MySQL 讀寫分離與負載均衡

MySQL 數據庫的讀寫分離和負載均衡通常是經過第三方軟件來實現的。 也能夠經過mysql驅動程序來實現，如com.mysql.jdbc.ReplicationDriver。

詳細文檔參見：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");
 
     .......
  }
}

讀寫分離：

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對象，當設置這個connection.readOnly=true時，鏈接slave，當connection.readOnly=false時，鏈接master

負載均衡：

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

問題：

讀寫分離時可能會碰到剛寫完master，再立刻到slave進行查詢的狀況，而主從複製的時候有延遲，這時怎麼解決呢？有兩個辦法：

1. 好比增長頁面保存數據後立刻跳轉到列表頁面，這時可能出不來數據，由於複製還沒完成，這時能夠在前臺添加一些成功的提示，成功頁面等進行一些頁面跳轉延遲處理，讓服務器有時間去複製（複製延遲通常在毫秒級，而這種提示處理在秒級，因此時間上通常是足夠的）

2. 第1種辦法可能部分場景是可行的，可是有些場景要求比較高，須要實時的，這時能夠在讀取的時候進行處理，強制從master中讀取，能夠經過註解，加參數/標識等來指定從master讀取數據