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The service Element

Compound service Elements - Serving Datasets with Multiple Methods

Datasets can be made available through more than one access method by defining and then referencing a compound service element. The following:

<service name="all" serviceType="Compound" base="" >
    <service name="odap" serviceType="OpenDAP" base="/thredds/dodsC/" />
    <service name="wcs" serviceType="WCS" base="/thredds/wcs/" />
</service>

defines a compound service named "all" which contains two nested services. Any dataset that reference the compound service will have two access methods. For instance:

<dataset name="cool data" urlPath="so/cool/data.nc" >
    <serviceName>all</serviceName>
</dataset>

would result in these two access URLs:

/thredds/dodsC/so/cool/data.nc

/thredds/wcs/so/cool/data.nc

Note: The contained services can still be referenced independently. For instance:

<dataset name="more cool data" urlPath="more/cool/data.nc" >
    <serviceName>odap</serviceName>
</dataset>

results in a single access URL:

/thredds/dodsC/more/cool/data.nc

Unique Names Required for each service Element in a Catalog

Service names are used by datasets to reference the service element that represents the available service(s) for that dataset. So that the service reference resolves to a unique service element, all service names within a given catalog must be unique.

Here's an example:

<service name="any" serviceType="Compound" base="" >
    <service name="service1" serviceType="OpenDAP" base="/thredds/dodsC/" />
    <service name="service2" serviceType="HTTPServer" base="/thredds/fileServer/" />
</service>
<service name="grid" serviceType="Compound" base="" >
    <service name="service1" serviceType="OpenDAP" base="/thredds/dodsC/" />
    <service name="service2" serviceType="WCS" base="/thredds/wcs/" />
    <service name="service3" serviceType="WMS" base="/thredds/wms/" />
    <service name="service4" serviceType="HTTPServer" base="/thredds/fileServer/" />
</service>

Notes:

• A dataset that references "service1" will be fine. But only because both "service1" instances have the same serviceType and base URL.
• A dataset that references "service2" may find either the HTTP file service or the WCS service.

THREDDS Metadata

Linking to Metadata

<metadata xlink:title="some good metadata" xlink:href="http://my.server/md/data1.xml" />

Linking to Human Readable Metadata

<description xlink:title="My Data" xlink:href="http://my.server/md/data1.html" />

Inherited Metadata

...
    <dataset name="TDS Tutorial: example inherited metadata">
        <metadata inherited="true">
            <serviceName>odap</serviceName>
            <description>Really great data.</description>
            <keyword>Ocean</keyword>
            <keyword>Temperature</keyword>
            <creator>Ethan</creator>
            <publisher>Ethan</publisher>
            <date type="created">2008-10-30T14:22</date>
            <dataFormat>netCDF</dateFormat>
        </metadata>

        <dataset name="TDS Tutorial: example data 1" urlPath="test/example1.nc" />
        <dataset name="TDS Tutorial: example data 2" urlPath="test/example2.nc" />
        <dataset name="TDS Tutorial: example data 3" urlPath="test/example3.nc" />
        <dataset name="TDS Tutorial: example data 4" urlPath="test/example4.grib2">
            <dataFormat>GRIB-2</dataFormat>
        </dataset>

    </dataset>
...

Notes:

• Inherited metadata can be overridden.

TDS Requirements for the service Elements

The TDS provides data access services at predefined URL base paths. Therefore, it is required that service base URLs must exactly match the values given here according to service type:

OPeNDAP

<service name="odap" serviceType="OPeNDAP" base="/thredds/dodsC/" />

NetCDF Subset Service

<service name="ncss" serviceType="NetcdfSubset" base="/thredds/ncss/grid/" />

WCS

 <service name="wcs" serviceType="WCS" base="/thredds/wcs/" />

WMS

 <service name="wms" serviceType="WMS" base="/thredds/wms/" />

HTTP Bulk File Service

<service name="fileServer" serviceType="HTTPServer" base="/thredds/fileServer/" />

Notes: These base URLs are relative to the server so your catalogs are independent of your servers hostname or port.

You can check that a data file is recognized as "gridded" with netCDF-Java ToolsUI (available for download from the netCDF-Java home page or you can use webstart).

TDS Configuration Catalogs and Metadata

The datasetScan element is an extension of the dataset element and so can contain metadata.

