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

[LDM #OQD-608743]: ldm failover question

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 Note that the archive is no longer being updated. We provide the archive for reference; many of the answers presented here remain technically correct, even if somewhat outdated.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 #OQD-608743]: ldm failover question
  • Date: Wed, 28 Oct 2009 12:24:13 -0600
Sue,

> Can you tell me how the downstream
> LDM makes the determination of which upstream host is PRIMARY
> and which is ALTERNATE ?   Does this determination happen
> on a product by product basis ? Is it traceable by the LDM user?

A downstream LDM process keeps track of its success in adding data-products to 
the product-queue and also knows how many other downstream LDM peer-processes 
have requested the same data.

A downstream LDM process in alternate mode will disconnect and reconnect in 
primary mode if and only if it has been more successful in adding data-products 
to the product-queue than any other peer-process over the last minute.

Similarly, a downstream LDM process in primary mode will disconnect and 
reconnect in alternate mode if and only if it has not been more successful in 
adding data-products to the product-queue than any other peer-process over the 
last minute.

Downstream LDM processes log when they switch modes.

The rtstats(1) web-pages can be used to determine the relative efficiency of 
routes.

> I am unclear on what it means to "ensure that the data-product signatures
> (i.e., MD5 checksums) [are] computed identically. "
> How does the LDM user do this?

The product-signature is computed by the LDM ingester responsible for creating 
the data-product and adding it to the product-queue.

> Certainly the script which creates product ids, feed-type,
> and the insert command are indentical on upstream hosts.

If the data-products are inserted using the pqinsert(1) utility in identical 
ways, then the product-signatures will be identical.

> It is possible that the underlying data for a particular
> product id  could differ if one host is experiencing
> some data latency issues that effect product generation.
> Is that an issue that could cause duplicate products( product ids)
> in the queue?

The only way I can think of that this would happen is if the pqinsert(1) 
utility was told to use the product-identifier rather than the product data as 
the basis for the product-signature and the identifier encoded the time.

Regards,
Steve Emmerson

Ticket Details
===================
Ticket ID: OQD-608743
Department: Support LDM
Priority: Normal
Status: Closed