[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[LDM #FOQ-439834]: Question about PRIMARY and SECONDARY

This archive contains answers to questions sent to Unidata support through mid-2025. Note that the archive is no longer being updated. We provide the archive for reference; many of the answers presented here remain technically correct, even if somewhat outdated. For the most up-to-date information on the use of NSF Unidata software and data services, please consult the Software Documentation first.

  • Subject: [LDM #FOQ-439834]: Question about PRIMARY and SECONDARY
  • Date: Tue, 14 Jun 2011 17:17:58 -0600
Niles,

> We have a machine, alpha, which only has resources to allow one
> downstream machine to connect to its LDM without getting overloaded, and
> we have two machines, beta and gamma, that need data.
> 
> We want gamma to request from beta, so the flow goes :
> 
> alpha -> beta -> gamma
> 
> But if the LDM on beta stops, we want gamma to failover and request
> direct from alpha :
> 
> alpha -> gamma
> 
> and then we want gamma to go back to requesting from beta when beta
> recovers.
> 
> 
> My question is, if we put the following in the LDM's ldmd.conf file on
> gamma :
> 
> request NLDN    ".*"    beta      PRIMARY
> request NLDN    ".*"    alpha     ALTERNATE
> 
> Will that do what we want? In the event of beta failing and then
> recovering, will gamma immediately switch back to using beta or will
> there be an interim in which alpha will be feeding both beta and gamma?
> And if there is an interim, how long will it be?

Unless you have an old version of LDM 6, the words PRIMARY and ALTERNATE don't 
matter: the first entry for a given feedtype and pattern will start in PRIMARY 
mode and all other entries for the same data will start in ALTERNATE mode.

The given entries will cause Gamma to receive data from both Alpha and Beta. 
The upstream system that is able to get the data to Gamma quicker will be the 
one that is in PRIMARY mode (regardless of the keyword in the entry). If the 
connection from Beta happens to be in Primary mode and Beta goes offline, then 
the ALTERNATE connection from Alpha will be reestablished in PRIMARY mode. If 
and when Beta comes back online, the connection that becomes the primary mode 
connection will depend solely on which upstream host is able to deliver the 
data-products more quickly.

I recommend having both Beta and Gamma request from Alpha as well as from each 
other. I haven't tried this, but I expect that it will work even though there 
might be a lot of mode shifting.

If you try this, please keep me apprised.

> Thanks,
> 
> Niles.

Regards,
Steve Emmerson

Ticket Details
===================
Ticket ID: FOQ-439834
Department: Support LDM
Priority: Normal
Status: Closed