[thredds] Fwd: Fwd: [uaf_tech] Re: time start / end

This is a message from Bob Simons, the developer of ERDDAP, that I think is of general interest, in case others want to comment also.

-------- Original Message --------
Subject:        Fwd: [uaf_tech] Re: time start / end
Date:   Fri, 11 Jun 2010 09:58:50 -0700
From:   Bob Simons <Bob.Simons@xxxxxxxx>
Organization:   NOAA/ERD
To:     John Caron <caron@xxxxxxxxxxxxxxxx>

In case you aren't on the uaf_tech mail list, this is my pitch for
adding a subscription service to THREDDS. (I understand you are very
busy and this is likely low priority.)

P.S. I also wonder if there is another solution.  Why does THREDDS need
metadata from the catalog.xml files to determine the dataset's start and
end time? Doesn't it have this information from when it does the
dataset's aggregation?
For that matter, it seems like a lot of the metadata we put in the
catalog.xml files could be gathered by THREDDS from the dataset's
metadata and data.  Could THREDDS be modified to use the dataset's
metadata and data so we wouldn't have to duplicate the information in
the catalog.xml files?

Thank you.

-------- Original Message --------
Subject: [uaf_tech] Re: time start / end
Date: Thu, 10 Jun 2010 10:42:03 -0700
From: Bob Simons<Bob.Simons@xxxxxxxx>
Organization: NOAA/ERD
To: _OAR PMEL UAF Tech List<uaf_tech@xxxxxxxx>

My $0.02:

The Ideal - The ideal situation is to have Start and End have specific
dates and times, e.g.,
 Start: 2010-06-03 12:00:00Z
 End: 2010-06-10 12:00:00Z
and to have this always perfectly up-to-date.

Statement of Intent - Something like
  Start: present - 7 days
  End: present
is pretty good as is, because it is a statement of intent.

Not Really Right - If it gets translated to some instantaneous values, e.g.,
 Start: 2010-06-03 12:04:57Z
 End: 2010-06-10 12:04:57Z
then it is less desirable. It implies accuracy and precision, but isn't
correct (e.g., perhaps the dataset is just updated daily sometime every

Polling - Having a downstream server (e.g., RAMADA or ERDDAP) frequently
check with TDS to find out the actual Start and End times isn't ideal.
The extremes cases are
* The downstream server polls infrequently, and so is usually way
* The downstream server polls frequently, and so is closer, but never
perfectly up-to-date. The problem with polling is: if lots of downstream
servers are polling 100's of datasets frequently, it can be a burden on
the TDS.  So polling is never an ideal solution.
(Note that one implementation of polling is RSS.)

Subscriptions: it would be great if TDS had a subscription service, so
that TDS would automatically send an email or ping some client specified
URL whenever the dataset changed. This is *much* more efficient than
polling, and the downstream servers are notified within seconds when a
dataset changes. With subscriptions, the downstream server could display
accurate (up-to-date) and precise Start and End times.  And people would
find other uses for a general purpose subscription system.

As an example of how subscriptions are useful:
* A subscription system
  and more specifically
* A flag system
If one ERDDAP is pointing to a dataset at a remote ERDDAP, it subscribes
to the remote ERDDAP's dataset (humans have to confirm the subscriptions).
Whenever the remote dataset changes, the remote ERDDAP contacts a
special URL on the first ERDDAP to set a flag, which indicates that a
specific dataset should be reloaded/checked because it has changed.
As soon as possible, the first ERDDAP reloads the dataset.
So the two ERDDAPs stay in synch, usually within a few seconds.
It would be great if TDS could offer a similar subscription system so
other TDS installations, ERDDAP, RAMADA, and other clients could be
notified immediately whenever a specific TDS dataset changes.

On 6/10/2010 9:03 AM, Kevin O'Brien wrote:

 Below is a bounced message from John Caron.....


