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.

New features:

  • New handle_unfinished() activity sets a drop-out trajectory for unfinished arrivals, i.e., those dropped from a resource (due to preemption, resource shrinkage or a rejected seize) or those that leave a trajectory (#178 addressing #177).
  • New release_all() and release_selected_all() activities automatically retrieve the amount of resources seized and release it (#180 addressing #25).
  • New get_seized() and get_seized_selected() getters allow an arrival to retrieve the amount of resources seized (#180 addressing #179).
  • New stop_if() activity sets a conditional breakpoint (#181 addressing #100).

Minor changes and fixes:

  • Fix performance issues in data sources (#176).
  • Update CITATION.
  • Fix monitored activity for preempted arrivals (as part of #178).
  • Fix seizes/releases with a null amount (as part of #180).
  • Rename internal status codes (as part of #181).
  • Provide more context on error or warning (as part of #181).
  • Extend the Queueing Systems vignette with a section about state-dependent service rates.
  • Fix performance issues in getters (#183).
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