Binary Search Tree

Timo Ewalds timo at
Fri Sep 7 17:10:18 UTC 2007

I don't quite follow what you're doing there, but you can always use a 
different key as a lock to get rid of the race conditions. Just use an 
add operation on the lock key instead of set. If the add worked, you 
have the lock, if not, someone else does, and you can act appropriately 
(skip, wait, whatever). The other thing you may care to know, is there 
are incr and decr commands which are good for counters.


Chris Goffinet wrote:
> I was wondering if anyone had any suggestions of trying to implement a 
> binary search tree using memcache? I know it wouldn't be a 
> good candidate if we had multiple memcache servers, but I was thinking 
> it might be doable if we partition the tree on 1 memcache node. 
> Our biggest concern at the moment is race conditions of the tree 
> updating. I thought a binary tree might solve the problem were having 
> and that is the following scenario:
> 1) We have hundreds of memcache objects being created (imagine rows in 
> a database of users visiting a site)
> 2) We need to have a running list of last 10 users for the hour, 
> without duplication of a users record. So we implemented the idea of a 
> counter:
>   if(!memcache->get("userid:$hourofday")
>   {
>     $counter = memcache->set("counter",1);
>     memcache->set("counter:$counter", payload);
>     memcache->set("userid:$hourofday",1);
>   }
> This would create only 1 record per hour of day, increment the 
> counter, so we could build the key list in our application to pull 
> last 10 items for the hour. The issue we run into is what happens when 
> the following scenario happens:
>   Hour 1 - > User A - Counter 1
>   Hour 2 - > User A - Counter 2
>   Hour 3 - > User A - Counter 3
>   Hour 4 - > User A - Counter 4
>   Hour 5 - > User A - Counter 5
> In this case we would only just want to show User A one time 
> especially since its a list of user activity. We can't store a hash 
> and just push/pop because of race conditions (our first thought). If 
> anyone has any ideas, I'd really appreciate it!
> -Chris
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