Basic data reduction

Guide specification
Guide type:	Wasm code
Requirements:	None
Recommended reading:	None

Introduction

In this guide we look at different ways of doing data reduction through sampling. We will illustrate the data reduction techniques on streams from the built-in synthetic stream generator simstream(), but the techniques apply just as well to data streams from real sensors.

Input data

The function simstream(pace) generates a stream with a new simulated numerical element every pace seconds. We can look at the stream from the first 1.0 seconds of simstream(0.1):

timeout(simstream(0.1), 1.0)

Count-based sampling

The easiest way to reduce the data rate is to sample the output of simstream(pace). One way of sampling is to use the function winagg(s,size,stride). For each stride elements of stream s, winagg() returns a temporal window as a vector of the last size elements. Choosing a window size less than the stride will return a down-sampled stream window based on selecting every 10th element.

winagg(simstream(0.1), 1, 10)

The preceding example samples from simstream() a window with one element every ten elements. We use vector indexing to get the first element in each result stream window:

select Stream of v[1]
  from Vector v
 where v in winagg(simstream(.1), 1, 10)

Exercise

Set a larger value of size and see how it affects the output!

This was an easy way of reducing the data stream through down sampling.

Time-based sampling

In certain applications it is meaningful to sample elements in temporal windows rather than the above counting windows formed by winagg(). The built-in stream function twinagg(s,size,stride) is similar to winagg(s,size,stride), but the input parameters size and stride are specified in seconds rather than number of stream elements. The function returns a stream of time stamped windows containing the last size elements in stream s each stride seconds. Note that twinagg() requires the stream s to be of a time stamped stream.

We can timestamp any stream using ts():

timeout(ts(simstream(0.1)), 1.0)

Now that we have a time-stamped stream we can use twinagg() on the stream:

twinagg(ts(simstream(0.1)), 1.0, 1.0)

In the preceding example, each window contains the elements in simstream(0.1) received each second (size=1.0), and the stride is also one second (stride=1.0), so all elements in simstream(0.1) are present in the output, which is called a temporal tumbling window.

We see that twinagg(s,size,stride) forms a stream of temporal windows of the elements in s. A temporal window consists of a time stamp and a vector that represents the elements of the window. To get the window elements we use the value() function. The following query extracts the window elements from the twinagg() result and returns the first element in the window vector, thereby sampling one element from the stream each second:

select value(tsv)[1]
  from Timeval of Vector tsv, Stream of Timeval s
 where s = ts(simstream(.1))
   and tsv in twinagg(s, 1.0, 1.0)

We can extract the timestamp from the time stamped window with the function timestamp(). So if we parameterize the example above with the variables streamrate and samplingrate we can adjust the frequency of the stream and how often the stream is sampled:

select timestamp(tsv), value(tsv)[1]
  from Timeval of Vector tsv, Stream of Timeval s,
       Number streamrate, Number samplingrate
 where streamrate = .02 and samplingrate = .5
   and s = ts(simstream(streamrate))
   and tsv in twinagg(s, samplingrate, samplingrate);

Conclusion

This guide has shown how to do data reduction by sampling. As next step we would recommend reading the Data reduction on edge devices guide where we try these concepts on a real edge device.