Client Side Transparent Application Failover for Oracle RAC

TAF – – Transparent Application Failover is a feature of the OCI driver, and TAF cannot be used with thin driver. The client side TAF can be configured through client connection definitions in TNS connect descriptors.

The tests are held under the following environment,  both 11gR2 database and client are used to test the behaviors of client side TAF by using SCAN.

                    GI: 12.1.0.2.0
4 nodes RAC: 11.2.0.4
Oracle Client: 11.2.0.4

The service name is RAC_TEST running on node1 and node4, while available nodes are node2 and node3.

$ srvctl status service -s RAC_TEST -d TESTDB
Service RAC_TEST is running on instance(s) TESTDB1,TESTDB4

There are two TYPES of TAF available, SESSION and SELECT.

SESSION: Session Failover re-creates the connections and sessions to the surviving instance.

SELECT: In addition to recreating the session, Select Failover also replays the queries that were in progress.

There are two METHODS in which TAF establishes the failover connection, BASIC and PRECONNECT.

BASIC: The second connection is re-established only after the first connection to the instance or service failed.

PRECONNECT: Two connections are established when the client logs in to the database. A login to database will create two connection at the same time. For this to work, clusterware actually starts two services. One main service and another shadow service.

Client Side TAF Configuration — BASIC

For non-TAF database connection, by query gv$session to get client side TAF connection details, which demonstrates the server side TAF is not configured. That is what we expected in this case.

SQL>select INST_ID,USERNAME,FAILOVER_TYPE,FAILOVER_METHOD,FAILED_OVER
      from gv$session 
     where username='TESTUSER';

INST_ID USERNAME  FAILOVER_TYPE  FAILOVER_M   FAILED_OVER
------- --------- -------------- -----------  ------------- 
4       TESTUSER  NONE            NONE        NO

Use the following TNS entry to test client side TAF with BASIC method.

CLIENT_TAF =
( DESCRIPTION =
  (ADDRESS = (PROTOCOL = TCP)(HOST =CLU-SCAN)(PORT = 1521))
             (CONNECT_DATA =
             (SERVICE_NAME = RAC_TEST)
             (FAILOVER_MODE= (TYPE=select)(METHOD=basic)
                             (RETRIES=20)(DELAY=15))
  )
)

1) Make a database connection by using TNS entry CLIENT_TAF, and run a long query :

SQL>select INST_ID,sid,serial#,USERNAME,FAILOVER_TYPE,
           FAILOVER_METHOD,FAILED_OVER 
      from gv$session 
     where username='TESTUSER';

INST_ID   SID  SERIAL#  USERNAME  FAILOVER_TYPE  FAILOVER_M  FAI
-------- ----- -------- --------- -------------  ----------- ---
 4         70   11171   TESTUSER  SELECT         BASIC        NO

2) Manually kill the session ( only works when session is ACTIVE, otherwise it is not working for killing from testing ).

SQL> alter system kill session '70,11171,@4';

System altered.

 Query the session status, we can see a new failed over session with different ‘SID, serial#’ created, and the query is still going until completed.

 SQL> select INST_ID,sid,serial#,USERNAME,FAILOVER_TYPE,
             FAILOVER_METHOD,FAILED_OVER 
        from gv$session 
       where username='TESTUSER';

INST_ID  SID  SERIAL# USERNAME FAILOVER_TYPE FAILOVER_M FAILED_OVER
-------- ---- ------- -------- ------------- --------- ------------
 4      366   14271   TESTUSER      SELECT     BASIC    YES

3) Instead of killing session in step 2, we manually stop the service on node 4 to simulate instance crash. (  it works for both ACTIVE and INACTIVE sessions ).

INST_ID SID SERIAL# USERNAME FAILOVER_TYPE  FAILOVER_M FAI
------- --- ------- -------- -------------- ---------- ---
4        70  11185  TETSUSER  SELECT        BASIC       NO

Now stop the service on node4, expect the session will failover onto survival node1 .

$ srvctl stop service -s RAC_TEST -i TESTDB4 -d TESTDB -f

SQL> select INST_ID,sid,serial#,USERNAME,FAILOVER_TYPE,
             FAILOVER_METHOD,FAILED_OVER 
        from gv$session 
        where username='TESTUSER';

INST_ID SID SERIAL#  USERNAME  FAILOVER_TYPE  FAILOVER_M FAI
------- --- -------- --------- -------------- ---------- ---
1       367 8663     TETSUSER   SELECT        BASIC      YES

restart the service on node4 for more tests.

$ srvctl start service -s RAC_TEST -i TESTDB4 -d TESTDB

Client Side TAF Configuration — PRECONNECT

Two tnsname entries are prepared with following contents:

PRIMARY=
      ( DESCRIPTION=(ADDRESS=
                     (PROTOCOL=tcp)(HOST=clu-scan)(PORT=1521))
                     (CONNECT_DATA=(SERVICE_NAME=RAC_TEST)
                     (FAILOVER_MODE=(BACKUP=SECONDARY)(TYPE=select)
                     (METHOD=preconnect))
                    )
       )

SECONDARY=(DESCRIPTION=
             (ADDRESS=(PROTOCOL=tcp)(HOST=clu-scan)(PORT=1521))
               (CONNECT_DATA=
               (SERVICE_NAME=RAC_TEST)
               (FAILOVER_MODE=
               (BACKUP=PRIMARY)
               (TYPE=select)
               (METHOD=preconnect))
             )
          )

1) Make a database connection by using TNS entry above, and run a long query :

SQL> select INST_ID,sid,serial#,USERNAME,FAILOVER_TYPE,
            FAILOVER_METHOD,FAILED_OVER,status 
       from gv$session 
       where username='TESTUSER';

INST_ID SID  SERIAL# USERNAME  FAILOVER_TYPE FAILOVER_M FAI STATUS
------- ---- ------ ---------- ------------ ----------- --- --------
 1      42     8485  TESTUSER  NONE          NONE        NO   ACTIVE
 4     366    14375  TESTUSER  SELECT        PRECONNECT  NO INACTIVE

2) Manually stop service RAC_TEST on node 4.

$ srvctl stop service -s RAC_TEST -i TESTDB4 -d TESTDB -f

3) The preconnected session takes over the session and continue the query until completion.

SQL> select INST_ID,sid,serial#,USERNAME,FAILOVER_TYPE,
            FAILOVER_METHOD,FAILED_OVER,status 
       from gv$session 
      where username='TESTUSER';

INST_ID SID SERIAL#   USERNAME FAILOVER_TYPE FAILOVER_M   FAI STATUS
------- --- ------- ---------- ------------ ------------- --- --------
 1       42  8485     TESTUSER       SELECT    PRECONNECT YES ACTIVE

Restart service on node4 for next test.

$ srvctl start service -s RAC_TEST -i -i TESTDB4 -d TESTDB

4) if you kill this PRECONNECT session again , TAF still works again as preconnect.

SQL> alter system kill session '42,8485,@1';

System altered.

SQL> select INST_ID,sid,serial#,USERNAME,FAILOVER_TYPE,
            FAILOVER_METHOD,FAILED_OVER,status 
       from gv$session 
      where username='TESTUSER';

INST_ID SID SERIAL# USERNAME  FAILOVER_TYPE FAILOVER_M FAI STATUS
------- --- ------- --------- ------------ ----------- --- ------
4       265  1563   TESTUSER  SELECT       PRECONNECT  YES ACTIVE

TAF – Transparent Application Failover is a very useful feature in mission critical environment, and it should be used whenever it is available and possible.