On Mon, Sep 13, 2010 at 08:06:37AM -0400, Karl Baum wrote: > Hi Willy. The "balance url_param" looks like what I need. > > In regards to setting a cookie, in my case each of the http clients is > actually an email fetching worker calling an imap api which will eventually > sit behind HAProxy. Because each api node will have a connection pool of > imap connections, depending on which email address the worker is processing, > i want the request to be directed to the server which already has a pool of > connections open for that email address. If i didn't do this, the more api > nodes behind HAProxy, the more connections i will have open to the imap > server and imap limits open connections to each email account. Each worker > will be serving multiple email accounts and workers will process the same > email account in parallel so i don't think the cookie based routing applies > to this use case (but i could be wrong).
OK, so what you describe perfectly matches the typical usage of a URL parameter hash. > Is there significant overhead in the url_param hashing algorithm? No, it's still quite cheap. You should not notice it on a mailing system. > Will the load be spread evenly across all nodes? yes, the hashing algorithm used tends to be quite smooth, but for that you obviously need a somewhat large population. I'd say that if your number of possible keys is about 100 times bigger than the number of nodes, you'll get quite a smooth distribution. > Also, in the case of node failure, will the "balance url_param" pick another > node? Yes, that's the same for all algorithms, provided that you enable health checks of course. The point to note with hashing algorithms is that when a node goes down, the farm size changes and everyone gets redispatched. This may or may not be desired. If this is a problem, you could use "hash-type consistent" to change the hashing algorithm to a consistent one : only the users of the failed node will be uniformly redispatched to remaining nodes. The downside is that consistent hashing is not as smooth at the normal one, but it might still be acceptable in your case. Regards, Willy
