Memcached::cas

(PECL memcached >= 0.1.0)

Memcached::cas — Compare and swap an item

Descrizione

public Memcached::cas(
    string|int|float $cas_token,
    string $key,
    mixed $value,
    int $expiration = 0
): bool

Memcached::cas() performs a "check and set" operation, so that the item will be stored only if no other client has updated it since it was last fetched by this client. The check is done via the cas_token parameter which is a unique 64-bit value assigned to the existing item by memcache. See the documentation for Memcached::get*() methods for how to obtain this token. Note that the token is represented as a float due to the limitations of PHP's integer space.

Elenco dei parametri

cas_token: Unique value associated with the existing item. Generated by memcache.
key: La chiave in cui salvare il valore.
value: Il valore da salvare.
expiration: La data di scadenza, il default è 0. Vedere Date di scadenza per ulteriori informazioni.

Valori restituiti

Restituisce true in caso di successo, false in caso di fallimento. The Memcached::getResultCode() will return Memcached::RES_DATA_EXISTS if the item you are trying to store has been modified since you last fetched it.

Esempi

Example #1 Memcached::cas() example

<?php
$m = new Memcached();
$m->addServer('localhost', 11211);

do {
    /* fetch IP list and its token */
    $ips = $m->get('ip_block', null, $cas);
    /* if list doesn't exist yet, create it and do
       an atomic add which will fail if someone else already added it */
    if ($m->getResultCode() == Memcached::RES_NOTFOUND) {
        $ips = array($_SERVER['REMOTE_ADDR']);
        $m->add('ip_block', $ips);
    /* otherwise, add IP to the list and store via compare-and-swap
       with the token, which will fail if someone else updated the list */
    } else { 
        $ips[] = $_SERVER['REMOTE_ADDR'];
        $m->cas($cas, 'ip_block', $ips);
    }   
} while ($m->getResultCode() != Memcached::RES_SUCCESS);

?>

Vedere anche:

Memcached::casByKey() - Compare and swap an item on a specific server

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User Contributed Notes 4 notes

down

abodera at gmail dot com ¶

14 years ago


Watch out!



When using binary protocol, the expected result after cas() is 21 (Memcached::RES_END).



For example, to make the above example #1 work with binary protocol, use the following:

<?php

$m = new Memcached();

$m->addServer('localhost', 11211);

$m->setOption(Memcached::OPT_BINARY_PROTOCOL,true)



// [...]



    } else { 

        $ips[] = $_SERVER['REMOTE_ADDR'];

        $m->cas($cas, 'ip_block', $ips);

    }   

} while ($m->getResultCode() != Memcached::RES_END);

?>

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Haravikk ¶

7 years ago


I'm not sure whether this remains true in the newer versions of the Memcached module (v3.0 onwards) but in the version shipped with PHP 5.6 the return value and result code when using this method with OPT_BINARY_PROTOCOL enabled are entirely useless.

Setting a value successful may return true, with a result code of RES_END, but it may also return true with a result code of RES_SUCCESS.

However, *unsuccessfully* setting a value likewise seems to return true and RES_SUCCESS, effectively rendering this function's return value useless with the binary protocol enabled as it is impossible to distinguish success from failure.

If you need to rely on the return value of this method then I strongly recommend disabling the binary protocol under PHP 5.6, as in its current state the common memcached module is too broken otherwise for CAS usage.

Hopefully someone else can weigh in on whether this is still broken in newer versions or not.

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php at sergentet dot fr ¶

7 years ago


To prevent a perpetual loop on any Memcached error, you can add a simple counter :

$security_count = 0;

do {
        //[]....
        $security_loop++
        if ($security_loop > 10) {
                break;     //( or return "your return value" on a function )
        }
} while ($m->getResultCode() != Memcached::RES_SUCCESS);

down

sparcbr at gmail dot com ¶

8 years ago


Do not check command success in a while loop with something like

    
$memCached->getResultCode() != Memcached::RES_SUCCESS

Memcached::RES_SERVER_ERROR or anything like this and your script will loop forev

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