...
      <service name="odap" serviceType="OpenDAP" base="/thredds/dodsC/" />

2)    <datasetScan name="Test all files in a directory" ID="testDatasetScan"
                   path="my/test/all" location="/my/data/testdata">
          <metadata inherited="true">
              <serviceName>odap</serviceName>
              <keyword>Ocean</keyword>
              <keyword>Temperature</keyword>
              <creator>Ethan</creator>
              <publisher>Ethan</publisher>
              <date type="created">2008-10-30T14:22</date>
          </metadata>
      </datasetScan>
...

The client view of the above datasetScan element will be a catalogRef element which will also contain any metadata contained in the datasetScan element. It will look something like:

<catalogRef xlink:href="/thredds/catalog/my/test/all/catalog.xml"
               xlink:name="Test all files in a directory" ID="testDatasetScan" name="">
    <metadata inherited="true">
        ...
    </metadata>
</catalogRef >

All generated catalogs that are descendants of this datasetScan will contain all inherit-able (inherited="true") metadata contained in the datasetScan element. For instance, given that the testdata directory contained three files (afile.nc, testData.nc, and junk.nc) and one directory (grib), the resulting child catalog will look like:

...
      <service name="odap" serviceType="OpenDAP" base="/thredds/dodsC/" />

      <dataset name="Test all files in a directory" ID="testDatasetScan" >
          <metadata inherited="true">
              <serviceName>odap</serviceName>
              <keyword>Ocean</keyword>
              <keyword>Temperature</keyword>
              <creator>Ethan</creator>
              <publisher>Ethan</publisher>
              <date type="created">2008-10-30T14:22</date>
          </metadata>
          <dataset name="afile.nc" ID="testDatasetScan/afile.nc" urlPath="my/test/all/afile.nc">
          <dataset name="testData.nc" ID="testDatasetScan/testData.nc" urlPath="my/test/all/testData.nc">
          <dataset name="junk.nc" ID="testDatasetScan/junk.nc" urlPath="my/test/all/junk.nc">

          <catalogRef xlink:title="grib" ID="testDatasetScan/grib" name=""
                      xlink:href="grib/catalog.xml" />
      </dataset>
...

TDS Root Catalog

At startup, the TDS reads the root catalog

${TOMCAT_HOME}/content/thredds/catalog.xml

and recursively all configuration catalogs that are linked to it through a relative catalogRef element . The resulting tree of catalogs are used as the top-level catalogs served by the TDS. In the case of our distributed root catalog, the tree looks like:

catalog.xml
    |
    |-- enhancedCatalog.xml

The tree of configuration catalogs can be as deeply nested as desired.

Additional Root Catalogs

Additional root configuration catalogs can be defined in

${TOMCAT_HOME}/content/thredds/threddsConfig.xml 

file. For instance, to add a test catalog add the following line:

<catalogRoot>myTestCatalog.xml</catalogRoot>

Each additional root configuration catalog can be the root of another tree of configuration catalogs.

Tools to Manage Configuration Catalogs

First, the TDS catalog initialization log

${TOMCAT_HOME}/content/thredds/logs/catalogInit.log 

contains all warning and error messages from parsing the configuration catalogs. As such, it is a great place to look for information if you run into problems with your TDS configuration catalogs

Second, the TDS Remote Management page provides access to a list of all the configuration catalogs the TDS has successfully read:

• From the TDS Remote Management page [http://localhost:8080/thredds/admin/debug]:
  
  
  
• Click on the "Show static catalogs" link:
  
  
  

Managing datasetRoot and datasetScan Elements

You can have as many datasetRoot and datasetScan elements as you want, for example

<datasetRoot path="model" location="/data/ncep" />
<datasetRoot path="obs" location="/data/raw/metars" />
<datasetRoot path="cases/001" location="C:/casestudy/data/001" />
<datasetScan path="myData" location="/data/ncep/run0023" name="NCEP/RUN 23" serviceName="myserver" />
<datasetScan path="myData/gfs" location="/pub/ldm/gfs" name="NCEP/GFS" serviceName="myserver" />

The datasetRoot and datasetScan are said to define a data root.

