问题描述
当前设置：
服务主机（Java）连接到（JDBC）主数据库（MysqL RW），并具有工作程序（只读）以确保可靠性。连接管理由C3PO处理
场景：
在翻转的情况下，我需要将一个工作线程提升为主数据库。当前的主数据库变为只读，新的主数据库被提升为可读写。我希望可靠，自动将C3P0连接池刷新到新的主数据库。现在哪种方法有效，但在某些情况下会失效。
技术：服务的数据库主机名为cname，将在翻转时进行更新。在DNS更改传播且新数据库为RW之前，所有连接将失败。尝试在连接池上无限设置C3PO重试，直到通过（acquireRetryAttempts）进行连接RW
现有的C3PO配置：
Generic configuration,inherited by primary and worker DB
<property name="maxIdleTime" value="44000"/>
<property name="idleConnectionTestPeriod" value="30"/>
<property name="maxStatements" value="50"/>
<property name="minPoolSize" value="3"/>
<property name="maxPoolSize" value="3"/>
<property name="acquireIncrement" value="2"/>
<property name="checkoutTimeout" value="15000"/>
<property name="acquireRetryDelay" value="1000"/>
Primary DB specific config
<property name="acquireRetryAttempts" value="0"/>
<property name="minPoolSize" value="3"/>
<property name="maxPoolSize" value="5"/>
<property name="testConnectionOnCheckout" value="true"/>
<property name="connectionTesterClassName" value="${Custom.connectionTesterClassName}"/>
问题：
自定义连接测试器检查连接是否可写（@@ global.read_only），并返回无效或有效的连接。 C3PO遇到了一个问题，即它会按照中间日志中所述的那样翻新（杀死和重新初始化）连接池。最终，它会在4-5分钟内检出所有（max_pool_size，5个）连接，并在连接未检回到池的状态下挂起。如最终日志中所述，这将导致所有后续的数据库连接检出超时并失败。强制重新启动服务，以便重新初始化C3PO池。
理想情况下，如果connectionTesterClassName返回false或在idleConnectionTestPeriod上或在某个随机周期返回，我需要连接池来保持终止状态并重新初始化。由于连接已签出，因此它们不属于空闲连接，也不会刷新。
如C3PO documentation所述，尝试使用unreturnedConnectionTimeout和debugUnreturnedConnectionStackTraces进行调试，但是没有运气。
日志：
中间日志
com.mchange.v2.resourcepool.BasicResourcePool: Resource [com.mchange.v2.c3p0.impl.NewPooledConnection@24e7ebec] Could not be refurbished in preparation for checkout. Will try to find a better resource.
(C3P0PooledConnectionPoolManager[identityToken->2tyqoiac1gfqk68krloa8|7e2c64]-HelperThread-#6) com.mchange.v2.resourcepool.BasicResourcePool: Preparing to destroy resource: com.mchange.v2.c3p0.impl.NewPooledConnection@24e7ebec
23 Sep 2020 22:06:23,780 [DEBUG]com.mchange.v2.resourcepool.BasicResourcePool: acquire test -- pool size: 0; target_pool_size: 3; desired target? 1
23 Sep 2020 22:06:23,780 [DEBUG] com.mchange.v2.resourcepool.BasicResourcePool: awaitAvailable(): com.mchange.v2.c3p0.impl.NewPooledConnection@4b52cf0c
23 Sep 2020 22:06:23,780 [DEBUG] (C3P0PooledConnectionPoolManager[identityToken->2tyqoiac1gfqk68krloa8|7e2c64]-HelperThread-#6) com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool: Preparing to destroy PooledConnection: com.mchange.v2.c3p0.impl.NewPooledConnection@24e7ebec
23 Sep 2020 22:06:23,780 [DEBUG] (C3P0PooledConnectionPoolManager[identityToken->2tyqoiac1gfqk68krloa8|7e2c64]-HelperThread-#6) com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool: Successfully destroyed PooledConnection: com.mchange.v2.c3p0.impl.NewPooledConnection@24e7ebec
23 Sep 2020 22:06:23,780 [DEBUG] (C3P0PooledConnectionPoolManager[identityToken->2tyqoiac1gfqk68krloa8|7e2c64]-HelperThread-#6) com.mchange.v2.resourcepool.BasicResourcePool: Successfully destroyed resource: com.mchange.v2.c3p0.impl.NewPooledConnection@24e7ebec
23 Sep 2020 22:06:23,781 [DEBUG] com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool: Testing PooledConnection [com.mchange.v2.c3p0.impl.NewPooledConnection@7fff041e] on CHECKOUT.
23 Sep 2020 22:06:23,782 [INFO] ReadWriteConnectionTester: Connection com.mchange.v2.c3p0.impl.NewProxyConnection@21eb35f6 [wrapping: com.MysqL.jdbc.JDBC4Connection@44732406] is read only!
23 Sep 2020 22:06:23,782 [DEBUG] com.mchange.v2.c3p0.impl.NewProxyConnection: com.mchange.v2.c3p0.impl.NewProxyConnection@21eb35f6 [wrapping: null]: close() called more than once.
