simmer 4.2.1

The 4.2.1 release of simmer, the Discrete-Event Simulator for R, is on CRAN with quite interesting new features and fixes. As discussed in the mailing list, there is a way to handle the specific case in which an arrival is rejected because a queue is full:

library(simmer)

reject <- trajectory() %>%
  log_("kicked off...")

patient <- trajectory() %>%
  seize("nurse", continue=FALSE, reject=reject) %>%
  log_("nurse seized") %>%
  timeout(5) %>%
  release("nurse") %>%
  log_("nurse released")

env <- simmer() %>%
  add_resource("nurse", 1, 0) %>%
  add_generator("patient", patient, at(0, 1)) %>%
  run()

## 0: patient0: nurse seized
## 1: patient1: kicked off...
## 5: patient0: nurse released

But as Tom Lawton pointed out, until now, there was no way of handling any alternative path for an arrival that was preempted and “kicked off” from a resource. This mechanism has been implemented into the new handle_unfinished() activity:

patient <- trajectory() %>%
  handle_unfinished(reject) %>%
  seize("nurse") %>%
  log_("nurse seized") %>%
  timeout(5) %>%
  release("nurse") %>%
  log_("nurse released")

env <- simmer() %>%
  add_resource("nurse", 1, 0, preemptive=TRUE, queue_size_strict=TRUE) %>%
  add_generator("LowPrio_patient", patient, at(0), priority=0) %>%
  add_generator("HighPrio_patient", patient, at(1), priority=10) %>%
  run()

## 0: LowPrio_patient0: nurse seized
## 1: HighPrio_patient0: nurse seized
## 1: LowPrio_patient0: kicked off...
## 6: HighPrio_patient0: nurse released

Note that such a mechanism is more general, because it also covers the first scenario:

env <- simmer() %>%
  add_resource("nurse", 1, 0) %>%
  add_generator("patient", patient, at(0, 1)) %>%
  run()

## 0: patient0: nurse seized
## 1: patient1: kicked off...
## 5: patient0: nurse released

Whenever rejection (or preemption) happens and it is catched by the appropriate handler, the new getter get_seized() may be useful to know which resource was abandoned.

Finally, the readership may find interesting the new section about the implementation of state-dependent service rates in the Queueing Systems vignette. See below for a complete list of changes.