Chapter 15 . Perl Development 407 Figure 15-16: Various methods for specifying the username portion of the credentials when creating a connection Figure 15-17: Different methods for specifying a password, one using the undef placeholder and one specifying a blank password
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406 Part IV . Development Other portions of the Datasource include the optional host and port. If no host or port is supplied, the host is assumed to be localhost and the port is assumed to be the default port, 3306. For example, Figure 15-15 shows different methods for supplying the host and/or port for connecting to the database. Figure 15-15: Methods for specifying host and port for the Datasource portion of the database handle Notice the word undef within the program in Figure 15-14. The undef is a placeholder where the username would normally go. In this instance, undef is there; the DBI takes the value of the user environment variable and puts it in the place of the username credential. If you want to supply the username, you should enclose the username in single or double quotation marks. However, if you supply the username as a variable, the DBI interpolates the variable without requiring you to use quotes. Figure 15-16 shows a few examples of correct methods for specifying the username credential as part of the $dbhcreation. If you supply undef as the password or don t use a password at all, it is assumed that no password is used. This is as opposed to specifying a blank password. Figure 15-17 shows examples of two scripts and the errors that result from running them with the different password methods. Notice the error from the first script (where MySQL believes no password is specified) and from the second script (where a blank password is specified).
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Chapter 15 . Perl Development 405 The authentication scheme is the same for a DBI-based Perl application as it is for any other connection to the database. The username supplied must be allowed access from the host where the program is running; the username must be allowed to access the database; the password supplied with the username must match; and other credentials must be correctly supplied, as is the case with any other connection to the database server. Recall that the basic syntax for a connection is $dbh = DBI->connect ($datasource, $credentials); The $datasource for MySQL contains a reference to the MySQL DBD followed by the optional name of the database you want to connect to, followed by the optional hostname of the database server that houses the database. Here s an example: $dbh = DBI->connect ( DBI:mysql:ecommerce , $credentials); would connect to the ecommerce database on the local host and create a database handle ($dbh), assuming the credentials supplied in $credentialsare correct. For example, Figure 15-14 shows a small example application that, when executed, issues a simple Connection successful output message. Figure 15-14: A small application for connecting to the ecommerce database As stated previously, the database itself is optional within the Datasource portion of the DBI connect operation that creates the object. The following command (which creates a database handle) is valid as well: $dbh = DBI->connect ( DBI:mysql: , $credentials);
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404 Part IV . Development Statement handles Statement handles are the means by which actual SQL and related statements are executed against the MySQL server. The statement handle is the child of the database handle; can issue multiple statements using the same database handle. Each statement you issue gets its own statement handle, usually referred to by the $sth variable name. Think of this process as issuing multiple statements from within the CLI. You don t have to reconnect to the CLI for each SELECT statement (for example); you can issue multiple statements. Connecting to and disconnecting from the database with the DBI Interaction with the database can only be done after a connection is made. The connection is known as a database handle and usually referred to by the $dbh variable name. The driver handle is created in the background when a connection is made. Use the connect method to create a database handle and connect to the database. When using the connect method, you must supply credentials as you would if you were connecting through the CLI. For example, if you have to specify a password (and you should), then connecting through the CLI means you have to specify that same password when you connect through the DBI. Figure 15-13 shows some failed connections attempted with a DBI-based Perl program; they fail when proper credentials aren t supplied. Figure 15-13: Errors when a DBI-based application doesn t have proper credentials to connect to the database requested
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Chapter 15 . Perl Development 403 and statements are run, and the connections are destroyed. Whenever you create an object, the DBI identifies it as a driver, a database, or a statement by associating it with one of three logical markers (known as handles) that correspond to those three types of objects: (drivers, databases, or statements). The handles are actually pointers to specific instances of objects. From a programming standpoint, you declare that you d like to use the DBI in your program by adding the following line near the beginning of your program: use DBI; Driver handles DBI driver handles are available for many database types. For the purposes of this book, you need be concerned only with the MySQL driver handle. Tip You can use more than one driver handle within the same program to connect to different database types. You can only use one driver handle per database. (Then again, why would you need more than one handle to the same driver?) You initialize the driver handle (or more properly, the DBI does so) behind the scenes. For example, the driver handle for MySQL is created when you call the connect method for the DBI. Database handles Database handles are the controls you use to manipulate database-related programs. When a database handle is created, you connect to the MySQL database and provide credentials such as username, password, and database name. These credentials are the same as those you would supply to connect with the MySQL CLI (or the mysqladmin command and so forth). The standard naming scheme for a database handle is the $dbh variable (which I use throughout this chapter). A database handle is created automatically whenever you call the connect method; the syntax looks like this: $dbh = DBI->connect($datasource, $credentials) The parent driver handle is also created automatically when the database handle is created. (Connection and disconnection get a closer look in an upcoming section of this chapter.) MySQL allows multiple simultaneous connections when using methods such as the CLI; connections using the DBI have the same capability. Therefore, multiple database handles can be created that attach to the same database. Further, each database handle is a completely separate object; you need not worry about commands or results colliding with each other.
