Memcache::add

(PECL memcache >= 0.2.0)

Memcache::add — Add an item to the server

Description

bool Memcache::add ( string $key , mixed $var [, int $flag [, int $expire ]] )

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_COMPRESSED to 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);

?>

User Contributed Notes

ktamas77 at gmail dot com

7 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;



?>

Davide Renzi

6 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;

}

?>

vasiliy at hotger dot com

2 years ago


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

matt