Laravel 5 配置读写分离和源码分析

一，配置过程：

Laravel5读写分离配置比较简单，只需修改config/database.php，下面以MySQL数据库为例
内容如下

'mysql' => [
    'read' => [
        'host' => '192.168.1.1'
    ],
    'write' => [
        'host' => '196.168.1.2'
    ],
    'driver'    => 'mysql',
    'database'  => 'database',
    'username'  => 'root',
    'password'  => '',
    'charset'   => 'utf8',
    'collation' => 'utf8_unicode_ci',
    'prefix'    => '',
]

设置完毕之后，Laravel5默认将select的语句让read指定的数据库执行，insert/update/delete则交给write指定的数据库，达到读写分离的作用。
这些设置对原始查询raw queries，查询生成器query builder，以及对象映射 Eloquent 都生效。
官网解释如下：
Sometimes you may wish to use one database connection for SELECT statements, and another for INSERT, UPDATE, and DELETE statements. Laravel makes this a breeze, and the proper connections will always be used whether you are using raw queries, the query builder, or the Eloquent ORM

二，实现原理

Laravel5读写分离主要有两个过程:
第一步，根据database.php配置，创建写库和读库的链接connection
第二步，调用select时先判断使用读库还是写库，而insert/update/delete统一使用写库

三，源码分析：根据database.php配置，创建写库和读库的链接connection

主要文件：Illuminate/Database/Connectors/ConnectionFactory.php
来看看几个重要的函数：

1，判断database.php是否配置了读写分离数据库

/**
 * Establish a PDO connection based on the configuration.
 *
 * @param  array   $config
 * @param  string  $name
 * @return \Illuminate\Database\Connection
 */
public function make(array $config, $name = null)
{
    $config = $this->parseConfig($config, $name);
    // 如果配置了读写分离，则同时创建读库和写库的链接
    if (isset($config['read'])) {
        return $this->createReadWriteConnection($config);
    }
    // 如果没有配置，默认创建单个数据库链接
    return $this->createSingleConnection($config);
}

2，看看如何创建读库和写库的链接

/**
 * Create a single database connection instance.
 *
 * @param  array  $config
 * @return \Illuminate\Database\Connection
 */
protected function createReadWriteConnection(array $config)
{
    // 获取写库的配置信息，并创建链接
    $connection = $this->createSingleConnection($this->getWriteConfig($config));
    // 创建读库的链接
    return $connection->setReadPdo($this->createReadPdo($config));
}

3，多个读库会选择哪个呢

/**
 * Get the read configuration for a read / write connection.
 *
 * @param  array  $config
 * @return array
 */
protected function getReadConfig(array $config)
{
    $readConfig = $this->getReadWriteConfig($config, 'read');

    // 如果数组即多个读库，那么通过随机函数array_rand()挑一个，默认取第一个
    if (isset($readConfig['host']) && is_array($readConfig['host'])) {
        $readConfig['host'] = count($readConfig['host']) > 1
            ? $readConfig['host'][array_rand($readConfig['host'])]
            : $readConfig['host'][0];
    }
    return $this->mergeReadWriteConfig($config, $readConfig);
}

4，写库也是随机选择的

/**
 * Get a read / write level configuration.
 *
 * @param  array   $config
 * @param  string  $type
 * @return array
 */
protected function getReadWriteConfig(array $config, $type)
{

    // 如果多个，那么通过随机函数array_rand()挑一个
    if (isset($config[$type][0])) {
        return $config[$type][array_rand($config[$type])];
    }
    return $config[$type];
}

总结：

1，可以设置多个读库和多个写库，或者不同组合，比如一个写库两个读库

2，每次只创建一个读库链接和一个写库链接，从多个库中随机选择一个；

四，源码分析：调用select时先判断使用读库还是写库，而insert/update/delete统一使用写库

主要文件：Illuminate/Database/Connection.php
看看几个重要的函数

1，select函数根据第三个输入参数判断使用读库还是写库

 /**
 * Run a select statement against the database.
 *
 * @param  string  $query
 * @param  array  $bindings
 * @param  bool  $useReadPdo
 * @return array
 */
public function select($query, $bindings = [], $useReadPdo = true)
{
    return $this->run($query, $bindings, function ($me, $query, $bindings) use ($useReadPdo) {
        if ($me->pretending()) {
            return [];
        }

        // For select statements, we'll simply execute the query and return an array
        // of the database result set. Each element in the array will be a single
        // row from the database table, and will either be an array or objects.
        // 根据$useReadPdo参数，判断使用读库还是写库；
        // true使用读库，false使用写库；默认使用读库
        $statement = $this->getPdoForSelect($useReadPdo)->prepare($query);

        $statement->execute($me->prepareBindings($bindings));

