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As you can see, all the data problems are due to concurrent access you can safely assume that a transaction executed in isolation will always leave the database in a consistent state when the transaction completes. If the database permits concurrent access, then you need to consider the cumulative effect of all the transactions on database consistency. To do this, the database uses a schedule, which is a sequence of operations from one or more transactions. If all the transactions executed serially, one after another, the schedule would also be serial. If the database can produce a schedule that is equivalent in its effect to a serial schedule, even though it may be derived from a set of concurrent transactions, it is called a serializable schedule. The serializable schedule consists of a series of intermingled database operations drawn from several transactions, the final outcome of which is a consistent database. As you can surmise, deriving a schedule is not easy in practice. However, users don t have to concern themselves with the mechanics of serialization when they use their transactions. The Oracle database automatically derives serializable schedules through the use of isolation levels and the management of undo data. Let s look at these important concepts next.

You know that one way to avoid data anomalies is to prevent more than one user from viewing or changing data at the same time. However, this defeats our main purpose of providing concurrent access to users. To control this trade-off between concurrency and isolation, you specify an isolation level for each transaction. The ISO (http://www.iso.ch) standard for transactions rests on the two key transaction-ending statements: COMMIT and ROLLBACK. All transactions, according to the ISO standard, begin with a SELECT, UPDATE, INSERT or DELETE statement. No transaction can view another transaction s intermediate results. Results of a second transaction are available to the first transaction only after the second transaction completes. The ISO transaction standards ensure the compliance of transactions with the atomic and isolation properties, and help avoid the concurrency problems explained in the previous section. All transactions must ensure that they preserve database consistency. A database is consistent before a transaction begins, and it must be left in a consistent state at the end of the transaction. If you can devise a method to avoid the problems mentioned in the previous section, you can ensure a high degree of concurrent interactions among transactions in the database. There is a price to pay for this, however. Attempts to reduce the anomalies will result in reduced concurrency. You can achieve consistency by enforcing serial use of the database, but it s impractical. Therefore, the practical goal is to find those types of concurrent transactions that don t interfere with each other in other words, to find transactions that guarantee a serializable schedule. Proper ordering of the transactions becomes very important, unless they re all read-only transactions.

For an Oracle DBA, no task is more fundamental and critical than protecting the database itself. The Oracle DBA is the person the information departments entrust with safeguarding the organization s data, and this involves preventing unauthorized use of and access to the database. The DBA has several means to ensure the database s security, and based on the company s security guidelines, he or she needs to maintain the database security policy (and to create the policy if it doesn t already exist). A more complex issue is the authorization of users actions within the database itself, after access has already been granted. I address this topic in depth in 12.

7. Daniel J. Barrett, Lori A. Clarke, Peri L. Tarr, and Alexander E. Wise, A Framework for Event-Based Software Integration, ACM Transactions on Software Engineering and Methodology, October 1996. 8. Richard N. Taylor, Nenad Medvidovic, Kenneth M. Anderson, E. James Whitehead Jr., and Jason E. Robbins, A Component- and Message-Based Architectural Style for GUI Software (proceedings of the 17th International Conference on Software Engineering, Seattle, WA, April 1995).

Note Some organizations don t have a general security policy in place. This is particularly true of smaller companies. In that case, it s usually up to the DBA to come up with the security policy and then enforce it within the database.

Every database has users, and it s the DBA s job to create them based on requests from the appropriate people. A DBA is expected to guide the users use of the database and ensure the database s security by using proper authorization schemes, roles, and privileges. Of course, when users are locked out of the database because of password expiration and related issues, the DBA needs to take care of them. It s also the responsibility of the DBA to monitor the resource usage by individual users and to flag the heavy resource users.

Routers can also be employed in distributed system to connect the various systems together. In this capacity, you usually deploy routers at both ends of a connection, as shown in Figure 10-56.