Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 8 ■ TRANSACTIONS 277 3 Autonomous_Insert; 4 rollback; 5 end; 6 / PL/SQL procedure successfully completed. ops$tkyte@ORA10G> select * from t; MSG ------------------------- Autonomous Insert Here, only the work done by and committed in the autonomous transaction persists. The INSERT done in the anonymous block was rolled back by the rollback statement on line 4. The autonomous transaction procedure’s COMMIT has no effect on the parent transaction started in the anonymous block. In a nutshell, this captures the essence of autonomous transactions and what they do. To summarize, if you COMMIT inside a “normal” procedure, it will make durable not only its own work but also any outstanding work performed in that session. However, a COMMIT performed in a procedure with an autonomous transaction will make durable only that procedure’s work. When to Use Autonomous Transactions The Oracle database has supported autonomous transactions internally for quite a while. We see them all of the time in the form of recursive SQL. For example, a recursive transaction may be performed when selecting from a sequence, in order for you to increment the sequence immediately in the SYS.SEQ$ table. The update of the SYS.SEQ$ table in support of your sequence was immediately committed and visible to other transactions, but your transaction was not committed as yet. Additionally, if you roll back your transaction, the increment to the sequence remained in place; it is not rolled back with your transaction, as it has already been committed. Space management, auditing, and other internal operations are performed in a similar recursive fashion. This feature has now been exposed for all to use. However, I have found that the legitimate real-world use of autonomous transactions is very limited. Time after time, I see them used as a work-around to such problems as a mutating table constraint in a trigger. This almost always leads to data integrity issues, however, since the cause of the mutating table is an attempt to read the table upon which the trigger is firing. Well, by using an autonomous transaction you can query the table, but you are querying the table now without being able to see your changes (which is what the mutating table constraint was trying to do in the first place; the table is in the middle of a modification, so query results would be inconsistent). Any decisions you make based on a query from that trigger would be questionable—you are reading “old” data at that point in time. A potentially valid use for an autonomous transaction is in custom auditing, but I stress the words “potentially valid.” There are more efficient ways to audit information in the database than via a custom written trigger. For example, you can use the DBMS_FGA package or just the AUDIT command itself.
278 CHAPTER 8 ■ TRANSACTIONS A question that application developers often pose to me is, “How can I audit every attempt to modify secure information and record the values they were trying to modify?” They want to not only prevent the attempted modification from taking place, but also create a permanent record of the attempt. Before the advent of autonomous transactions, many developers tried (and failed) to do this using standard triggers without an autonomous transaction. The trigger would detect the UPDATE and, upon discovering a user modifying data she should not, it would create an audit record and fail the UPDATE. Unfortunately, when the trigger failed the UPDATE, it also rolled back the audit record—it was an all-or-nothing failure. With autonomous transactions, it is now possible to securely capture the audit of an attempted operation as well as roll back that operation. In the process, we can inform the end user that she has attempted to modify data that she does not have permission to modify and that a record of the attempt has been made. It is interesting to note that the native Oracle AUDIT command provided the ability to capture unsuccessful attempts to modify information, using autonomous transactions, for many years. The exposure of this feature to Oracle developers allows us to create our own, more customized auditing. Here is a small example. Let’s place an autonomous transaction trigger on a table that captures an audit trail, detailing who tried to update the table and when that person tried to do it, along with a descriptive message of which data the person tried to modify. The logic behind this trigger will be that it will prevent any attempt to update the record of an employee who does not (directly or indirectly) report to you. First, we make a copy of the EMP table from the SCOTT schema to use as our example table: ops$tkyte@ORA10G> create table emp 2 as 3 select * from scott.emp; Table created. ops$tkyte@ORA10G> grant all on emp to scott; Grant succeeded. We also create an AUDIT_TAB table in which to store the audit information. Note that we’re using the DEFAULT attribute of the columns to have the currently logged in username and the current date/time logged into our audit trail as well: ops$tkyte@ORA10G> create table audit_tab 2 ( username varchar2(30) default user, 3 timestamp date default sysdate, 4 msg varchar2(4000) 5 ) 6 / Table created. Next, we create an EMP_AUDIT trigger to audit UPDATE activity on the EMP table: ops$tkyte@ORA10G> create or replace trigger EMP_AUDIT 2 before update on emp 3 for each row 4 declare
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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