1. Writing a WDSS-II Application using w2algcreator
V Lakshmanan
National Severe Storms Laboratory & University of Oklahoma
11/14/2018

2. WDSS-II – application development
WDSS-II provides a easy environment to develop new applications or algorithms.
Any algorithm developed in WDSS-II can be run either in archive mode or in real-time.
provided that the algorithm is fast enough to run in real-time and the machine can handle it!
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3. WDSS-II for Applications
What does WDSS-II provide your application?
Easy ingest of data from multiple sensors
Easy ways to write out and visualize intermediate and final outputs.
A common framework for accessing data.
The framework is written in C++.
It’s best if you can write your code in C++ too.
But you can call Fortran or C code from C++
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4. Creating a new application
WDSS-II has a tool to create a new application quickly.
w2algcreator
Need to create an input XML file specifying the inputs to your application.
Will create C++ main and a template for your scientific code.
Your then write the scientific code.
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5. How WDSS-II works WDSS-II applications connect to a source of data.
Called an Index.
Applications listen for new products in that Index.
Decide whether or not to process that product.
Create and process the data if needed.
Write out outputs (in netcdf or XML).
Notify an Index about the new products available.
For other applications listening to that Index
May be a different index from the source.
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6. WDSS-II Applications WDSS-II Applications are just executables.
launched on the command line.
In deployed systems through scripts.
Inputs are specified on the command-line.
w2vil –i xmllb:/data/realtime/radar/KTLX/code_index.lb \
-R ReflectivityQC \
-o /data/realtime/radar/KTLX/ \ -r
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7. Command-line options
w2vil – algorithm executable name
VerticalIntegratedLiquid – algorithm full name (used in source code)
Input RadarIndex specified by –i option
Input data type is specified by –R option
Output directory is specified by –o option
Real-time is specified by –r option
The options –r, -l (the letter ell), -o are reserved so that the deployment scripts work correctly.
w2vil –i xmllb:/data/realtime/radar/KTLX/code_index.lb \ -R ReflectivityQC \ -o /data/realtime/radar/KTLX/ \ -r
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8. Some conventions
If you need to provide domain extents, use the following letters:
t: “top” to specify the north-west-top corner
b: “bottom” to specify the south-east-bottom corner
s: to specify the grid spacing
If you need to process only a certain sub-type, use colons as a separator:
-R ReflectivityQC:00.50
-R ReflectivityMaximum:vol
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9. Creating a new application
Need to create an XML file.
Use your favorite text editor. Save it somewhere.
A template is available as example_alg.xml – you can edit this if you want.
What is the algorithm’s descriptive name?
Used to generate class names in template.
What is the executable name?
What is the namespace that generated code should be in? Pick your group (casa) or name (tj) i.e. something that will distinguish your code from something someone else might create.
<algorithmcreator
name=“VerticalIntegratedLiquid
exec=“w2vil” namespace=“w2polar”>
</algorithmcreator>
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10. Inputs Next you need to specify inputs.
Done within an “inputs” section.
How many indexes do you need to connect to?
Each index or group of indexes will get a new index tag.
Give each index a name (RadarIndex, ModelIndex, etc.) so that a user can understand what needs to be provided to your algorithm.
Supply a mnemonic option letter as well.
The most important input index is by convention given the option letter i.
Specify the history (in minutes) that needs to be maintained for each index (or group of indexes).
Can be just 1 minute if you will never search backwards in your algorithm (only new data).
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11. Input Index specification
<algorithmcreator …. >
<inputs>
<index history=“5”>
<optionvalue name=“RadarIndexes”
letter=“i” />
</index>
<index … > ….
</inputs>
</algorithmcreator>
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12. Input Data specification
The user may specify either just a single index or a bunch of index URLs. So each “index” tag actually corresponds to a group of indexes in general.
For each index, specify the data that you will be listening to in that index.
For each product listened to, specify:
How you will refer to the product (e.g: dbz)
The default name of the product (e.g: ReflectivityQC)
This is the “official” name of the product that you can see on the display.