        $fetchArgument = $me->getFetchArgument();

        return isset($fetchArgument) ?
            $statement->fetchAll($me->getFetchMode(), $fetchArgument, $me->getFetchConstructorArgument()) :
            $statement->fetchAll($me->getFetchMode());
    });
}

/**
 * Get the PDO connection to use for a select query.
 *
 * @param  bool  $useReadPdo
 * @return \PDO
 */
protected function getPdoForSelect($useReadPdo = true)
{
    // 根据$useReadPdo参数，选择PDO即判断使用读库还是写库；
    // true使用读库getReadPdo，false使用写库getPdo； 
   return $useReadPdo ? $this->getReadPdo() : $this->getPdo();
}

2， insert/update/delete统一使用写库

/**
 * Run an insert statement against the database.
 *
 * @param  string  $query
 * @param  array   $bindings
 * @return bool
 */
public function insert($query, $bindings = [])
{
    return $this->statement($query, $bindings);
}

/**
 * Run an update statement against the database.
 *
 * @param  string  $query
 * @param  array   $bindings
 * @return int
 */
public function update($query, $bindings = [])
{
    return $this->affectingStatement($query, $bindings);
}

/**
 * Run a delete statement against the database.
 *
 * @param  string  $query
 * @param  array   $bindings
 * @return int
 */
public function delete($query, $bindings = [])
{
    return $this->affectingStatement($query, $bindings);
}

/**
 * Execute an SQL statement and return the boolean result.
 *
 * @param  string  $query
 * @param  array   $bindings
 * @return bool
 */
public function statement($query, $bindings = [])
{
    return $this->run($query, $bindings, function ($me, $query, $bindings) {
        if ($me->pretending()) {
            return true;
        }

        $bindings = $me->prepareBindings($bindings);
        // 直接调用写库
        return $me->getPdo()->prepare($query)->execute($bindings);
    });
}

/**
 * Run an SQL statement and get the number of rows affected.
 *
 * @param  string  $query
 * @param  array   $bindings
 * @return int
 */
public function affectingStatement($query, $bindings = [])
{
    return $this->run($query, $bindings, function ($me, $query, $bindings) {
        if ($me->pretending()) {
            return 0;
        }

        // For update or delete statements, we want to get the number of rows affected
        // by the statement and return that back to the developer. We'll first need
        // to execute the statement and then we'll use PDO to fetch the affected.
        // 直接调用写库
        $statement = $me->getPdo()->prepare($query);

        $statement->execute($me->prepareBindings($bindings));

        return $statement->rowCount();
    });
}

总结：

1，getReadPdo()获得读库链接，getPdo()获得写库链接；

2，select()函数根据第三个参数判断使用读库还是写库；

五，强制使用写库

有时候，我们需要读写实时一致，写完数据库后，想马上读出来，那么读写都指定一个数据库即可。
虽然Laravel5配置了读写分离，但也提供了另外的方法强制读写使用同一个数据库。

实现原理：上面$this->select()时指定使用写库的链接，即第三个参数useReadPdo设置为false即可

有几个方法可实现
1，调用方法
DB::table('posts')->selectFromWriteConnection('*')->where('id', $id);

源码解释：通过selectFromWriteConnection()函数
主要文件:Illuminate/Database/Connection.php

/**
 * Run a select statement against the database.
 *
 * @param  string  $query
 * @param  array   $bindings
 * @return array
 */
public function selectFromWriteConnection($query, $bindings = [])
{
    ，
    // 上面有解释$this->select()函数的第三个参数useReadPdod的意义
    // 第三个参数是 false，所以 select 时会使用写库，而不是读库
    return $this->select($query, $bindings, false);
}

2，调用方法

User::onWriteConnection()->find($id);

源码解释：通过onWriteConnection()函数
主要文件:Illuminate/Database/Eloquent/Model

/**
 * Begin querying the model on the write connection.
 *
 * @return \Illuminate\Database\Query\Builder
 */
public static function onWriteConnection()
{
    $instance = new static;
    // query builder 指定使用写库
    return $instance->newQuery()->useWritePdo();
}

看看query builder如何指定使用写库
主要文件：Illuminate/Database/Query/Builder

/**
 * Use the write pdo for query.
 *
 * @return $this
 */
public function useWritePdo()
{
    // 指定使用写库，useWritePdo 为true
    $this->useWritePdo = true;

    return $this;
}

/**
 * Run the query as a "select" statement against the connection.
 *
 * @return array
 */
protected function runSelect()
{
    // 执行select时，useWritePdo原值为true，这里取反，被改成false；
    // 即$this->select()函数第三个参数为false，所以使用写库；
    return $this->connection->select($this->toSql(), $this->getBindings(), ! $this->useWritePdo);
}

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