On Fri, 9 Sep 2016 11:20:59 +0530 Mithun Cy <mithun...@enterprisedb.com> wrote:
> If concern is only about excluding address which was tried > previously. Then I think we can try this way, randomly permute given > address list (for example fisher-yates shuffle) before trying any of > those address in it. After that you can treat the new list exactly > like we do for sequential connections. I think this makes code less > complex. Random permutation is much more computationally complex than random picking. Alexander suggests to use other random pick algorithm than I use. I use algorithm which works with lists of unknown length and chooses line of such list with equal probability. This algorithm requires one random number for each element of the list, but we are using C library rand function which uses quite cheap linear congruental pseudo random numbers, so random number generation is infinitely cheap than network connection attempt. As this algorithm doesn't need beforehand knowledge of the list length, it is easy to wrap this algorithm into loop, which modifies the list, moving already tried elements out of scope of next picking. (it is really quite alike fisher-yates algorithm). Alexander suggests to store somewhere number of elements of the list, than generate one random number and pick element with this number. Unfortunately, it doesn't save much effort in the linked list. At average we'll need to scan half of the list to find desired element. So, it is O(N) anyway. Now, the main point is that we have two different timeframes of operation. 1. Attempt to connect. It is short period, and we can assume that state of servers doesn't change. So if, server is down, we should ignore it until the end of this attempt to connect. 2. Attempt to failover. If no good candidate was found, we might need to retry connection until one of standbys would be promoted to master (with targetServerType=master) or some host comes up. See documentation of failover_timeout configuration parameter. It means that we cannot just throw away those hosts from list which didn't respond during this connect attempt. So, using random permutation instead of random pick wouln't help. And keeping somewhere number of elements in the list wouldn't help either unless we change linked list to completely different data structure which allows random access. -- -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
