Re: [Netconf] Is there a problem with confirmed commits?

"Jonathan Hansford" <jonathan@hansfords.net> Mon, 14 January 2019 16:05 UTC

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From: "Jonathan Hansford" <jonathan@hansfords.net>
To: "Robert Wilton" <rwilton@cisco.com>, "netconf@ietf.org" <netconf@ietf.org>
Date: Mon, 14 Jan 2019 16:05:03 +0000
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References: <em106ef27b-c989-4e0b-b819-413fef852d53@morpheus> <20190114135056.t6sow7dbcyow6qcn@anna.jacobs.jacobs-university.de> <em5dfb175c-7835-43eb-a767-38e270601427@morpheus> <20190114154026.tbevjbcdn3oh34uz@anna.jacobs.jacobs-university.de> <2492d27d-d64f-58bd-6006-2b10128f2813@cisco.com>
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Archived-At: <https://mailarchive.ietf.org/arch/msg/netconf/RbAeQt_DDy7XjtC-gwZ5ntpkCx4>
Subject: Re: [Netconf] Is there a problem with confirmed commits?
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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
>>>>>
>>>>>   ---
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>>>>>   _______________________________________________
>>>>>   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/>