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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