This is just two minor things about memcache that might not be perfectly clear, the limits on key and data sizes and what happen to flags in the memcache protocol.
* There is a max key size of 250 anything bigger gets truncated. There is also a (1MB - 42 bytes) limit on the data.
* In the memcache protocol there is a 16bit, 32bit in newer version, flag that you can set to whatever you want because memcache doesn't do anything with the flags. The php api doesn't let you get the flags because php uses the flags for php's own use such as "MEMCACHE_COMPRESSED" and I decided to test if it was doing something because it wasn't part of the memcache protocol.
<?php
$memcache = new Memcache();
$memcache->connect("127.0.0.1", 11211);
// Since memcache truncates the keys at 250 bytes both the get "250 a's" and "251 a's" will find the key in the cache
echo "*** Truncate key test ***<br>";
echo "set 251: " . ($memcache->set(str_repeat("a", 251), "value", 0, 1) ? "t" : "f") . "<br>";
echo "get 249: " . (($ret = $memcache->get(str_repeat("a", 249))) !== false ? "'$ret'" : "f") . "<br>";
echo "get 250: " . (($ret = $memcache->get(str_repeat("a", 250))) !== false ? "'$ret'" : "f") . "<br>";
echo "get 251: " . (($ret = $memcache->get(str_repeat("a", 251))) !== false ? "'$ret'" : "f") . "<br>";
echo "delete: " . ($memcache->delete(str_repeat("a", 250)) ? "t" : "f") . "<br><br>";
echo "*** Compress value test ***<br>";
echo "set 1024*1024-42: " . ($memcache->set("test", str_repeat("a", 1024*1024-42), 0, 1) ? "t" : "f") . "<br>";
echo "set 1024*1024-41: " . ($memcache->set("test", str_repeat("a", 1024*1024-41), 0, 1) ? "t" : "f") . "<br>";
echo "set 1024*1024 compressed: " . ($memcache->set("test", str_repeat("a", 1024*1024), MEMCACHE_COMPRESSED, 1) ? "t" : "f") . "<br>";
echo "delete: " . ($memcache->delete("test") ? "t" : "f") . "<br>";
$memcache->close();
?>
Output:
*** Truncate key test ***
set 251: t
get 249: f
get 250: 'value'
get 251: 'value'
delete: t
*** Compress value test ***
set 1024*1024-42: t
set 1024*1024-41: f
set 1024*1024 compressed: t
delete: t
Memcache::set
(PECL memcache >= 0.2.0)
Memcache::set — Stocke des données dans le serveur de cache
Description
Memcache::set() stocke l'élément var
avec la clé key sur le serveur de cache.
Le paramètre expire représente le délai en secondes
d'expiration de l'élément. S'il vaut 0, l'élément n'expirera jamais (mais le serveur
de cache ne garantie pas que cet élément sera toujours stocké, il peut être
effacé du cache pour faire de la place à d'autres éléments).
Vous pouvez utiliser la constante MEMCACHE_COMPRESSED
comme valeur du paramètre flag
si vous voulez utiliser la compression à la volée (utilisation de la bibliothèque zlib).
Vous pouvez également utiliser la fonction memcache_set().Note:
Souvenez-vous que les ressources (i.e. identifiant de fichiers ou de connexion) ne peuvent pas être stockées dans le cache, car elles ne peuvent pas être représentées linéairement.
Liste de paramètres
-
key -
La clé qui sera associée avec l'élément.
-
var -
La variable à enregistrer. Les chaînes de caractères et les entiers sont enregistrés comme tels, les autres types sont enregistrés de manière sérialisée.
-
flag -
Utilisez
MEMCACHE_COMPRESSEDpour enregistrer l'élément compressé (utilise zlib). -
expire -
Temps d'expiration pour l'élément. S'il égal
0, l'élément n'expirera jamais. Vous pouvez aussi utiliser un timestamp Unix ou un nombre de seconde en commençant par la date d'aujourd'hui, mais dans le dernier cas, le nombre de secondes ne doit pas excéder 2592000 (30 jours).
Valeurs de retour
Cette fonction retourne true en cas de succès ou false si une erreur survient.