23 Sep 2020 22:06:23,782 [DEBUG] com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool: Test of PooledConnection [com.mchange.v2.c3p0.impl.NewPooledConnection@7fff041e] on CHECKOUT has Failed.
java.sql.sqlException: Connection is invalid
最终日志：
org.hibernate.engine.jdbc.internal.LogicalConnectionImpl: Obtaining JDBC connection
com.mchange.v2.resourcepool.BasicResourcePool: acquire test -- pool is already maxed out. [managed: 5; max: 5]
com.mchange.v2.resourcepool.BasicResourcePool: awaitAvailable(): com.mchange.v2.c3p0.impl.NewPooledConnection@4b52cf0c
com.mchange.v2.sql.sqlUtils: Converting Throwable to sqlException...
com.mchange.v2.resourcepool.TimeoutException: A client timed out while waiting to acquire a resource from com.mchange.v2.resourcepool.BasicResourcePool@43aeb5e0 -- timeout at awaitAvailable()
at com.mchange.v2.resourcepool.BasicResourcePool.awaitAvailable(BasicResourcePool.java:1467) ~[c3p0-0.9.5.1.jar:0.9.5.1]
at com.mchange.v2.resourcepool.BasicResourcePool.prelimCheckoutResource(BasicResourcePool.java:644) ~[c3p0-0.9.5.1.jar:0.9.5.1]
at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:554) ~[c3p0-0.9.5.1.jar:0.9.5.1]
at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutAndMarkConnectionInUse(C3P0PooledConnectionPool.java:758) ~[c3p0-0.9.5.1.jar:0.9.5.1]
at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:685) ~[c3p0-0.9.5.1.jar:0.9.5.1]
at com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource.getConnection(AbstractPoolBackedDataSource.java:140) ~[c3p0-0.9.5.1.jar:0.9.5.1]
org.hibernate.engine.jdbc.spi.sqlExceptionHelper: Could not open connection [n/a]
java.sql.sqlException: An attempt by a client to checkout a Connection has timed out.
at com.mchange.v2.sql.sqlUtils.tosqlException(sqlUtils.java:118) ~[mchange-commons-java-0.2.10.jar:0.2.10]
at com.mchange.v2.sql.sqlUtils.tosqlException(sqlUtils.java:77) ~[mchange-commons-java-0.2.10.jar:0.2.10]
at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:690) ~[c3p0-0.9.5.1.jar:0.9.5.1]
at com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource.getConnection(AbstractPoolBackedDataSource.java:140) ~[c3p0-0.9.5.1.jar:0.9.5.1]
at org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource.getConnection(AbstractRoutingDataSource.java:164) ~[spring-jdbc-4.3.24.RELEASE.jar:4.3.24.RELEASE]
at org.hibernate.service.jdbc.connections.internal.DatasourceConnectionProviderImpl.getConnection(DatasourceConnectionProviderImpl.java:141) ~[hibernate-core-4.2.2.Final.jar:4.2.2.Final]
at org.hibernate.internal.AbstractSessionImpl$NonContextualJdbcConnectionAccess.obtainConnection(AbstractSessionImpl.java:292) ~[hibernate-core-4.2.2.Final.jar:4.2.2.Final]
at org.hibernate.engine.jdbc.internal.LogicalConnectionImpl.obtainConnection(LogicalConnectionImpl.java:214) ~[hibernate-core-4.2.2.Final.jar:4.2.2.Final]
at org.hibernate.engine.jdbc.internal.LogicalConnectionImpl.getConnection(LogicalConnectionImpl.java:157) ~[hibernate-core-4.2.2.Final.jar:4.2.2.Final]
at org.hibernate.internal.SessionImpl.connection(SessionImpl.java:550) ~[hibernate-core-4.2.2.Final.jar:4.2.2.Final]
Caused by: com.mchange.v2.resourcepool.TimeoutException: A client timed out while waiting to acquire a resource from com.mchange.v2.resourcepool.BasicResourcePool@43aeb5e0 -- timeout at awaitAvailable()
at com.mchange.v2.resourcepool.BasicResourcePool.awaitAvailable(BasicResourcePool.java:1467) ~[c3p0-0.9.5.1.jar:0.9.5.1]
at com.mchange.v2.resourcepool.BasicResourcePool.prelimCheckoutResource(BasicResourcePool.java:644) ~[c3p0-0.9.5.1.jar:0.9.5.1]
at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:554) ~[c3p0-0.9.5.1.jar:0.9.5.1]
at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutAndMarkConnectionInUse(C3P0PooledConnectionPool.java:758) ~[c3p0-0.9.5.1.jar:0.9.5.1]
at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:685) ~[c3p0-0.9.5.1.jar:0.9.5.1]
... 51 more
org.hibernate.engine.jdbc.spi.sqlExceptionHelper: sql Error: 0,sqlState: null
org.hibernate.engine.jdbc.spi.sqlExceptionHelper: An attempt by a client to checkout a Connection has timed out
解决方法
因此，这不是“翻新（杀死并重新初始化）连接池”的正确解释。
这是一个个人Connection，该池正在此处尝试“翻新”。它正在尝试刷新Connection，以供客户端使用。由于您将testConnectionOnCheckout设置为true，并且数据库已“翻转”，因此您的ReadWriteConnectionTester将声明当前池中的每个Connection无效（因为只读），直到该池用尽Connections尝试或用尽为止客户超时导致的时间。
当发现ConnectionTester.CONNECTION_IS_INVALID无效时，您的ReadWriteConnectionTester返回ConnectionTester.DATABASE_IS_INVALID或Connection吗？您应该希望它返回ConnectionTester.DATABASE_IS_INVALID才能重置整个池。