Shared Transaction between different OracleDB Connections
<p>After several days passed to investigate about the issue, I decided to submit this question because there is no sense apparently in what is happening.</p>

<p><strong>The Case</strong></p>

<p>My computer is configured with a local Oracle Express database.
I have a JAVA project with several JUnit Tests that extend a parent class (I know that it is not a "best practice") which opens an OJDBC Connection (using a static Hikari connection pool of 10 Connections) in the @Before method and rolled Back it in the @After.</p>

<pre><code>public class BaseLocalRollbackableConnectorTest {
private static Logger logger = LoggerFactory.getLogger(BaseLocalRollbackableConnectorTest.class);
protected Connection connection;

@Before
public void setup() throws SQLException{
    logger.debug("Getting connection and setting autocommit to FALSE");
    connection = StaticConnectionPool.getPooledConnection();
}

@After
public void teardown() throws SQLException{ 
    logger.debug("Rollback connection");
    connection.rollback();
    logger.debug("Close connection");
    connection.close();
}
</code></pre>

<p>StacicConnectionPool</p>

<pre><code>public class StaticConnectionPool {

private static HikariDataSource ds;

private static final Logger log = LoggerFactory.getLogger(StaticConnectionPool.class);

public static Connection getPooledConnection() throws SQLException {

    if (ds == null) {
        log.debug("Initializing ConnectionPool");
        HikariConfig config = new HikariConfig();
        config.setMaximumPoolSize(10);
        config.setDataSourceClassName("oracle.jdbc.pool.OracleDataSource");
        config.addDataSourceProperty("url", "jdbc:oracle:thin:@localhost:1521:XE");
        config.addDataSourceProperty("user", "MyUser");
        config.addDataSourceProperty("password", "MyPsw");
        config.setAutoCommit(false);
        ds = new HikariDataSource(config);

    }
    return ds.getConnection();

}
</code></pre>

<p>}</p>

<p>This project has hundreds tests (not in parallel) that use this connection (on localhost) to execute queries (insert/update and select) using Sql2o but transaction and clousure of connection is managed only externally (by the test above). 
The database is completely empty to have ACID tests.</p>

<p>So the expected result is to insert something into DB, makes the assertions and then rollback. in this way the second test will not find any data added by previous test in order to maintain the isolation level.</p>

<p><strong>The Problem</strong>
Running all tests together (sequentially), 90% of times they work properly. the 10% one or two tests, randomly, fail, because there is dirty data in the database (duplicated unique for example) by previous tests. looking the logs, rollbacks of previous tests were done properly. In fact, if I check the database, it is empty) 
If I execute this tests in a server with higher performance but the same JDK, same Oracle DB XE, this failure ratio is increased to 50%. </p>

<p>This is very strange and I have no idea because the connections are different between tests and the rollback is called each time. The JDBC Isolation level is READ COMMITTED    so even if we used the same connection, this should not create any problem even using the same connection.
So my question is:
Why it happen? do you have any idea? Is the JDBC rollback synchronous as I know or there could be some cases where it can go forward even though it is not fully completed?</p>

<p>These are my main DB params:
processes   100
sessions    172
transactions    189</p>