Question

See this simple piece of code in PHP:

//Documentation:
//memcache_set ( string $key , mixed $var [, int $flag [, int $expire ]] )
//memcache_increment ( string $key [, int $value = 1 ] )

//part 1
memcache_set ( 'id' , 1 , 0 , 60 );

//part 2
$id = memcache_increment ( 'id' , 1 );

And now imagine that the incrementing “part 2” is called from many independent clients and every is getting its unique ID.

Qestion is: How extend expiration of value ‘id’ with keeping consistency of value? Keep in mind that everytime some client can come to increment the value.

Some ideas how solve this problem:

Try to be fast

memcache_set( 'id' , memcache_get( 'id'  ) , 0 , 60 );

But here is time hole between get and set and other client at time can change the value.(?)

Use semaphore e.g.:

not much effective…

memcache_set ( 'lock' , 1 , 0 , 60 );
memcache_set( 'id' , memcache_get( 'id'  ) , 0 , 60 );
memcache_delete( 'lock' ); 
//client will not increment if the lock is present and wait for lock will get out

use memcache_delete:

memcache::delete  ( 'id' , 60 );

documentation is lack of what happened if you call it twice. Should it extend Expiration of first call or not?

Last surprising example (try it your self):

$memcache_obj = new Memcache;
$memcache_obj->connect('127.0.0.1', 11211);
$memcache_obj->set('id', '1', 0 , 30);

echo "*" . $memcache_obj->get('id') . "\n" ;

$memcache_obj->increment('id');
echo "*" . $memcache_obj->get('id') . "\n" ;

echo " now delete with timeout..." . "\n";
$memcache_obj->delete('id' , 10 )  . "\n" ;
echo "*" . $memcache_obj->get('id')  . "\n" ;

sleep(11);
echo "*" . $memcache_obj->get('id')  . "\n" ;

Returns me:

*1
*2
 now delete with timeout...
*
*

Answer 1

Use the memcached extension instead of memcache, which supports cas tokens: http://php.net/manual/en/memcached.cas.php The example there is pretty much self-explanatory.