4.3. Data Source NamesWhen connecting to a database via the DBI, you need to tell the DBI where to find the database to connect to. For example, the database driver might require a database name, or a physical machine name upon which the database resides. This information is termed a data source name, and of all the aspects of DBI, this is possibly the most difficult to standardize due to the sheer number and diversity of connection syntaxes. The DBI requires the data source name to start with the characters dbi:, much like a URL begins with http:, and then the name of the driver, followed by another colon -- for example, dbi:Oracle:. Any text that follows is passed to the driver's own connect() method to interpret as it sees fit. Most drivers expect either a simple database name or, more often, a set of one or more name/value pairs separated with semicolons. Some common examples are listed later in this section. For example, mSQL requires the hostname, database name, and potentially, the TCP/IP port number for connecting to the database server. However, Oracle may require only a single word that is an alias to a more complicated connection identifier that is stored in separate Oracle configuration files. DBI offers two useful methods for querying which data sources are available to you for each driver you have installed on your system. Firstly, you can get a list of all the available drivers installed on your machine by using the DBI->available_drivers() method. This returns a list with each element containing the data source prefix of an installed driver,[36] such as dbi:Informix:.
Secondly, you can invoke the DBI->data_sources() method against one or more of the drivers returned by the DBI->available_drivers() method to enumerate which data sources are known to the driver.[37] Calling the data_sources() method will actually load the specified driver and validate that it is completely and correctly installed. Because DBI dies if it can't load and initialize a driver, this method should be called inside an eval{} block if you need to catch that error.
The following script lists all the drivers and data sources for each driver on your system: #!/usr/bin/perl -w # # ch04/listdsns: Enumerates all data sources and all installed drivers # use DBI; ### Probe DBI for the installed drivers my @drivers = DBI->available_drivers(); die "No drivers found!\n" unless @drivers; # should never happen ### Iterate through the drivers and list the data sources for each one foreach my $driver ( @drivers ) { print "Driver: $driver\n"; my @dataSources = DBI->data_sources( $driver ); foreach my $dataSource ( @dataSources ) { print "\tData Source is $dataSource\n"; } print "\n"; } exit; The output from this script on my machine looks like: Driver: ADO Driver: CSV Data source is DBI:CSV:f_dir=megaliths Data source is DBI:CSV:f_dir=pictish_stones Driver: ExampleP Data Source is dbi:ExampleP:dir=. Driver: File Data Source is DBI:File:f_dir=megaliths Data Source is DBI:File:f_dir=pictish_stones Driver: ODBC Driver: Proxy Driver: XBase Data Source is dbi:XBase:. which tells us that we have the standard drivers DBD::Proxy, DBD::ADO, DBD::File, and DBD::ExampleP installed, as well as DBD::ODBC, DBD::XBase, and DBD::CSV. While this may be interesting in theory, in practice you rarely need to use these methods. Most applications are written to use one data source name, either hardcoded into the application or passed in as a parameter in some way. When specifying a data source name for a database, the text following the driver prefix should be of the form that is appropriate for the particular database that you wish to connect to. This is very database-specific, but the following table shows some examples.[38]
In the case of the $data_source argument, an empty or undefined value will result in the environment variable DBI_DSN being checked for a valid value. If this environment variable is not defined, or does not contain a valid value, the DBI will call die(). Copyright © 2001 O'Reilly & Associates. All rights reserved. |
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