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Gary D. Gregory resolved DBCP-585. ---------------------------------- Fix Version/s: 2.10.0 Resolution: Fixed > 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 > Fix For: 2.10.0 > > Attachments: 0001-DBCP-585-idea-clarification-1.patch, > conn_instance_attrs.png, connections_attrs.png, ds_attrs.png, final.png > > Time Spent: 1h 20m > Remaining Estimate: 0h > > 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. -- This message was sent by Atlassian Jira (v8.20.7#820007)