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).