For example, using the above data roots, the following matches would be made:

urlPath	file
model/run0023/mydata.nc	/data/ncep/run0023/mydata.nc
obs/test.nc	/data/raw/metars/test.nc
myData/mydata.nc	/data/ncep/run0023/mydata.nc
myData/gfs/mydata.nc	/pub/ldm/gfs/mydata.nc
cases/001/test/area/two	C:/casestudy/data/001/test/area/two

The structure of a full OPeNDAP URL for the first urlPath above would look like:

http://hostname:port/thredds/dodsC/model/run0023/mydata.nc
|<---  server   --->|<----->|<--->|<--->|<-   filename  ->|
                        |      |     |
           webapp name -|      |     |- data root
                               |
                      service -|

where:

• http://hostname:port is the server's hostname and port. By using relative service base URLs, you never have to specify this explicitly in your catalogs. This means you can change hosts or ports without having to rewrite your catalogs.
• /thredds is the name of the web application, taken from the thredds.war file.
• /dodsC maps to the servlet inside the web application, here it would be the OPeNDAP servlet.
• /model is the path, associated with the directory location /data/ncep/.
• /run0023/mydata.nc is the relative filename, and so is mapped to /data/ncep/run0023/mydata.nc.

TDS Remote Management - List of Dataset Roots

The TDS Remote Management page has a link to list all known dataset roots:

• Go to the TDS Remote Management page [http://localhost:8080/thredds/admin/debug]
• Click on the "Show data roots" link
  
  
  

Including Only the Desired Files

A datasetScan element can specify which files and directories it will include with a filter element (see spec for more details). When no filter element is given, all files and directories are included in the generated catalog(s). Adding a filter element to your datasetScan element allows you to include (and/or exclude) the files and directories as desired. The datasetScan element for the NAM_12km example included the following:

<filter>
    <include wildcard="*.grib2" />
</filter>

To exclude the analysis data, the filter could be modified to:

<filter>
    <include wildcard="*.grib2" />
    <exclude wildcard="*f000.grib2" />
</filter>

The include and exclude elements both determine which datasets they match on whether their wildcard pattern (given by the wildcard attribute) or regular expression (given by the regExp attribute) match the dataset name. By default, includes and excludes apply only to regular files (atomic datasets). You can specify that they apply to directories (collection datasets) as well by using the atomic and collection attributes. For example, if the nam_12km directory contained a badData directory, I could exclude it by adding the following to the filter:

<exclude wildcard="badData" atomic="false" collection="true" />

Sorting Datasets

By default, datasets are listed in decreasing lexigraphic order by the dataset name. A sort element can be added to a datasetScan element to specify an increasing lexigraphic order:

<sort>
    <lexigraphicByName increasing="true" />
</sort>

Currently, the lexigraphic increasing or decreasing sort algorithm is the only one supported.

Dataset IDs

All generated datasets are given an ID. The IDs are simply the path of the dataset appended to the datasetScan path value or, if one exists, the ID of the datasetScan element. So, for the nam_12km directory and our current configuration:

<datasetScan name="NCEP NAM 12km" ID="NAM_12km"
             path="nam_12km" location="/data/ldm/nam_12km">

and for the data file 2008110406f018.grib2, the value of the dataset ID would be "NAM_12km/2008110406f018.grib2".

Dataset Size Information

As of TDS 4.2.8, file size metadata is added to all atomic datasets, for instance:

<dataSize units="Kbytes">6.08</dataSize>

Prior to TDS 4.2.8, file size metadata was not added unless an addDatasetSize element was added to a datasetScan element, e.g.:

<addDatasetSize />

Naming Datasets

By default, all datasets are named with the name of their underlying file. By adding a namer element, you can specify more human readable dataset names. For instance, the following namer element causes any dataset named "NCEP NAM_12km" to be renamed with the value of replaceString:

<namer>
  <regExpOnName regExp="NCEP NAM 12km" replaceString="NCEP NAM 12km model data" />
</namer>

More complex renaming is possible as well. The namer uses a regular expression match on the dataset name. If the match succeeds, any regular expression capturing groups are used in the replacement string.

A capturing group is a part of a regular expression enclosed in parenthesis. When a regular expression with a capturing group is applied to a string, the substring that matches the capturing group is saved for later use. The captured strings can then be substituted into another string in place of capturing group references,"$n", where "n" is an integer indicating a particular capturing group. (The capturing groups are numbered according to the order in which they appear in the match string.) For example, the regular expression "Hi (.*), how are (.*)?" when applied to the string "Hi Fred, how are you?" would capture the strings "Fred" and "you". Following that with a capturing group replacement in the string "$2 are $1." would result in the string "you are Fred."