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402 Part IV . Development Finally, install the MySQL Perl DBD by typing the following command: make install The Normal installation dialog box should appear while the DBD is installed in the proper locations (see Figure 15-12). Figure 15-12: Normal installation output when you run the make install command to install the MySQL DBD Other specific drivers and tools can be installed to work with MySQL and Perl; their installation process looks much the same as this one. Introducing the DBI The Perl DBI is the name for a database-independent set of software programs that a programmer can use with many different databases using the same functions in every case. The DBI saves having to account for the multitude of access methods used for different databases. For example, the DBI enables the programmer to use different database drivers (DBDs) for databases such as Oracle, mSQL, Sybase, and others without having to learn an entirely new syntax to program each one. The upcoming portion of the chapter introduces the DBI itself in particular, the basics of DBI use (such as connecting to a database and disconnecting), as well as the DBI s error-handling capability. Basic DBI Usage The DBI is built around an object-oriented programming style. Within that context, the database driver is initialized, connections to the database are created, commands
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Chapter 15 . Perl Development 401 Figure 15-10: The testing phase of the DBD installation fails if you provided incorrect information during the initial stage of the installation. Figure 15-11: Successful testing of the DBD
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400 Part IV . Development The next two questions have no defaults. The first asks for the username to use for testing the driver; the second asks for the password for that user. I ll be using the root user account for testing and giving that password as well, as in Figure 15-9. Figure 15-9: Using the root user account for testing of the MySQL DBD The questions you just answered create files that serve as the basis for the next step in the process: the actual creation of the DBD. Warnings about a missing library and Data-Showtablecan be ignored. Begin this next phase by typing make. You begin the testing phase by typing the following command: make test If you happen to mistype (or specify a user, password, or test database that doesn t exist) during the initial stage of the installation, this testing phase fails miserably and shows errors that resemble Figure 15-10. However, if you provided correct information for the testing portion of the initial phase the tests should complete successfully, as shown in Figure 15-11.
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Chapter 15 . Perl Development 399 (and probably is) in a different location. As stated in the prompt in Figure 15-7, the location should be where the Makefilecan find the include directory of the MySQL installation. Figure 15-7: The location of the MySQL server files is necessary. The default (/usr/local) is correct for my example server. The next series of questions sets the parameters for testing of the driver. The first question is the name of the database that the installation process should use for testing. The MySQL server usually includes a database called test ; therefore this is an easy choice to leave at the default of test at the prompt. The second question in this series asks for the name of the server to use for testing. (The test server shown in Figure 15-8 runs on the local machine; the default of localhostis acceptable.) Figure 15-8: Using the test database for testing, with the server running on the local machine. Both defaults are accepted if you press Return or Enter.
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398 Part IV . Development Installing the MySQL DBD The MySQL Database Driver or DBD is a combined with the driver for the mSQL database system. As stated previously, the DBD can be obtained at your local CPAN mirror or at http://www.cpan.org. The installation process for the DBD is somewhat similar to the DBI however there are some questions that must be answered during the installation. Use the tar command to unpack the MySQL DBD archive, typically called MsqlMysql- modules-
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