LOCKS

         *  DB2 uses locks to control concurrence and prevent lost updates.

         *  Locks prevent one program from accessing data that another  program has changed but not yet committed.

         *  DB2 acquires all locks implicitly under DB2 control.

         Lock can be obtained  ==>

            *  Implicitly - by the system

            *  Explicitly - by the user

         There are 2 kinds of locks :

            Shared             S

            Exclusive         X

SHARE MODE Lock :

            *  Is used for querying consistent data

            *  Prevents other users from updating data, you are reading

            *  Prevents other users from putting EXCLUSIVE mode lock  on the table

            *  Lets other users read the data

            *  Locks the whole table

         Example  :

            LOCK TABLE EMPLOYEE

                        IN  SHARE  MODE ;

            EXCLUSIVE MODE lock :

            *  Is used for changing data

            *  Prevents other users from updating data, you’re reading

            *  Prevents other users from putting ANY lock on the table

            *  Lets other users read the data

            *  Does not guarantee current data

            *  Locks the whole table

         Example :

         LOCK TABLE EMPLOYEE       IN                          EXCLUSIVE MODE ;

Locks can be obtained implicitly or explicitly.

         Implicit lock is issued by the system itself.

         DB2 provides facilities for explicit locking :

         *  The SQL statement LOCK TABLE

         *  The ISOLATION parameter on the BIND command

         *  The tablespace LOCKSIZE parameter

         *  The ACQUIRE/RELEASE parameters on the BIND command