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Applies to:

Purpose

Scope and Application

How To Resolve Stranded DBA_2PC_PENDING Entries

Contents

1. Problem Description
2. Solutions
   2.1 Dba_2pc entries without a corresponding transaction
   2.2 Distributed transaction without corresponding dba_2pc entries
   2.3 How to PURGE the DISTRIBUTED transaction in PREPARED state, when COMMIT
       or ROLLBACK FORCE hangs ?, where we have entries for both Distributed
       transaction and dba_2pc entries.

1. Problem Description:
=======================

As a result of a failed commit of a distributed transaction, some entries can
be left in dba_2pc views, i.e. dba_2pc_pending and dba_2pc_neighbors. The RECO 
process checks these views to recover the failed txn. However, in some cases 
RECO cannot perform. the recovery. One cause is that all sites involved in the
transaction not being  accessible at the same  time. Another cause is dba_2pc 
views being  inconsistent with the  transaction table, which  is the topic of 
this article. This cause can further be classified as follows:

   1. dba_2pc views have entries for a non-existent distributed transaction

   2. There is a distributed transaction for which there are no entries in
      dba_2pc views

   3. How to PURGE the DISTRIBUTED transaction in PREPARED state, when COMMIT
      or ROLLBACK FORCE hangs ?, where we have entries for both Distributed
      transaction and dba_2pc entries.

Solutions to each subclass is provided in the rest of the article.


2. Solutions:


2.1 Dba_2pc entries without a corresponding transaction


In this case dba_2pc views show distributed transactions but there are no txns
in reality. If the state of the  transaction is committed,  rollback forced or 
commit forced then this is normal and it can be cleaned up using 

   dbms_transaction.purge_lost_db_entry

However, if the state of the transaction is PREPARED and there is no entry in 
the transaction table for it then this entry can be cleaned up manually as 
follows:



    set transaction use rollback segment SYSTEM;
    delete from sys.pending_trans$ where local_tran_id = ;
    delete from sys.pending_sessions$ where local_tran_id = ;
    delete from sys.pending_sub_sessions$ where local_tran_id = ;
    commit;

    Example:
    --------
    The following query reports a dist. txn. in prepared state
       select local_tran_id, state from dba_2pc_pending;
           LOCAL_TRAN_ID          STATE
           ---------------------- ----------------
           1.92.66874             prepared

    Given that a transaction id is composed of  triple,
    '1.92.66874' is located in rollback segment# 1. To find out the list of 
    active transactions in that rollback segment, use:

      SELECT KTUXEUSN, KTUXESLT, KTUXESQN, /* Transaction ID */
             KTUXESTA Status,
             KTUXECFL Flags
      FROM x$ktuxe
      WHERE ktuxesta!='INACTIVE'
            AND ktuxeusn= 1; <== this is the rollback segment#

      no rows selected

    It is not possible to rollback force or commit force this transaction.



      rollback force '1.92.66874';

      ORA-02058: no prepared transaction found with ID 1.92.66874

    Hence, we have to manually cleanup that transaction:

      

set transaction use rollback segment SYSTEM;

      delete from sys.pending_trans$
        where local_tran_id = '1.92.66874';

      delete from sys.pending_sessions$ where local_tran_id = '1.92.66874';

      delete from sys.pending_sub_sessions$ where local_tran_id = '1.92.66874';

      commit;


2.2 Distributed transaction without corresponding dba_2pc entries

In this case dba_2pc views are empty but users are receiving distributed txn
related errors, e.g. ORA-2054, ORA-1591. Normally such a case should not appear
and if it is reproducible a bug should be filed. Here is the list of several
alternative solutions that can be used in this case:

     a. Perform. incomplete recovery
     b. Truncate the objects referenced by that transaction and import them
     c. Use _corrupted_rollback_segments parameter to drop that rollback segment
     d. Insert dummy entries into dba_2pc views and either commit or rollback
        force the distributed transaction

The first three solutions are discussed in Backup and Recovery manuals and in
the notes referred above. In the 4th solution a dummy entry is inserted into
the dictionary so that the transaction can be manually committed or rolled back. 
Note that RECO will not be able to process this txn and distributed txn recovery 
should be disabled before using this method. Furthermore, please take a BACKUP
of your database before using this method.

The stranded entries is the cause of ORA-01591 so we need to 
clear the stranded entries by purging them using 

execute DBMS_TRANSACTION.PURGE_LOST_DB_ENTRY('transanction_id');



The following example describes how to diagnose and resolve this case. Suppose
that users are receiving

 

ORA-1591: lock held by in-doubt distributed transaction 1.92.66874

and the following query returns no rows:

   

 select local_tran_id, state from dba_2pc_pending 
    where local_tran_id='1.92.66874';
    
    no rows selected

Furthermore querying the rollback segment shows that 1.92.66874 remains in 
prepared state

   

 SELECT KTUXEUSN, KTUXESLT, KTUXESQN, /* Transaction ID */
           KTUXESTA Status,
           KTUXECFL Flags
    FROM x$ktuxe
    WHERE ktuxesta!='INACTIVE'
          AND ktuxeusn= 1;  /* <== Replace this value with your txn undo seg#
                                   Which is displayed in the first part of
                                   the transaction ID */

      KTUXEUSN   KTUXESLT   KTUXESQN STATUS           FLAGS
    ---------- ---------- ---------- ---------------- ------------------------
             1         92      66874 PREPARED         SCO|COL|REV|DEAD


