Oracle JDBC ReadTimeout QueryTimeout

If a query exceeds the oracle.jdbc.ReadTimeout without receiving any data, an exception is thrown and the connection is terminated by the Oracle driver on the client. Unfortunately the session will still be queued on the database and continue to wait for locks, hold any current locks, and complete any DML/PL*SQL procedures that are pending on the server-side of the orphaned connection.

If an application, on a another connection, due to ReadTimeout exception, retries DML/PL*SQL  which requires locks, those queries will queue behind the initial DML/PL*SQL. This will increase the workload exacerbating the situation. On applications with retries, this can be observed by querying the v$session table or gv$session on RAC and noting new sessions started periodically based on the ReadTimeout interval. The sessions may often have the same SQL_ID and/or SQL_HASH_VALUE.

If stmt.setQueryTimeout(Seconds) is issued and the statement exceeds the timeout, it will attempt to cancel the associated in-progress or queued DML/PL*SQL. Assuming the connection is still alive, it will not leave locks and pending or in-progress DML/PL*SQL procedures on the database. One potential down side with QueryTimeout is that if data is trickling in from the server, it won't flexibly extend the timeout like ReadTimeout.

Whenever a ReadTimeout occurs, an attempt should be made to cleanup the orphaned server-side session to eliminate locks, and pending and in-progress DML/PL*SQL. Of course this would have to occur out-of-band on another connection. You will need a connection ID.

When connections are abruptly terminated (no TCP fin/ack), they will continue to be queued, execute, hold locks, and block other queries until detected and terminated which by default is ~2 hours. In order to detect and terminate orphaned database connection/sessions, sqlnet.ora sqlnet.expire_time and/or the OS level tcp_keepalive_interval should be set. The sqlnet.ora sqlnet.expire_time is not set by default and the tcp_keepalive_interval is set to ~2 hours. To set tcp_keepalive settings, see below or follow this link: How to set tcp_keepalive options. The expire_time and tcp_keepalive will not detect and terminate connections killed by JDBC Connection ReadTimeout.

Abnormal TCP connection termination can occur due to OS or VM crashes, network issues, or Firewall failures. You can easily simulate this scenario by pulling the network cable on the JDBC client system. Issuing Ctrl+C or kill -9 does not cause the issues as they both appear to cause the TCP connection to close.

Oracle® Database JDBC Developer's Guide
11g Release 2 (11.2)

Similar issues have been documented by others

Howto set tcp_keepalive settings:

Persistent changes:

Edit /etc/sysctl.conf and add/update

net.ipv4.tcp_keepalive_time=7200
net.ipv4.tcp_keepalive_intvl=75
net.ipv4.tcp_keepalive_probes=9

sudo /etc/rc.d/init.d/network restart

Temporary changes:

sudo sysctl -w net.ipv4.tcp_keepalive_time=60 net.ipv4.tcp_keepalive_intvl=6 net.ipv4.tcp_keepalive_probes=10

cat /proc/sys/net/ipv4/tcp_keepalive_time
cat /proc/sys/net/ipv4/tcp_keepalive_intvl
cat /proc/sys/net/ipv4/tcp_keepalive_probes

OR

sudo su

echo 7200 > /proc/sys/net/ipv4/tcp_keepalive_time
echo 75 > /proc/sys/net/ipv4/tcp_keepalive_intvl
echo 9 > /proc/sys/net/ipv4/tcp_keepalive_probes

Sqlnet.ora timeout setting:

Typical path and format:

/oracle_ee/app/product/11.2.0/dbhome_1/network/admin/sqlnet.ora

SQLNET.EXPIRE_TIME=1


Tests:


