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Table 7-9. A Comparison of Transaction Isolation Levels and Locking Behaviour in Oracle Versus Databases That Employ Read Locking Writes Block Reads No Reads Block Writes No DeadlockSensitive Reads No Incorrect Query Results Yes

Isolation Level READ UNCOMMITTED READ COMMITTED READ COMMITTED REPEATABLE READ SERIALIZABLE SERIALIZABLE

Concurrency controls and how the database implements them are definitely things you want to understand. I ve been singing the praises of multi-versioning and read consistency, but like everything else in the world, they are double-edged swords. If you don t understand that multi-versioning is there and how it works, you will make errors in application design. Consider the resource scheduler example from 1. In a database without multi-versioning and its associated non-blocking reads, the original logic employed by the program may very well have worked. However, this logic would fall apart when implemented in Oracle. It would allow data integrity to be compromised. Unless you know how multi-versioning works, you will write programs that corrupt data. It is that simple.

Transactions are one of the features that set databases apart from file systems. In a file system, if you are in the middle of writing a file and the operating system crashes, that file will probably be corrupted, though there are journaled file systems and the like that may be able to recover your file to some point in time. However, if you need to keep two files synchronized, such a system won t help if you update one file and the system fails before you finish updating the second, your files won t be synchronized. This is the main purpose of transactions they take the database from one consistent state to the next. That is their function. When you commit work in the database, you are assured that either all of your changes, or none of them, have been saved. Furthermore, you are assured that the various rules and checks that protect data integrity were implemented. In the previous chapter, Concurrency and Multi-versioning, we discussed transactions in terms of concurrency control and how, as a result of Oracle s multi-versioning, read-consistent model, Oracle transactions can provide consistent data every time, under highly concurrent data access conditions. Transactions in Oracle exhibit all of the required ACID characteristics: Atomicity: Either all of a transaction happens or none of it happens. Consistency: A transaction takes the database from one consistent state to the next. Isolation: The effects of a transaction may not be visible to other transactions until the transaction has committed. Durability: Once the transaction is committed, it is permanent.

In particular, we discussed how Oracle obtains consistency and isolation in the previous chapter. Here we ll focus most of our attention on the concept of atomicity and how that is applied in Oracle. In this chapter, we ll discuss the implications of atomicity and how it affects statements in Oracle. We ll cover transaction control statements such as COMMIT, SAVEPOINT, and ROLLBACK, and we ll discuss how integrity constraints are enforced in a transaction. We ll also look at why you may have some bad transaction habits if you ve been developing in other databases. We ll look at distributed transactions and the two-phase commit (2PC). Lastly, we ll examine autonomous transactions, what they are, and the role they play.

You don t need a begin transaction statement in Oracle. A transaction implicitly begins with the first statement that modifies data (the first statement that gets a TX lock). You can explicitly begin a transaction using SET TRANSACTION or the DBMS_TRANSACTION package, but it is not a necessary step, unlike in some other databases. Issuing either a COMMIT or ROLLBACK statement explicitly ends a transaction.