Transcript Data::Dumper - Monash University

Topic 8: Databases
CSE2395/CSE3395
Perl Programming
Llama3 chapter 16, pages 221-224
Camel3 pages 363-398
Programming the Perl DBI chapters 2-5, pages 7-135
perlfunc, perlmod, perlmodlib, Data::Dumper, perltie,
DB_File, DBI manpages
In this topic
 Modules
►
►
►
►
use Module;
Perl standard library
CPAN
writing modules
 Databases
►
►
►
persistent data
Data::Dumper
DBM
– databases in files
►
pack and unpack
– flat-file databases
►
DBI
– interfacing with SQL databases
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Modules
 Much of Perl’s functionality is contained in
optional modules
 A module is a file which contains re-usable Perl
code for a particular task
Your Perl
code
Module
imported
module
Your code is importing;
module is exporting
Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
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Modules
 Modules can be
►
found in the Perl Standard Library
– supplied with Perl and already installed
►
downloaded from CPAN
– tar xzf Module.tar.gz
– cd Module
– perl Makefile.PL
– make
– make install
►
written by you
– using package keyword and saving file as Module.pm
Camel3 pages 299-307, 831-915
perlmod, perlmodlib, perlfunc manpages
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Using a module
 Import a module into your code with use
keyword at top of your program
 use Data::Dumper;
 use CGI ":all";
 use Dog qw(bark bite);
 Read module documentation to see how to
access module’s functionality
►
►
perldoc Data::Dumper
perldoc CGI
Llama3 pages 282-283
Camel3 pages 299-300, 822-823
perlfunc manpage
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Writing a module
 To create a module called Dog:
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start file with package Dog;
– package keyword ensures that Dog’s variables and functions don’t
interfere with anyone else’s
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►
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use Exporter;
@ISA = qw(Exporter);
set @EXPORT_OK to a list of function names (bark, bite)
– these two lines allow Perl to put Dog’s functions into code that says
use Dog ("bark", "bite");
►
►
write the functions for Dog (bark, bite)
end file with a nonzero expression
– needed or else use Dog will fail
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save file as Dog.pm in current directory
– or any directory named in special Perl @INC array
Camel3 pages 301-305
perlmod, perlfunc manpages
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Timeout
package DayOfWeek;
use Exporter; @ISA = qw(Exporter);
@EXPORT_OK = "weekday"; # Allow import of &weekday.
# weekday(y,m,d): Return weekday of date (0 = Sun).
sub weekday
{
my ($y, $m, $d) = @_;
# Uses array references (topic 10).
( [[6,2,2,5,0,3,5,1,4,6,2,4],
[6,2,3,6,1,4,6,2,5,0,3,5]] -> [
((! ($y % 400) || ($y % 100)) && (! ($y % 4)))
|| 0 ][ $m-1 ] + $d + $y + int(($y-1)/4)
- int(($y-1)/100) + int(($y-1)/400)
) % 7;
}
1; # Indicate successful definition of module.
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Timeout
# Import the weekday function from DayOfWeek.pm
use DayOfWeek qw(weekday);
# Names of days of week.
@days = qw(Sunday Monday Tuesday
Wednesday Thursday Friday Saturday);
# Check usage.
die "Usage: $0 year month day\n"
if @ARGV != 3;
$daynum = weekday(@ARGV[0..2]);
print "That day is $days[$daynum]\n";
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Databases
 A database
contains organized data
► contains relations between data
► is persistent
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 A hash
contains organized data
► has a key/value relationship
► ceases to exist at end of scope
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– never beyond end of program
 A persistent hash would be great
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but how?
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Persistent data
 To make a variable persistent, need a way to make it last
between invocations of program
►
by saving its state in a file
 Write-back (“freeze/thaw”, serialization) strategy
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►
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dump the contents of variable to file at program exit (“freeze”)
initialize variable with dumped contents at program entry (“thaw”)
file is up-to-date only when program is not running
for example: Data::Dumper, Storable
 Write-through (“tie”) strategy
►
►
►
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update file every time the variable is changed
requires help from Perl to tie the variable to the file
file is always up-to-date
for example: DBM files
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Data::Dumper
 Perl Standard Library module
►
use Data::Dumper;
 Example of write-back (freeze/thaw) strategy
 Writes Perl code to file
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when later evaluated by Perl, data is reconstructed
 To dump (freeze) a variable %hash:
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open the persistent file for writing
Data::Dumper->Dump([\%hash], [qw(*hash)]);
– [...] is array reference, see topic 10.
