Introduction To Sybase, Part 3
Welcome Back! In the first two parts of this series, we installed the Sybase database and configured the Perl language to work with the database. In part three, we will create a web application using our newly installed database server. Before you can manage an application using the Sybase server, some background information is in order. I will assume you have read the two previous articles in the series.
The example program is a bookstore on the Web. We will discuss all aspects of creating and maintaining a client-server application using the Sybase SQL server. You won't be an expert after reading the article, but you'll have a good idea of what needs to be done for client-server applications. The example application will not be able to compete with Amazon.com, but it will give you an idea of how to design a client-server application. We'll first take a look at some Sybase SQL server basics.
The Sybase SQL Server is a transactional database server. A transaction is a single piece of work. For example, placing an order would be considered one transaction. Moving money from one account to another is one transaction. Even if a transaction affects two tables in our database, the entire transaction should be completed or should not happen at all—it should never be half-completed. For example, if you want to transfer $100 from your savings account to your checking account, you either want the transfer to happen or not happen. You don't want the $100 to be removed from your savings account but not placed in your checking account. The SQL to accomplish this transfer would look something like this:
update accounts_t set balance = balance - 100 where account_nbr = 'mysaving' update accounts_t set balance = balance + 100 where account_nbr = 'mychecking'
The bank wouldn't want the first update to execute without the second one. If it were my account, I know I wouldn't. The Sybase SQL Server allows the developer to denote where a transaction begins and where it ends.
begin transaction update accounts_t set balance = balance - 100 where account_nbr = 'mysaving' if @@error != 0 begin rollback transaction return end update accounts_t set balance = balance + 100 where account_nbr = 'mychecking' if @@error != 0 begin rollback transaction return end commit transaction if @@error != 0 begin rollback transaction endIf you do not use the begin transaction and commit transaction commands, the database server will assume each SQL command is a single transaction. Make sure you use transaction control in your application where it is needed. For more information on transactions, see Chapter 17 in the Transact-SQL User's Guide.
The SQL Server uses a log to keep track of all transactions. Each database has its own log. The log is part of the database and is not human readable. Each change made to a database is saved in the database's transaction log. Some databases save their data and log information in the same area. Most databases have separate areas for the data and their log. Each area of a database is called a segment. By default, a database has three segments:
System: stores the database's system tables.
Log: stores the database's transaction log.
Default: stores all other database objects.
You can create a database where all three segments are on the same device. If you do this, you won't have as much control over the database as you'd like. The best way to create a database is to separate the log segment from the system and default segments. When you create the database, you specify which devices to put the database on. For more information on these segments and on creating your own custom segments, look in chapter 16 of the System Administration Guide (see Resources). For more information on creating databases, look in chapter 14. In our example program, a database called book_d is created with 20MB allocated for data and 10MB for log.
All transactions that modify a database are saved in the database's transaction log. The data in this log will keep increasing until you back up the log to tape or disk. The data in the log will remain even after all transactions are committed. It stays until the log segment is backed up. This means you need to back up your database often, or this log segment will fill up. When the log segment fills up, you cannot make changes to data in this database until you back up the log segment.
A function is available that will allow the database server to automatically call a stored procedure when a segment is filling up. So, you can write a stored procedure to automatically back up the log when the segment is close to full. A stored procedure is an object in the database containing SQL code. An example stored procedure called sp_thresholdaction that will back up the log to disk is included in the application. For more information about Threshold procedures, see chapter 21 of the System Administration Guide.
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