Re: [Netconf] Is there a problem with confirmed commits?
Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de> Mon, 14 January 2019 16:19 UTC
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Date: Mon, 14 Jan 2019 17:19:19 +0100
From: Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de>
To: Jonathan Hansford <jonathan@hansfords.net>
CC: Robert Wilton <rwilton@cisco.com>, "netconf@ietf.org" <netconf@ietf.org>
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Subject: Re: [Netconf] Is there a problem with confirmed commits?
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Someone can modify candidate between your discard_changes() and lock(). I have to check whether lock requires <candidate/> to match <running/>. /js On Mon, Jan 14, 2019 at 04:05:03PM +0000, Jonathan Hansford wrote: > And maybe a discard_changes before the lock is better behaviour (or is there > something that prevents that?) to ensure the lock will succeed. > > ------ Original Message ------ > From: "Robert Wilton" <rwilton@cisco.com> > To: "netconf@ietf.org" <netconf@ietf.org> > Cc: "Jonathan Hansford" <jonathan@hansfords.net> > Sent: 14/01/2019 15:48:58 > Subject: Re: [Netconf] Is there a problem with confirmed commits? > > > Hi Juergen, > > > > On 14/01/2019 15:40, Juergen Schoenwaelder wrote: > > > It seems the <candidate> datastore should not be allowed to be used as > > > long as a persistent confirmed commit is still ongoing. I leave it to > > > Martin to check whether this is said somewhere or an omission. > > > > > > In general, an application can't assume that <candidate> contains > > > anything sensible. Hence, the proper way is to lock <candidate> and > > > then to make sure it contains something sensible, i.e., issuing a > > > discard_changes. > > > > But the text that you quote below states that a client cannot acquire a lock on candidate if it contains any changes. Doesn't this implies that discard_changes after acquiring the lock should be unnecessary? > > > > Thanks, > > Rob > > > > > > > And I think implementations should not allow an > > > application to obtain a lock on <candidate> while a commit is active. > > > The text on page 45 already says: > > > > > > A lock MUST NOT be granted if any of the following conditions is > > > true: > > > > > > [...] > > > > > > * The target configuration is <candidate>, it has already been > > > modified, and these changes have not been committed or rolled > > > back. > > > > > > I think this covers the case of an ongoing but not completed > > > persistent confirmed commit, no? > > > > > > /js > > > > > > On Mon, Jan 14, 2019 at 03:14:02PM +0000, Jonathan Hansford wrote: > > > > If a persistent confirmed commit has not timed out, the running > > > > configuration datastore will be the same as the candidate and > > > > <discard-changes> won't change its contents. Any edit of candidate will be > > > > based on the configuration resulting from the persistent confirmed commit. > > > > > > > > If the persistent confirmed commit has timed out, the running configuration > > > > datastore will have reverted and <discard-changes> will change candidate. > > > > Any edit of candidate in this case will be based on the configuration prior > > > > to the start of the persistent confirmed commit. > > > > > > > > ------ Original Message ------ > > > > From: "Juergen Schoenwaelder" <j.schoenwaelder@jacobs-university.de> > > > > To: "Jonathan Hansford" <jonathan@hansfords.net> > > > > Cc: "netconf@ietf.org" <netconf@ietf.org> > > > > Sent: 14/01/2019 13:50:56 > > > > Subject: Re: [Netconf] Is there a problem with confirmed commits? > > > > > > > > > Hi, > > > > > > > > > > I have not yet understood where you see a problem. In general, > > > > > <candidate/> contains arbitrary stuff and hence it is the client's > > > > > responsibility to clear any arbitrary stuff found in <candidate/> > > > > > after obtaining a lock. If does not really matter whether there has > > > > > been a failed confirmed commit before or something else. I think the > > > > > general safe pattern is: > > > > > > > > > > lock(candidate) > > > > > discard_changes() > > > > > push_whatever_needed() > > > > > commit() > > > > > unlock(candidate) > > > > > > > > > > If you do a confirmed commit and the session disappears, then the lock > > > > > will disappear as well. But I do not think this creates a race > > > > > condition, or I am just not yet seeing it. Perhaps it helps to write > > > > > down the sequence of actions that leads to a race. > > > > > > > > > > /js > > > > > > > > > > On Mon, Jan 14, 2019 at 12:50:38PM +0000, Jonathan Hansford wrote: > > > > > > Hi, > > > > > > > > > > > > No one seems to be responding to my email and proposed erratum around > > > > > > the subject of confirmed commits (apart from Martin), but I would really > > > > > > like to know it I am missing something here. As far as I can tell, > > > > > > session termination during a confirmed commit leads to unpredictable > > > > > > behaviour and I would like to know whether anyone is using confirmed > > > > > > commits and how (if at all) they address the issues outlined below. My > > > > > > assumptions are that locks are used and :writable-running is not > > > > > > supported. > > > > > > > > > > > > If