How To Distinguish Between a Crashed Server and Non-Existent Memcached Key Value?

Jason Edgecombe jedgecombe at
Sun Jun 4 16:04:52 UTC 2006

Andreas Vierengel wrote:
> Ian Kallen wrote:
>> On 6/4/06, Andreas Vierengel <avierengel at> wrote:
>>> that's right, but it could lead to a nice DoS-Attack especially if (for
>>> example) a web-request leads to a specific key which cannot be filled
>>> from the original source, because the original source produces 'undef'
>>> as well.
>> If there is a high frequency of null result lookup cases on the data
>> origin and you're concerned with the costs of those lookups, then
>> implement a negative cache.  The negative cache interval should be
>> sized by how tolerant your applications are of having no result when
>> in fact there may be one.
>> Here's some babytalk code to illustrate how to typically handle it:
>> rv = getMemcacheVal(key);
>> if (rv == null) { // memcache said "nope"
>>    rv = getExpensiveVal(key); // poor performing mysql indices, etc
>>    if (rv == null) { // rdbms said "nope"
>>        putNegativeCache(key); // has appropriate TTL
>>    } else { // rdbms said "yep"
>>        putPositiveCache(key);
>>    }
>> }
>> return rv; // may be undef
>>> One solution could be that you never let this happen and always 
>>> generate
>>> a "reasonable" value for this key, or you could hack 
>>> Cache::Memcached to
>>>   add a undef-state using the flags-field.
>> Hacking the mecached client to handle the negative case, given the
>> various tolerances that applications will have for "false negatives",
>> sounds like a bad idea. When I fetch null from memcache, I really
>> expect that to mean there's no cache entry for it and have that be
>> distinct from a negative cache. But I'd be curious to see a general
>> solution if you have one.
> I use a self written client in perl, which stores undef values as the 
> string "UNDEF" and set some bitfield in the flag-header to true. If I 
> fetch a key from memcached an my client returns undef, i question 
> another method which returns if the client returned undef because of a 
> general error or if the value was simply undef. So I can distinguish 
> between negative hits and errors.
> --Andy
>> -Ian
What if you store a known value?

if get("keepalive")  {
# server is up!

storing the last time that you used the cache might be helpful as well.


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