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https://issues.apache.org/jira/browse/DBCP-585?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17516403#comment-17516403
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Kurtcebe Eroglu commented on DBCP-585:
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Thanks for your comment [~ggregory],
It makes perfect sense to handle this with as little disruption as possible to
existing users, and dropping an existing feature may indeed be a no go for a
minor version.
Would you agree that even if we don't have multiple examples (from different
vendors and versions) at the moment, the existence of clear guidance from
multiple major DB/ JDBC driver vendors, telling that their Connection objects
are not thread-safe and shall not be accessed as such, may justify an optional
config param for 2.x line? JMX instrumentation is already optional via
"jmxName" parameter, but we'd love to hang on to the DataSource MBeans, which
are providing immense value on monitoring. For example, an additional config
option "registerConnectionMBean", "true" by default, hence completely
transparent to existing users, may give users who have problems, or want to
follow the guidance from their DB vendors a way to opt-out from Connection
monitoring, only when this parameter is explicitly set to "false"?
> Connection level JMX queries result in concurrent access to connection
> objects, causing errors
> ----------------------------------------------------------------------------------------------
>
> Key: DBCP-585
> URL: https://issues.apache.org/jira/browse/DBCP-585
> Project: Commons DBCP
> Issue Type: Bug
> Affects Versions: 2.9.0
> Reporter: Kurtcebe Eroglu
> Priority: Major
>
> As we expose Connection objects over JMX, they may be accessed by multiple
> threads concurrently;
> a) an application thread that borrows the Connection and uses it business as
> usual,
> b) another thread simultaneously performing a JMX query, which in turn calls
> getters on the same connection object via the MBean interface.
> Also, calls to Connection object getters are mostly delegated to the
> underlying vendor-specific connection provided by the JDBC driver. For
> example, when we make the JMX query to get the "schema" attribute of the JMX
> connection object, this is translated into a
> "java.sql.Connection.getSchema()", and passed to the vendor-specific
> Connection object by DBCP. In the case of Postgres, for example, this is
> further translated to a query "select current_schema()" and sent to the
> server.
> Hence, querying connections over JMX result in concurrent access by multiple
> threads to the underlying Connection provided by the vendors, to the point
> that these two threads may be running queries simultaneously on the same
> connection.
> However, this is not supported by any of the major database vendors. Vendor
> links on Connection objects not being threadsafe:
> - [Postgres|https://jdbc.postgresql.org/documentation/head/thread.html]
> {quote}The PostgreSQL™ JDBC driver is not thread safe. The PostgreSQL server
> is not threaded. Each connection creates a new process on the server; as such
> any concurrent requests to the process would have to be serialized. The
> driver makes no guarantees that methods on connections are synchronized. It
> will be up to the caller to synchronize calls to the driver.
> {quote}
> -
> [Oracle|https://docs.oracle.com/en/database/oracle/oracle-database/19/jjdbc/JDBC-coding-tips.html#GUID-EE479007-D105-4F82-8D51-000CBBD4BC77]
>
> {quote}Oracle strongly discourages sharing a database connection among
> multiple threads. Avoid allowing multiple threads to access a connection
> simultaneously.
> {quote}
> - [Microsoft SQL
> Server|https://www.javadoc.io/doc/com.microsoft.sqlserver/mssql-jdbc/latest/com.microsoft.sqlserver.jdbc/com/microsoft/sqlserver/jdbc/SQLServerConnection.html]
> {quote}SQLServerConnection is not thread safe, however multiple statements
> created from a single connection can be processing simultaneously in
> concurrent threads.
> {quote}
> Another interesting point to note, also to do justice to previous committers
> who have put this feature in place, is that this was not always the case. In
> the following links, you may see the same links to the older versions of the
> same pages. In the past, all vendors indicated that Connection is fully
> thread-safe; [Postgres|https://www.postgresql.org/docs/7.1/jdbc-thread.html],
> [Oracle|https://docs.oracle.com/cd/A97335_02/apps.102/a83724/tips1.htm],
> [MSSQL
> Server|https://www.javadoc.io/doc/com.microsoft.sqlserver/mssql-jdbc/6.1.0.jre7/com/microsoft/sqlserver/jdbc/SQLServerConnection.html].
>
> Hence, it was once safe to expose Connection objects via JMX given the
> thread-safety guarantees for the underlying vendor connection were in place.
> But as Vendors dropped the thread-safety guarantee one by one, it is not safe
> anymore, and may actually cause convoluted errors that pop up intermittently
> due to thread races in the JDBC driver code (see an example in the comments
> section below). Accordingly, exposing Connections via JMX shall be retired
> along with dropped support from most vendors.
> Note: the Datasource MBeans, which provide a vital set of metrics have no
> such problems as they don't depend on the underlying JDBC provider.
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