Provided by: libmldbm-sync-perl_0.30-4_all 
NAME
MLDBM::Sync - safe concurrent access to MLDBM databases
SYNOPSIS
use MLDBM::Sync; # this gets the default, SDBM_File
use MLDBM qw(DB_File Storable); # use Storable for serializing
use MLDBM qw(MLDBM::Sync::SDBM_File); # use extended SDBM_File, handles values > 1024 bytes
use Fcntl qw(:DEFAULT); # import symbols O_CREAT & O_RDWR for use with DBMs
# NORMAL PROTECTED read/write with implicit locks per i/o request
my $sync_dbm_obj = tie %cache, 'MLDBM::Sync' [..other DBM args..] or die $!;
$cache{"AAAA"} = "BBBB";
my $value = $cache{"AAAA"};
# SERIALIZED PROTECTED read/write with explicit lock for both i/o requests
my $sync_dbm_obj = tie %cache, 'MLDBM::Sync', '/tmp/syncdbm', O_CREAT|O_RDWR, 0640;
$sync_dbm_obj->Lock;
$cache{"AAAA"} = "BBBB";
my $value = $cache{"AAAA"};
$sync_dbm_obj->UnLock;
# SERIALIZED PROTECTED READ access with explicit read lock for both reads
$sync_dbm_obj->ReadLock;
my @keys = keys %cache;
my $value = $cache{'AAAA'};
$sync_dbm_obj->UnLock;
# MEMORY CACHE LAYER with Tie::Cache
$sync_dbm_obj->SyncCacheSize('100K');
# KEY CHECKSUMS, for lookups on MD5 checksums on large keys
my $sync_dbm_obj = tie %cache, 'MLDBM::Sync', '/tmp/syncdbm', O_CREAT|O_RDWR, 0640;
$sync_dbm_obj->SyncKeysChecksum(1);
my $large_key = "KEY" x 10000;
$sync{$large_key} = "LARGE";
my $value = $sync{$large_key};
DESCRIPTION
This module wraps around the MLDBM interface, by handling concurrent access to MLDBM
databases with file locking, and flushes i/o explicity per lock/unlock. The new
[Read]Lock()/UnLock() API can be used to serialize requests logically and improve
performance for bundled reads & writes.
my $sync_dbm_obj = tie %cache, 'MLDBM::Sync', '/tmp/syncdbm', O_CREAT|O_RDWR, 0640;
# Write locked critical section
$sync_dbm_obj->Lock;
... all accesses to DBM LOCK_EX protected, and go to same tied file handles
$cache{'KEY'} = 'VALUE';
$sync_dbm_obj->UnLock;
# Read locked critical section
$sync_dbm_obj->ReadLock;
... all read accesses to DBM LOCK_SH protected, and go to same tied files
... WARNING, cannot write to DBM in ReadLock() section, will die()
... WARNING, my $v = $cache{'KEY'}{'SUBKEY'} will trigger a write so not safe
... to use in ReadLock() section
my $value = $cache{'KEY'};
$sync_dbm_obj->UnLock;
# Normal access OK too, without explicity locking
$cache{'KEY'} = 'VALUE';
my $value = $cache{'KEY'};
MLDBM continues to serve as the underlying OO layer that serializes complex data
structures to be stored in the databases. See the MLDBM BUGS section for important
limitations.
MLDBM::Sync also provides built in RAM caching with Tie::Cache md5 key checksum
functionality.
INSTALL
Like any other CPAN module, either use CPAN.pm, or perl -MCPAN "-e" shell, or get the file
MLDBM-Sync-x.xx.tar.gz, unzip, untar and:
perl Makefile.PL
make
make test
make install
LOCKING
The MLDBM::Sync wrapper protects MLDBM databases by locking and unlocking around read and
write requests to the databases. Also necessary is for each new lock to tie() to the
database internally, untie()ing when unlocking. This flushes any i/o for the dbm to the
operating system, and allows for concurrent read/write access to the databases.
