[ldm-users] example of LDM 6.11 disallowing overlapping data-feed requests


I just had a report sent-in about a downstream site no longer receiving METAR data from its upstream site. I think the situation is a good example of the new feature that was added to LDM 6.11 and want to share it with you so that you'll know what to do if you get bitten by this new feature.

The situation was this: the downstream site had REQUEST entries for both the WMO and DDS data-feeds. The DDS data-feed is a subset of the WMO data-feed. Consequently, the upstream LDM 6.11 rejected the DDS request:

Sep 26 17:00:06 upstream-host downstream-host[15832] NOTE: [uldb.c:255] 
feedtypes: requested=DDS, extant=WMO
Sep 26 17:00:06 upstream-host downstream-host[15832] NOTE: [uldb.c:1206] 
Upstream LDM
is disallowed
Sep 26 17:00:06 upstream-host downstream-host[15832] NOTE: [uldb.c:1242] 
request from downstream-host
Sep 26 17:00:06 upstream-host downstream-host[15832] NOTE: [uldb.c:1771] 
Couldn't add
entry to database
Sep 26 17:00:06 upstream-host downstream[15832] NOTE: [ldm6_server.c:335] New
upstream LDM process is disallowed

This situation is both inefficient and potentially dangerous. It's inefficient because the DDS data-products would be sent twice (not too bad unless the DDS request was actually for something like CONDUIT data or the bandwidth to the downstream site was low). It's potentially dangerous because a DDS data-product could conceivably be processed twice -- leading to unknown or unexpected behavior.

The solution was to remove the REQUEST entry for DDS data at the downstream site. The DDS data-products are included in the WMO data-feed, so nothing will be lost.

Having non-overlapping REQUEST entries has always been something that was strongly recommended but never enforced. The addition of the new anti-denial-of-service feature means that mis-configured LDM configuration-files will now be caught.

You should check your LDM configuration-file for overlapping requests to the same upstream LDM.

Please send any questions to <support-ldm@xxxxxxxxxxxxxxxx>. I'll copy them to this list if I think they're of general interest.

Steve Emmerson

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