Hi Wenli:
You're right that dynamic catalog generation is (sort of) the problem.
As a practical matter, its not hard to figure out how often the datasets come
in, and use that as a heuristic on how often to crawl.
In principle, "Last Modified" HTTP header could tell you if the catalog has been
modified, or perhaps "Expires" is better. The problem is that the server doesn't actually
know what that should be, but perhaps we can figure out a way to add that. This would only be
approximate, but I assume that would be good enough for your purposes?
Realtime data is challenging. For gridded data, the granularity is large enough that
"Expires" is probably useful. For datasets like radar and surface obs, the answer is
always "yes, it changed since last time you asked".
John
Wenli Yang wrote:
Hello,
We are doing a project on ingesting THREDDS catalogs to OGC catalogs
(Catalog Service for Web, or CSW). We find that we have to go through
an entire THREDDS catalog to update an ingested CSW server, because we
don't know if the THREDDS catalog has been modified before exhaust it.
There is a "date" element in the
threddsMetadataGroup. The element can be used to
identify the modified (or created, valid, issued, available,
etc)date/time of a individual and/or collection dataset. This
element is very useful not only at individual dataset level but also at
data collection level. For example, suppose a data collection A
contains another collection AA which contains another collection AAA
which contains datasets a,b,c,and d (i.e.,
A>AA>AAA>a,b,c,d). If the "modified" date stamp
is applied to all the dataset nodes, individual as well as collection, a
returned user would not need to follow the complete path to find out if a
new dataset is added/modified/etc in data collection AAA and/or another
other collections in the hierarchy.
However, it seems that this "date" element is not used widely,
if any, at the data collection level. In fact, I randomly browsed
some of the data paths in Unidata's motherlode catalog
(
http://motherlode.ucar.edu:8080/thredds/catalog.html)and didn't find
any "Last Modified" information until I got to the final
dataset level.
I guess that the reason THREDDS catalog does not show modified date/time
at collection level is that the catalog is not automatically updated
when a new dataset is inserted into the database/file system connected
to the catalog. Once a user browses down to the catalog, the server
will scan the immediate child nodes to get all the available
datasets/data collections. Thus, a user browsing down the hierarchy
will always be presented the most currently available datasets although
the catalog does not update itself upon new datasets being inserted.
The disadvantage of the approach is that a user always needs to go to
the bottom level to find out if any new datasets has been inserted.
Similarly, in order to update our CSW catalog, our THREDDStoCSW ingestor
will have to scan through an entire THREDDS catalog, which can be very
large, such as the Unidata catalog.
Any comments/suggestions will be highly appreciated.
Wenli Yang
George Mosaon University
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