.NET/ASP.NET/C#/VB.NET PDF Document SDK

you see that nonuse of bind variables results in the use of almost 9 times the CPU and takes 3.3 times the execution time when compared to the bind variable solution. The more users are added over time, the longer each user spends waiting for these latches. We went from an average of 28 seconds/session (142 seconds of wait/5 sessions) of wait time for latches with 5 users to an average of 127 seconds/session of wait time with 10 users. However, the implementation that avoided overuse of the latch suffered no ill effects as it scaled up.

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A mutex is a serialization device much like a latch is, in fact, the name mutex stands for mutual exclusion. It is another serialization tool used by the database; it was introduced in Oracle 10g Release 1 and is used in place of traditional latches in many places in the server. A mutex differs from a latch in that it is even more lightweight in its implementation. It requires less code to implement, approximately one-fifth of the instructions (which results in less CPU to request in general) and it requires less memory, approximately one-seventh of the size, to implement. A mutex, in addition to being lighter weight, is a little less functional in some respects. Just like an enqueue lock is much heavier than a latch, a latch is heavier than a mutex. But, like the enqueue to latch comparison, the latch can do more than a mutex in some cases (like an enqueue can do more than a latch in some cases). This means that not every latch will be, or should be, replaced by a mutex, just as every enqueue lock will not be, or should not be, replaced by a latch. When reading about mutexes in various reports, just remember that they are lighter-weight serialization devices. They enable possibly more scalability than a latch (just as latches are more scalable than enqueues), but they are still a serialization device. If you can avoid doing something that requires a mutex, in general, you should, for the same reason you would avoid requesting a latch if possible.

Let s take a closer look at Listing 14-2. ASP.NET files can initially look like a sea of strange symbols. Table 14-3 shows the meanings of some of the symbolic forms you ll see in ASP.NET files.

So far, we have looked mostly at locks that Oracle places for us transparently. When we update a table, Oracle places a TM lock on it to prevent other sessions from dropping that table (or performing most DDL, in fact). We have TX locks that are left on the various blocks we modify so others can tell what data we own. The database employs DDL locks to protect objects from change while we ourselves are changing them. It uses latches and locks internally to protect its own structure. Next, let s take a look at how we can get involved in some of this locking action. Our options are as follows: Manually lock data via a SQL statement. Create our own locks via the DBMS_LOCK package.

We have, in fact, already seen a couple of cases where we might want to use manual locking. The SELECT...FOR UPDATE statement is the predominant method of manually locking data. We used it in previous examples to avoid the lost update issue whereby one session would overwrite another session s changes. We ve seen it used as a method to serialize access to detail records to enforce business rules (e.g., the resource scheduler example from 1 Developing Successful Oracle Applications ). We can also manually lock data using the LOCK TABLE statement. This statement is used rarely, because of the coarseness of the lock. It simply locks the table, not the rows in the table. If you start modifying the rows, they will be locked as normal. So, this is not a method to save on resources (as it might be in other RDBMSs). You might use the LOCK TABLE IN EXCLUSIVE MODE statement if you were

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writing a large batch update that would affect most of the rows in a given table and you wanted to be sure that no one would block you. By locking the table in this manner, you can be assured that your update will be able to do all of its work without getting blocked by other transactions. It would be the rare application, however, that has a LOCK TABLE statement in it.