RE: Multiple sets in one pdu

Dave,

Sorry coming back to this topic again, how do I walk through the list in
5.x? for instance, the nsLogging example has handle_nsLoggingTable(), is
that the forth input parameter: 'requests', has all incoming varbinds? I
just go through it via "request->next"?

Actually I'm doing SET for a table in UCD 4.x style, then how do I walk
through the incoming varbinds for a SET?

Thanks a lot,
Yong Chen


The usual basic technique is to walk through the full list of
assignments, checking them individually and making any preparations for
acting on them. RESERVE1 ?
Then walk through the list again, updating the individual assignments.
RESERVE2 ?
Then walk through the list *again*, checking that the final state is
consistent. ACTION ?
Then walk through the list again, tidying up. COMMIT ?


-----Original Message-----
From: Dave Shield [mailto:D.T.Shield@csc.liv.ac.uk]=20
Sent: Thursday, October 20, 2005 1:29 AM
To: Yong Chen (yongche)
Cc: net-snmp-users@lists.sourceforge.net
Subject: RE: Multiple sets in one pdu

On Wed, 2005-10-19 at 10:52 -0700, Yong Chen (yongche) wrote:
> But in ChangeLog, it says:
>=20
> Fixes:
> - the agent should properly handle requests in the right order.
> (e.g., restrict the agent to processing only one SET at a time)

Yes - that's concerned with the processing of two *different* SET
requests. Not the processing of two varbind assignments in the
*same* SET request.


If an agent receives:

SET oneFish =3D xxx
SET twoFish =3D yyy

then it should finish handling the "oneFish" assignment *before*
starting to look at the "twoFish" assignment.

If it receives:

SET redFish =3D xxx, blueFish =3D yyy

then it needs to process *both* assignments "at the same time".


> Also the ordering of varbinds in a SET request may have some=20
> significance because of dependency between the varbinds.

That's why handling SET requests properly is Non-Trivial!

The usual basic technique is to walk through the full list of
assignments, checking them individually and making any preparations for
acting on them.
Then walk through the list again, updating the individual assignments.
Then walk through the list *again*, checking that the final state is
consistent.
Then walk through the list again, tidying up.


If one assignment ("oldFish") relies on another ("newFish"), it is *NOT*
valid to assume that the varbinds occur in the most helpful order. The
agent *MUST* be able to process both
SET oldFish =3D xxx, newFish =3D yyy
and
SET newFish =3D yyy, oldFish =3D xxx

with *exactly* the same results in either case.



> Where can I find doc regarding the simultaneously processing SET=20
> assignments?

RFC 3416, section 4.2.5
See p22, second para.


> And I've been reading the code, but not finding the "as if=20
> simultaneously" processing of SETs. Can you please point me to the=20
> code?

There is no One Single Place where this is handled.
Each MIB will have different requirements for what assignments rely on
other ones, so the "as if simultaneous" code will be specific to each
particular MIB.

For an example, have a look at 'agent/mibgroup/agent/nsLogging'.
This shows one possible approach to ensuring that SEt requests will be
processed correctly, independent of varbind order.
I probably wouldn't write this in quite the same way now, but it show
the basic ideas.

Dave


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