> On 5 Mar 2019, at 08:30, Mark Reynolds <mreynolds@xxxxxxxxxx> wrote: > > > On 2/22/19 11:46 AM, Mark Reynolds wrote: >> I want to start a brief discussion about a major problem we have backend transaction plugins and the entry caches. I'm finding that when we get into a nested state of be txn plugins and one of the later plugins that is called fails then while we don't commit the disk changes (they are aborted/rolled back) we DO keep the entry cache changes! >> >> For example, a modrdn operation triggers the referential integrity plugin which renames the member attribute in some group and changes that group's entry cache entry, but then later on the memberOf plugin fails for some reason. The database transaction is aborted, but the entry cache changes that RI plugin did are still present :-( I have also found other entry cache issues with modrdn and BE TXN plugins, and we know of other currently non-reproducible entry cache crashes as well related to mishandling of cache entries after failed operations. >> >> It's time to rework how we use the entry cache. We basically need a transaction style caching mechanism - we should not commit any entry cache changes until the original operation is fully successful. Unfortunately the way the entry cache is currently designed and used it will be a major change to try to change it. >> >> William wrote up this doc: http://www.port389.org/docs/389ds/design/cache_redesign.html >> >> But this also does not currently cover the nested plugin scenario either (not yet). I do know how how difficult it would be to implement William's proposal, or how difficult it would be to incorporate the txn style caching into his design. What kind of time frame could this even be implemented in? William what are your thoughts? >> >> If William's design is too huge of a change that will take too long to safely implement then perhaps we need to look into revising the existing cache design where we use "cache_add_tentative" style functions and only apply them at the end of the op. This is also not a trivial change. >> >> And what impact would changing the entry cache have on Ludwig's plugable backend work? >> >> Anyway we need to start thinking about redesigning the entry cache - no matter what approach we want to take. If anyone has any ideas or comments please share them, but I think due to the severity of this flaw redesigning the entry cache should be one of our next major goals in DS (1.4.1?). > > We are actually seeing more of these cases popping up now, so we need to do something soon. I had proposed we could always just flush the entire cache when a backend txn op fails, but Ludwig had a much better idea that we could implement a type of csn in the entry cache. So when a backend txn plugin fails, we flush the entry cache entries with a csn >= start of the parent operation. > > So until LMDB or a new caching mechanism is implemented this could be a viable/realistic option. Well, the cache currently works by having a version number (I think watermark?) in the cache. That’s how old content is removed (entries < watermark). So perhaps we could get the watermark id at the start of an operation and anything where watermark >= current op, we flush on rc != 0. (disclaimer, this is from my memory and may not represent real cache behaviour, so could be wildly wrong). > > Mark > >> >> Thanks, >> >> Mark >> _______________________________________________ >> 389-devel mailing list -- 389-devel@xxxxxxxxxxxxxxxxxxxxxxx >> To unsubscribe send an email to 389-devel-leave@xxxxxxxxxxxxxxxxxxxxxxx >> Fedora Code of Conduct: https://getfedora.org/code-of-conduct.html >> List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines >> List Archives: https://lists.fedoraproject.org/archives/list/389-devel@xxxxxxxxxxxxxxxxxxxxxxx > _______________________________________________ > 389-devel mailing list -- 389-devel@xxxxxxxxxxxxxxxxxxxxxxx > To unsubscribe send an email to 389-devel-leave@xxxxxxxxxxxxxxxxxxxxxxx > Fedora Code of Conduct: https://getfedora.org/code-of-conduct.html > List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines > List Archives: https://lists.fedoraproject.org/archives/list/389-devel@xxxxxxxxxxxxxxxxxxxxxxx — Sincerely, William Brown Software Engineer, 389 Directory Server SUSE Labs _______________________________________________ 389-devel mailing list -- 389-devel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to 389-devel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://getfedora.org/code-of-conduct.html List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/389-devel@xxxxxxxxxxxxxxxxxxxxxxx