A mnemonic letter for this input
What type of data is it? WDSS-II data are usually one of these types:
RadialSet
LatLonGrid
LatLonHeightGrid
DataTable
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13. Input Data Specification
<index history=“5”>
<optionvalue …. />
<data type=“RadialSet”>
<optionvalue name=“dbz”
letter=“R”
default=“ReflectivityQC” />
</data>
<data … > …
</index>
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14. Option The optionvalue tag: Has a letter, name and default
The default tag is optional – if you leave it out the user has to specify a value on the command-line.
If the default is “false”, then the option is a boolean option i.e. yes/no.
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15. Optional data
To specify an optional index, specify that the default value is an empy string.
E.g. you will use a model index if one is available, otherwise, you will run only on radar data.
You need to specify a default name for all input data types
otherwise we won’t know what to listen for.
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16. Other options
Besides the input, do you need any other parameters specified for your algorithm?
For example, maybe you need to know the radar name.
For each such option:
Specify the name of the option (e.g: RadarName)
Specify a mnemonic (e.g: ‘S’ for sourceRadar, since –r and –R are both taken by realtime and Reflectivity respectively …)
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17. Other options <algorithmcreator …> <options>
<optionvalue letter=“s”
name=“sourceRadar” />
<optionvalue …/>
</options>
</algorithmcreator>
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18. Use the w2algcreator w2algcreator –i name-of-your-XML-file
Now that you have the XML file:
Run the w2algcreator program
It will create a w2vil_main.cc and w2vil_VerticalIntegratedLiquid.h file in the output directory.
Plug in the code for the two functions specified in the .h file:
One function tells you the radar name, the output directory to write your outputs to and the LB to notify after you have written the outputs.
The other function is called processdbz()
Your scientific code goes in there.
w2algcreator –i name-of-your-XML-file
-o output-directory
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19. Final steps Fill in the scientific code. Compiling:
Use the example_Makefile that is provided to compile and link against the WDSS-II libraries.
Or put your source code into a subdirectory of the w2algs repository
Need to create Makefile.am
See w2algs/w2algcreator/README for details.
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20. The autogenerated code
What does the autogenerated code do?
It creates “listeners” to listen for the products your algorithm needs
The user tells the algorithm the Index where those products may be found
Also specifies that ReflectivityQC_smoothed should be used where your algorithm expects a reflectivity
May tell your algorithm to provide heartbeat messages
The code has fault handling
If you lose connection to an Index on a remote machine, the algorithm will reconnect.
If your algorithm hangs or starts to eat up the CPU, it’ll stop providing heartbeat messages, thus allowing an external monitoring program to kill it and restart it.
A good idea to specify “initOnStart=true” for any product that should re-read if your algorithm is restarted.
As soon as it receives notification of a product, it tells your algorithm class about it
Then, you do your thing!
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21. On a timer W2algcreator is for data-driven applications
Best to process data as it comes in
So, design your application to be data-driven as much as possible.
To do things on a timer
Use a TimedEventHandler (see API documentation)
Provide a listener that will be called every N milliseconds.
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22. Exercises
The best way to learn is to write simple data-driven algorithms.
The next module on data formats will help you accomplish these tasks.
May need to get input data from some source first.
These examples should get you started:
Read in single-radar reflectivity data and write out an indexed scan with values below a user-defined threshold set to MissingData.
Hint: PolarGrid is an indexed RadialSet
User-defined implies you need a command-line parameter
Use ProcessTimer to determine how long certain steps take.
Read in both reflectivity and velocity data from the same radar and index the reflectivity data at the velocity azimuths.
You will need to operate with RadialSet data
Wait for matching Reflectivity and Velocity scans (use the elevation angle to check)
Form a histogram of VIL values to satellite IR temperatures.
Use DataConverter and/or DataRemapper
A multi-source algorithm. Makes a difference only when running, not when writing the algorithm!
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23. Questions? Email me: lakshman@ou.edu
More documentation (including a more up-to-date version of this document) is at:
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