This paper presents Poker, a visual instrumentation platform for reactive programs. Similar to other platforms, Poker features a visual dashboard that allows the programmer to inspect the flow of values through the reactive program. The novelty of Poker is that: (a) It features a canvas of so-called probes that can be dynamically wired into a running reactive program in order to instrument the running system. (b) In addition to focusing on the values flowing through the program, a probe can measure a particular property about the way these values flow through the instrumented program. (c) The set of probes is open because a probe is programmed in the same language as the instrumented program.

Poker is implemented for Stella, an experimental reactive programming language. The paper uses an application written in Stella to motivate the concepts provided by Poker. We show 4 different probes that help us understand the behaviour of the application and we measure the overhead of using Poker on the running application with some preliminary benchmarks.