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Conference azur::mcc

Title:DECmcc user notes file. Does not replace IPMT.
Notice:Use IPMT for problems. Newsletter location in note 6187
Moderator:TAEC::BEROUD
Created:Mon Aug 21 1989
Last Modified:Wed Jun 04 1997
Last Successful Update:Fri Jun 06 1997
Number of topics:6497
Total number of notes:27359

6256.0. "DNS confidence level not working" by CSC32::J_RYER (MCI Mission Critical Support Team) Fri Mar 24 1995 03:57

    How does MCC use DNS confidence levels?
    
    My customer is running MCC V1.3 on OpenVMS V5.5-2.  On the node running
    MCC, the logical MCC_DNS_CONF is set to "LOW".  The DNS server with the
    master replica of the MCC directories is running on another VAX on the
    same Ethernet, running OpenVMS V6.1.  Last night, the DNS server went
    down for a while, and during that time, the MCC node was flooded with
    exception messages from MCC alarm rules.  The customer is questioning
    whether this means that the alarm fm or possibly the decnet_V am is
    ignoring the confidence level that they have set and instead looking
    at the server for every alarm rule firing.
    
    The alarm rule exceptions looked like this:
    	Exception:	The requested operation cannot be completed
    			%MCC-E-DNSERROR, unknown DNS error
    
    
    Any comments, answers, or advice?
    
    Thanks,
    Jane Ryer
T.RTitleUserPersonal
Name		DateLines
6256.1Current algorithm of MCC's DNS usageAZUR::KOREVAARDerk Korevaar, CBS Engineering, ValbonneFri Mar 24 1995 11:3141
Hi,

I'll explain you the current usage of DNS by MCC.

First, from you note, it is not clear whether MCI is using only a master for the
namespace or whether they have replicas. In any case there is a QAR report about
MCC's usage of DNS which makes it impossible to work if the namespace master is
down.

Let's assume you're working in low confidence mode and you need information
about an MCC entity.

o MCC requests DNS for the attributes of the entity

o DNS looks in the local cache. If the data is there, so far so good. All
  finished.

o However, some attributes are not stored by default, for instance references
  and domain members may not be there (or in the case of domains, the MCC
  information is split over several DNS entities, not all of which have the
  member information). DNS is unable to decide from the local cache whether
  these attributes have never been cached or simply do not exist. So it is
  obliged to look at the next level (replica if available else the master). If
  the entity exist, there is definite proof whether the attribute is missing or
  not. So in a way, this 'missing' information forces DNS to read the server
  over the network as there is no way to cache 'no data', diminishing the value
  of low confidence. This is a feature of DNS, not easily defeated except by
  changing the MCC way of storing attributes such that even 'missing' elements
  are always stored, making the DNS database that much bigger.

o But there is another 'feature', this time in MCC. If reading an attribute
  returns 'empty' MCC increases the confidence level and reads again. If you
  started at low confidence DNS will have tried to read at medium level anyway
  to satisfy the request for this missing attribute. But MCC will ask DNS to
  read again at confidence high, bypassing all replicas and going straight for
  the master. It is this feature of MCC that is planned for withdrawal in the
  next version of the product.

I hope this clears things up for you,

Derk Korevaar.
6256.2exCSC32::J_RYERMCI Mission Critical Support TeamFri Mar 31 1995 21:4116
    Thanks for your explanation, Derk.
    
    Two follow-on questions:
    
    1. Can you provide any more information about the QAR that you mention
    in your first paragraph?  ("about MCC's usage of DNS which makes it
    impossible to work if the namespace master is down")  In particular,
    we'd like to know if there will be a patch to MCC to correct this
    and/or will it work differently in a future release of MCC ?
    
    2. Is the last "feature" that you mention (if reading an attribute
    returns 'empty' MCC increases the confidence level and reads again)
    related to the QAR referenced above?
    
    Thanks,
    Jane
6256.3More answersAZUR::KOREVAARDerk Korevaar, CBS Engineering, ValbonneThu Apr 06 1995 11:4215
Hi,

I have not looked for the QAR number, but I'm told it exists. I know that for
British Telecom a special fix has been made by the MCC support group to avoid
the DNS retry based on an addtional environment variable.

In the OSF/1 team we are looking at a more comprehensive solution that will be
implemented in TeMIP Framework V3.

For any information about the status of this issue in the MCC maintenance
environment, you should ask Hubert Langensteiner.

Regards,

Derk.