 BOUNCE uaf_tech@xxxxxxxx: Non-member submission from [John Caron
 Thu, 10 Jun 2010 05:13:15 -0700


 Date: Thu, 10 Jun 2010 06:13:06 -0600
 From: John Caron<caron@xxxxxxxxxxxxxxxx>
 Subject: Re: [uaf_tech] Next UAF telcon: June 10th, 12:30pm EDT
 To: Rich Signell<rsignell@xxxxxxxx>
 Cc: Ted Habermann<ted.habermann@xxxxxxxx>,
          Steve Hankin<Steven.C.Hankin@xxxxxxxx>,
          David Neufeld<David.Neufeld@xxxxxxxx>,
          _OAR PMEL UAF Tech List<uaf_tech@xxxxxxxx>,
          Ethan Davis<edavis@xxxxxxxxxxxxxxxx>,

 Hi Rich, et al:

 I agree that modifying NcML in the TDS when files arrive is not a viable solution. You 
need to use a scan element for this, although we are replacing<scan>   elements 
with<collection>   elements (in FMRC right now, will be extended to other 
aggregations in 4.3).

 1) Specifying the time range in the catalog for this case is possible. Heres 
how we do it on motherlode:

            <duration>7 days</duration>

 this means that the starting time is "present" - 7 days. The TDS generates the 
actual ISO dates in the catalog, eg at this moment:


 Start: 2010-06-03 12:04:57Z
 End: 2010-06-10 12:04:57Z
 Duration: 7 days

 A bit more detail at:


 2) One can also generate time ranges from the filename, see "Adding 
timeCoverage" in


 this is used when you have files with the starting time embedded in the 
filename and a known duration.

 3) We are moving towards automatic generation of the time coverage, as Rich 
mentioned, we do that now in the FMRC, and we will try to extend that to other 
aggregations where the time coordinate can be extracted

 Not sure if I covered all the issues.


 Rich Signell wrote:


 Sorry to sent this twice, but I wanted to cc John Caron and Ethan
 Davis to allow them to comment.


 On Wed, Jun 9, 2010 at 6:02 PM, Rich Signell<rsignell@xxxxxxxx>   wrote:


 With time aggregations, the virtual dataset is served dynamically via
 THREDDS as new data arrives without modifying the underlying catalog
 that specifies the aggregation.    We don't want to be modifying NcML
 in the catalog every time a file arrives.   So it seems we have two
 choices:  (1) have the crawler actually read the last time value and
 since it's CF-compliant, this is easy (there is a NetCDF-Java function
 for this).   I think both ncISO and RAMADDA already do this.   (2) we
 ask Unidata to modify the TDS so that it automatically generates the
 stop time as THREDDS metadata.  It already does this for FRMC
 aggregations.   On the plus side, this ensures that we get the right
 time without reading the time values.  The disadvantage is that it
 would only work for TDS served data.


 On Wed, Jun 9, 2010 at 5:42 PM, Ted Habermann<ted.habermann@xxxxxxxx>   wrote:

 Rich et al.,

 Seems to me our first choice should be to use an existing standard for
 describing time periods. In my experience the most commonly used is ISO
 8601. Describing time periods of known duration is straightforward if we
 know the starting point. For example a period with duration 7 days starting
 today would be: 20100609/P7D. There are probably a couple ways to expressing
 this explicitly in NcML:

 <attribute name="time_coverage_start" value="2010-06-09"/>
 <attribute name="time_coverage_duration" value="P7D"/>

 or, it may make sense to just calculate the end time and write it into the

 <attribute name="time_coverage_start" value="2010-06-09"/>
 <attribute name="time_coverage_end" value="2010-06-16"/>

 If we are dealing with collection level NcML (?), one could say
 <attribute name="time_coverage_start" value="present"/>
 <attribute name="time_coverage_duration" value="P7D"/>

 I'm not sure off hand how this would get translated to ISO. Maybe
        <gml:TimePeriod gml:id="t3">
          <gml:beginPosition indeterminatePosition="now"/>


 On 6/9/2010 12:34 PM, Steve Hankin wrote:

 David Neufeld wrote:

 Hi Rich, Steve,

 I think if we move toward a model where metadata is handled as a service as
 opposed to a static file this problem starts to go away.

 Agree in principle.  I have argued this same pov with Ted -- that we should
 not insist that metadata be inserted into files, if that metadata is
 derivable from information already contained in the file.

