When I started college, everybody was talking about “Information Technology.” At that point I had been programming for quite a while and it wasn’t clear to me what coding had to do with that fancy terminology. After a few more years of coding, I realized the connection: all I do, day in and day out, is move bytes (information) from one memory location to another. Copying the contents of a struct into the socket buffer and sending it out; getting the bytes from the socket buffer and deserializing them into a structure to pass them to the application logic. Well, that’s part of what communication middleware does for you!

RTI Connext DDS implements the OMG DDS Standard. DDS is data-centric middleware. We call those bytes being transferred data, and we assign a type to them. The type describes each byte in your data, so you (and your application) can make good use of them.

Designing a good distributed system means having a good type system in place. Often though, those types carry a lot of information, and it can become pretty difficult to deal with them. Also, you may not need all the information all the time. This often happens when you have heterogeneous systems with nodes that have different capabilities, such as bandwidth constraints or resource limits. In this case, some of the nodes in the system may not be able to handle a whole sample for certain data types, but they still need to be able to receive part of the information.

Let me present a possible scenario first and then suggest one way to solve it.

Figure 1: Scenario

Let’s say a new testing module for the International Space Station (ISS) has been shipped to space. The module has a device that collects thousands of data points, puts them all in a DDS sample with a specific type, and writes it on the topic, ComplexDataTopic. We will call this device the Emitter.

One of the many different data points this device collects is the temperature from 10 different sensors. It puts all the collected temperature values into a sequence.

enum Unit { F, C, K }; struct Temp { long sensorId; double value; Unit unit; } struct Measurements { long deviceId; // thousand of more fields. sequence<Temp,10> temperatures; }; // Open file

The module also comes with a FancyReceiver (what we call a topic subscriber) that runs a UI; it gets all the data contained in each sample and creates statistics and charts so astronauts can easily understand the data. For example, looking that the temperature data, it could calculate the average (mean) or the median, or spot significant differences between sensors.

Let’s now say that back on Earth, NASA scientists need to know what the temperature is on the testing module. They’re not interested in pressure and humidity. Just the temperature. But, since the bandwidth between Earth and the ISS “is what it is”, it’s considered acceptable to receive aggregates of the date. For example, instead of sending all the values for the temperature the testing module will only send the average, which will save the ISS some bandwidth on the link to ground. Basically, they want something that looks like this:

enum Unit { F, C, K }; struct SimpleMeasurements { long deviceId; double avgTmp; Unit unit; } // Open file

We have many options to achieve this result:

We can create an ad-hoc application: it will subscribe to the ComplexDataTopic, get the data we need, create another topic, another type, calculate what we need, and send it over. We can use RTI Routing Service: write a custom transformation library and be done with it. We can use RTI Connector via the RTI Prototyper with Lua. We can use RTI Connector via Python or node.js.

I will now explain option 3: Use RTI Prototyper with Lua to easily transform your types. If you are already an RTI Connext user, you will have what you need in your distribution (under the “bin” directory if you are using RTI Connext 5.2, or under the “scripts” directory for older versions). Otherwise, check out this page to learn how to get your free copy of RTI Prototyper.

We will create a new component, the Transformer. The Transformer will subscribe to ComplexDataTopic (with type Measurements) and will Publish to SimpleDataTopic (with type SimpleMeasurements).

The Types

For simplicity, let’s say that the complex type is the following:

The simple type looks like this:

The Topics and the Entities

All we need to do now is define an XML application file with a data reader for topic ComplexDataTopic and a writer to SimpleDataTopic:

<!-- Domain Library --> <domain_library name="MyDomainLibrary"> <domain name="MyDomain" domain_id="0"> <register_type name="Measurement" kind="dynamicData" type_ref="Measurement"/> <register_type name="SimpleMeasurement" kind="dynamicData" type_ref="SimpleMeasurement"/> <topic name="ComplexDataTopic" register_type_ref="Measurement"/> <topic name="SimpleDataTopic" register_type_ref="SimpleMeasurement"/> </domain> </domain_library>

The Actual Logic

Once you have defined what your types are and how your entities are called, you just have to write a simple chunk of Lua code that will be executed by RTI Prototyper. Let’s have a look:

local myComplexReader = CONTAINER.READER['MySubscriber::MyComplexReader'] local mySimpleWriter = CONTAINER.WRITER['MyPublisher::MySimpleWriter'] local instance = mySimpleWriter.instance myComplexReader:take() for i, sample in ipairs(myComplexReader.samples) do if (not myComplexReader.infos[i].valid_data) then print("\t invalid data!") else local deviceId = sample['deviceId'] local avgTmp = 0 local sum = 0; for i=1, sample['temperatures#'] do sum = sum + sample['temperatures[' .. i .. '].value']; end avgTmp = sum / sample['temperatures#'] --setting the instance instance['deviceId'] = deviceId instance['avgTmp'] = (avgTmp - 32) * (5/9); instance['unit'] = 1 -- C -- writing the simple instance print("Writing sample with avgTmp = " .. instance['avgTmp'] ) mySimpleWriter:write() end end -- Open File

As you can see, you have to write very little code to transform your complex data type into a simple one using RTI Prototyper.

In the first 3 lines we are just getting the complex reader and the simple writer. We are then assigning the pre-allocated instance of the simple writer to the variable called instance.

Next we are doing a take and, if the received samples are valid, we iterate over them, getting only the field we care about and aggregating the data. (In this example, we calculate the average of all the temperatures sent in the original sample, and we ignore humidity and pressure.) We can also do some more intelligent transformations, such as transforming the incoming temperature from Fahrenheit to Celsius (line 20)!

Once we have what we need, we assign the values to writer instance (lines 19-21) and we write the sample (line 25 ). It’s that simple!!!

For your convenience I uploaded all the code and files to a GitHub repository here.

In the repo you will find the following directories:

Transformer: Contains both the XML and the Lua file described in this blog post. To run it just execute:

rtiddsprototyper -cfgFile Transformer.xml -luaFile Transformer.lua

EmitterEmu: Contains an XML file and a Lua file to emulate the behavior of your ISS module Emitter. To run it just execute:

rtiddsprototyper -cfgFile EmitterEmu.xml -luaFile EmitterEmu.lua

FancyReceiver: Contains an XML file and a Lua file to emulate your fancy dashboard running on board the ISS. To run it, just execute:

rtiddsprototyper -cfgFile FancyReceiver.xml -luaFile FancyReceiver.lua

LimitedReceiver: Contains an XML file and a Lua file to emulate your limited subscriber running back on earth. To run it, just execute:

rtiddsprototyper -cfgFile LimitedReceiver.xml -luaFile LimitedReceiver.lua

So, clone the repo, play with the examples, read the few lines of code, and you will right away understand the power of RTI Prototyper with Lua. If you want more information on RTI Prototyper check out the Getting Started Guide.

And if you have some time left check out the other solution for doing scripting with RTI Connext DDS using Python and node.js here.