> > However, memcached semantics don't quite give you what you want. > Depending on whether you can reasonably get a configuration to do what > you want, it might be easier to think of memcached as a bloom filter > than as a set in this case. That is, if you negatively cache things > that *aren't* part of your list, then the presence of a key will tell > you for certain that a particular key is not a member, but the absence > of a key would mean that you don't know (or perhaps memcached *did* > know, but had since forgotten).
I had thought of doing a bloom filter on it as well. The problem here is, the membership list might change sometimes, and reading info on bloom filters, it's not really well-suited for dynamically changing lists? > You could optionally preload objects that are likely to be used if > you think the natural population wouldn't do it effectively (you can > measure this with stats). > Suppose I cache 10,000 recently-logged in members. Also, suppose 50% of traffic actually come from these users. Then, this cache would have a high hit ratio when testing for membership. However, what about the non-members? For instance let's say 40% of the traffic come from non-members. This would mean there'd need to be a full listing of members to check? Hmmm an interesting thought did just come across my mind. Let me hear your thoughts: - Cache 10,000 most recently logged-in members - Bloom filter on the entire list This way, you can test for negatives (bloom filter), and if there's a positive, check the 10,000 most recently logged-in users. If that still yields nothing, then do a database query. In effect, only a small minority of checks would require a trip to the DB. > > On Nov 4, 2007, at 23:32, J A wrote: > > > I have a fairly large members list that I want to keep in memcache. > > What I do with this list is query it against particular user IDs to > > see if they are a member of that list or not. If they are they get > > certain priviledges. > > > > The problem is, this list has gotten to the point of saturating the > > PHP's memory when fetching the MySQL query the first time. > > > > Is there a way to do this more effectively, for instance, > > partitioning the list into separate smaller lists, grouped by time > > of login? I'm thinking of this, as users who have logged in in the > > past 3 months are more likely to be in the list anyway. > > > It'd be easier to not think of it as a list if you're just testing > for membership. All you want to know is if a particular object is an > element of a particular set. You could do this by key convention if > you batch populate the records. > > However, memcached semantics don't quite give you what you want. > Depending on whether you can reasonably get a configuration to do what > you want, it might be easier to think of memcached as a bloom filter > than as a set in this case. That is, if you negatively cache things > that *aren't* part of your list, then the presence of a key will tell > you for certain that a particular key is not a member, but the absence > of a key would mean that you don't know (or perhaps memcached *did* > know, but had since forgotten). > > You could, of course, record the status either way so as to tell > the > difference between not knowing and knowing whether it's a member or > not. This is probably best suited to your needs. > > You could optionally preload objects that are likely to be used if > you think the natural population wouldn't do it effectively (you can > measure this with stats). > > -- > Dustin Sallings > > > >
