin_conn, connect)(/* ... */) {
  /* ... */
}
]]>
</programlisting>
  <para>
   <emphasis role="bold">Task analysis: from C to userspace</emphasis>
  </para>
<programlisting>
<![CDATA[
 class proxy extends mysqlnd_plugin_connection {
  public function connect($host, ...) { .. }
}
mysqlnd_plugin_set_conn_proxy(new proxy());
]]>
</programlisting>
  <para>
   Process:
  </para>
  <orderedlist>
   <listitem>
    <para>
     PHP: user registers plugin callback
    </para>
   </listitem>
   <listitem>
    <para>
     PHP: user calls any PHP MySQL API to connect to MySQL
    </para>
   </listitem>
   <listitem>
    <para>
     C: ext/*mysql* calls mysqlnd method
    </para>
   </listitem>
   <listitem>
    <para>
     C: mysqlnd ends up in ext/mysqlnd_plugin
    </para>
   </listitem>
   <listitem>
    <para>
     C: ext/mysqlnd_plugin
     <orderedlist>
      <listitem>
       <para>
        Calls userspace callback
       </para>
      </listitem>
      <listitem>
       <para>
        Or original <literal>mysqlnd</literal> method, if userspace
        callback not set
       </para>
      </listitem>
     </orderedlist>
    </para>
   </listitem>
  </orderedlist>
  <para>
   You need to carry out the following:
  </para>
  <orderedlist>
   <listitem>
    <para>
     Write a class "mysqlnd_plugin_connection" in C
    </para>
   </listitem>
   <listitem>
    <para>
     Accept and register proxy object through
     "mysqlnd_plugin_set_conn_proxy()"
    </para>
   </listitem>
   <listitem>
    <para>
     Call userspace proxy methods from C (optimization -
     zend_interfaces.h)
    </para>
   </listitem>
  </orderedlist>
  <para>
   Userspace object methods can either be called using
   <literal>call_user_function()</literal> or you can operate at a level
   closer to the Zend Engine and use
   <literal>zend_call_method()</literal>.
  </para>
  <para>
   <emphasis role="bold">Optimization: calling methods from C using zend_call_method</emphasis>
  </para>
  <para>
   The following code snippet shows the prototype for the
   <literal>zend_call_method</literal> function, taken from
   <filename>zend_interfaces.h</filename>.
  </para>
<programlisting>
<![CDATA[
 ZEND_API zval* zend_call_method(
  zval **object_pp, zend_class_entry *obj_ce,
  zend_function **fn_proxy, char *function_name,
  int function_name_len, zval **retval_ptr_ptr,
  int param_count, zval* arg1, zval* arg2 TSRMLS_DC
);
]]>
</programlisting>
  <para>
   Zend API supports only two arguments. You may need more, for example:
  </para>
<programlisting>
<![CDATA[
 enum_func_status (*func_mysqlnd_conn__connect)(
  MYSQLND *conn, const char *host,
  const char * user, const char * passwd,
  unsigned int passwd_len, const char * db,
  unsigned int db_len, unsigned int port,
  const char * socket, unsigned int mysql_flags TSRMLS_DC
);
]]>
</programlisting>
  <para>
   To get around this problem you will need to make a copy of
   <literal>zend_call_method()</literal> and add a facility for
   additional parameters. You can do this by creating a set of
   <literal>MY_ZEND_CALL_METHOD_WRAPPER</literal> macros.
  </para>
  <para>
   <emphasis role="bold">Calling PHP userspace</emphasis>
  </para>
  <para>
   This code snippet shows the optimized method for calling a userspace
   function from C:
  </para>
<programlisting>
<![CDATA[ 
/* my_mysqlnd_plugin.c */

MYSQLND_METHOD(my_conn_class,connect)(
  MYSQLND *conn, const char *host /* ... */ TSRMLS_DC) {
  enum_func_status ret = FAIL;
  zval * global_user_conn_proxy = fetch_userspace_proxy();
  if (global_user_conn_proxy) {
    /* call userspace proxy */
    ret = MY_ZEND_CALL_METHOD_WRAPPER(global_user_conn_proxy, host, /*...*/);
  } else {
    /* or original mysqlnd method = do nothing, be transparent */
    ret = org_methods.connect(conn, host, user, passwd,
          passwd_len, db, db_len, port,
          socket, mysql_flags TSRMLS_CC);
  }
  return ret;
}
]]>
</programlisting>
  <para>
   <emphasis role="bold">Calling userspace: simple arguments</emphasis>
  </para>
<programlisting>
<![CDATA[
/* my_mysqlnd_plugin.c */