 Ideas for implementing this approach?  The most appealing to me is that TDS,
 itself, would generate data discovery metadata such as

 time_coverage_start = "present minus 30 days";   // a running archive
 time_coverage_end = "present plus 10 days";   // a forecast

 based upon coordinates and use metadata found inside the dataset, and
 perhaps some new ncML directives that govern the "metadata service".  But
 the questions remain: who would do this work and when?  And what should UAF
 do in the interim (i.e. now)?

      - Steve

 So for example, if we generate metadata dynamically and it contains the
 standard static attributes along side of dynamically retrieved values for
 geographic and temporal bounds then we're in good shape at the catalog
 level.  There is still the issue of how often to harvest the metadata in
 other clearinghouses like RAMADDA or Geonetwork, but that can be left more
 for the portal provider to determine.


 On 6/9/2010 10:39 AM, Steve Hankin wrote:

 Rich Signell wrote:

 UAF Folks,

 I can't make the 12:30 ET/9:30 PT meeting tomorrow, but here are my two

 Hi Rich,

 Sorry you cannot make it.   With that in mind have started the conversations
 here by email ...

 1) How to handle temporal metadata for time aggregated datasets that are
 changing every day (or perhaps every 15 min for the HF Radar measurements).
 I got bit by this when I did a temporal/ geospatial search in RAMADDA for
 UAF data in the Gulf of Mexico during the last week and turned up no
 datasets.  It should have turned up the NCOM Region 1 model data, HF radar
 data and USGS COAWST model results.   I'm pretty sure the problem is that
 RAMADDA harvested the data from the clean catalog more than a week ago, so
 the "stop dates" in the metadata database are older than one week ago.   How
 should this best be fixed?

 Might this be best addressed by using the Unidata Discover Attribute
 They offer the global attribute:

     time_coverage_end = "present"

 Arguably within UAF we should insert such global attributes into the
 relevant datasets and also work to communicate the need back to the data
 providers to do so on their own THREDDS servers.  An alternative to consider
 is putting this information into the THREDDS metadata instead of into the
 ncML of the dataset.

 btw: A seeming omission in the Unidata recommendations is any way to
 represent "3 months ago" as the start time.  A start time of this style is
 pretty common in operational outputs.

 2) How to represent FMRC data.   If we scan a catalog with a Forecast Model
 Run Collection we currently get hundreds of datasets, because the FRMC
 automatically produces datasets for the daily forecasts as well as the "Best
 Time Series" dataset that most people are interested in.   In the latest
 version of the Thredds Data Server (4.2 beta), the provider can specify they
 only want the best time series dataset to be exposed.   This will help
 significantly, but it will take a while to get everybody with FMRCs
 retrofit.   I will bring this up on the Model Data Interoperability Google

 Might be best to hold off this topic until you are on the phone, since you
 are our resident expert.  No?

     - Steve

 ==== Ted Habermann ===========================
       Enterprise Data Systems Group Leader
       NOAA, National Geophysical Data Center
       V: 303.497.6472   F: 303.497.6513
       "I entreat you, I implore you, I exhort you,
       I challenge you: To speak with conviction.
       To say what you believe in a manner that bespeaks
       the determination with which you believe it.
       Because contrary to the wisdom of the bumper sticker,
       it is not enough these days to simply QUESTION AUTHORITY.
       You have to speak with it, too."
       Taylor Mali,www.taylormali.com
 ====Ted.Habermann@xxxxxxxx  ==================

 Dr. Richard P. Signell   (508) 457-2229
 USGS, 384 Woods Hole Rd.
 Woods Hole, MA 02543-1598

 Kevin O'Brien                   UW/JISAO       
 Research Scientist              NOAA/PMEL/TMAP

 "The contents of this message are mine personally and do
 not necessarily reflect any position of the Government
 or the  National Oceanic and Atmospheric Administration."


Bob Simons
IT Specialist
Environmental Research Division
NOAA Southwest Fisheries Science Center
1352 Lighthouse Ave
Pacific Grove, CA 93950-2079
Phone: (831)658-3205
Fax:   (831)648-8440
Email: bob.simons@xxxxxxxx

The contents of this message are mine personally and
do not necessarily reflect any position of the
Government or the National Oceanic and Atmospheric
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