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So far in this chapter, you ve seen the components of the Oracle database system: the necessary files and memory allocations and how you can adjust them. It s time now to look into how Oracle processes users queries and how it makes changes to data. It s important to understand the mechanics of SQL transaction processing because all interaction with an Oracle database occurs either in the form of SQL queries that read data or SQL (or PL/SQL) operations that modify, insert, or delete data. A transaction is a logical unit of work in an Oracle database, and consists of one or more SQL statements. A transaction begins with the first executable SQL statement and terminates when you commit or roll back the transaction. Committing a transaction will make your changes permanent, and rolling back the changes will, of course, undo them. Once you commit the transaction, all other users transactions that start subsequently will be able to see the changes made by your transactions. When a transaction fails to execute completely (say, due to a power failure), the entire transaction must be undone. Oracle will roll back any changes made by earlier SQL statements in the transaction, leaving the data in its original (pretransaction) state. The whole process is designed to maintain data consistency a transaction is an all-or-nothing concept. The following simple example of a row being inserted outlines how Oracle processes transactions: 1. A user requests a connection to the Oracle server through a 3-tier or an n-tier web-based client using Oracle Net Services. 2. Upon validating the request, the server starts a new dedicated server process for that user.

The UNDO_RETENTION parameter specifies the amount of redo information to be saved in the undo tablespace before it can be overwritten. The appropriate value for this parameter depends on the size of the undo tablespace and the nature of the queries in your database. If the queries aren t huge, they don t need to have large snapshots of data, and you could get by with a low undo retention interval. Similarly, if there is plenty of free space available in the undo tablespace, transactions won t be overwritten, and you are less likely to run into the snapshot-too-old problem. If you plan on using the Flashback Query feature extensively, you will have to figure out how far back in time your Flashback queries will go, and specify the UNDO_RETENTION parameter accordingly. Example: UNDO_RETENTION = 14400 (4 hours)

Default value: 900 (seconds) Parameter type: Dynamic. You can use the ALTER SYSTEM command to increase the value to a practically unlimited time period.

Signals delivered with procedure calls can be handled synchronously or asynchronously. Signals delivered as messages can also be synchronous or asynchronous, depending on the communication protocol used. For example, using low-level TCP socket operations, the sender can send data and continue without waiting for any confirmation from the receiver. If HTTP is used, the sender might use POST verbs to send data. When the POST is sent, the sender can block, waiting for data to be returned. Whether messages are sent synchronously or not depends on how the communication layer is designed. You can use differently shaped arrowheads on wiring diagrams to show the synchronicity of signals. A similar convention is used in UML Interaction diagrams. If the arrowhead is complete, the signal is synchronous, as shown in Figure 7-12. If the arrowhead is missing the upper portion, the signal is asynchronous, as shown in Figure 7-13.

The use of Oracle software is limited by the license agreement between Oracle and the customer. There are two important parameters that control the usage of Oracle licenses, LICENSE_MAX_USERS and LICENSE_MAX_SESSIONS, the first setting a limit on the number of concurrent Oracle users and the second, the maximum number of users you can create in a database.

The LICENSE_MAX_SESSIONS parameter sets the maximum number of concurrent sessions you can have at any given time. Once your database reaches this limit, new sessions can t connect, getting a warning about the database reaching its maximum user capacity. If you set a zero value for this parameter, the database doesn t enforce session licensing. Example: LICENSE_MAX_SESSIONS=250 Default value: 0 Parameter type: Dynamic. You can issue the ALTER SYSTEM command to change the value of this parameter.

ErrorDetected()

You must set either the LICENSE_MAX_SESSIONS parameter or the LICENSE_MAX_USERS parameter, but not both. That is, you must set one of the parameters to zero and the other to a positive number.