 MYSQLND_METHOD(my_conn_class,connect)(
  /* ... */, const char *host, /* ...*/) {
  /* ... */
  if (global_user_conn_proxy) {
    /* ... */
    zval* zv_host;
    MAKE_STD_ZVAL(zv_host);
    ZVAL_STRING(zv_host, host, 1);
    MY_ZEND_CALL_METHOD_WRAPPER(global_user_conn_proxy, zv_retval, zv_host /*, ...*/);
    zval_ptr_dtor(&zv_host);
    /* ... */
  }
  /* ... */
}
]]>
</programlisting>
  <para>
   <emphasis role="bold">Calling userspace: structs as arguments</emphasis>
  </para>
<programlisting>
<![CDATA[
/* my_mysqlnd_plugin.c */

MYSQLND_METHOD(my_conn_class, connect)(
  MYSQLND *conn, /* ...*/) {
  /* ... */
  if (global_user_conn_proxy) {
    /* ... */
    zval* zv_conn;
    ZEND_REGISTER_RESOURCE(zv_conn, (void *)conn, le_mysqlnd_plugin_conn);
    MY_ZEND_CALL_METHOD_WRAPPER(global_user_conn_proxy, zv_retval, zv_conn, zv_host /*, ...*/);
    zval_ptr_dtor(&zv_conn);
    /* ... */
  }
  /* ... */
}
]]>
</programlisting>
  <para>
   The first argument of many <literal>mysqlnd</literal> methods is a C
   "object". For example, the first argument of the connect() method is
   a pointer to <literal>MYSQLND</literal>. The struct MYSQLND
   represents a <literal>mysqlnd</literal> connection object.
  </para>
  <para>
   The <literal>mysqlnd</literal> connection object pointer can be
   compared to a standard I/O file handle. Like a standard I/O file
   handle a <literal>mysqlnd</literal> connection object shall be linked
   to the userspace using the PHP resource variable type.
  </para>
  <para>
   <emphasis role="bold">From C to userspace and back</emphasis>
  </para>
<programlisting>
<![CDATA[
 class proxy extends mysqlnd_plugin_connection {
  public function connect($conn, $host, ...) {
    /* "pre" hook */
    printf("Connecting to host = '%s'\n", $host);
    debug_print_backtrace();
    return parent::connect($conn);
  }

  public function query($conn, $query) {
    /* "post" hook */
    $ret = parent::query($conn, $query);
    printf("Query = '%s'\n", $query);
    return $ret;
  }
}
mysqlnd_plugin_set_conn_proxy(new proxy());
]]>
</programlisting>
  <para>
   PHP users must be able to call the parent implementation of an
   overwritten method.
  </para>
  <para>
   As a result of subclassing it is possible to refine only selected
   methods and you can choose to have "pre" or "post" hooks.
  </para>
  <para>
   <emphasis role="bold">Built-in class: mysqlnd_plugin_connection::connect()</emphasis>
  </para>
<programlisting>
<![CDATA[
/*  my_mysqlnd_plugin_classes.c */

 PHP_METHOD("mysqlnd_plugin_connection", connect) {
  /* ... simplified! ... */
  zval* mysqlnd_rsrc;
  MYSQLND* conn;
  char* host; int host_len;
  if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "rs",
    &mysqlnd_rsrc, &host, &host_len) == FAILURE) {
    RETURN_NULL();
  }
  ZEND_FETCH_RESOURCE(conn, MYSQLND* conn, &mysqlnd_rsrc, -1,
    "Mysqlnd Connection", le_mysqlnd_plugin_conn);
  if (PASS == org_methods.connect(conn, host, /* simplified! */ TSRMLS_CC))
    RETVAL_TRUE;
  else
    RETVAL_FALSE;
}
]]>
</programlisting>
 </section>
</chapter>
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