Here's an example namer:

<namer>
    <regExpOnName regExp="([0-9]{4})([0-9]{2})([0-9]{2})([0-9]{2})f([0-9]{3}).grib2"
                  replaceString="NCEP NAM 12km $1-$2-$3 $4 GMT - Forecast hour: $5"/>
</namer>

the regular expression has five capturing groups

1. The first capturing group, "([0-9]{4})", captures four digits, in this case the year.
   
2. The second capturing group, "([0-9]{2})", captures two digits, in this case the month.
   
3. The third capturing group, "([0-9]{2})", captures two digits, in this case the day of the month.
   
4. The fourth capturing group, "([0-9]{2})", captures two digits, in this case the hour of the day.
5. The fifth capturing group, "([0-9]{3})", captures three digits, in this case the forecast hour.

When applied to the dataset name "2008110406f018.grib2", the strings "2008", "11", "04", "06", and "018" are captured. After replacing the capturing group references in the replaceString attribute value, we get the name "NCEP NAM 12km 2008-11-04 06 GMT - Forecast hour: 018".

Adding timeCoverage Elements

A datasetScan element may contain an addTimeCoverage element. The addTimeCoverage element indicates that a timeCoverage metadata element should be added to each dataset in the collection and describes how to determine the time coverage for each datasets in the collection.

Currently, the addTimeCoverage element can only construct start/duration timeCoverage elements and uses the dataset name to determine the start time. As described in the "Naming Datasets" section above, the addTimeCoverage element applies a regular expression match to the dataset name. If the match succeeds, any regular expression capturing groups are used in the start time replacement string to build the start time string. The values of the following attributes are used to determine the time coverage:

1. Either the datasetNameMatchPattern or the datasetPathMatchPattern attribute gives a regular expression used to match on the dataset name or path, respectively. If a match is found, a timeCoverage element is added to the dataset. The match pattern should include capturing groups which allow the match to save substrings from the dataset name.
2. The startTimeSubstitutionPattern attribute value has all capture group references ("$n") replaced by the corresponding substring that was captured during the match. The resulting string is used as the start value of the resulting timeCoverage element.
3. The duration attribute value is used as the duration value of the resulting timeCoverage element.
   

For instance, adding

<addTimeCoverage datasetNameMatchPattern="([0-9]{4})([0-9]{2})([0-9]{2})([0-9]{2})f[0-9]{3}.grib2"
                 startTimeSubstitutionPattern="$1-$2-$3T$4:00:00"
                 duration="60 hours" />

to a datasetScan element and given a data file named

2005071812f006.grib2

results in the following timeCoverage element:

<timeCoverage>
    <start>2005-07-18T12:00:00</start>
    <duration>60 hours</duration>
</timeCoverage>

Adding a "Latest" Proxy Datasets

With a real-time archive, it is convenient to define a "proxy" dataset that always points to the most recent dataset in a collection. Other types of proxy datasets may be useful as well and the addProxies element provides a place for describing proxy datasets. Currently, only two addProxies child elements are defined. They are both "Latest" proxy elements. The simpleLatest element adds a proxy dataset which proxies the existing dataset whose name is lexigraphically greatest (which finds the latest dataset assuming a timestamp is part of the dataset name). The latestComplete element behaves similarly to simpleLatest except that the proxied dataset does not include any datasets that have been modified more recently than a given time limit, e.g., you could specify you want the most recent (lexigraphically) dataset that hasn't been modified for 60 minutes. Both the simpleLatest and latestComplete elements must point to an existing service element.

To add a "Latest" dataset to our "NAM_12km" dataset, we could add:

<service name="latest" serviceType="Resolver" base="" />

to our catalog and

<addProxies>
    <latestComplete name="latestComplete.xml" top="true" serviceName="latest" lastModifiedLimit="60" />
</addProxies>

to our "NAM_12km" datasetScan element. This would result in the following dataset being at the top of the "NAM_12km" collection of datasets:

<dataset name="latestComplete.xml" serviceName="latest" urlPath="latestComplete.xml" />

The latestComplete element includes a name attribute which provides the name of the proxy dataset, the serviceName attribute that references the service used by the proxy dataset, the top attribute which indicates if the proxy dataset should appear at the top or bottom of the list of datasets in this collection, and the lastModifiedLimit which feeds into the algorithm which determines which dataset is being proxied.

The simpleLatest element allows for the same attributes as the latestComplete element minus the lastModifiedLimit attribute. In this case, all the attributes have default values: the name attribute defaults to "latest.xml", the top attribute defaults to "true", and the serviceName attribute defaults to "latest".

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