  Reactive software calls for instrumentation methods that uphold the reactive
attributes of systems. Runtime verification imposes another demand on the
instrumentation, namely that the trace event sequences it reports to monitors
are sound -- that is, they reflect actual executions of the system under
scrutiny. This paper presents RIARC, a novel decentralised instrumentation
algorithm for outline monitors meeting these two demands. The asynchronous
setting of reactive software complicates the instrumentation due to potential
trace event loss or reordering. RIARC overcomes these challenges using a
next-hop IP routing approach to rearrange and report events soundly to
monitors.
  RIARC is validated in two ways. We subject its corresponding implementation
to rigorous systematic testing to confirm its correctness. In addition, we
assess this implementation via extensive empirical experiments, subjecting it
to large realistic workloads to ascertain its reactiveness. Our results show
that RIARC optimises its memory and scheduler usage to maintain latency
feasible for soft real-time applications. We also compare RIARC to inline and
centralised monitoring, revealing that it induces comparable latency to inline
monitoring in moderate concurrency settings, where software performs
long-running, computationally-intensive tasks, such as in Big Data stream
processing.
