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 — データをサーバーに格納する
説明
Memcache::set() は、キー
key に var という値を
関連付け、memcached サーバーに格納します。パラメータ expire
は、データの有効期限を秒単位で指定します。もし 0 を指定した場合は
その項目が期限切れになることはありません (これは、その項目のデータが
memcached サーバー上にずっと残り続けることを保証するものではありません。
他の項目をキャッシュするための場所を確保するためにサーバーから
削除されてしまうこともあります)。
(zlib を使用して) その場でのデータの圧縮を行いたい場合は、
flag の値として、定数
MEMCACHE_COMPRESSED を指定します。
memcache_set() 関数を使用することも可能です。注意:
リソース型の変数 (たとえばファイル記述子や接続記述子など) はキャッシュに保存できないことを覚えておきましょう。これは、 シリアライズした状態ではそれらのデータを適切に表すことが できないためです。
パラメータ
-
key -
項目に関連付けられたキー。
-
var -
格納する値。文字列および整数値はそのままの形式で、それ以外の型は シリアライズされて格納されます。
-
flag -
項目を圧縮して格納する場合に
MEMCACHE_COMPRESSEDを使用します (zlib を使用します)。 -
expire -
項目の有効期限。ゼロの場合は有効期限なし (いつまでも有効) となります。Unix タイムスタンプ形式、あるいは現在からの 秒数で指定することが可能ですが、後者の場合は秒数が 2592000 (30 日) を超えることはできません。
戻り値
成功した場合に true を、失敗した場合に false を返します。
例
例1 Memcache::set() の例
<?php
/* 手続き型の API */
/* memcached サーバーに接続します */
$memcache_obj = memcache_connect('memcache_host', 11211);
/*
キー 'var_key' の項目の値を設定します。
flag の値として 0 を使用し、圧縮は使用しません。
有効期限は 30 秒です。
*/
memcache_set($memcache_obj, 'var_key', 'some variable', 0, 30);
echo memcache_get($memcache_obj, 'var_key');
?>例2 Memcache::set() の例
<?php
/* オブジェクト指向の API */
$memcache_obj = new Memcache;
/* memcached サーバーに接続します */
$memcache_obj->connect('memcache_host', 11211);
/*
キー 'var_key' に対応する値を設定します。その際、データの圧縮を行います。
有効期限は 50 秒です。
*/
$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 ¶
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).
