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
PHP - Manual: Memcache::set
2024-12-22
Memcache::set
(PECL memcache >= 0.2.0)
Memcache::set — Store data at the server
说明
Memcache::set()向key存储一个元素值为
var。参数expire是以秒为单位的失效时间，
如果设置为0表明该元素永不过期（但是它可能会因为为了给其他项分配空间而被删除）。如果你希望存储的元素
经过压缩（使用zlib），你可以设置flag的值为MEMCACHE_COMPRESSED。
同样你也可以使用函数memcache_set()。注意:
谨记：资源类型变量（比如文件或连接）不能被存储在缓存中，因为它们在序列化状态不能被完整描述。
参数
-
key -
要设置值的key。
-
var -
要存储的值，字符串和数值直接存储，其他类型序列化后存储。
-
flag -
使用
MEMCACHE_COMPRESSED指定对值进行压缩(使用zlib)。 -
expire -
当前写入缓存的数据的失效时间。如果此值设置为0表明此数据永不过期。你可以设置一个UNIX时间戳或 以秒为单位的整数（从当前算起的时间差）来说明此数据的过期时间，但是在后一种设置方式中，不能超过 2592000秒（30天）。
返回值
成功时返回 true， 或者在失败时返回 false。
范例
示例 #1 Memcache::set() 示例
<?php
/* procedural API */
/* connect to memcached server */
$memcache_obj = memcache_connect('memcache_host', 11211);
/*
设置'var_key'对应存储的值
flag参数使用0,值没有经过压缩
失效时间为30秒
*/
memcache_set($memcache_obj, 'var_key', 'some variable', 0, 30);
echo memcache_get($memcache_obj, 'var_key');
?>
示例 #2 Memcache::set() 示例
<?php
/* OO API */
$memcache_obj = new Memcache;
/* connect to memcached server */
$memcache_obj->connect('memcache_host', 11211);
/*
设置'var_key'对应值，使用即时压缩
失效时间为50秒
*/
$memcache_obj->set('var_key', 'some really big variable', MEMCACHE_COMPRESSED, 50);
echo $memcache_obj->get('var_key');
?>
add a note
User Contributed Notes 10 notes
Sc00bz ¶
14 years ago
wbonde at yakabod dot com ¶
11 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 ¶
13 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 );
}
?>
Stephen from veedow.com ¶
14 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.
duerra at nospam dot yahoo dot com ¶
11 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);
?>
winmutt ¶
2 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 ¶
4 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,$value,0)
effeesse gmail com ¶
12 years ago
if you want to cache an image created on-the-fly you can do:
<?php
ob_start();
imagepng($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 ¶
9 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 ¶
14 years ago
The lowest byte of the int is reserved for pecl/memcache internal usage (e.g. to indicate compression and serialization status).
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