NAME

Hash::SharedMem::Handle - handle for efficient shared mutable hash

SYNOPSIS

    use Hash::SharedMem::Handle;

    if(Hash::SharedMem::Handle->referential_handle) { ...

    $shash = Hash::SharedMem::Handle->open($filename, "rwc");

    if($shash->is_readable) { ...
    if($shash->is_writable) { ...
    $mode = $shash->mode;

    if($shash->exists($key)) { ...
    $length = $shash->length($key);
    $value = $shash->get($key);
    $shash->set($key, $newvalue);
    $oldvalue = $shash->gset($key, $newvalue);
    if($shash->cset($key, $chkvalue, $newvalue)) { ...

    if($shash->occupied) { ...
    $count = $shash->count;
    $size = $shash->size;
    $key = $shash->key_min;
    $key = $shash->key_max;
    $key = $shash->key_ge($key);
    $key = $shash->key_gt($key);
    $key = $shash->key_le($key);
    $key = $shash->key_lt($key);
    $keys = $shash->keys_array;
    $keys = $shash->keys_hash;
    $group = $shash->group_get_hash;

    $snap_shash = $shash->snapshot;
    if($shash->is_snapshot) { ...

    $shash->idle;
    $shash->tidy;

    $tally = $shash->tally_get;
    $shash->tally_zero;
    $tally = $shash->tally_gzero;

    tie %shash, "Hash::SharedMem::Handle", $shash;
    tie %shash, "Hash::SharedMem::Handle", $filename, "rwc";

    $shash = tied(%shash);
    if(exists($shash{$key})) { ...
    $value = $shash{$key};
    $shash{$key} = $newvalue;
    $oldvalue = delete($shash{$key});

DESCRIPTION

An object of this class is a handle referring to a memory-mapped shared hash object of the kind described in Hash::SharedMem. It can be passed to the functions of that module, or the same operations can be performed by calling the methods described below. Uses of the function and method interfaces may be intermixed arbitrarily; they are completely equivalent in function. They are not equivalent in performance, however, with the method interface being somewhat slower.

This class also supplies a tied-hash interface to shared hashes. The tied interface is much slower than the function and method interfaces. The behaviour of a tied hash more resembles the function and method interfaces to shared hashes than it resembles the syntactically-similar use of ordinary Perl hashes. Using a non-string as a key will result in an exception, rather than stringification of the key. Using a string containing a non-octet codepoint as a key will also result in an exception, rather than merely referring to an absent hash element.

CLASS METHODS

Hash::SharedMem::Handle->referential_handle

Returns a truth value indicating whether each shared hash handle contains a first-class reference to the shared hash to which it refers. See "Filesystem referential integrity" in Hash::SharedMem for discussion of the significance of this.

CONSTRUCTOR

Hash::SharedMem::Handle->open(FILENAME, MODE)

Opens and returns a handle referring to a shared hash object, or dies if the shared hash can't be opened as specified. See "shash_open" in Hash::SharedMem for details.

METHODS

$shash->is_readable

$shash->is_writable

$shash->mode

$shash->exists(KEY)

$shash->getd(KEY)

$shash->length(KEY)

$shash->get(KEY)

$shash->set(KEY, NEWVALUE)

$shash->gset(KEY, NEWVALUE)

$shash->cset(KEY, CHKVALUE, NEWVALUE)

$shash->occupied

$shash->count

$shash->size

$shash->key_min

$shash->key_max

$shash->key_ge(KEY)

$shash->key_gt(KEY)

$shash->key_le(KEY)

$shash->key_lt(KEY)

$shash->keys_array

$shash->keys_hash

$shash->group_get_hash

$shash->snapshot

$shash->is_snapshot

$shash->idle

$shash->tidy

$shash->tally_get

$shash->tally_zero

$shash->tally_gzero

These methods are each equivalent to the corresponding "shash_"-prefixed function in Hash::SharedMem. See that document for details.

TIE CONSTRUCTORS

tie(VARIABLE, "Hash::SharedMem::Handle", SHASH)

VARIABLE must be a hash variable, and SHASH must be a handle referring to a shared hash object. The call binds the variable to the shared hash, so that the variable provides a view of the shared hash that resembles an ordinary Perl hash. The shared hash handle is returned.

tie(VARIABLE, "Hash::SharedMem::Handle", FILENAME, MODE)

VARIABLE must be a hash variable. The call opens a handle referring to a shared hash object, as described in "shash_open" in Hash::SharedMem, and binds the variable to the shared hash, so that the variable provides a view of the shared hash that resembles an ordinary Perl hash. The shared hash handle is returned.

