rbed

Data gaps

A Channel keeps its last value until the next Sample brings a new one. If a sensor stops reporting, that last value would hold forever, and an Event could run on stale data. maxGap bounds how far apart two Samples may be.

maxGap

Rule running = Rule.named("Running").when(rpm.above(100)).maxGap(Duration.ofSeconds(10));

When the next Sample (or time advance) comes more than 10 seconds after the previous Sample:

  • a Pending Event is Discarded;
  • a Confirmed Event Ends with a Truncated end, at the last moment evaluated before the gap;
  • an Event that starts right after the gap has a Truncated start, since its Rule has not seen its Conditions not holding since data resumed.

Without maxGap, gaps of any length are accepted.

A silent sensor

A machine reports its rpm once a second and runs from 6 s to 31 s. For six seconds its sensor is silent: the Samples inside the gap are missing. Running is an Event while rpm is above 100.

The live chart and its controls need JavaScript. The tables show the Events at the default parameters.

The site cannot run the default parameters: (JavaScript) TypeError: Cannot read properties of undefined (reading 'length')

import io.github.romanhaendler.rbed.*;
import java.time.Duration;
import java.util.Map;

Channel rpm = Channel.named("rpm");

Rule running = Rule.named("Running")
    .when(rpm.above(100)).maxGap(Duration.ofSeconds(5));
Detector detector = Detector.builder()
    .rules(running)
    .build();

for (int s = 0; s < 40; s++) {
  if (s > 12 && s < 12 + 6) continue;   // the sensor is silent for 6 s
  detector.push(s * 1_000L, Map.of(rpm, s >= 6 && s <= 31 ? 1500.0 : 0.0));
}
detector.close();

At the defaults, the gap runs from 12 s to 18 s, one second longer than the maxGap of 5 s. Running Ends at 12 s, the last Sample before the gap, with a Truncated end: nobody knows whether the machine kept running. A second Event starts at 18 s with a Truncated start, since nobody knows when it started either.

Things to try

  • Raise maxGap to 6 s: the gap is no longer too long. The Samples at 12 s and 18 s are exactly 6 seconds apart, not more, and a single, complete Event runs from 6 s to 32 s.
  • Move the gap to 33 s, after the machine has stopped: it changes nothing.
  • Move the gap to 0 s, so that it ends just as the machine starts: the Event starts at 6 s, the first Sample after the gap, and its start is Truncated although rpm was 0 before the gap. After a gap, the Rule has to see its Conditions not holding again before it knows where an Event begins.

Reference

Methods used: Channel.named, Rule.named, Rule.when, Expression.above, Rule.maxGap, Detector.builder, Detector.Builder.rules, Detector.Builder.build, Detector.push, Detector.close.