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
代码语言：java复制@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
代码语言：yaml复制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
代码语言：java复制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：
代码语言：java复制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