the <candidate> and <running> configuration datastores are locked to > > > > > > prevent concurrent access, and a confirmed commit sequence is > > > > > > interrupted by the session terminating, the locks will automatically be > > > > > > released but the server MUST NOT accept a lock on <running> from any > > > > > > session if another session has an ongoing confirmed <commit>. > > > > > > Consequently, after session termination no client can acquire a <lock> > > > > > > on <running>, not even the one that initiated the confirmed <commit>, > > > > > > until after the confirmed <commit> has timed out. However, if the > > > > > > confirmed <commit> included the <persist> parameter, the original client > > > > > > could still issue a <commit> using the persist-id to complete the > > > > > > sequence prior to the timeout, even without a lock. > > > > > > > > > > > > Of course, the problem now is the race for the new lock on <candidate>. > > > > > > If the original client is successful then all is good. But if a new > > > > > > client locks <candidate> before the timeout on the confirmed commit, > > > > > > whether or not they precede <lock> with <discard-changes>, <candidate> > > > > > > will be the same as <running> and the new client will pick up everything > > > > > > from the previous session. However, the client won’t be able to lock > > > > > > <running> until after the timeout, at which point <running> reverts but > > > > > > <candidate> still represents the previous session. If the client tries > > > > > > to lock <candidate> after the timeout, <running> will have reverted and > > > > > > the lock will only be granted after a <discard-changes> which will cause > > > > > > the <candidate> to revert. So, depending on when the lock on <candidate> > > > > > > occurs relative to the confirmed commit timeout, the client could be > > > > > > editing <candidate> in one of two states. Further, before the timeout on > > > > > > the confirmed commit, even if the new client has locked candidate, the > > > > > > original client could still issue a confirming commit (they don’t need a > > > > > > lock on <candidate> to do so) which would persistently commit any edits > > > > > > made by the new client. NOTE: it is not the use of the persist-id that > > > > > > introduces this behaviour; a new client would have the same problem even > > > > > > if a confirmed commit was not intended to persist beyond a session > > > > > > termination. > > > > > > > > > > > > If the server also supports the :startup capability then, if the session > > > > > > termination was due to the server rebooting, the behaviour above would > > > > > > be further complicated by <running> now containing the configuration > > > > > > from the <startup> configuration datastore. > > > > > > > > > > > > Am I right? > > > > > > > > > > > > Jonathan > > > > > > > > > > > > --- > > > > > > This email has been checked for viruses by Avast antivirus software. > > > > > > https://www.avast.com/antivirus > > > > > > _______________________________________________ > > > > > > Netconf mailing list > > > > > > Netconf@ietf.org > > > > > > https://www.ietf.org/mailman/listinfo/netconf > > > > > > > > > > -- > > > > > Juergen Schoenwaelder Jacobs University Bremen gGmbH > > > > > Phone: +49 421 200 3587 Campus Ring 1 | 28759 Bremen | Germany > > > > > Fax: +49 421 200 3103 <https://www.jacobs-university.de/> > > _______________________________________________ > Netconf mailing list > Netconf@ietf.org > https://www.ietf.org/mailman/listinfo/netconf -- Juergen Schoenwaelder Jacobs University Bremen gGmbH Phone: +49 421 200 3587 Campus Ring 1 | 28759 Bremen | Germany Fax: +49 421 200 3103 <https://www.jacobs-university.de/>
- [Netconf] Is there a problem with confirmed commi… Jonathan Hansford
- Re: [Netconf] Is there a problem with confirmed c… Martin Bjorklund
- Re: [Netconf] Is there a problem with confirmed c… Juergen Schoenwaelder
- Re: [Netconf] Is there a problem with confirmed c… Jonathan Hansford
- Re: [Netconf] Is there a problem with confirmed c… Juergen Schoenwaelder
- Re: [Netconf] Is there a problem with confirmed c… Jonathan Hansford
- Re: [Netconf] Is there a problem with confirmed c… Jonathan Hansford
- Re: [Netconf] Is there a problem with confirmed c… Juergen Schoenwaelder
- Re: [Netconf] Is there a problem with confirmed c… Robert Wilton
- Re: [Netconf] Is there a problem with confirmed c… Jonathan Hansford
- Re: [Netconf] Is there a problem with confirmed c… Jonathan Hansford
- Re: [Netconf] Is there a problem with confirmed c… Juergen Schoenwaelder
- Re: [Netconf] Is there a problem with confirmed c… Juergen Schoenwaelder
- Re: [Netconf] Is there a problem with confirmed c… Andy Bierman
- Re: [Netconf] Is there a problem with confirmed c… jonathan
- Re: [Netconf] Is there a problem with confirmed c… Juergen Schoenwaelder
- Re: [Netconf] Is there a problem with confirmed c… Jonathan Hansford
- Re: [Netconf] Is there a problem with confirmed c… Juergen Schoenwaelder
- Re: [Netconf] Is there a problem with confirmed c… Jonathan Hansford
- Re: [Netconf] Is there a problem with confirmed c… Juergen Schoenwaelder
- Re: [Netconf] Is there a problem with confirmed c… Jonathan Hansford