这里因为涉及到公司的业务问题不进行深入讨论，下面换成通用的一些业务场景就是举例。
众所周知，所谓锁的产生本质上是想解决资源竞争问题，在MySQL的前提下，MySQL为了解决事务并发独写的问题，在进行insert、update、delete的时候会对数据库的记录加上行级锁或者间隙锁，正因为有锁，才会导致死锁的发生。
以常见的商城订单系统，列举一个业务场景来说明我这次遇到的问题。
假设每个用户都有自己的订单。如订单号为800001、800002、800003
假设我们有一个这样的需求（暂不考虑需求合理性），我们通过cancel监听上游数据库的用户积分变更，我们要对这个用户的所有单据做更新。这样我们的业务场景对应下的技术架构图如下：
我们可以很明显的看到我这边会在不同的分区上的处理逻辑是：
查询出这批用户的单据，然后更新每一个单据的优惠金额。
很明显，我们在不同的分区上执行了同一段逻辑，那么就有很大的可能出现两个事务同事操作一条数据的可能性，从而出现了锁的竞争，又因为我这里是批量操作，那么就可能出现我等待你锁释放、你等待我的锁释放的情况，也就是所谓的死锁的发生。如下图的竞争图所示：
然后我们的DB就出现死锁，线上监控出现死锁报错。
### SQL: UPDATE foo SET name=? WHERE id=?
### Cause: com.mysql.cj.jdbc.exceptions.MySQLTransactionRollbackException: Deadlock found when trying to get lock; try restarting transaction
; Deadlock found when trying to get lock; try restarting transaction; nested exception is com.mysql.cj.jdbc.exceptions.MySQLTransactionRollbackException: Deadlock found when trying to get lock; try restarting transaction
at org.springframework.jdbc.support.SQLErrorCodeSQLExceptionTranslator.doTranslate(SQLErrorCodeSQLExceptionTranslator.java:266) ~[spring-jdbc-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:72) ~[spring-jdbc-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.mybatis.spring.MyBatisExceptionTranslator.translateExceptionIfPossible(MyBatisExceptionTranslator.java:88) ~[mybatis-spring-2.0.5.jar:2.0.5]
at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:440) ~[mybatis-spring-2.0.5.jar:2.0.5]
at com.sun.proxy.$Proxy90.update(Unknown Source) ~[na:na]
at org.mybatis.spring.SqlSessionTemplate.update(SqlSessionTemplate.java:287) ~[mybatis-spring-2.0.5.jar:2.0.5]
at com.baomidou.mybatisplus.core.override.MybatisMapperMethod.execute(MybatisMapperMethod.java:65) ~[mybatis-plus-core-3.4.1.jar:3.4.1]
at com.baomidou.mybatisplus.core.override.MybatisMapperProxy$PlainMethodInvoker.invoke(MybatisMapperProxy.java:148) ~[mybatis-plus-core-3.4.1.jar:3.4.1]
at com.baomidou.mybatisplus.core.override.MybatisMapperProxy.invoke(MybatisMapperProxy.java:89) ~[mybatis-plus-core-3.4.1.jar:3.4.1]
at com.sun.proxy.$Proxy91.updateById(Unknown Source) ~[na:na]
at com.guorou.foo.service.impl.FooServiceImpl.updateSort1(FooServiceImpl.java:67) ~[classes/:na]
at com.guorou.foo.service.impl.FooServiceImpl$$FastClassBySpringCGLIB$$1.invoke(<generated>) ~[classes/:na]
at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218) ~[spring-core-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:746) ~[spring-aop-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163) ~[spring-aop-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:294) ~[spring-tx-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:98) ~[spring-tx-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:688) ~[spring-aop-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at com.guorou.foo.service.impl.FooServiceImpl$$EnhancerBySpringCGLIB$$1.updateSort1(<generated>) ~[classes/:na]
at com.guorou.foo.web.controller.FooController.getById(FooController.java:47) ~[classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_201]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_201]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_201]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_201]
at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:215) ~[spring-web-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:142) ~[spring-web-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:102) ~[spring-webmvc-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895) ~[spring-webmvc-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:800) ~[spring-webmvc-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1038) ~[spring-webmvc-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:942) ~[spring-webmvc-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:998) [spring-webmvc-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:890) [spring-webmvc-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:634) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:875) [spring-webmvc-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) [tomcat-embed-websocket-9.0.12.jar:9.0.12]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) [spring-web-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:92) [spring-web-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93) [spring-web-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) [spring-web-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.1.2.RELEASE.jar:5.1.2.