Exemples
Exemple #1 Exemple avec Memcache::set()
<?php
/* API procédurale */
/* connexion au serveur de cache */
$memcache_obj = memcache_connect('memcache_host', 11211);
/*
définie la valeur de l'élément identifié par la clé 'var_key' ;
utilisation de la valeur 0 pour le flag ;
la compression n'est pas utilisée ;
le délai d'expiration vaut 30 secondes
*/
memcache_set($memcache_obj, 'var_key', 'quelques variables', 0, 30);
echo memcache_get($memcache_obj, 'var_key');
?>Exemple #2 Exemple avec Memcache::set()
<?php
/* API orientée objet */
$memcache_obj = new Memcache;
/* connexion au serveur de cache */
$memcache_obj->connect('memcache_host', 11211);
/*
définie la valeur de l'élément identifié par la clé 'var_key' ;
utilisation de la compression à la volée ;
le délai d'expiration vaut 50 secondes
*/
$memcache_obj->set('var_key', 'quelques grosses variables', MEMCACHE_COMPRESSED, 50);
echo $memcache_obj->get('var_key');
?>Voir aussi
- Memcache::add() - Ajoute un élément dans le server
- Memcache::replace() - Remplace une valeur d'un élément existant
add a note
User Contributed Notes 10 notes
Sc00bz ¶
17 years ago
wbonde at yakabod dot com ¶
13 years ago
The max time for expiration (without having to worry about deletions when necessary as with 0 seconds) is 2,592,000 seconds (30 days).
Specifying an expiration value above that will return false, but will NOT throw in error so it is easy to miss.
argyleblanket ¶
16 years ago
Using set more than once for the same key seems to have unexpected results - it does not behave as a "replace," but instead seems to "set" more than one value for the same key. "get" may return any of the values.
This was tested on a multiple-server setup - behaviour may be different if you only have one server.
Remedy is to use a combination of replace and set:
<?php
$result = $memcache->replace( $key, $var );
if( $result == false )
{
$result = $memcache->set( $key, $var );
}
?>
duerra at nospam dot yahoo dot com ¶
14 years ago
If you're interested in using compression, please note that, at least for PHP version 5.3.2 and Memcache version 3.0.4, when retrieving a key who's value is a numeric or boolean type, PHP throws a notice of the following:
Message: MemcachePool::get(): Failed to uncompress data
The way around this is to test your variable type before setting or adding it to Memcache, or even cast it as a string.
<?php
$key = 'mc_key';
$value = 12345;
$compress = is_bool($value) || is_int($value) || is_float($value) ? false : MEMCACHE_COMPRESSED;
$mc= new Memcache;
$mc->connect('localhost', 11211);
$mc->set($key, $value, $compress);
echo $mc->get($key);
//Alternative is to cast the variable
$value = is_scalar($value) ? (string)$value : $value;
$mc->set($key, $value, MEMCACHE_COMPRESSED);
?>
Stephen from veedow.com ¶
16 years ago
I ran into problems using the MEMCACHE_COMPRESSED flag when storing small amounts of data, such as an integers.
For expample.
<?php
Memcache::set('integer', 123456, MEMCACHE_COMPRESSED);
//would return true
Memcache::get('integer');
//would return false
?>
This problem went away when I removed the MEMCACHE_COMPRESSED flag for values that were small.
winmutt ¶
5 years ago
The note here about replace and set is no longer valid in my testing. You can call set as many times as you want on the same key and reliably get the last written value. I tested this with 3 memcache nodes over 10000 keys.
jcastromail at yahoo dot es ¶
7 years ago
If you get the next message
"The lowest two bytes of the flags array is reserved for pecl/memcache internal use"
Then try the next operations:
a) try to use Memcached instead of Memcache.
b) switch the compressed value
$memcache->set($key,$value,MEMCACHE_COMPRESSED)
or
$memcache->set($key,$value,0)
effeesse gmail com ¶
14 years ago
if you want to cache an image created on-the-fly you can do:
<?php
ob_start();
imagepng($image);
$memcache->set("my_image", ob_get_contents(), false, $cache_time);
ob_end_clean();
?>
then you could access the chached image as simple variable:
<?php $my_image = $memcache->get("my_image"); ?>
so, in short, you have to buffer the output
aamthor at advertzoom dot de ¶
12 years ago
to put some things right:
max expiration time: RTFM, it's written here.
max amount of data: almost unlimited as long as your server can bear it.
speed and pace:
well, thats another thing. We had a couple of data records which for application reasons must be kept in memory. Since the bunch of data is big and doesn't change very often, we considered caching it to memcache instead of retrieving it from the DB each and every time.
This isn't a general advice nor any quality statement, but we did a couple of tests with serialized arrays (50 MB), compressed and uncompressed and it turned out that in our particular scenario, memcache is much slower than the DB (mySql).
In general, one can not predict on the behavior of memcache in certain scenarios but always need to make some testing and benchmarking upfront before starting to deploy things to a live system.
Despite of the tests above, we are still using memcache for session caching instead of file system, since there are certain other things to consider and the amount of data is always small (few KB)
jakub dot lopuszanski at nasza-klasa dot pl ¶
16 years ago
The lowest byte of the int is reserved for pecl/memcache internal usage (e.g. to indicate compression and serialization status).