Trying to manually commit or rollback this transaction 

    

commit force '1.92.66874';

    ORA-02058: no prepared transaction found with ID 1.92.66874

raises ORA-02058 since dba_2pc views are empty. In order to use commit force or 
rollback force a dummy record should be inserted into pending_trans$ as follows:

    

alter system disable distributed recovery;

    insert into pending_trans$ (
        LOCAL_TRAN_ID,
        GLOBAL_TRAN_FMT,
        GLOBAL_ORACLE_ID,
        STATE,
        STATUS,
        SESSION_VECTOR,
        RECO_VECTOR,
        TYPE#,
        FAIL_TIME,
        RECO_TIME)
    values( '1.92.66874', /* <== Replace this with your local tran id */
        306206,                  /*                                         */
        'XXXXXXX.12345.1.2.3',   /*  These values can be used without any    */
        'prepared','P',          /*  modification. Most of the values are   */
        hextoraw( '00000001' ),  /*  constant.                              */
        hextoraw( '00000000' ),  /*                                         */
        0, sysdate, sysdate );

    insert into pending_sessions$ 
    values( '1.92.66874',/* <==Replace only this with your local tran id */
        1, hextoraw('05004F003A1500000104'), 
        'C', 0, 30258592, '', 
        146
      );

    commit;

    commit force '1.92.66874';


    If commit force raises an error then note the errormessage and execute the 
    following:
      

delete from pending_trans$ where local_tran_id='1.92.66874';
      delete from pending_sessions$ where local_tran_id='1.92.66874';
      commit;
      alter system enable distributed recovery;

     Otherwise run purge the dummy entry from the dictionary, using     

alter system enable distributed recovery;
connect / as sysdba
COMMIT;
Use following query to retrieve the value for such _smu_debug_mod parameter:

col Parameter for a20
col "Session Value" for a20
col "Instance Value" for a20

SELECT a.ksppinm "Parameter",b.ksppstvl "Session Value",c.ksppstvl "Instance Value"
FROM x$ksppi a, x$ksppcv b, x$ksppsv c
WHERE a.indx = b.indx AND a.indx = c.indx AND a.ksppinm = '_smu_debug_mode'
/


-- set it temporarily to 4:


alter system set "_smu_debug_mode" = 4;  /* if automatic undo management 
                                                   is being used */
-- in 9.2x alter session can be used instead.

commit;  /* this is to prevent the ORA-01453 in purge_lost_db_entry call */

exec dbms_transaction.purge_lost_db_entry( '1.92.66874' );

SQL> commit;

SQL> alter system set "_smu_debug_mode" = ;

SQL> commit;


2.3 How to PURGE the DISTRIBUTED transaction in PREPARED state, when COMMIT
or ROLLBACK FORCE hangs ?, where we have entries for both Distributed
transaction and dba_2pc entries.


ORA-01591: lock held by in-doubt distributed transaction 44.88.85589 

The row exist from dba_2pc_pending & Rollback segment 



SQL> SELECT LOCAL_TRAN_ID,STATE FROM DBA_2PC_PENDING; 

LOCAL_TRAN_ID STATE 
----------------- ----------- 
44.88.85589 prepared 


SQL> SELECT KTUXEUSN, KTUXESLT, KTUXESQN, /* Transaction ID */ 
KTUXESTA Status, 
KTUXECFL Flags 
FROM x$ktuxe 
WHERE ktuxesta!='INACTIVE' 
AND ktuxeusn= 44; /* <== Replace this value with your txn undo seg# 
Which is displayed in the first part of 
the transaction ID */ 

KTUXEUSN KTUXESLT KTUXESQN STATUS FLAGS 
---------- ---------- ---------- ---------------- ------------------------ 
44 88 85589 PREPARED SCO|COL|REV|DEAD


SQL> Commit force 44.88.85589; 
SQL> rollback force 44.88.85589; 


Executing COMMIT or ROLLBACK FORCE hangs 

The wait event is ""free global transaction table entry" 


Purging the transaction should fail with below error: 



EXECUTE DBMS_TRANSACTION.PURGE_LOST_DB_ENTRY('44.88.85589'); 
BEGIN DBMS_TRANSACTION.PURGE_LOST_DB_ENTRY('44.88.85589'); END; 

* 
ERROR at line 1: 
ORA-06510: PL/SQL: unhandled user-defined exception 
ORA-06512: at "SYS.DBMS_TRANSACTION", line 94 
ORA-06512: at line 1 



Solution:
--------

You have to implement both the solution :

2.1 Dba_2pc entries without a corresponding transaction 
2.2 Distributed transaction without corresponding dba_2pc entries 

1. 

delete from sys.pending_trans$ where local_tran_id = '44.88.85589'; 
   delete from sys.pending_sessions$ where local_tran_id = '44.88.85589'; 
   delete from sys.pending_sub_sessions$ where local_tran_id ='44.88.85589'; 
   commit; 


2. Now insert the dummy record as explained in section: 

   2.2 Distributed transaction without corresponding dba_2pc entries 
       commit; 

3. Commit force '44.88.85589' 

4. Purge the transaction: 

   exec dbms_transaction.purge_lost_db_entry('44.88.85589');


Note:126069.1   Manually Resolving In-Doubt Transactions: Different Scenario's

References