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This topic has been translated from a Chinese forum by GPT and might contain errors.Original topic: tidb 客户端断掉，连接无法释放
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username: TiDBer_OJg7HeFu
The TiDB connections cannot be released, causing the connections to be full and the service to crash.
You can track the top SQL through the dashboard to see which SQL queries are slow and consuming resources, and then optimize them.
What exactly does it mean that the connection cannot be released?
Sorry, I can’t translate images. Please provide the text you need translated.
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
at com.mysql.cj.jdbc.exceptions.SQLError.createCommunicationsException(SQLError.java:174)
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:64)
at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:836)
at com.mysql.cj.jdbc.ConnectionImpl.(ConnectionImpl.java:456)
at com.mysql.cj.jdbc.ConnectionImpl.getInstance$original$cIghTEv0(ConnectionImpl.java:246)
at com.mysql.cj.jdbc.ConnectionImpl.getInstance$original$cIghTEv0$accessor$piZyFsI3(ConnectionImpl.java)
at com.mysql.cj.jdbc.ConnectionImpl$auxiliary$3DwVSHHx.call(Unknown Source)
at org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.StaticMethodsInter.intercept(StaticMethodsInter.java:83)
at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java)
at com.mysql.cj.jdbc.NonRegisteringDriver.connect$original$n4kNkdPl(NonRegisteringDriver.java:197)
at com.mysql.cj.jdbc.NonRegisteringDriver.connect$original$n4kNkdPl$accessor$ODCq0Wvr(NonRegisteringDriver.java)
at com.mysql.cj.jdbc.NonRegisteringDriver$auxiliary$8pSE0BcZ.call(Unknown Source)
at org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.InstMethodsInter.intercept(InstMethodsInter.java:86)
at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java)
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1657)
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1723)
at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2838)
Caused by: com.mysql.cj.exceptions.CJCommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
at sun.reflect.GeneratedConstructorAccessor89.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:61)
at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:105)
at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:151)
at com.mysql.cj.exceptions.ExceptionFactory.createCommunicationsException(ExceptionFactory.java:167)
at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:91)
at com.mysql.cj.NativeSession.connect(NativeSession.java:144)
at com.mysql.cj.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:956)
at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:826)
… 14 common frames omitted
Caused by: java.net.ConnectException: Connection refused (Connection refused)
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at com.mysql.cj.protocol.StandardSocketFactory.connect(StandardSocketFactory.java:155)
at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:65)
… 17 common frames omitted
While you can’t connect here, try connecting with other clients.
After restarting, high concurrency comes in and it doesn’t work again.
Find a way to prevent requests from coming in. For example, change the password of the user initiating the request, and change it back after optimization.
TiDB can support how many connections? It seems like there’s no connection pool reuse effect, and the connections get maxed out quickly. It’s quite disappointing.
When you can’t connect, use netstat to check how many connections there are in the system. Also, check the memory and CPU status.
The default maximum connections are 151, you can try setting it higher.
Intuitively, your tidb-server’s memory usage is too high, causing the tidb-server to not establish new connections.
Isn’t the default 0 supposed to mean unlimited connections?
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