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When the number of notification types is small When the number of senders and receivers is small When notifications carry no payload You can combine semaphores with out-of-band data to support notifications carrying a payload.

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Note that a zero value under the WAIT_TIME column means that the session is waiting for a specific wait event. A nonzero value represents the time waited for the last event.

The V$SESSION_WAIT view tells you what resource a session is waiting for. The V$SESSION view also provides significant wait information for active sessions. However, neither of these views provides you with historical information about the waits in your instance. Once the wait is over, you can no longer view the wait information using the V$SESSION_WAIT view. The waits are so fleeting

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that by the time you query the views, the wait in most times is over. The new Active Session History (ASH) feature, by recording session information, enables you to go back in time and review the history of a performance bottleneck in your database. Although the AWR provides hourly snapshots of the instance by default, you won t be able to analyze events that occurred five or ten minutes ago, based on AWR data. This is where the ASH information comes in handy. ASH samples the V$SESSION view every second and collects the wait information for all active sessions. An active session is defined as a session that s on the CPU or waiting for a resource. You can view the ASH session statistics through the view V$ACTIVE_SESSION_HISTORY, which contains a single row for each active session in your instance. ASH is a rolling buffer in memory, with older information being overwritten by new session data. Every 60 minutes, the MMON background process flushes filtered ASH data to disk, as part of the hourly AWR snapshots. If the ASH buffer is full, the MMNL background process performs the flushing of data. Once the ASH data is flushed to disk, you won t be able to see it in the V$ACTIVE_ SESSION_HISTORY view. You ll now have to use the DBA_HIST_ACTIVE_SESS_HISTORY view to look at the historical data. In the following sections, I show how you can query the V$ACTIVE_SESSION_HISTORY view to analyze current (recent) active session history.

Each major platform has its own flavor of semaphore implementations. For example, the .NET Framework has a binary semaphore class named AutoResetEvent. The class exposes the two methods Set and WaitOne, to respectively signal and test the semaphore. The diagram in Figure 8-18 shows how you might use an AutoResetEvent semaphore to send notifications with no payload. Listing 8-20 shows a C# implementation.

The V$ACTIVE_SESSION_HISTORY view provides a window on the ASH data held in memory by the Oracle instance before it s flushed as part of the hourly AWR snapshots. You can use it to get information on things such as the SQL that s consuming the most resources in the database, the particular objects causing the most waits, and the identities of the users who are waiting the most. In the following sections I show how to use the ASH information to gain valuable insights into the nature of the waits in your instance, including answering such questions as the objects with the highest waits, the important wait events in your instance, and the users waiting the most.

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The following query identifies the objects causing the most waits and the type of events the objects waited for during the last 15 minutes: SQL> 2 3 4 5 6 7 8 9* SELECT o.object_name, o.object_type, a.event, SUM(a.wait_time + a.time_waited) total_wait_time FROM v$active_session_history a, dba_objects o WHERE a.sample_time between sysdate - 30/2880 and sysdate AND a.current_obj# = o.object_id GROUP BY o.object_name, o.object_type, a.event ORDER BY total_wait_time;

Figure 8-18. Using a binary semaphore to deliver notifications Listing 8-20. Using C# Binary Semaphores to Fire Events Between Threads public class Sender { AutoResetEvent semaphore; public Sender(AutoResetEvent theSemaphore) { semaphore = theSemaphore; } void FireEvent() { semaphore.Set(); } }

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