Memcache::get

jakub dot lopuszanski at nasza-klasa dot pl (2013-06-27 14:12:08)

The comment by bijay dot rungta at gmail dot com could saved us a lot of time, money and users if we knew about this bug before.
Consider a usual scenario in which you first check something in a cache and only if it is not in cache, you check in the database.
Something like (in pseudocode):
<?php
$value = $cache->get("resultFor/$x");
if($value is missing){
   $value = $database->get($x);
   $cache->set("resultFor/$x",$value);
}
return $value;
?>
Now, consider a call to the above function for "dooms day" followed by "dooms_day". As both keys silently clash, you will get answers to the first query as results to second query... In particular this missfeature caused our short URLs ( such as nk.pl/dooms_day) to stop working as as soon as someone typed  "nk.pl/dooms%20day" or "nk.pl/dooms day" or nk.pl/dooms+day", because we only had version with underscore in our database, yet all of these spellings used the same cache key.

bijay dot rungta at gmail dot com (2013-05-21 06:31:48)

Note that if you include spaces in your keys when saving Data and use array of keys to get the Data, The returned array will replace spaces with underscores in the keys. 

<?php

$memcache = new Memcache;
$memcache->connect('localhost');
$memcache->set('my-key', 'value1', 0, 300);
$memcache->set('key with space', 'value 2', 0, 300);

print_r($memcache->get(array('my-key', 'key with space'))); // Array ( [my-key] => value1 [key_with_space] => value 2 )

?>

Memcache replaces spaces with underscores when saving, it does so when doing a get too, but when you do a get with single key (string) you don't notice this as it merely returns the value. But when you do a get for array of keys, you would expect the same keys in the returned array but it replaces spaces by underscores in them.

clover at fromru dot com (2013-04-27 00:14:37)

$flags stays untouched if $key was not found on the server, it's helpfull to determine if bool(false) was stored:

<?php

$memcache = new Memcache();

$memcache->set('test', false); // 
$flags = false;
var_dump($memcache->get('test', $flags)); // bool(false)
var_dump($flags); // int(256) - changed to int

$memcache->delete('test');

$flags = false;
var_dump($memcache->get('test', $flags)); // bool(false)
var_dump($flags); // bool(false) - untouched

?>

Michael Brenden (2011-02-17 02:46:20)

Beware, if there's a backslash present in the key name then the results are unpredictable and random.
I was generating keys like this:
$mc_key = get_called_class(). $_COOKIE['crumb'];
This works fine when get_called_class() returns CLASSNAME.
But then I began using namespaces, and get_class_class() of course returned NAMESPACE\CLASSNAME
Well that backslash sends Memcache into quite the tizzy.
No errors, mind you, just insanity.

janis at unrepublic dot com (2010-03-22 14:00:00)

For me it was the case that if such key doesn't exist, null is returned not false.

nate (2008-10-14 08:18:17)

Be aware that when using the multi-key version, Memcache::get returns bool false if no servers are configured for the pool (and possibly if other errors occur as well while attempting to fetch). Also, Memcache class throws annoying warnings on every get/set/delete-type calls if you have no servers added to the pool.

The following snippet var_dump's bool false, not an empty array like you might expect.

<?php
$cache = new Memcache;

// no $cache->addServer calls (for example,
due to temporarily disabling use of cache)

// use @ symbol to ignore warning

var_dump(
    @$cache->get(array('one', 'two'))
);
?>

marko at nastja dot com (2008-07-30 05:07:08)

It looks like memcache take only first 256 characters. So if you want to cache some (large) queries do md5 or similar before caching.

jakub dot lopuszanski at nasza-klasa dot pl (2008-07-07 07:39:49)

If deserialization fails for some reason, that is when memcache server returned flag 1 set, but the value was not a correctly serialized PHP data,
then Memcache::get acts in a following way:
If it was called with a single key to retrieve, then a warning is raised, but since it was not actually a bug of a server, the warning says something confusing like "Memcached Server Error: null" and the function returns bool(false).
If it was called by passing an array (even with a single element in it), then the warning is not raised and the resulting array contains a value bool(false).
Since there are some buffer overrun bugs present in Memcached Server, which from time to time cause overwriting of [part of] data and therefore rendering it impossible to deserialize, make sure to check if the result of Memcache::get contains only string, or deserialized structure. If the result is bool,dobule or long, then something went wrong.