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https://issues.apache.org/jira/browse/DBCP-585?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17516575#comment-17516575
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Kurtcebe Eroglu commented on DBCP-585:
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Hi [~ggregory],
We register two different sets of information to JMX. Below are screenshots
from JavaMissionControl for quick visualization.
When we configure the data source as
{code:java}
BasicDataSource ds = new BasicDataSource();
ds.setJmxName("com.example:name=BasicDataSource1");
{code}
# The selected object in the screenshot is
"{{{}com.example:name=BasicDataSource1{}}}".
It gives the datasource config and runtime metrics as can be seen above.
!ds_attrs.png!
We also register a subset of this data at
"{{{}com.example:connectionpool=connections,name=BasicDataSource1{}}}"
!connections_attrs.png!
# And we have the MBeans pointing to the connection object instances. These
are registered at {{{}org.apache.commons.dbcp2.PoolableConnection{}}}, as a
part of the connection lifecycle. This is the problematic one as values are
fetched from underlying vendor-specific connection objects (we extend
{{{}DelegatingConnection{}}}).
These are registered with the name
"{{{}com.example:connectionpool=connections,connection=X,name=BasicDataSource1{}}}",
where "X" changes with each new connection creation.
!conn_instance_attrs.png!
The first two sets of data are essential for monitoring, as it gives us both
DataSource runtime config options, and metrics like connection pool size, idle
and active connections etc. Setting {{setJmxEnabled(false)}} would disable
everything. However, we can hold on to the essential metrics and just drop out
the connection instance metrics (to be honest, in my personal opinion, these
are not adding too much value to monitoring anyways).
Nothing changes with default settings, but if we set up the pool specifying the
new flag like below, we end up disabling only the problematic part, and can
still reach all the essential monitoring information (notice the connections
disappear);
{code:java}
BasicDataSource ds = new BasicDataSource();
ds.setJmxName("com.example:name=BasicDataSource1");
ds.setRegisterConnectionMBean(false);
{code}
!final.png!
Below is the patch file (a quick one), for testing the above parameter.
[^0001-DBCP-585-idea-clarification.patch]
> 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
> Attachments: 0001-DBCP-585-idea-clarification.patch,
> conn_instance_attrs.png, connections_attrs.png, ds_attrs.png, final.png
>
>
> 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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