TIED OPERATORS

For all of these operators, the key of interest (KEY parameter) and values can each be any octet (Latin-1) string. Strings containing non-octets (Unicode characters above U+FF) and items other than strings cannot be used as keys or values. If a dualvar (scalar with independent string and numeric values) is supplied, only its string value will be used.

tied(%SHASH)

Returns the handle via which %SHASH is bound to the shared hash. This is a shared hash handle that can be used by calling the methods described above or by passing it to the functions of Hash::SharedMem.

exists($SHASH{KEY})

Returns a truth value indicating whether the specified key is currently present in the shared hash.

$SHASH{KEY}

Returns the value currently referenced by the specified key in the shared hash, or undef if the key is absent.

$SHASH{KEY} = NEWVALUE

Modifies the shared hash so that the specified key henceforth references the specified value. The new value must be a string.

delete($SHASH{KEY})

Modifies the shared hash so that the specified key is henceforth absent, and returns the value that the key previously referenced, or undef if the key was already absent. This swap is performed atomically.

scalar(%SHASH)

From Perl 5.25.3 onwards, returns the number of items that are currently in the shared hash. This matches the behaviour of untied hashes on these Perl versions. Prior to Perl 5.25.3, from Perl 5.8.3 onwards, returns a truth value indicating whether there are currently any items in the shared hash. Does not supply any additional information corresponding to the hash bucket usage information that untied hashes supply in this situation. Prior to Perl 5.8.3, returns a meaningless value, due to a limitation of the tying system.

If the hash is evaluated in a truth value context, with the expectation of this testing whether the shared hash is occupied, there is a performance concern. Prior to Perl 5.25.3 only the truth value would be determined, quite cheaply. From Perl 5.25.3 onwards, a more expensive operation is performed, counting all the keys. If this is a problem, one can evaluate tied(%SHASH)->occupied to explicitly invoke the truth-value-only operation. However, if performance is a concern then the tied interface is best entirely avoided.

scalar(keys(%SHASH))

scalar(values(%SHASH))

Returns the number of items that are currently in the shared hash.

Due to a limitation of the tying system, the item count is not extracted atomically, but is derived by means equivalent to a loop using each. If the set of keys in the shared hash changes during this process, the count of keys visited (which is what is actually returned) does not necessarily match any state that the shared hash has ever been in.

each(%SHASH)

Iterates over the shared hash. On each call, returns either the next key (in scalar context) or the next key and the value that it references (in list context). The iterator state, preserved between calls, is attached to %SHASH.

The iteration process always visits the keys in lexicographical order. Unlike iteration of untied hashes, it is safe to make any changes at all to the shared hash content between calls to each. Subsequent calls see the new content, and the iteration process resumes with whatever key (in the new content) follows the key most recently visited (from the old content).

When using each in list context, the fetching of the next key and its corresponding value is not an atomic operation, due to a limitation of the tying system. The key and value are fetched as two separate operations (each one individually atomic), and it is possible for the shared hash content to change between them. This is noticeable if the key that was fetched gets deleted before the value is fetched: it will appear that the value is undef, which is not a permitted value in a shared hash.

keys(%SHASH)

values(%SHASH)

%SHASH

Enumerates the shared hash's content (keys alone, values alone, or keys with values), and as a side effect resets the iterator state used by each. Always returns the content in lexicographical order of key.

Due to a limitation of the tying system, the content is not extracted atomically, and so the content returned as a whole does not necessarily match any state that the shared hash has ever been in. The content is extracted by means equivalent to a loop using each, and the inconsistencies that may be seen follow therefrom.

%SHASH = LIST

Setting the entire content of the shared hash (throwing away the previous content) is not supported.

BUGS

Due to details of the Perl implementation, this object-oriented interface to the shared hash mechanism is somewhat slower than the function interface, and the tied interface is much slower. The functions in Hash::SharedMem are the recommended interface.

Limitations of the tying system mean that whole-hash operations (including iteration and enumeration) performed on shared hashes via the tied interface are not as atomic as they appear. If it is necessary to see a consistent state of a shared hash, one must create and use a snapshot handle. A snapshot may be iterated over or enumerated at leisure via any of the interfaces.

SEE ALSO

Hash::SharedMem

AUTHOR

Andrew Main (Zefram) <zefram@fysh.org>

COPYRIGHT

Copyright (C) 2014, 2015 PhotoBox Ltd

Copyright (C) 2014, 2015, 2017 Andrew Main (Zefram) <zefram@fysh.org>

LICENSE

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