Web hosting comparisons - Why Use an RDBMS? 3 every time the
Why Use an RDBMS? 3 every time the location or layout of the files changes, you are required to change your software. Once the datasets your software is storing become numerous or large, the storage management process becomes cumbersome. Using a database system gives you storage transparency. Your software does not care where and how the data is stored. The data can even be stored on some other computer and accessed via networking protocols. Transactions When you have more than one user accessing and changing your data, you want to make these changes transactional. Transactions group operations on your data into units of work that meet the ACID test. The ACID test concept is best illustrated with a commonly used example from the banking industry. Jack and Jill share a joint checking account with a balance of $1000. They are both performing various operations, such as deposits, withdrawals, and transfers, on the account. Let s see how the four aspects of the ACID test come into play: Atomicity: All changes made during a transaction are made successfully, or in the case of failure, none are made. If any operation fails during the transaction, then the entire transaction is rolled back, leaving your data in the state it was before the transaction was started. For example, suppose Jack is making a transfer of $500 from his checking account to a savings account. Sometime between the withdrawal of the $500 from the checking account and the deposit of $500 to the savings account, the software running the banking system crashes. Jack s $500 has disappeared! With atomicity, either the entire transfer would have happened, or none of it would have happened, leaving Jack a much happier customer than he is now. Consistency: All operations transform the database from one consistent state to another consistent state. Consistency is defined by how the database schema is designed and whether integrity constraints such as foreign keys are used. The database management system is responsible for ensuring that transactions do not violate the database schema or integrity constraints. For example, the bank s database developers have declared in the database schema that the balance of an account cannot be empty, or null. If any transaction attempts to set the balance to an empty value, the transaction will be aborted and any changes rolled back. Isolation: A transaction s changes are not made visible to other transactions until they are committed under the atomicity rule described earlier. This is best demonstrated by what happens when month-end reports are generated. Let s say that Jack is performing the transfer transaction outlined in the atomicity example, and at the same time you are generating his
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