MySQL 读写分离与负载均衡

MySQL 数据库的读写分离和负载均衡一般是通过第三方软件来实现的。 也可以通过mysql驱动程序来实现，如com.mysql.jdbc.ReplicationDriver。

详细文档参见：http://dev.mysql.com/doc/refman/5.5/en/connector-j-info.html

import java.sql.Connection;
import java.sql.ResultSet;
import java.util.Properties;

import com.mysql.jdbc.ReplicationDriver;

public class ReplicationDriverDemo {

  public static void main(String[] args) throws Exception {
    ReplicationDriver driver = new ReplicationDriver();

    Properties props = new Properties();

    // We want this for failover on the slaves
    props.put("autoReconnect", "true");

    // We want to load balance between the slaves
    props.put("roundRobinLoadBalance", "true");

    props.put("user", "foo");
    props.put("password", "bar");

    //
    // Looks like a normal MySQL JDBC url, with a
    // comma-separated list of hosts, the first
    // being the 'master', the rest being any number
    // of slaves that the driver will load balance against
    //

    Connection conn =
        driver.connect("jdbc:mysql:replication://master,slave1,slave2,slave3/test",
            props);

    //
    // Perform read/write work on the master
    // by setting the read-only flag to "false"
    //

    conn.setReadOnly(false);
    conn.setAutoCommit(false);
    conn.createStatement().executeUpdate("UPDATE some_table ....");
    conn.commit();

    //
    // Now, do a query from a slave, the driver automatically picks one
    // from the list
    //

    conn.setReadOnly(true);

    ResultSet rs =
      conn.createStatement().executeQuery("SELECT a,b FROM alt_table");

     .......
  }
}

读写分离：

jdbc:mysql:replication://master:3306,slave1:3306,slave2:3306/dbname

When using the following connection string: jdbc:mysql:replication://dbmaster:3306,dbslave1:3306,dbslave2:3306/dbname

dbmaster is used for all write connections as expected and dbslave1 is used for all read connections, but dbslave2 is never used. I would have expected distributed reads between dbslave1 and dbslave2.

原理是：ReplicationDriver生成代理的connection对象，当设置这个connection.readOnly=true时，连接slave，当connection.readOnly=false时，连接master

负载均衡：

jdbc:mysql:loadbalance://master:3306,slave1:3306,slave2:3306/dbname

When using the following connection string: jdbc:mysql:loadbalance://dbmaster:3306,dbslave1:3306,dbslave2:3306/dbname
connections are load-balanced between all three servers for both read and write connections.

问题：

读写分离时可能会碰到刚写完master，再马上到slave进行查询的情况，而主从复制的时候有延迟，这时怎么解决呢？有两个办法：

1. 比如增加页面保存数据后马上跳转到列表页面，这时可能出不来数据，因为复制还没完成，这时可以在前台添加一些成功的提示，成功页面等进行一些页面跳转延迟处理，让服务器有时间去复制（复制延迟一般在毫秒级，而这种提示处理在秒级，所以时间上一般是足够的）

2. 第1种办法可能部分场景是可行的，但是有些场景要求比较高，需要实时的，这时可以在读取的时候进行处理，强制从master中读取，可以通过注解，加参数/标识等来指定从master读取数据

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文章标题:  MySQL 读写分离与负载均衡
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