Discrete-event simulation (DES) is a specific technique for modelling stochastic, dynamic and discretely evolving systems. As opposed to continuous simulation, which typically uses smoothly-evolving equational models, DES is characterised by sudden state changes at precise points of (simulated) time. Customers arriving at a bank, products being manipulated in a supply chain, or packets traversing a network are common examples of such systems. The discrete nature of a given system arises as soon as its behaviour can be described in terms of events, which is the most fundamental concept in DES. An event is an instantaneous occurrence that may change the state of the system, while, between events, all the state variables remain constant.
The applications of DES are vast, including, but not limited to, areas such as manufacturing systems, construction engineering, project management, logistics, transportation systems, business processes, healthcare and telecommunications networks. The simulation of such systems provides insights into the process’ risk, efficiency and effectiveness. Also, by simulation of an alternative configuration, one can proactively estimate the effects of changes to the system. In turn, this allows one to get clear insights into the benefits of process redesign strategies (e.g., extra resources). A wide range of practical applications is prompted by this, such as analysing bottlenecks in customer services centres, optimising patient flows in hospitals, testing the robustness of a supply chain or predicting the performance of a new protocol or configuration of a telecommunications network.
simmer is a DES package for R which enables high-level process-oriented modelling, in line with other modern simulators. But in addition, it exploits the novel concept of trajectory: a common path in the simulation model for entities of the same type. In other words, a trajectory consist of a list of standardised actions which defines the life cycle of equivalent processes. This design pattern is flexible and simple to use, and takes advantage of the chaining/piping workflow introduced by the magrittr package.