 To restore (thaw) from a file file:
►
►
do "file";
Perl evaluates the statements in file, reconstructing the
variable %hash
Camel3 pages 702, 882
Data::Dumper, perlfunc manpages
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Timeout
# Freezing and thawing some variables.
use Data::Dumper;
# Thaw saved values, if they exist.
if (-f "storage") {
do "storage"; # Restore previous values.
} else {
$important = 0; %valuable = (); # First run.
}
# ... do stuff with $important and %valuable ...
# Freeze values for next time before program exit.
open STORAGE, ">storage";
print STORAGE Data::Dumper->Dump(
[$important, \%valuable],
[qw(important *valuable)]);
Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
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DBM files
 DBM files are a simple database format common
to Unix
►
also available on other platforms
 Data stored in a binary file
►
usually with a sophisticated data structure like a Btree
 Roughly equivalent to a hash
can look up value given key
► keys are strings, values are strings
► may be arbitrary limits on data size
►
Llama3 pages 221-222
Camel3 pages 696-697, 883-884, 985
perlmod manpage
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
DBM files
 Perl provides a simple interface to DBM files
using dbmopen or tie functions
► connects a hash to the DBM file
► accessing the hash transparently accesses the file
instead
►
– all hash operations supported
►
example of write-through (tie) strategy
 To tie a hash %hash to a file file:
►
either dbmopen %hash, "file", 0600
– mode (0600) only used if file needs to be created
►
or tie %hash, "DB_file", "file"
Llama3 pages 222-224
Camel3 pages 363-398, 697-698
perltie, perlfunc manpages
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Timeout
# Use DBM to track mail aliases.
# Use Berkeley-DB format for DBM file.
# This is the most flexible one.
use DB_File;
dbmopen %alias, "aliases", 0600
or die "Can't tie to DBM file: $!\n";
# Define functions that modify file
# through %alias hash.
sub get_alias { return $alias{$_[0]}; }
sub set_alias { $alias{$_[0]} = $_[1]; }
sub delete_alias { delete $alias{$_[0]}; }
sub exists_alias { return exists $alias{$_[0]}; }
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
pack and unpack
 Files store only strings
 If data other than strings are to be stored, need to
encode into strings before storing
►
and decode from strings when fetching
 To encode data into a string, use pack
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$string = pack "a10 l", $word, $int;
 To decode data from a string, use unpack
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($word, $int) = unpack "a10 l", $string;
 Format string ("a10 l") specifies how to pack or
unpack
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dozens of different specifiers
Llama3 page 224
Camel3 pages 757-762, 819-821
perlfunc manpage
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Random-access files
 Can store fixed-length records in a file
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e.g., 30 bytes per record
– 20 for name, 8 for ID number, 1 for age, 1 for mark
bytes 0 to 29 for first record
► bytes 30 to 59 for second record
► etc.
►
record 1
record 2
record 3
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Random-access files
 Open file for both reading and writing
 To modify or create record
create record with pack
► seek to position in file with seek
► write packed record to file with print
►
 To read record
seek to position in file with seek
► read appropriate number of bytes with read
► extract fields from string with unpack
►
Llama3 pages 225-227
Camel3 pages 769, 780
perlfunc manpage
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Timeout
open FILE, "+< database"; # Open for read/write.
# write_record(recno, name, id, age, mark)
sub write_record {
my $recno = shift;
my $string = pack "a20 a8 c c", @_;
seek FILE, ($recno-1)*30, 0;
print FILE $string;
}
# (name, id, age, mark) = read_record(recno)
sub read_record {
my $recno = shift;
seek FILE, ($recno-1)*30, 0;
read FILE, $string, 30;
return unpack "a20 a8 c c", $string;
}
Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
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Client/server databases
 Sometimes necessary to interact with an existing
database
►
database is kept on a server
– e.g., Oracle, MySQL, Microsoft Access (ODBC), etc.
