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,外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，明天咱们就来聊聊如何触发这个切换动作？

1、计划一：利用异样机制

伪代码如下：


首先这个计划是不可行的，因为每次申请，还是先走到数据库逻辑，而后等抛出异样，这个工夫会挺长的，业务上是无奈承受的

2、计划二：被动进行mysql探活

实现思路： 能够利用数据库连接池检测无效连贯的思路

实现计划

1、形式一：利用druid连接池的ValidConnectionChecker进行扩大

外围逻辑如下

@Slf4jpublic 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配置咱们自定义的检测器

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:            # 指定连贯的无效查看类            valid-connection-checker-class-name: com.github.lybgeek.db.check.test.user.check.MysqlConnectionCheck

这个计划也是不大行的，前面翻了一下druid源码。当数据库抛出不可复原的异样时，比方网络抖动，异样断开，druid会触发exceptionSorter，摈弃连贯。而CreateConnectionThread会检测是否须要创立连贯，如果不须要，他就会进行期待。当连贯不够时，会调用

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

进行创立，同时isValidConnection也是在这个办法外面进行连贯验证，但这边就有问题，就是当数据库宕机了，就创立不了连贯，因而就进入异样流程，isValidConnection是没法执行的

2、形式二：参考druid的检测连贯逻辑，额定编写定时器触发检测逻辑

外围代码块：

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的探活实现形式有很多种，本文的实现检测逻辑是间接套用druid的检测连贯逻辑，之前对druid的应用，基本上就是停留在配置上，没过多关注。

为了写这篇文章，顺便翻了一下druid的源码，次要是因为之前认为扩大ValidConnectionChecker就行了，前面发现行不通。就看了一下源码，发现druid的设计思路挺好的，有些实现思维是咱们在日常开发中，能够借鉴应用的。还有druid外面有些跟数据库相干的util，也是能够间接拿过去用的。

demo链接

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