On (28/07/16 13:56), Jakub Hrozek wrote: >On Thu, Jul 28, 2016 at 01:33:32PM +0200, Lukas Slebodnik wrote: >> On (28/07/16 12:06), thierry bordaz wrote: >> >On 07/28/2016 09:39 AM, Jakub Hrozek wrote: >> >> On Wed, Jul 27, 2016 at 04:09:07PM +0200, thierry bordaz wrote: >> >> > >> >> > On 07/27/2016 03:36 PM, Jakub Hrozek wrote: >> >> > > On Wed, Jul 27, 2016 at 02:55:37PM +0200, thierry bordaz wrote: >> >> > > > On 07/27/2016 01:56 PM, Jakub Hrozek wrote: >> >> > > > > On Wed, Jul 27, 2016 at 01:03:59PM +0200, Jakub Hrozek wrote: >> >> > > > > > On Wed, Jul 27, 2016 at 12:22:46PM +0200, Lukas Slebodnik wrote: >> >> > > > > > > On (27/07/16 12:08), Jakub Hrozek wrote: >> >> > > > > > > > On Wed, Jul 27, 2016 at 12:02:24PM +0200, Jakub Hrozek >> >> > > > > > > > wrote: >> >> > > > > > > > > On Wed, Jul 27, 2016 at 11:54:16AM +0200, Lukas Slebodnik >> >> > > > > > > > > wrote: >> >> > > > > > > > > > ehlo, >> >> > > > > > > > > > >> >> > > > > > > > > > attached patch fixes acces denied after activating user >> >> > > > > > > > > > in 389ds. >> >> > > > > > > > > > Jakub had some comments/ideas in ticket but I think >> >> > > > > > > > > > it's better to discuss >> >> > > > > > > > > > about virtual attributes and timestamp cache on mailing >> >> > > > > > > > > > list. >> >> > > > > > > > > Yes, so the comment I have is that while this works, it >> >> > > > > > > > > might break some >> >> > > > > > > > > strange LDAP servers. >> >> > > > > > > > > >> >> > > > > > > > > We use modifyTimestamp as a 'positive' indicator that the >> >> > > > > > > > > entry has not >> >> > > > > > > > > changed -- if the modifyTimestamp didn't change, we >> >> > > > > > > > > consider the cached >> >> > > > > > > > > entry the same as what is on the server and only bump the >> >> > > > > > > > > timestamp >> >> > > > > > > > > cache. If the timestamp is different, we do a >> >> > > > > > > > > deep-comparison of cached >> >> > > > > > > > > attribute values with what is on the LDAP server and >> >> > > > > > > > > write the sysdb >> >> > > > > > > > > cache entry only if the attributes differ. >> >> > > > > > > > > >> >> > > > > > > > > I was wondering if we can use the modifyTimestamp at all, >> >> > > > > > > > > then, because >> >> > > > > > > > > even if it's the same, we might want to check the >> >> > > > > > > > > attributes to see if >> >> > > > > > > > > some of the values are different because some of the >> >> > > > > > > > > attributes might be >> >> > > > > > > > > this operational/virtual attribute.. >> >> > > > > > > > Sorry, sent too soon. >> >> > > > > > > > >> >> > > > > > > > I think the questions are -- 1) can we enumerate the >> >> > > > > > > > virtual attributes? >> >> > > > > > > That might be a question for 389-ds developers. >> >> > > > > > > But it's very likely it will be different on other LDAP >> >> > > > > > > servers. >> >> > > > > > > >> >> > > > > > > > 2) Would different LDAP servers have different virtual >> >> > > > > > > > attributes. >> >> > > > > > > > >> >> > > > > > > > For 2) maybe a possible solution might be to set a >> >> > > > > > > > non-existing >> >> > > > > > > > modifyTimestamp attribute value, but I would consider that >> >> > > > > > > > only a >> >> > > > > > > > kludge, we shouldn't break existing setups.. >> >> > > > > > > I am not satisfied with this POC solution either. >> >> > > > > > > >> >> > > > > > > So should we remove usage of modifyTimestamp for detecting >> >> > > > > > > changes? >> >> > > > > > I would prefer to ask the DS developers before removing it >> >> > > > > > completely. >> >> > > > > > >> >> > > > > > At least for large groups it might take a long time to compare >> >> > > > > > all attribute >> >> > > > > > values and IIRC we don't depend on any virtual attributes for >> >> > > > > > groups. Maybe >> >> > > > > > we could parametrize that part of the code and enable the fast >> >> > > > > > way with >> >> > > > > > modifyTimestamps for 'known' server types, that is for setups >> >> > > > > > with AD and >> >> > > > > > IPA providers. >> >> > > > > > >> >> > > > > > For users, there is typically not as many attributes so we >> >> > > > > > should be >> >> > > > > > fine deep-comparing all attributes. >> >> > > > > I'm adding Thierry (so please reply-to-all to keep him in the >> >> > > > > thread). >> >> > > > > >> >> > > > > Thierry, in the latest sssd version we tried to add a performance >> >> > > > > improvement related to how we store SSSD entries in the cache. >> >> > > > > The short >> >> > > > > version is that we store the modifyTimestamp attribute in the >> >> > > > > cache and >> >> > > > > when we fetch an entry, we compare the entry modifyTimestamp with >> >> > > > > what >> >> > > > > is on the server. When the two are the same, we say that the >> >> > > > > entry did >> >> > > > > not change and don't update the cache. >> >> > > > > >> >> > > > > This works fine for most attributes, but not for attributes like >> >> > > > > nsAccountLock which do not change modifyTimestamp when they are >> >> > > > > modified. So when an entry was already cached but then >> >> > > > > nsAccountLock >> >> > > > > changed, we treated the entry as the same and never read the new >> >> > > > > nsAccountLock value. >> >> > > > > >> >> > > > > To fix this, I think we have several options: >> >> > > > > 1) special-case the nsAccountLock. This seems a bit >> >> > > > > dangerous, >> >> > > > > because I'm not sure we can say that some other attribute >> >> > > > > we are >> >> > > > > interested in behaves the same as nsAccountLock. >> >> > > > > 2) drop the modifyTimestamp optimization completely. Then >> >> > > > > we fall >> >> > > > > back to comparing the attribute values, which might work, >> >> > > > > but for >> >> > > > > huge