– server handles mundane aspects like storage, access control,
file/record locking, etc.
►
your program is client
– makes requests to server on database’s behalf
 Need to interact with server using its interface
►
usually requires making a network connection
 Commonly done with SQL
►
►
Structured Query Language, a standard way of phrasing queries
and database transformations
SQL is not examinable in this subject!
 But
SQL dialects vary slightly
► network protocols and interfaces vary
Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
►
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DBI
 Perl provides a unified interface for all database
servers
►
►
called Database Interface, or DBI
use DBI;
 Client uses DBI to access database
DBI speaks to database through a system-specific
database driver (DBD)
► DBD translates client’s SQL request for specific
server dialects
► DBD massages server responses into standard form
for client
►
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
DBI
DBD::Oracle
(driver)
Your Perl
program
Oracle
database
Oracle
database
DBI
(interface)
DBD::CSV
(driver)
CSV
database
DBI system
Always one
instance of DBI
One driver per type
of database
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
DBI handles
 DBI system uses handles to manage
connections to database system
►
database handle
– one per database
– used to perform instantaneous actions and to initiate longer
statements
►
statement handle
– one per SQL statement
– usually used to perform lengthy SELECT queries
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Connecting to a database
 Need to know database’s data source name
Always
DBI
Database driver
DBI:mSQL:frotz.org:base:4910
Database identifier; format differs
for each driver (here: hostname,
database name, port number
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Timeout
use DBI;
# Connect to the database.
# OO: class->method() notation is how to call
# a class method (in this case, a constructor).
my $db_handle = DBI->connect(
"DBI:Ingres:data", $username, $passwd,
{ PrintError => 1 } # Hash ref: topic 10
) or die "Can't connect: $DBI::errstr\n";
# ... do stuff ...
# Now disconnect.
# OO: $object->method() notation is how to call
# an object method (here, a destructor).
$db_handle->disconnect()
or warn "Disconnection error: $DBI::errstr\n";
Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
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Executing DBI statements
 Actions on database are performed through database
handle
 Simple (atomic) statements can be performed using do
method
►
►
$db_handle->do("DELETE FROM ...");
return value indicates success or failure
 Complex statements, which may produce several results
in sequence, require a statement handle
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►
$st_handle = $db_handle->prepare("SELECT...");
statement handle used to iterate over results
– @fields = $st_handle->fetchrow_array()
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Timeout
# Insert a name into a phone database
# usage: thisprogram name number
use DBI;
# ... connect to the database ...
# quote() method correctly escapes strings
# with metacharacters in them.
$db_handle->do("
INSERT INTO phonedir (name, phone_number)
VALUES ( " . $db_handle->quote($ARGV[0]) .
", " . $db_handle->quote($ARGV[1]) .
")"
) or warn "Insert error: $DBI::errstr\n";
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Timeout
# Extract entries from the phone database
# usage: thisprogram name
# ... connect to the database ...
$st_handle = $db_handle->prepare("
SELECT name, phone_number
FROM phonedir
WHERE name LIKE " . $db_handle->quote($ARGV[0])
);
$st_handle->execute();
# Iterate over the data returned by the query.
while (($name, $num) = $st_handle->fetchrow_array())
{
print "$name: $num\n";
}
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Covered in this topic
 Modules
►
►
►
►
use Module;
Perl standard library
CPAN
writing modules
 Databases
►
►
►
persistent data
Data::Dumper
DBM
– databases in files
►
pack and unpack
– flat-file databases
►
DBI
– interfacing with SQL databases
29
Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Going further
 Tying
adding new behaviour to built-in-types
► perldoc -f tie
►
 Transactions
protecting databases from corruption
► supported by some DBD modules
►
 Multi-level DBM files
storing nested data structures in a DBM file
► MLDBM module
►
 ODBC
Microsoft’s database standard
► DBI::ODBC and Win32::ODBC modules
►
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University
Next topic
 The World Wide Web
 Writing a Perl web client
►
LWP module
 Dynamic web pages
►
Common Gateway Interface (CGI)
LWP, CGI manpages
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Original Slides by Debbie Pickett, Modified by David Abramson, 2006, Copyright Monash University