Re: [netmod] a question about 'when'

Ladislav Lhotka <lhotka@nic.cz> Wed, 07 August 2019 10:15 UTC

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From: Ladislav Lhotka <lhotka@nic.cz>
To: "Rob Wilton (rwilton)" <rwilton@cisco.com>, Andy Bierman <andy@yumaworks.com>, "Fengchong (frank)" <frank.fengchong@huawei.com>, "netmod@ietf.org" <netmod@ietf.org>, "Zhangxiaoping (C)" <zhang.xiaoping@huawei.com>, liuzhiying <liuzhiying@huawei.com>
Date: Wed, 07 Aug 2019 12:15:05 +0200
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Subject: Re: [netmod] a question about 'when'
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On Wed, 2019-08-07 at 09:07 +0000, Rob Wilton (rwilton) wrote:
> I can see that 'when automatic deletion' processing can be useful if the
> configuration is being manipulated by a human.  E.g. if I delete a VRF then
> all the configuration that references that VRF can magically
> disappear.  Assuming the server supports config rollback then even if I make a
> catastrophic mistake, it isn't usually that hard to recover from.
> 
> But for a fully automated client, then I agree with Lada, in that I see the
> server side 'when automatic deletion' processing as unhelpful.  The client
> logically needs to know/understand the full configuration anyway, so it should
> be able to generate the complete configuration change required to update the
> server with a new valid configuration state.  In these scenarios, having the
> server perform 'when automatic deletion' processing seems to increase the risk
> that that client and server views of the configuration could end up out of
> sync.  Some clients simplify the protocol operations by always doing a config
> replace on every config change to guarantee that the copy of the configuration
> on the server matches what is in the client.
> 
> For clients that exist somewhere between no automation and full automation,
> then I can imagine that for some cases 'when automatic deletion' processing
> might be useful, and other cases where it is unhelpful.
> 
> Personally, I would have preferred that the 'when automatic deletion'
> processing was controlled via an explicit protocol option, with the default
> behaviour to just validate when statements (equivalently to must statements)
> and not perform any automatically config deletion.

I agree. In any case, protocol behaviour like this should not be a part of YANG
specification. This is one of the things that need to be removed in 7950bis.

Lada

> 
> Thanks,
> Rob
> 
> 
> -----Original Message-----
> From: netmod <netmod-bounces@ietf.org>; On Behalf Of Ladislav Lhotka
> Sent: 07 August 2019 08:39
> To: Andy Bierman <andy@yumaworks.com>;; Fengchong (frank) <
> frank.fengchong@huawei.com>;; netmod@ietf.org; Zhangxiaoping (C) <
> zhang.xiaoping@huawei.com>;; liuzhiying <liuzhiying@huawei.com>;
> Subject: Re: [netmod] a question about 'when'
> 
> Andy Bierman <andy@yumaworks.com>; writes:
> 
> > On Mon, Aug 5, 2019 at 2:49 AM Ladislav Lhotka <lhotka@nic.cz>; wrote:
> > 
> > > "Fengchong (frank)" <frank.fengchong@huawei.com>; writes:
> > > 
> > > > Hi all,
> > > > 
> > > > I encounter a question about 'when', when I implement yang model
> > > associated when condition.
> > > > Yang model:
> > > > 
> > > > leaf password-type {
> > > >    type enumeration {
> > > >       enum null;
> > > >       enum simple;
> > > >       enum cipher;
> > > >    }
> > > > }
> > > > 
> > > > leaf password-text {
> > > > type string;
> > > > when "../password-type != null";
> > > > }
> > > > 
> > > > I config these two leafs as below:
> > > > <password-type>simple</password-type>
> > > > <password-text>123456</password-text>
> > > > 
> > > > And I changed password-type to null, I get the config like below:
> > > > <password-type>null</password-type>
> > > > 
> > > > And then, I reconfig the password-type to simple, what data should 
> > > > be
> > > returned?
> > > > Is
> > > >   <password-type>simple</password-type>
> > > 
> > > According to RFC 7950, sec. 8.2, the server deleted "password-text" 
> > > after you changed "password-type" to null but the original value 
> > > isn't recovered after you change the type back.
> > > 
> > > This server behaviour means that a typo or similar trivial error may 
> > > have catastrophic consequences such as auto-deletion of entire 
> > > configuration subtrees. That's why our RESTCONF implementation 
> > > (jetconf) does something
> > > else: it won't permit you to change "password-type" to null as long 
> > > as the "password-text" exists.
> > > 
> > > 
> > It seems odd to optimize the server for client mistakes.
> 
> This is just the principle of least embarrassment. The problem is that it is
> not indicated in the data model that deleting or changing something may have
> far-reaching consequences.
> 
> > It is far more likely (99 to 1?) that the client knows what it is 
> > doing and expects the standard to be followed.  Consider the burden on 
> > the client deleting all the "false-when" nodes manually. This is
> 
> If it is a significant burden, then it's also quite likely that the client may
> not be completely aware of what's going to be auto-deleted.
> 
> > also inconsistent with the standard behavior for choice-stmt (new case 
> > deletes the old case automatically).
> 
> This is quite different in that the impact is localized: one can easily see
> that a given leaf is a case in a choice so that it cannot exist along with
> another case.
> 
> Lada
> 
> > Lada
> > > 
> > Andy
> > 
> > 
> > > > Or
> > > > 
> > > >   <password-type>simple</password-type>
> > > >   <password-text>123456</password-type>
> > > > _______________________________________________
> > > > netmod mailing list
> > > > netmod@ietf.org
> > > > https://www.ietf.org/mailman/listinfo/netmod
> > > 
> > > --
> > > Ladislav Lhotka
> > > Head, CZ.NIC Labs
> > > PGP Key ID: 0xB8F92B08A9F76C67
> > > 
> > > _______________________________________________
> > > netmod mailing list
> > > netmod@ietf.org
> > > https://www.ietf.org/mailman/listinfo/netmod
> > > 
> > _______________________________________________
> > netmod mailing list
> > netmod@ietf.org
> > https://www.ietf.org/mailman/listinfo/netmod
> 
> --
> Ladislav Lhotka
> Head, CZ.NIC Labs
> PGP Key ID: 0xB8F92B08A9F76C67
> 
> _______________________________________________
> netmod mailing list
> netmod@ietf.org
> https://www.ietf.org/mailman/listinfo/netmod
-- 
Ladislav Lhotka
Head, CZ.NIC Labs
PGP Key ID: 0xB8F92B08A9F76C67