objects like groups with thousands of members, this >> >> > > > > might be >> >> > > > > too expensive. >> >> > > > > 3) only use the modifyTimestamp optimization for cases >> >> > > > > where we know >> >> > > > > we don't read any virtual attributes. >> >> > > > > >> >> > > > > And my question is -- can we, in general, know if the >> >> > > > > modifyTimestamp >> >> > > > > way of detecting changes is realiable for all LDAP servers? Or do >> >> > > > > you >> >> > > > > think it should only be used for cases where we know we are not >> >> > > > > interested in any virtual attributes (that would mostly be storing >> >> > > > > groups from servers where we know exactly what is on the server >> >> > > > > side, >> >> > > > > like IPA or AD). >> >> > > > Hello, >> >> > > > >> >> > > > Relying on modifytimestamp looks a good idea. Any MOD/MODRDN will >> >> > > > update it, >> >> > > > except I think it is unchanged when updating some password policy >> >> > > > attributes. >> >> > > > >> >> > > > Regarding virtual attribute, the only one I know in IPA is >> >> > > > nsaccountlock. >> >> > > > nsaccountlock is an operational attribute (you need to request it >> >> > > > to see it) >> >> > > > and is also a virtual attribute BUT only for 'staged' and 'deleted' >> >> > > > users. >> >> > > > It is a stored attribute for regular users and we should update >> >> > > > modifytimestamp when it is set. >> >> > > > >> >> > > > thanks >> >> > > > thierry >> >> > > OK, in that case it seems like we can special-case it. But do you know >> >> > > about any other attributes in any other LDAP servers? >> >> > Any LDAP server following standard should provide modifytimestamp that >> >> > reflect the last update of the entry. Now virtual attribute values may >> >> > be >> >> > "attached" to the entry and its value change without modification of >> >> > modifytimestamp. >> >> > For 389-ds and IPA it is fine as virtual value of nsaccountlock is >> >> > changed >> >> > only when the DN change. >> >> > For others LDAP servers I suppose it exists the same ability to define >> >> > service providers that return virtual attribute values. The difficulty >> >> > is >> >> > that the schema may not give any hint if the retrieved attributes values >> >> > were stored or computed and consequently trust modifytimestamp to know >> >> > if >> >> > the values changed or not. >> >> > For example in ODSEE, memberof is a virtual attribute. >> >> Thank you, for the explanation Thierry. >> >> >> >> Then to be on the safe side I propose: >> >> 1) We add an (probably undocumented?) flag that says whether to use >> >> modifyTimestamp to detect entry changes or not >> >> 2) for the generic LDAP provider we always really compare the >> >> attribute values, in other words the option would be set to >> >> false. If there is anyone with performance issues with a generic >> >> setup, we tell them to flip the option. >> >> 3) For the IPA and AD providers, we set this option to true and use >> >> the modifyTimestamp value to detect changes >> >> 4) We special case nsAccountLock >> >> >> >> Lukas, do you agree? >> >Hi Jakub, >> > >> >digging further into the server, it appears that a DS plugin 'acctpolicy' >> >updates an entry without changing the mofidytimestamp. >> >The updated attribute is 'lastlogintime' (by default) but i think can be any >> >attribute configured in the entry account policy. >> >I need to do further tests to confirm this. >> > >> >> IMHO, the problems with nsAccountLock just revealed the fact that >> it might happen that the attribute modifyTimestamp/whenChanged needn't be >> a reliable way how to determine change in entry for any LDAP Server. >> >> We improved the performance but there might be other corner cases with >> other LDAP servers. >> >> Jakub proosed in 2) that we should always really compare the the attibutes >> from sssd cache and from LDAP search. But it would mean that we would not >> improve a performance for generic LDAP providers. > >We would still avoid the cache writes, "only" after comparing the >attribute values. So essentially by using the modifyTimestamp we save a >single LDB base search and a number of attribute-value comparisons, >depending on how large the object is. > >> >> We cannot generally detect virtual attributes for any LDAP server. >> What about adding an option where user could list virtual attributes. >> It would be a kind of proposed solution 4) but it would not be a >> special case for nsAccountLock but for more attributes which could be changed >> in configuration. > >OK, so your proposal is to keep the more aggressive cache optimization? It was just a proposal.
We can still decide to use more conservative way and do not rely on modifyTimestamp. We can also used your 3rd proposal. Add option for enabling detection for modifyTimestamp which would be disabled by default for generic LDAP servers. The question is what is the best/the safest for downstream and what is a performance enhancement. I did't do any measurements. >I think I'm mostly afraid about the more exotic LDAP servers, like IBM >Tivoli or Novell eDirectory which I already know from experience don't >follow the established standards closely. I guess I'm fine with that, we >can always flip the default back. Worst case for people who start using >1.14, the admin can always define modifyTimestamp to some non-existing >attribute and force the attribute value comparison check. > >Yes, we can make the list configurable. We also need to be >very careful about printing the reason for (not) updating the cache to >the admin (#3060). LS _______________________________________________ sssd-devel mailing list sssd-devel@lists.fedorahosted.org https://lists.fedorahosted.org/admin/lists/sssd-devel@lists.fedorahosted.org
