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A blocking lock occurs when a lock placed on an object by a user prevents or blocks other users from accessing the same object or objects. The DBA_BLOCKERS table is useful in getting this information it tells you which sessions are currently holding locks on objects for which some other object is presently waiting. You can combine the information in the DBA_BLOCKERS table with that in the V$SESSION tables, to find out who is holding the blocking session. Here is the SQL statement: SQL> SELECT a.username, a.program, a.sid, a.serial# 2 FROM v$session a, dba_blockers b 3 WHERE a.sid = b.holding_session; SQL> The following is a simple example of a blocking session: user nick alapati issues the following DML statement, but doesn t commit it: SQL> DELETE FROM emp WHERE name='samalapati'; 1 row deleted. SQL>

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User nina alapati, in the meanwhile, issues an identical statement, but when she executes it, it hangs: SQL> DELETE FROM emp WHERE name='samalapati'; The second user s DML statement will hang because the first user hasn t committed yet, and thus holds a row-level lock on the row the second user is trying to change. When the first user rolls back or commits, the second user s session automatically moves forward and finishes. You can use the V$SESSION view to find out which sessions are blocking other sessions. Here s a simple query using the view that shows the blocking lock caused by the previous two SQL statements: SQL> SELECT username, blocking_session blocking_session_status FROM V$SESSION WHERE blocking_session_status='VALID'; When you do find a blocking session, and it is blocking another session from doing its job, you may have to terminate the blocking session by using the ALTER SYSTEM KILL SESSION command. If the process or the session still won t go away, go to the operating system level and kill the process or the thread that spawned the Oracle session.

!SampleClass() { ... this function is called by GC for last-chance cleanup ... } ... }; Finalizers are integrated into the Dispose pattern described in 6. The following pseudocode shows what the compiler generates for a class that contains a destructor (~T) and a finalization function (!T): // pseudocode public ref class SampleClass : IDisposable { public: virtual void Dispose() sealed // implements IDisposable::Dispose { Dispose(true); GC::SuppressFinalize(this); } protected: virtual void Finalize() override { Dispose(false); }

ystem process monitors can be a vital tool in determining the health of a running machine Ensuring that the required processes are running and that the total number of each type of running process is appropriate is a good way to maintain system stability The downside of these types of monitors is that they let you know only which processes are running and how many there are They don t give you an indication of the health of each individual process This script dives a little deeper into the condition of processes By using the ps command with a customized format, we ll be able to monitor the age, proportion of CPU usage, virtual-memory consumption, and amount of CPU time consumed by a particular process If you are monitoring multiple instances of any given process, each instance will be held up to the standard being monitored.

Deadlocks occur in any RDBMS when two sessions block each other while each waits for a resource that the other session is holding. This is a catch-22 situation, because the stalemate can t be broken by either session unilaterally. In such circumstances, Oracle steps in, kills one of the sessions, and rolls back its transaction. Oracle quickly recognizes that two sessions are deadlocked and terminates the transaction that holds the most recently applied lock. This will release the object locks that the other session is waiting for. You don t really need to do anything when there are deadlocks, although you ll see messages in your dump directory that deadlocks are currently in the database. When Oracle encounters a deadlock between transactions, it records in the trace file (in the directory location specified by the USER_DUMP_DEST initialization parameter) the session IDs involved, the SQL statements issued in the transactions, and the specific object name and the rows on which locks are held in each session involved in the deadlock. Oracle further informs you that the deadlock is not an Oracle error, but is due to errors in application design or is a result of issuing ad hoc SQL. Application designers must write exception handlers in the code to roll back the aborted transaction and restart it. You can avoid deadlocks by paying attention in the design phase and ensuring the proper locking order of the objects. Given that writers block other writers, deadlocks in Oracle are a rare phenomenon.

The V$BUFFER_POOL view shows you multiple buffer pool information. By default there s just one pool, named the default pool, but you can configure other pools, called the recycle and keep buffer pools. The following query uses the V$BUFFER_POOL view:

SQL> 2 3 4 5*