Description of bugs caused by MySQL-JDBC driver

Problem Background

The company is engaged in e-commerce systems, and the entire system is built on Huawei Cloud. When designing the system, considering that the number of subsequent users and orders is relatively large, some large database components need to be used. For the relational database, considering the rapid growth of subsequent data volume, instead of directly writing to MySQL, Huawei Cloud's distributed database middleware DDM was used. After using DDM, you can directly increase the number of MySQL read instances without the business being aware of it, linearly improving the read performance. It also supports sub-databases and tables at the middleware level, providing operations for massive relational databases. It is simply customized for e-commerce systems.

DDM itself provides services in the form of a cluster, and multiple connection IP addresses are open to the business. A layer of load balancing is required. If you use the traditional method of adding LB for load balancing, there will be an extra layer of transit, which will cause performance losses. Therefore, the client load balancing capability provided by MySQL-JDBC is directly used.

The logical structure is shown in the figure below:

▲The business can access multiple DDM nodes through the Loadbalance of MySQL-JDBC. MySQL-JDBC provides load balancing capabilities.

Problem Description

The client load balancing capability of MySQL JDBC driver has been running well and the performance is amazing. But a while ago, the business request failed for no reason. I am responsible for the e-commerce order module, which involves real Money. This problem scared me into a cold sweat...

So I quickly checked the background log and found that there was an exception when accessing DDM. Without saying anything, I directly Submitted a work order to Huawei Cloud DDM Service.

I have to say that Huawei Cloud's service is still very good. Within half an hour, a dedicated staff member contacted me and worked with me to troubleshoot the problem.

Took down the logs of our business and analyzed them with DDM support staff, and found that the error was reported as follows: The root cause turned out to be a bug in the MySQL driver, which caused the local stack overflow of StackOverflow. ……It turned out to be a murder caused by a bug. I misunderstood the DDM service. I’m really sorry.

It can be seen from the stack , an exception triggered a bug in MySQL-JDBC, causing loop calls until the stack overflowed. At the suggestion of Huawei DDM support staff, the driver code was decompiled. From the decompilation, we can see that there is indeed the possibility of loop nesting.

Loadbalance polling connection -> Synchronize the status of new and old connections -> Send sql to the server -> Loadbalance polling connection.

The relevant code is as follows:

## Such an obvious bug is not very I believe MySQL will not find it. We are currently using the 5.1.44 version of the driver. After checking the latest 5.1.66 code, we found that this problem has indeed been fixed. The code is as follows:

By filtering out SET and SHOW statements, loop nesting is avoided.

But 5.1.66 has introduced a new bug. Since not every place where postProcess is called has SQL, the code here will throw a null pointer exception. Don’t the developers of MySQL JDBC do testing...

No way, I analyzed the code of 5.1.44 and found that by appropriately adjusting the value of the loadBalanceAutoCommitStatementThreshold parameter, loop nesting can also be avoided. . Our environment was changed to 5. After the modification, it ran smoothly for a week without any problems.

Modification plan

loadBalanceAutoCommitStatementThreshold was modified to 5, but the problem introduced is that if the business contains some time-consuming SQL, it may cause DDM load imbalance . However, judging from the current situation, the performance of DDM is still relatively strong~

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