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

Provided by: libhash-sharedmem-perl_0.005-1_amd64

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 "die"s 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.

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.