We describe the design and implementation of \texttt{Scheduler}, a new library for Elixir which provides a user-level scheduler. The goal is to improve the control over scheduling decisions, i.e., which process runs at which time, in order to obtain executions that are more random, repeatable, modifiable, and moreover provide a detailed explanation of the scheduling decisions taken. This work is inspired by the Pulse user-level scheduler for Erlang programs, as well as other related tools. Our library is agnostic with regards to what other testing/execution/formal verification tool uses the scheduler, and instruments Elixir code running under the scheduler through use of the Elixir macro facility. Moreover the library provides a number of algorithms to explore the state space of the concurrent programs under study, including random search, depth-first search (potentially capable of exploring the whole state space of the program-under-study), and a novel search algorithm which selects schedules randomly.

As an example the Scheduler library is applied to the task of checking whether a number of snapshot algorithms are correct.