It looks like add() function is truly 100% atomic, and safeadd bicycle mentioned in the other comment is useless. There are few links where developers of Memcahed explained it deeper
http://lists.danga.com/pipermail/memcached/2008-March/006647.html
http://www.serverphorums.com/read.php?9,214222
Memcache::add
(PECL memcache >= 0.2.0)
Memcache::add — Add an item to the server
Description
Memcache::add() stores variable
var with key only if such
key doesn't exist at the server yet.
Also you can use memcache_add() function.
Parameters
-
key -
The key that will be associated with the item.
-
var -
The variable to store. Strings and integers are stored as is, other types are stored serialized.
-
flag -
Use
MEMCACHE_COMPRESSEDto store the item compressed (uses zlib). -
expire -
Expiration time of the item. If it's equal to zero, the item will never expire. You can also use Unix timestamp or a number of seconds starting from current time, but in the latter case the number of seconds may not exceed 2592000 (30 days).
Return Values
Returns true on success or false on failure.
Returns false if such key already exist. For the rest
Memcache::add() behaves similarly to
Memcache::set().
Examples
Example #1 Memcache::add() example
<?php
$memcache_obj = memcache_connect("localhost", 11211);
/* procedural API */
memcache_add($memcache_obj, 'var_key', 'test variable', false, 30);
/* OO API */
$memcache_obj->add('var_key', 'test variable', false, 30);
?>See Also
- Memcache::set() - Store data at the server
- Memcache::replace() - Replace value of the existing item
add a note
User Contributed Notes 5 notes
vasiliy at hotger dot com ¶
10 years ago
Davide Renzi ¶
14 years ago
Race conditions happen on an heavy load server when more than one thread tries to execute memcache_add.
For example if thread A and thread B try to save the same key you can test that sometimes both return TRUE.
To have the right behaviour you can verify that the correct value is in the assigned key:
<?php
function memcache_safeadd(&$memcache_obj, $key, $value, $flag, $expire)
{
if (memcache_add($memcache_obj, $key, $value, $flag, $expire))
{
return ($value == memcache_get($memcache_obj, $key));
}
return FALSE;
}
?>
ktamas77 at gmail dot com ¶
15 years ago
skeleton of a thread safe updater for an incremental counter:
<?php
$key = "counter";
$value = $memcache->increment($key, 1);
if ($value === false) {
// --- read from DB ---
$query = "SELECT value FROM database";
$result = mysql_query($query);
$row = mysql_fetch_assoc($result);
$db_value = $row["value"];
$add_value = $memcache->add($key, $db_value + 1, 0, 0);
if ($add_value === false) {
$value = $memcache->increment($key, 1)
if ($value === false) {
error_log ("counter update failed.");
}
} else {
$value = $db_value + 1;
}
}
// --- display counter value ---
echo $value;
?>
roberto at spadim,com dot br ¶
17 years ago
[c.2007]
if you read source code for MMC_SERIALIZED you will see at line ~1555 that [a line ~1560]
!(is_string,is_long,is_double,is_bool)
[is] serialized and that serialized values are flaged as MMC_SERIALIZED for return (fetch) code unserialize these values again