Without any extra effort from the developer, an existing MLDBM database will benefit from
MLDBM::sync.
my $dbm_obj = tie %dbm, ...;
$dbm{"key"} = "value";
As a write or STORE operation, the above will automatically cause the following:
$dbm_obj->Lock; # also ties
$dbm{"key"} = "value";
$dbm_obj->UnLock; # also unties
Just so, a read or FETCH operation like:
my $value = $dbm{"key"};
will really trigger:
$dbm_obj->ReadLock; # also ties
my $value = $dbm{"key"};
$dbm_obj->Lock; # also unties
However, these lock operations are expensive because of the underlying tie()/untie() that
occurs for i/o flushing, so when bundling reads & writes, a developer may explicitly use
this API for greater performance:
# tie once to database, write 100 times
$dbm_obj->Lock;
for (1..100) {
$dbm{$_} = $_ * 100;
...
}
$dbm_obj->UnLock;
# only tie once to database, and read 100 times
$dbm_obj->ReadLock;
for(1..100) {
my $value = $dbm{$_};
...
}
$dbm_obj->UnLock;
CACHING
I built MLDBM::Sync to serve as a fast and robust caching layer for use in multi-process
environments like mod_perl. In order to provide an additional speed boost when caching
static data, I have added an RAM caching layer with Tie::Cache, which regulates the size
of the memory used with its MaxBytes setting.
To activate this caching, just:
my $dbm = tie %cache, 'MLDBM::Sync', '/tmp/syncdbm', O_CREAT|O_RDWR, 0640;
$dbm->SyncCacheSize(100000); # 100000 bytes max memory used
$dbm->SyncCacheSize('100K'); # 100 Kbytes max memory used
$dbm->SyncCacheSize('1M'); # 1 Megabyte max memory used
The ./bench/bench_sync.pl, run like "bench_sync.pl "-c"" will run the tests with caching
turned on creating a benchmark with 50% cache hits.
One run without caching was:
=== INSERT OF 50 BYTE RECORDS ===
Time for 100 writes + 100 reads for SDBM_File 0.16 seconds 12288 bytes
Time for 100 writes + 100 reads for MLDBM::Sync::SDBM_File 0.17 seconds 12288 bytes
Time for 100 writes + 100 reads for GDBM_File 3.37 seconds 17980 bytes
Time for 100 writes + 100 reads for DB_File 4.45 seconds 20480 bytes
And with caching, with 50% cache hits:
=== INSERT OF 50 BYTE RECORDS ===
Time for 100 writes + 100 reads for SDBM_File 0.11 seconds 12288 bytes
Time for 100 writes + 100 reads for MLDBM::Sync::SDBM_File 0.11 seconds 12288 bytes
Time for 100 writes + 100 reads for GDBM_File 2.49 seconds 17980 bytes
Time for 100 writes + 100 reads for DB_File 2.55 seconds 20480 bytes
Even for SDBM_File, this speedup is near 33%.
KEYS CHECKSUM
A common operation on database lookups is checksumming the key, prior to the lookup,
because the key could be very large, and all one really wants is the data it maps too. To
enable this functionality automatically with MLDBM::Sync, just:
my $sync_dbm_obj = tie %cache, 'MLDBM::Sync', '/tmp/syncdbm', O_CREAT|O_RDWR, 0640;
$sync_dbm_obj->SyncKeysChecksum(1);
!! WARNING: keys() & each() do not work on these databases
!! as of v.03, so the developer will not be fooled into thinking
!! the stored key values are meaningful to the calling application
!! and will die() if called.
!!
!! This behavior could be relaxed in the future.
An example of this might be to cache a XSLT conversion, which are typically very
expensive. You have the XML data and the XSLT data, so all you do is:
# $xml_data, $xsl_data are strings
my $xslt_output;
unless ($xslt_output = $cache{$xml_data.'&&&&'.$xsl_data}) {
... do XSLT conversion here for $xslt_output ...
$cache{$xml_data.'&&&&'.xsl_data} = $xslt_output;
}
What you save by doing this is having to create HUGE keys to lookup on, which no DBM is
likely to do efficiently. This is the same method that File::Cache uses internally to
hash its file lookups in its directories.
New MLDBM::Sync::SDBM_File
SDBM_File, the default used for MLDBM and therefore MLDBM::Sync has a limit of 1024 bytes
for the size of a record.
