# Library(redis): report DNS failure **URL:** \ **Category:** General\ **Created:** [October 16, 2020, 4:16am UTC](https://swi-prolog.discourse.group/t/library-redis-report-dns-failure/3049 "2020-10-16T04:16:08Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![swi](https://avatars.discourse-cdn.com/v4/letter/s/51bf81/32.png) [@swi](https://swi-prolog.discourse.group/u/swi)\ **Post date:** [October 16, 2020, 4:16am UTC](https://swi-prolog.discourse.group/t/library-redis-report-dns-failure/3049/1 "2020-10-16T04:16:08Z")
In testing library(redis) I mistyped the redis host name in redis\_server/3, and when I ran the redis/2 command it simply blocked forever. Problably should throw an exception like ping does: ```prolog $ ping: nonexistent ping: nonexistent: No address associated with hostname ``` ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [October 21, 2020, 7:32am UTC](https://swi-prolog.discourse.group/t/library-redis-report-dns-failure/3049/2 "2020-10-21T07:32:56Z")
As is, redis\_server/3 does not check the server address. Calls to redis/3 and anything that depend on it are retried after 10ms, doubling on each failure up to one minute. It then keeps trying for a day. You can change these defaults using library(setting). The question is when to retry and when not. Is a DNS failure sufficiently different from a failed server which may also be permanently failed or inaccessible due to routing or firewall issues. We could of course let redis\_server/3 do a DNS check. On the other hand, a rebooting cluster may not always have its DNS right from the start. A possible intermediate could be to print some or all connect failures as warnings. ---
**Author:** ![swi](https://avatars.discourse-cdn.com/v4/letter/s/51bf81/32.png) [@swi](https://swi-prolog.discourse.group/u/swi)\ **Post date:** [October 22, 2020, 6:43pm UTC](https://swi-prolog.discourse.group/t/library-redis-report-dns-failure/3049/3 "2020-10-22T18:43:30Z")
> [@jan](#): > > A possible intermediate could be to print some or all connect failures as warnings. I think this is the proper solution, it keeps the behavior the same, but the user has a warning. (and if necessary the warnings can be silenced). Thanks for your great work. ---
**Author:** ![peter.ludemann](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/peter.ludemann/32/48_2.png) [@peter.ludemann](https://swi-prolog.discourse.group/u/peter.ludemann)\ **Post date:** [October 22, 2020, 6:56pm UTC](https://swi-prolog.discourse.group/t/library-redis-report-dns-failure/3049/4 "2020-10-22T18:56:29Z")
By “print”, do you mean writing to stderr, or calling a logger, or something else? On Windows, would you open a warning window? (I’m not a fan of printing warnings, but it’s fine to provide some kind of callback for handling error situations; or you might want to throw an exception) ---
**Author:** ![swi](https://avatars.discourse-cdn.com/v4/letter/s/51bf81/32.png) [@swi](https://swi-prolog.discourse.group/u/swi)\ **Post date:** [October 22, 2020, 7:10pm UTC](https://swi-prolog.discourse.group/t/library-redis-report-dns-failure/3049/5 "2020-10-22T19:10:03Z")
> [@peter.ludemann](#): > > but it’s fine to provide some kind of callback for handling error situations Something like `onerror(Callback)` as an option for redis\_server/3? Sounds good. ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [October 23, 2020, 8:23am UTC](https://swi-prolog.discourse.group/t/library-redis-report-dns-failure/3049/6 "2020-10-23T08:23:17Z")
> [@peter.ludemann](#): > > By “print”, do you mean writing to stderr, or calling a logger, or something else? _print_ means print\_message/2. That is designed for printing messages and can easily be linked to OS logging facilities (e.g., library(syslog) realizes sending such messages to Unix syslog). > [@swi](#): > > Something like `onerror(Callback)` as an option for [redis\_server/3](https://www.swi-prolog.org/pldoc/doc_for?object=redis_server/3)? Sounds good. I think we have something for errors, called exceptions 🙂 There is not much you can repair here. There are just two options: try again or give up and raise an exception. The repair needed is typically outside the reach of the client: fix the server or the network. I will add a sensible message. ---
**Author:** ![swi](https://avatars.discourse-cdn.com/v4/letter/s/51bf81/32.png) [@swi](https://swi-prolog.discourse.group/u/swi)\ **Post date:** [October 24, 2020, 3:06am UTC](https://swi-prolog.discourse.group/t/library-redis-report-dns-failure/3049/7 "2020-10-24T03:06:33Z")
> [@jan](#): > > I think we have something for errors, called exception 😁 > [@jan](#): > > I will add a sensible message. I think this is good; so that the user is informed about what is happening.