前言

先前写过一篇文章聊聊如何利用redis实现多级缓存同步,里面讲到业务部门因数据库宕机，有技术提出当数据库宕机，切换到redis，今天我们就来聊聊如何触发这个切换动作？

1、方案一：利用异常机制

伪代码如下：

首先这个方案是不可行的，因为每次请求，还是先走到数据库逻辑，然后等抛出异常，这个时间会挺长的，业务上是无法接受的

2、方案二：主动进行mysql探活

实现思路： 可以利用数据库连接池检测有效连接的思路

实现方案

1、方式一：利用druid连接池的ValidConnectionChecker进行扩展

核心逻辑如下

@Slf4j
public class MysqlConnectionCheck extends MySqlValidConnectionChecker {

    private ApplicationContext applicationContext;

    public MysqlConnectionCheck(ApplicationContext applicationContext) {
        this.applicationContext = applicationContext;
    }


    @Override
    public boolean isValidConnection(Connection conn, String validateQuery, int validationQueryTimeout) throws Exception {
      return checkMySQLCommunications(conn,validateQuery,validationQueryTimeout);

    }

    private boolean checkMySQLCommunications (Connection conn, String validateQuery, int validationQueryTimeout) {
        boolean validConnection = false;
        try {
            validConnection = super.isValidConnection(conn, validateQuery, validationQueryTimeout);
        } catch (Exception e) {
        }
        if(validConnection){
            boolean b = MySQLCommunicationsHolder.isMySQLCommunicationsException.compareAndSet(true, false);
            if(b){
                CommunicationsHealthEvent event = CommunicationsHealthEvent.builder().conn(conn).build();
                applicationContext.publishEvent(event);
            }
        }


        return validConnection;
    }
}

在yml配置我们自定义的检测器

spring:
    datasource:
        type: com.alibaba.druid.pool.DruidDataSource
        driverClassName: ${DRIVER_CALSS_NAME:com.mysql.cj.jdbc.Driver}
        url: ${DATASOURCE_URL:jdbc:mysql://localhost:3306/demo?useUnicode=true&characterEncoding=utf8&useSSL=false&serverTimezone=Asia/Shanghai}
        username: ${DATASOURCE_USERNAME:root}
        password: ${DATASOURCE_PWD:123456}
        druid:
            # 指定连接的有效检查类
            valid-connection-checker-class-name: com.github.lybgeek.db.check.test.user.check.MysqlConnectionCheck 

这个方案也是不大行的，后面翻了一下druid源码。当数据库抛出不可恢复的异常时，比如网络抖动，异常断开，druid会触发exceptionSorter，抛弃连接。而CreateConnectionThread会检测是否需要创建连接，如果不需要，他就会进行等待。当连接不够时，会调用

com.alibaba.druid.pool.DruidAbstractDataSource#createPhysicalConnection()

​进行创建，同时isValidConnection也是在这个方法里面进行连接验证，但这边就有问题，就是当数据库宕机了，就创建不了连接，因此就进入异常流程，isValidConnection是没法执行的

2、方式二：参考druid的检测连接逻辑，额外编写定时器触发检测逻辑

核心代码块：

public class ValidConnectionCheckerAdapter implements ValidConnectionChecker {

    private DbCheckProperies dbCheckProperies;

    public ValidConnectionCheckerAdapter(DbCheckProperies dbCheckProperies) {
        this.dbCheckProperies = dbCheckProperies;
    }

    @Override
    public boolean isValidConnection(Connection conn, String query, int validationQueryTimeout) throws SQLException {
        boolean valid = checkConnection(conn, query, validationQueryTimeout);

        // unexcepted branch
        if (valid && isMysql()) {
            long lastPacketReceivedTimeMs = MySqlUtils.getLastPacketReceivedTimeMs(conn);
            if (lastPacketReceivedTimeMs > 0) {
                long mysqlIdleMillis = System.currentTimeMillis() - lastPacketReceivedTimeMs;
                if (lastPacketReceivedTimeMs > 0
                        && mysqlIdleMillis >= dbCheckProperies.getTimeBetweenEvictionRunsMillis()) {
                    return false;
                }
            }
        }
        return valid;

    }


    private boolean checkConnection(Connection conn, String query, int validationQueryTimeout) throws SQLException {
        if (query == null || query.length() == 0) {
            return true;
        }

        if(conn == null){
            return false;
        }

        Statement stmt = null;
        ResultSet rs = null;
        try {
            stmt = conn.createStatement();
            if (validationQueryTimeout > 0) {
                stmt.setQueryTimeout(validationQueryTimeout);
            }
            rs = stmt.executeQuery(query);
            return true;
        } finally {
            JdbcUtils.close(rs);
            JdbcUtils.close(stmt);
        }
    }

 @Override
    public void afterPropertiesSet() throws Exception {
        executorService.scheduleWithFixedDelay(new DbCheckTask(),0,dbCheckProperies.getTimeBetweenEvictionRunsMillis(), TimeUnit.MILLISECONDS);
    }
    

    private class DbCheckTask implements Runnable{

        @Override
        public void run() {
            SQLException sqlException = null;
            Connection conn = dbConnManger.getConn();
            try {
                boolean validConnection = validConnectionChecker.isValidConnection(conn, dbCheckProperies.getValidationQuery(), dbCheckProperies.getValidationQueryTimeout());
                if(validConnection){
                    boolean b = MySQLCommunicationsHolder.isMySQLCommunicationsException.compareAndSet(true, false);
                    if(b){
                        CommunicationsHealthEvent event = CommunicationsHealthEvent.builder().conn(conn).build();
                        applicationContext.publishEvent(event);
                    }
                }else{
                   sqlException = new SQLException("connection is invalid","10040");
                }
            } catch (SQLException e) {
                 log.error("{}",e);
                 sqlException = e;
                 dbConnManger.closeConnection();
                 conn = null;
            }

            if(sqlException != null){
                MySQLCommunicationsHolder.isMySQLCommunicationsException.compareAndSet(false, true);
                CommunicationsUnHealthEvent event = CommunicationsUnHealthEvent.builder().sqlException(sqlException).build();
                applicationContext.publishEvent(event);
            }



        }
    } 

总结

其实mysql的探活实现方式有很多种，本文的实现检测逻辑是直接套用druid的检测连接逻辑，之前对druid的使用，基本上就是停留在配置上，没过多关注。

为了写这篇文章，特地翻了一下druid的源码，主要是因为之前以为扩展ValidConnectionChecker就行了，后面发现行不通。就看了一下源码，发现druid的设计思路挺好的，有些实现思想是我们在日常开发中，可以借鉴使用的。还有druid里面有些跟数据库相关的util，也是可以直接拿过来用的。

demo链接

https://github.com/lyb-geek/springboot-learning/tree/master/springboot-db-check