SDBM_File is also an order of magnitude faster for small records to use with MLDBM::Sync,
than DB_File or GDBM_File, because the tie()/untie() to the dbm is much faster.
Therefore, bundled with MLDBM::Sync release is a MLDBM::Sync::SDBM_File layer which works
around this 1024 byte limit. To use, just:
use MLDBM qw(MLDBM::Sync::SDBM_File);
It works by breaking up up the STORE() values into small 128 byte segments, and spreading
those segments across many records, creating a virtual record layer. It also uses
Compress::Zlib to compress STORED data, reducing the number of these 128 byte records. In
benchmarks, 128 byte record segments seemed to be a sweet spot for space/time efficiency,
as SDBM_File created very bloated *.pag files for 128+ byte records.
BENCHMARKS
In the distribution ./bench directory is a bench_sync.pl script that can benchmark using
the various DBMs with MLDBM::Sync.
The MLDBM::Sync::SDBM_File DBM is special because is uses SDBM_File for fast small
inserts, but slows down linearly with the size of the data being inserted and read.
The results for a dual PIII-450 linux 2.4.7, with a ext3 file system blocksize 4096
mounted async on a RAID-1 2xIDE 7200 RPM disk were as follows:
=== INSERT OF 50 BYTE RECORDS ===
Time for 100 writes + 100 reads for SDBM_File 0.16 seconds 12288 bytes
Time for 100 writes + 100 reads for MLDBM::Sync::SDBM_File 0.19 seconds 12288 bytes
Time for 100 writes + 100 reads for GDBM_File 1.09 seconds 18066 bytes
Time for 100 writes + 100 reads for DB_File 0.67 seconds 12288 bytes
Time for 100 writes + 100 reads for Tie::TextDir .04 0.31 seconds 13192 bytes
=== INSERT OF 500 BYTE RECORDS ===
(skipping test for SDBM_File 100 byte limit)
Time for 100 writes + 100 reads for MLDBM::Sync::SDBM_File 0.52 seconds 110592 bytes
Time for 100 writes + 100 reads for GDBM_File 1.20 seconds 63472 bytes
Time for 100 writes + 100 reads for DB_File 0.66 seconds 86016 bytes
Time for 100 writes + 100 reads for Tie::TextDir .04 0.32 seconds 58192 bytes
=== INSERT OF 5000 BYTE RECORDS ===
(skipping test for SDBM_File 100 byte limit)
Time for 100 writes + 100 reads for MLDBM::Sync::SDBM_File 1.41 seconds 1163264 bytes
Time for 100 writes + 100 reads for GDBM_File 1.38 seconds 832400 bytes
Time for 100 writes + 100 reads for DB_File 1.21 seconds 831488 bytes
Time for 100 writes + 100 reads for Tie::TextDir .04 0.58 seconds 508192 bytes
=== INSERT OF 20000 BYTE RECORDS ===
(skipping test for SDBM_File 100 byte limit)
(skipping test for MLDBM::Sync db size > 1M)
Time for 100 writes + 100 reads for GDBM_File 2.23 seconds 2063912 bytes
Time for 100 writes + 100 reads for DB_File 1.89 seconds 2060288 bytes
Time for 100 writes + 100 reads for Tie::TextDir .04 1.26 seconds 2008192 bytes
=== INSERT OF 50000 BYTE RECORDS ===
(skipping test for SDBM_File 100 byte limit)
(skipping test for MLDBM::Sync db size > 1M)
Time for 100 writes + 100 reads for GDBM_File 3.66 seconds 5337944 bytes
Time for 100 writes + 100 reads for DB_File 3.64 seconds 5337088 bytes
Time for 100 writes + 100 reads for Tie::TextDir .04 2.80 seconds 5008192 bytes
AUTHORS
Copyright (c) 2001-2002 Joshua Chamas, Chamas Enterprises Inc. All rights reserved.
Sponsored by development on NodeWorks http://www.nodeworks.com and Apache::ASP
http://www.apache-asp.org
This program is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.
SEE ALSO
MLDBM(3), SDBM_File(3), DB_File(3), GDBM_File(3)