Hi Yann, 2016-05-28 0:31 GMT+02:00 Yann Ylavic <ylavic....@gmail.com>:
> Hi Luca, > > On Mon, May 23, 2016 at 2:21 PM, Luca Toscano <toscano.l...@gmail.com> > wrote: > > > > I replied to the comment that you pointed out with Jim's answer and I > > updated the trunk documentation with a note about DNS resolution: > > > > http://httpd.apache.org/docs/trunk/mod/mod_proxy.html#workers > > > > HTML diff only: > > > http://svn.apache.org/viewvc/httpd/httpd/trunk/docs/manual/mod/mod_proxy.html.en?r1=1745170&r2=1745169&pathrev=1745170 > > I don't think this is correct. > When disablereuse is true (including the default workers), the DNS > resolution occurs for each connection. > But when mod_proxy is reusing connections (including addresses), each > worker/backend address is resolved only once per child process, and > reused for all further connections (until the child is recycled). > AIUI, this is not related to the TTL. > Thanks a lot for the explanation and for the info. So httpd does cache origin DNS resolution and there is no way to control it via mod-proxy settings (maybe the only thing that could be used is MaxConnectionsPerChild?). > > From ap_proxy_determine_connection(): > /* > * Worker can have the single constant backend adress. > * The single DNS lookup is used once per worker. > * If dynamic change is needed then set the addr to NULL > * inside dynamic config to force the lookup. > */ > AFAICT, nothing sets "addr to NULL" anywhere in httpd, that would be > racy anyway. > > There is indeed a worker 'is_address_reusable' flag distinct from > 'disablereuse', but it is not exposed to the admin and seems to be > internally mapped to 'disablereuse'. > > I have a patch which introduces 'addressTTL' if one is interested, but > it's not that simple because it addresses the race locklessly (and > leaklessly :), thus with refcounting, address pool... > > If I got everything correctly, the description of the TTL might be a bit misleading because it is not clear IMHO that the DNS resolution won't be performed: "Time to live for inactive connections and associated connection pool entries, in seconds. Once reaching this limit, a connection will not be used again; it will be closed at some later time." What would happen when an admin performs a change of the origin DNS A or AAAA record? If httpd doesn't happen to recycle workers at that time it will try to connect to the old IP address probably triggering a 502 due to timeouts or connection failures right? The perfect thing, IMHO, would be to force the DNS resolution every time a new connection to origin needs to be performed (transparently to all the options), but as you mentioned the implementation could be messy and I am not sure how many people would like this feature. Anyhow, I'll wait for some replies to check that I got everything correctly and then I'll updated the documentation (unless somebody wants to do it before me). Thanks! Luca
