/ 
Tie-File-FixedRecLen-2.112531
River stage zero No dependents
/ Tie::File::FixedRecLen

NAME

Tie::File::FixedRecLen - Fixed Length Record support for Tie:File

VERSION

version 2.112531

SYNOPSIS

# for typical read/write random access...

use Tie::File::FixedRecLen;

tie @array, 'Tie::File::FixedRecLen', $file, record_length => 20
    or die ...;

# or for faster, sequential write-only use...

use Tie::File::FixedRecLen::Store;

tie @array, 'Tie::File::FixedRecLen::Store', $file, record_length => 20
    or die ...;

DESCRIPTION

Use Tie::File::FixedRecLen as a drop-in replacement to Tie::File in order to add support for fixed length records within your tied files. When tieing to a file, you must specify the length of a record in the file. This length does not include the record separator character(s).

Apart from the configuration parameters mentioned below, you should use Tie::File::FixedRecLen in just the same way as Tie::File. This module is designed to create files which are read/write compatible with Tie::File;

Please take just a minute to read the "CAVEATS" section, below.

There is an ancilliary module, Tie::File::FixedRecLen::Store, which provides a subset of the features of Tie::File::FixedRecLen. It is designed for fast, write-only, sequential data logging. More information is given in the "STORE MODULE" section, below.

CAVEATS

  • Tie::File::FixedRecLen is written for Tie::File 0.97, and cannot be used with any other version of that module. This is because there is no formlized API into Tie::File, so it's quite likely things will break as Tie::File's internals are changed. Sorry about that.

  • Do not try using cacheing or deferred writing, at least not yet. Tie::File is quite a complicated beast, so to make life simpler for Tie::File::FixedRecLen it does not try to cope with cacheing or deferring.

  • In Tie::File you could include the record separator character(s) within a record, and although the module might get confused, the file would still be valid. In Tie::File::FixedRecLen this is a really bad thing to do, so please don't. Indeed, trailing multiple record separator character(s) on a field will be (sliently) stripped and replaced by a single record separator.

  • Anyone with multi-byte character set experience is very welcome to lend support in making this module work in those environments. Currently my best guess is that things will break if this module is used with multi-byte character set files.

CONFIGURATION

There are three configuration parameters you can pass when tieing to a file (in addition to those offered by Tie::File). This module does not support the fancy - prefix to option names that you have with Tie::File.

record_length

This parameter is required. It specifies the length (in bytes) of a record in the tied file. record_length must be an integer, and it must be greater than zero. Each time a record is read or written, it is compared to this length, and an error is raised if there is a mismatch.

When writing records to the tied file, they are padded out to record_length if necessary. Be aware that this length does not include the record separator.

pad_char

This parameter is optional.

Records will be padded with this character until they are record_length bytes in length. You should make this a single byte character, otherwise things are likely to break.

The default padding character is the space character. This allows the tied file to remain readable by a human. If you use leading or trailing space characters in your records, then select another character, and if you are not bothered about human readability, it could be a control character (e.g. ^G).

pad_dir

This parameter is optional.

Records may be padded out to the record length either before the first character or after the last character.

Set this option to "right" if you would prefer end padding; the default is to pad with the pad_char character before the first character of the record data. For example with "right" padding, a record length of 10 and pad character of '.':

data: "abc123"
written record: "abc123....\n"
returned data when read back: "abc123"

And with the same settings except we'll use the module's default "left" padding this time:

data: "abc123"
written record: "....abc123\n"
returned data when read back: "abc123"

DIAGNOSTICS

Tie::File::FixedRecLen written for Tie::File 0.97

The Tie::File programmers' API is not standardized, and may change in the future. You must have version 0.97 of Tie::File to use this version of Tie::File::FixedRecLen.

Useless use of Tie::File::FixedRecLen without a record_length

You have forgotten to provide the record_length parameter when tieing your file, or it is there but is not a positive integer.

Record '...' does not match set length (...)

When reading a record from the tied file, it is not the expected record_length in size. Are you sure the file was created and written by Tie::File::FixedRecLen?

Record '...' exceeds fixed record length (...)

When attempting to write a record to the tied file, you have passed data which exceeds record_length in size. Please don't do that.

File does not appear to be using fixed length records

Internally, Tie::File and Tie::File::FixedRecLen compute offset markers for each record in the file. This error indicates the file is not a whole multiple of record_length (+ recsep's length) in size, which probably means it is not a Tie::File::FixedRecLen file.

STORE MODULE

Rationale

The project for which Tie::File::FixedRecLen was written required very fast logging of polled SNMP data, of the order of thousands of variables every couple of minutes, to a remote networked server, for a period of many years.

This requires very fast writes indeed on the storage server, so you will find Tie::File::FixedRecLen to be a lot quicker than Tie::File (for most operations), at the obvious cost of storage space. However this module still suffers in that by using the core of Tie::File, its write time is still proportional to the size of the file. There is no easy way around this. Whilst the effect is measured in the milliseconds as file size grows, it is not suitable for use over a period of years.

Hence the ancilliary module Tie::File::FixedRecLen::Store was written, for really fast writes in a file format compatible with Tie::File and Tie::File::FixedRecLen, with some compromise in functionality.

Usage

Use Tie::File::FixedRecLen for write-only, sequential storage of fixed-length record data.

Records can only be written (not read), and only at the end of an array (push), although this may be at the immediate end or at some further point and the file will be suitably padded.

The module has a very simple interface:

use Tie::File::FixedRecLen::Store;

tie @store, 'Tie::File::FixedRecLen::Store', $filename, record_length => $record_length
   or die...

Note that Tie::File::FixedRecLen::Store accepts the record_length, recsep and pad_char options just like Tie::File::FixedRecLen. However, padding in the elements is always "left" (i.e. element start) and there is currently no option to change this.

Other than that, you can use any write method on the array, for example:

push @store, 'item';
push @store, 'item1', 'item2', 'etc';
$store[10] = 'value'; # only if $#store < 10
$#store = 20; # again, only if $#store < 20

If you try to operate on the array in any other fashion, for instance to pop an element, the module will die.

ACKNOWLEDGEMENTS

  • Naturally this would not be here without the excellent Tie::File module.

  • Tie::File::VERSION check bug - Tom Hukins

  • Thanks to my wife Suzanne, for her patience whilst I whined about not being able to get the performance I wanted out of this project.

AUTHOR

Oliver Gorwits <oliver@cpan.org>

COPYRIGHT AND LICENSE

This software is copyright (c) 2011 by University of Oxford.

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