use OpenLDAPs LMDB Key-Value store as a cache backend
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README.mkdn

NAME

CHI::Driver::LMDB - use OpenLDAPs LMDB Key-Value store as a cache backend.

VERSION

version 0.002005

SYNOPSIS

use CHI;

my $cache = CHI->new(
  driver => 'LMDB',
  root_dir => 'some/path',
  namespace => "My::Project",
);

See CHI documentation for more details on usage.

ATTRIBUTES

dir_create_mode

What mode (if any) to use when creating root_dir if it does not exist.

->new(
  # Default is 775 = rwxr-xr-x
  dir_create_mode => oct 666,
);

root_dir

The prefix directory the LMDB data store will be installed to.

->new(
  root_dir => 'some/path'
)

Default is:

OSTEMPDIR/chi-driver-lmdb-$EUID

cache_size

The size in bytes for each database.

This is a convenience wrapper for "mapsize" which supports suffixes:

cache_size => 5  # 5 bytes
cache_size => 5k # 5 Kilobytes
cache_size => 5m # 5 Megabytes ( default )

This is also designed for syntax compatibility with CHI::Driver::FastMmap

single_txn

single_txn => 1

SPEED: For performance benefits, have a single transaction that lives from the creation of the CHI cache till its destruction.

However, WARNING: this flag is currently a bit dodgy, and CHI caches being kept alive till global destruction WILL trigger a SEGV, and potentially leave your cache broken.

You can avoid this by manually destroying the cache with:

undef $cache

Prior to global destruction.

db_flags

Flags to pass to OpenDB/LMDB_File->open.

See LMDB_File's constructor options for details.

use LMDB_File qw( MDB_CREATE );

db_flags => MDB_CREATE # default

tx_flags

Flags to pass to LMDB::Env->new

See LMDB::Env's constructor options for details.

Default is 0

tx_flags => 0 # no flags

put_flags

Flags to pass to ->put(k,v,WRITE_FLAGS).

See LMDB_File->put options for details.

mapsize

Passes through to LMDB::Env->new( mapsize => ... )

Default value is taken from "cache_size" with some m/k math if its set.

maxreaders

Passes through to LMDB::Env->new( maxreaders => ... )

maxdbs

Passes through to LMDB::Env->new( maxdbs => ... )

Defines how many CHI namespaces ( Databases ) a path can contain.

Default is 1024.

mode

Passes through to LMDB::Env->new( mode => ... )

Defines the permissions on created DB Objects.

Defaults to oct 600 == -rw-------

flags

Passes through to LMDB::Env->new( flags => ... )

PERFORMANCE

If write performance is a little slow for you ( due to the defaults being a single transaction per SET/GET operation, and transactions being flushed to disk when written ), there are two ways you can make performance a little speedy.

Single Transaction Mode.

If you pass single_txn => 1 the cache will be given a single transaction for the life of its existence. However, pay attention to the warnings about cleaning up properly in "single_txn".

Also, this mode is less ideal if you want to have two processes sharing a cache, because the data won't be visible on the other one till it exits! ☺

NOSYNC Mode.

You can also tell LMDB NOT to call sync at the end of every transaction, and this will greatly improve write performance due to IO being greatly delayed.

This greatly weakens the databases consistency, but that seems like a respectable compromise for a mere cache backend, where a missing record is a performance hit, not a loss of data.

use LMDB_File qw( MDB_NOSYNC MDB_NOMETASYNC );
...
my $cache = CHI->new(
  ...
  flags => MDB_NOSYNC | MDB_NOMETASYNC
);

This for me cuts down an operation that takes 30 seconds worth of writes down to 6 ☺.

In Depth

For an in-depth comparison of the performance of various options, and how that compares to CHI::Driver::FastMmap, see http://kentnl.github.io/CHI-Driver-LMDB

AUTHOR

Kent Fredric kentnl@cpan.org

COPYRIGHT AND LICENSE

This software is copyright (c) 2017 by Kent Fredric kentfredric@gmail.com.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.