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.StandardContextValve.__invoke(StandardContextValve.java:96) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:41002) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:770) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1415) [tomcat-embed-core-9.0.12.jar:9.0.12]
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.12.jar:9.0.12]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_201]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_201]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.12.jar:9.0.12]
at java.lang.Thread.run(Thread.java:748) [na:1.8.0_201]
Caused by: com.mysql.cj.jdbc.exceptions.MySQLTransactionRollbackException: Deadlock found when trying to get lock; try restarting transaction
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:123) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.ClientPreparedStatement.executeInternal(ClientPreparedStatement.java:974) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.ClientPreparedStatement.execute(ClientPreparedStatement.java:391) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.zaxxer.hikari.pool.ProxyPreparedStatement.execute(ProxyPreparedStatement.java:44) ~[HikariCP-3.2.0.jar:na]
at com.zaxxer.hikari.pool.HikariProxyPreparedStatement.execute(HikariProxyPreparedStatement.java) ~[HikariCP-3.2.0.jar:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_201]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_201]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_201]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_201]
at org.apache.ibatis.logging.jdbc.PreparedStatementLogger.invoke(PreparedStatementLogger.java:59) ~[mybatis-3.5.6.jar:3.5.6]
at com.sun.proxy.$Proxy141.execute(Unknown Source) ~[na:na]
at org.apache.ibatis.executor.statement.PreparedStatementHandler.update(PreparedStatementHandler.java:47) ~[mybatis-3.5.6.jar:3.5.6]
at org.apache.ibatis.executor.statement.RoutingStatementHandler.update(RoutingStatementHandler.java:74) ~[mybatis-3.5.6.jar:3.5.6]
at com.baomidou.mybatisplus.core.executor.MybatisSimpleExecutor.doUpdate(MybatisSimpleExecutor.java:56) ~[mybatis-plus-core-3.4.1.jar:3.4.1]
at org.apache.ibatis.executor.BaseExecutor.update(BaseExecutor.java:117) ~[mybatis-3.5.6.jar:3.5.6]
at com.baomidou.mybatisplus.core.executor.MybatisCachingExecutor.update(MybatisCachingExecutor.java:85) ~[mybatis-plus-core-3.4.1.jar:3.4.1]
at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:197) ~[mybatis-3.5.6.jar:3.5.6]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_201]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_201]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_201]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_201]
at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:426) ~[mybatis-spring-2.0.5.jar:2.0.5]
... 72 common frames omitted发生死锁后，如果MySQL开启了死锁检测机制，会自动把权重小的事务进行释放。如果没有开启死锁检测机制，则两个线程都会回滚。（死锁检测比较消耗CPU，看具体情况开启）
MySQL的死锁检测参数
show variables like '%innodb_deadlock_detect%';更详细的死锁打印日志可以参考我的另外一篇博客，这里不详细展开说明
MySQL如何输出发生死锁的SQL到日志文件_小胖子——鑫的博客-CSDN博客
以上场景死锁发生的根本原因就是并发批量操作了数据，所以解决思路应该围绕并发、批量两个关键词展开。
死锁的解决思路无非就是几种
（1）处理逻辑串行化
（2）批量操作改成单条操作（跟串行化也有点关联，单条操作其实就是变像的串行化）
（3）程序里面加分布式锁（多条数据加锁可能会影响程序性能，需要慎重考虑）
我这边采用的解决方式为处理逻辑串行化
既然上游通过Kafka传递消息针对用户来说是一个没有顺序的情况，那么我可以在我这边把他变成顺序的，利用Kafka的分区机制，同一个key 经过一致性hash算法，最终会发送到同一个Kafka分区，然后Kafka同一个分区的数据是会进行顺序消费，相当于也是一种串行化的做法。技术的实现就会演变如下图所示：
因为是以用户ID作为Hash(Key)进行发送分区，所以不会出现多个线程处理同一个用户数据这种情况出现了，所以也解决了数据库的死锁问题。
1、死锁的解决三大要素：逻辑串行化、批量改单条、程序加锁（如Redis分布式锁之类）
2、针对死锁的问题要灵活变通，不仅要从技术的角度找到为什么会产生死锁，同时也要出业务上进行出发，深究业务的本质，可能在业务本身上进行优化的成本远大于技术上的优化。