[
https://issues.apache.org/jira/browse/DBCP-585?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17516009#comment-17516009
]
Kurtcebe Eroglu commented on DBCP-585:
--------------------------------------
It's hard to consistently repro race conditions with test classes, but just to
provide a sample of cases where concurrent access can actually break things, we
may use the class below. We seem to be able to generate such an error almost
50% of the time with the example below.
The class depends on commons-dbcp2 2.9.0 and Postgres JDBC driver 42.2.23. We
simulate external JMX queries using [Prometheus JMX
Exporter|https://github.com/prometheus/jmx_exporter] 0.16.1.
- download the dependencies and the javaagent
- create a new file called 'jmx-exporter-config.yml' in the same directory,
with the following contents. This config will query and report all MBeans.
{noformat}
---
startDelaySeconds: 0
{noformat}
- compile class with dependencies and run like below. Notice that we expose
port 9099 for JMX queries over HTTP. Replace the JDBC connection string
parameter as per your DB instance details.
- after starting a program use a browser or Curl to send a get request to the
port exposed by the agent using the address "http://localhost:9099";
{noformat}
java -cp
postgresql-42.2.23.jar:commons-dbcp2-2.9.0.jar:commons-pool2-2.10.0.jar:commons-logging/1.2/commons-logging-1.2.jar:.
-javaagent:jmx_prometheus_javaagent-0.16.1.jar=9099:jmx-exporter-config.yml
TestBasicDataSource
"jdbc:postgresql://localhost:5432/testdb?user=testuser&password=testpass&ssl=false"
{noformat}
~50% of the time, as soon as we browse "http://localhost:9099";, we can notice
the following errors in the logs. If the error does not happen, stop and
restart the test and try again.
{noformat}
Setting up data source.
Creating threads to use the datasource
waiting for error messages during JMX access...
org.postgresql.util.PSQLException: Cannot change transaction isolation level in
the middle of a transaction.
at
org.postgresql.jdbc.PgConnection.setTransactionIsolation(PgConnection.java:940)
at
org.apache.commons.dbcp2.DelegatingConnection.setTransactionIsolation(DelegatingConnection.java:958)
at
org.apache.commons.dbcp2.DelegatingConnection.setTransactionIsolation(DelegatingConnection.java:958)
at TestBasicDataSource$QueryTask.runOnce(TestBasicDataSource.java:54)
at TestBasicDataSource$QueryTask.run(TestBasicDataSource.java:35)
at java.lang.Thread.run(Thread.java:748)
Thread terminated on error
org.postgresql.util.PSQLException: Cannot change transaction isolation level in
the middle of a transaction.
at
org.postgresql.jdbc.PgConnection.setTransactionIsolation(PgConnection.java:940)
at
org.apache.commons.dbcp2.DelegatingConnection.setTransactionIsolation(DelegatingConnection.java:958)
at
org.apache.commons.dbcp2.DelegatingConnection.setTransactionIsolation(DelegatingConnection.java:958)
at TestBasicDataSource$QueryTask.runOnce(TestBasicDataSource.java:54)
at TestBasicDataSource$QueryTask.run(TestBasicDataSource.java:35)
at java.lang.Thread.run(Thread.java:748)
...
{noformat}
Test class
{code:java}
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import javax.sql.DataSource;
import org.apache.commons.dbcp2.BasicDataSource;
public class TestBasicDataSource {
static DataSource dataSource;
public static void main(String[] args) {
System.out.println("Setting up data source.");
dataSource = setupDataSource(args[0]);
System.out.println("Creating threads to use the datasource");
for (int i = 0; i < 20; i ++){
new Thread(new QueryTask()).start();
}
System.out.println("waiting for error messages during JMX access...");
}
public static DataSource setupDataSource(String connectURI) {
BasicDataSource ds = new BasicDataSource();
ds.setJmxName("com.example:name=BasicDataSource1");
ds.setDriverClassName("org.postgresql.Driver");
ds.setUrl(connectURI);
return ds;
}
static class QueryTask implements Runnable {
boolean runFlag = true;
public void run() {
while (runFlag) {
runOnce();
}
System.out.println("Thread terminated on error");
}
private void runOnce() {
Connection conn = null;
Statement stmt = null;
ResultSet rset = null;
try {
conn = dataSource.getConnection();
// Following setTransactionIsolation call will fail if we're
already in an active transaction.
//
// This happens intermittently, because the JMX thread
sometimes read the stale value of "autoCommit"
// flag on the Postgres connection object, and send the "select
current_schema()" query with a preceding
// "begin" statement, starting a transaction, which the
connection pool and JDBC driver are not aware of
//
// This never fails if we don't perform any JMX access
conn.setTransactionIsolation(Connection.TRANSACTION_READ_COMMITTED);
conn.setAutoCommit(false);
stmt = conn.createStatement();
rset = stmt.executeQuery("SELECT CURRENT_TIME(3) ;");
rset.next();
rset.getString(1);
conn.commit();
conn.setAutoCommit(true);
} catch(SQLException e) {
e.printStackTrace();
runFlag = false;
} finally {
try { if (rset != null) rset.close(); } catch(Exception e) { }
try { if (stmt != null) stmt.close(); } catch(Exception e) { }
try { if (conn != null) conn.close(); } catch(Exception e) { }
}
}
}
}{code}
My testing environment details:
{noformat}
- Mac 2,6 GHz 6-Core Intel Core i7
- Darwin C02DG16JMD6V 19.6.0 Darwin Kernel Version 19.6.0: Tue Feb 15 21:39:11
PST 2022; root:xnu-6153.141.59~1/RELEASE_X86_64 x86_64
- OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_292-b10)
{noformat}
> 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.
--
This message was sent by Atlassian Jira
(v8.20.1#820001)
