faq.pm

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

also the amount of resource each Oracle connection will consume. I<mSQL>
is lighter resource-wise and faster.

These views are not necessarily representative of anyone else's opinions,
and do not reflect any corporate sponsorship or views. They are provided
I<as-is>.

=head2 3.8 Is <I<insert feature here>> supported in DBI?

Given that we're making the assumption that the feature you have requested
is a non-standard database-specific feature, then the answer will be I<no>.

DBI reflects a I<generic> API that will work for most databases, and has
no database-specific functionality.

However, driver authors may, if they so desire, include hooks to database-specific
functionality through the C<func()> method defined in the DBI API.
Script developers should note that use of functionality provided I<via>
the C<func()> methods is very unlikely to be portable across databases.

=head1 Programming Questions

=head2 4.1 Is DBI any use for CGI programming?

In a word, yes! DBI is hugely useful for CGI programming! In fact, I would
tentatively say that CGI programming is one of two top uses for DBI.

DBI confers the ability to CGI programmers to power WWW-fronted databases
to their users, which provides users with vast quantities of ordered
data to play with. DBI also provides the possibility that, if a site is
receiving far too much traffic than their database server can cope with, they
can upgrade the database server behind the scenes with no alterations to
the CGI scripts.

=head2 4.2 How do I get faster connection times with DBD::Oracle and CGI?

    Contributed by John D. Groenveld

The Apache C<httpd> maintains a pool of C<httpd> children to service client 
requests.

Using the Apache I<mod_perl> module by I<Doug MacEachern>, the perl 
interpreter is embedded with the C<httpd> children. The CGI, DBI, and your 
other favorite modules can be loaded at the startup of each child. These 
modules will not be reloaded unless changed on disk.

For more information on Apache, see the Apache Project's WWW site:

    http://www.apache.org

The I<mod_perl> module can be downloaded from CPAN I<via>:

    http://www.perl.com/cgi-bin/cpan_mod?module=Apache

=head2 4.3 How do I get persistent connections with DBI and CGI?

    Contributed by John D. Groenveld

Using Edmund Mergl's I<Apache::DBI> module, database logins are stored in a 
hash with each of these C<httpd> child. If your application is based on a 
single database user, this connection can be started with each child. 
Currently, database connections cannot be shared between C<httpd> children.

I<Apache::DBI> can be downloaded from CPAN I<via>:

    http://www.perl.com/cgi-bin/cpan_mod?module=Apache::DBI

=head2 4.4 ``When I run a perl script from the command line, it works, but, when I run it under the C<httpd>, it fails!'' Why?

Basically, a good chance this is occurring is due to the fact that the user
that you ran it from the command line as has a correctly configured set of
environment variables, in the case of I<DBD::Oracle>, variables like
C<ORACLE_HOME>, C<ORACLE_SID> or C<TWO_TASK>.

The C<httpd> process usually runs under the user id of C<nobody>,
which implies there is no configured environment. Any scripts attempting to
execute in this situation will correctly fail.

One way to solve this problem is to set the environment for your database in a
C<BEGIN { }> block at the top of your script. Another technique is to configure
your WWW server to pass-through certain environment variables to your CGI 
scripts.

Similarly, you should check your C<httpd> error logfile for any clues,
as well as the ``Idiot's Guide To Solving Perl / CGI Problems'' and
``Perl CGI Programming FAQ'' for further information. It is
unlikely the problem is DBI-related.

The ``Idiot's Guide To Solving Perl / CGI Problems'' can be located at:

    http://www.perl.com/perl/faq/index.html

as can the ``Perl CGI Programming FAQ''. Read I<BOTH> these documents 
carefully!

=head2 4.5 How do I get the number of rows returned from a C<SELECT> statement?

Count them. Read the DBI docs for the C<rows()> method.

=head1 Miscellaneous Questions

=head2 5.1 Can I do multi-threading with DBI?

Perl version 5.005 and later can be built to support multi-threading.
The DBI, as of version 1.02, does not yet support multi-threading
so it would be unsafe to let more than one thread enter the DBI at
the same time.

It is expected that some future version of the DBI will at least be
thread-safe (but not thread-hot) by automatically blocking threads
intering the DBI while it's already in use.

For some OCI example code for Oracle that has multi-threaded C<SELECT>
statements, see:

    http://www.symbolstone.org/technology/oracle/oci/orathreads.tar.gz

=head2 5.2 How do I handle BLOB data with DBI?

Handling BLOB data with the DBI is very straight-forward. BLOB columns are
specified in a SELECT statement as per normal columns. However, you also
need to specify a maximum BLOB size that the <I>database handle</I> can
fetch using the C<LongReadLen> attribute.

For example:

    ### $dbh is a connected database handle
    $sth = $dbh->prepare( "SELECT blob_column FROM blobby_table" );
    $sth->execute;

would fail.

    ### $dbh is a connected database handle
    ### Set the maximum BLOB size...
    $dbh->{LongReadLen} = 16384;        ### 16Kb...Not much of a BLOB!

    $sth = $dbh->prepare( "..." );

would succeed <I>provided no column values were larger than the specified
value</I>.

If the BLOB data is longer than the value of C<LongReadLen>, then an
error will occur. However, the DBI provides an additional piece of
functionality that will automatically truncate the fetched BLOB to the
size of C<LongReadLen> if it is longer. This does not cause an error to
occur, but may make your fetched BLOB data useless.

This behaviour is regulated by the C<LongTruncOk> attribute which is 
defaultly set to a false value ( thus making overlong BLOB fetches fail ).

