Hi Mick, Depending on the application and purpose for wanting a shared cache, I might try forwarding all queries to a single "master" recursor which only serves the "front-end" servers, performing all root server queries. It would only receive the queries that the front-end servers cannot answer from their own respective caches, yet its cache would be the same as the collection of the individual caches combined. This provides both the load-sharing benefit of a pool of load-balanced servers and the desired shared cache.
There are several downsides to this setup. 1. If the particular network has a high volume of unique queries, the probability of the "front-end" recursors having the answers in their cache is low, so the load on the master recursor might be too great. However, in general that also means the probability of the master recursor having the answers cached is also low, so the benefit of a shared cache is diminished. 2. If the number of "front-end" recursors is very high, at some point it becomes impractical to try to hold the collection of all their caches on one server, and handle so many queries. Without testing I do not know exactly how high that would be, but it would seem to be a rare case. 3. It means a central point of failure takes away the redundancy benefit of load balancing. This can be handled simply with a trigger in your monitoring software which runs a script on the "front-end" servers when the master recursor fails. The script would swap out the recursor.conf for one without the forward-zones statement and execute 'rec_control reload-zones' so that they switch to doing their lookups on the root servers while the master recursor is unavailable, without clearing their caches. 4. If for any reason the master recursor's cache is cleared, the benefit of it will be temporarily diminished. A possible solution is to consider using a different recursor on it than PowerDNS, one which saves the cache to disk and re-loads it on start. Brendan _______________________________________________ Pdns-users mailing list [email protected] http://mailman.powerdns.com/mailman/listinfo/pdns-users
