Collection Contents Index Introduction to concurrency Correctness pdf/chap15.pdf

User's Guide
   PART 2. Relational Database Concepts
     CHAPTER 15. Using Transactions and Locks       

Typical inconsistencies


Adaptive Server Anywhere affords you control over the amount of locking it uses to isolate transactions. For example, you can eliminate inconsistencies such as the dirty read demonstrated in the previous example.

This section revisits the dirty read and introduces two other typical types of inconsistencies which you may encounter. Knowledge of these inconsistencies will help you select appropriate levels of isolation for the transactions in your own databases.

It then proceeds to introduce a fourth type of inconsistency known as lost updates. These are particularly interested to you if you use something known as cursors in your SQL programs.

Top of page  Three typical types of inconsistency

There are three typical types of inconsistency that can occur during the execution of concurrent transactions. This list is not exhaustive as other types of inconsistencies can also occur. These three types are mentioned in the ISO SQL/92 standard and are important because behavior at lower isolation levels is defined in terms of them.

You will recognize the first type as the one you demonstrated in the previous tutorial.

Other types of inconsistencies can also exist. These three were chosen for the ISO SQL/92 standard because they are typical problems and because it was convenient to describe amounts of locking between transactions in terms of them.

For Info     Amounts of locking will be described further in Isolation levels and consistency.

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Collection Contents Index Introduction to concurrency Correctness pdf/chap15.pdf