Oracle 11g R2 EE 11.2.0.1.0 64 bit with Partitioning
Oracle Linux Server Release 6.2
Kernel Linux 2.6.32-300.3.1.el6uek.x86_64
ojdbc6-11.2.0.2.0.jar

sqlnet.ora sqlnet.EXPIRE_TIME set to 1
tcp_keepalive set to defaults
   net.ipv4.tcp_keepalive_time=7200
   net.ipv4.tcp_keepalive_intvl=75
   net.ipv4.tcp_keepalive_probes=9


create table foo (name varchar2(50));
insert into foo(name) values('foo');
insert into foo(name) values('bar');
commit;

select * from foo where name='bar' for update;


set serveroutput on;
declare
  startDate date;
  tmpVar foo.name%TYPE;
begin
  select sysdate into startDate from dual;
  select name into tmpVar from foo where name='foo' for update;
  dbms_output.put_line('total time' || numtodsinterval(sysdate-startDate, 'day'));
end;



import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;

public class JdbcIdleTestB
{
  public static void main(String[] args) throws ClassNotFoundException, SQLException
  {
    String jdbcUrl = "jdbc:oracle:thin:@10.20.10.44:1521:orcl";
    String user = "system";
    String pwd = "fubar1";
 
    //see below for formmatted version
    String lockFooAndSleepSql =
        "DECLARE startDate date := sysdate; tmpVar1 foo.name%TYPE; tmpVar2 foo.name%TYPE; waitTimeSec NUMBER := ?; endDate date; curDate date; BEGIN select sysdate+(waitTimeSec*(1/86400)) into endDate from dual; SELECT name INTO tmpVar1 FROM foo WHERE name='foo' FOR UPDATE; SELECT name INTO tmpVar2 FROM foo WHERE name='bar' FOR UPDATE;  WHILE sysdate < endDate  LOOP select sysdate into curDate from dual; END LOOP; END;";
    String waitTimeSec = "600000";
    lockFooAndSleepSql = lockFooAndSleepSql.replace("?", waitTimeSec);
 
    String readTimeout = "10000"; // mS
    System.setProperty("oracle.jdbc.ReadTimeout", readTimeout);

    Class.forName("oracle.jdbc.OracleDriver");
    System.out.println("Loaded driver: oracle.jdbc.OracleDriver");

    Connection conn = null;
    Statement stmt = null;
    try
    {
      conn = DriverManager.getConnection(jdbcUrl, user, pwd);
      System.out.println("Connected");
      conn.setAutoCommit(false);

      stmt = conn.createStatement();

      System.out.println("Performing sql: " + lockFooAndSleepSql);
      stmt.executeUpdate(lockFooAndSleepSql);
    }
    finally
    {
      if (conn != null)
      {
        // System.out.println("stmt.close");
        // conn.close();
      }
    }
  }
}

/*
DECLARE
  startDate DATE := sysdate;
  tmpVar1 foo.name%TYPE;
  tmpVar2 foo.name%TYPE;
  waitTimeSec NUMBER := ?;
  endDate DATE;
  curDate DATE;
BEGIN
  SELECT sysdate+(waitTimeSec*(1/86400)) INTO endDate FROM dual;
  SELECT name INTO tmpVar2 FROM foo WHERE name='foo' FOR UPDATE;
  SELECT name INTO tmpVar1 FROM foo WHERE name='bar' FOR UPDATE;
  WHILE sysdate < endDate
  LOOP
    SELECT sysdate INTO curDate FROM dual;
  END LOOP;
END;
*/

Comments

Post a Comment

Popular posts from this blog

Sites, Newsletters, and Blogs

BLOGS Java.net Weblogs Martin Fowler's blog Paul Graham Ruminations of a Programmer Form Follows Function Newsletters Core Java Tech Tips Enterprise Java Tech Tips JavaSpecialists.EU TheServerSide.com Sites Java.net Today Hot Java Sites CSS Zen Garden Evernote Hacker News Pragmatic Programmer
Read more

Locks held on Oracle for hours after sessions abnormally terminated by node failure

If a session holds db locks and is abnormally terminated (no fin/ack), the locks will persist until the db session is closed, typically around 2 hours and 12 minutes with default network tcp_keepalive settings. Abnormally terminated does not include CTRL C or kill -9. The ojdbc6 driver apparently has a shutdown hook thread that closes the connections in graceful shutdowns. The OS apparently closes the connections when processes are killed. Abnormally terminated might include power failure, firewall failures, Out Of Memory, OS/kernel crash, network connection failure, JBoss or other server node failure, etc. This was easily reproduced by creating a  process that connected to the db and updated a record but did not commit. While the first process was waiting, a second process with a contending update was started which blocked on the first update. The first clients network cable was then disconnected. The second clients transaction waited for approximately 2 hours until the database O
Read more