[Netconf] Is there a problem with confirmed commits?

"Jonathan Hansford" <jonathan@hansfords.net> Mon, 14 January 2019 12:50 UTC

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From: "Jonathan Hansford" <jonathan@hansfords.net>
To: "netconf@ietf.org" <netconf@ietf.org>
Date: Mon, 14 Jan 2019 12:50:38 +0000
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Subject: [Netconf] Is there a problem with confirmed commits?
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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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