    ### Set BLOB handling such that it's 16Kb and can be truncated
    $dbh->{LongReadLen} = 16384;
    $dbh->{LongTruncOk} = 1;

Truncation of BLOB data may not be a big deal in cases where the BLOB
contains run-length encoded data, but data containing checksums at the end,
for example, a ZIP file, would be rendered useless.

=head2 5.3 How can I invoke stored procedures with DBI?

The DBI does not define a database-independent way of calling stored procedures.

However, most database that support them also provide a way to call
them from SQL statements - and the DBI certainly supports that.

So, assuming that you have created a stored procedure within the target
database, I<eg>, an Oracle database, you can use C<$dbh>->C<do()> to
immediately execute the procedure. For example,

    $dbh->do( "BEGIN someProcedure; END;" );   # Oracle-specific

You should also be able to C<prepare> and C<execute>, which is
the recommended way if you'll be calling the procedure often.

=head2 5.4 How can I get return values from stored procedures with DBI?

    Contributed by Jeff Urlwin

    $sth = $dbh->prepare( "BEGIN foo(:1, :2, :3); END;" );
    $sth->bind_param(1, $a);
    $sth->bind_param_inout(2, \$path, 2000);
    $sth->bind_param_inout(3, \$success, 2000);
    $sth->execute;

Remember to perform error checking, though! ( Or use the C<RaiseError>
attribute ).

=head2 5.5 How can I create or drop a database with DBI?

Database creation and deletion are concepts that are entirely too abstract
to be adequately supported by DBI. For example, Oracle does not support the
concept of dropping a database at all! Also, in Oracle, the database
I<server> essentially I<is> the database, whereas in mSQL, the
server process runs happily without any databases created in it. The
problem is too disparate to attack in a worthwhile way.

Some drivers, therefore, support database creation and deletion through
the private C<func()> methods. You should check the documentation for
the drivers you are using to see if they support this mechanism.

=head2 5.6 How can I C<commit> or C<rollback> a statement with DBI?

See the C<commit()> and C<rollback()> methods in the DBI Specification.

Chapter 6 of "Programming the Perl DBI" discusses transaction handling within
the context of DBI in more detail.

=head2 5.7 How are C<NULL> values handled by DBI?

C<NULL> values in DBI are specified to be treated as the value C<undef>.
C<NULL>s can be inserted into databases as C<NULL>, for example:

    $rv = $dbh->do( "INSERT INTO table VALUES( NULL )" );

but when queried back, the C<NULL>s should be tested against C<undef>.
This is standard across all drivers.

=head2 5.8 What are these C<func()> methods all about?

The C<func()> method is defined within DBI as being an entry point
for database-specific functionality, I<eg>, the ability to create or
drop databases. Invoking these driver-specific methods is simple, for example,
to invoke a C<createDatabase> method that has one argument, we would
write:

    $rv =$dbh->func( 'argument', 'createDatabase' );

Software developers should note that the C<func()> methods are
non-portable between databases.

=head2 5.9 Is DBI Year 2000 Compliant?

DBI has no knowledge of understanding of what dates are. Therefore, DBI
itself does not have a Year 2000 problem. Individual drivers may use date
handling code internally and therefore be potentially susceptible to the
Year 2000 problem, but this is unlikely.

You may also wish to read the ``Does Perl have a Year 2000 problem?'' section
of the Perl FAQ at:

    http://www.perl.com/CPAN/doc/FAQs/FAQ/PerlFAQ.html

=head1 Support and Training

The Perl5 Database Interface is I<FREE> software. IT COMES WITHOUT WARRANTY
OF ANY KIND. See the DBI README for more details.

However, some organizations are providing either technical support or
training programs on DBI. The present author has no knowledge as
to the quality of these services. The links are included for reference
purposes only and should not be regarded as recommendations in any way.
I<Caveat emptor>.

=head2 Commercial Support

=over 4

=item The Perl Clinic

The Perl Clinic provides commercial support for I<Perl> and Perl
related problems, including the I<DBI> and its drivers.  Support is
provided by the company with whom Tim Bunce, author of I<DBI> and
I<DBD::Oracle>, works and ActiveState. For more information on their
services, please see:

    http://www.perlclinic.com

=back

=head2 Training

=over 4

=item Westlake Solutions

A hands-on class for experienced Perl CGI developers that teaches
how to write database-connected CGI scripts using Perl and DBI.pm.  This
course, along with four other courses on CGI scripting with Perl, is
taught in Washington, DC; Arlington, Virginia; and on-site worldwide upon
request.

See:

    http://www.westlake.com/training

for more details.

=back

=head1 Other References

In this section, we present some miscellaneous WWW links that may be of
some interest to DBI users. These are not verified and may result in
unknown sites or missing documents.

    http://www-ccs.cs.umass.edu/db.html
    http://www.odmg.org/odmg93/updates_dbarry.html
    http://www.jcc.com/sql_stnd.html

=head1 AUTHOR

Alligator Descartes <I<http://www.symbolstone.org/descarte/contact.html>>. 
Portions are Copyright their original stated authors.

=head1 COPYRIGHT

This document is Copyright (c)1994-2000 Alligator Descartes, with portions
Copyright (c)1994-2000 their original authors. This module is released under
the 'Artistic' license which you can find in the perl distribution.

This document is Copyright (c)1997-2000 Alligator Descartes. All rights reserved.
Permission to distribute this document, in full or in part, via email,
Usenet, ftp archives or http is granted providing that no charges are involved,
reasonable attempt is made to use the most current version and all credits
and copyright notices are retained ( the I<AUTHOR> and I<COPYRIGHT> sections ).
Requests for other distribution rights, including incorporation into 
commercial products, such as books, magazine articles or CD-ROMs should be
made to Alligator Descartes <I<http://www.symbolstone.org/descarte/contact.html>>.

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