The default is to use synchronous futures, but this default can be overridden via R options, system environment variables and command-line options as explained below as well as in help("future.options", package = "future").

## R options

The default strategy for resolving futures can be controlled via R option future.plan. For instance, if we add

options(future.plan = "multisession")

to our ~/.Rprofile startup script, the future package will resolve futures in parallel (asynchronously using all available cores), i.e.

$Rscript -e "class(future::plan())" [1] "multisession" "future" "function" Environment variable R_FUTURE_PLAN is ignored if either option future.plan or command-line option --parallel (-p) is specified. ## Command-line options When loaded, the future package checks for the command-line option --parallel=ncores (short -p ncores) and sets the future strategy (via option future.plan) and the number of available cores (via option mc.cores) accordingly. This provides a convenient mechanism for specifying parallel future processing from the command line. For example, if we start R with $ R --quiet --args --parallel=2

then future will interpret this as we wish to resolve futures in parallel using 2 cores. More specifically, we get that

> availableCores()
mc.cores
2
> class(future::plan())
[1] "FutureStrategy" "tweaked"        "multisession"   "future"         "function"

We can use this command-line option also with Rscript, which provides a convenient mechanism for launching future-enhanced R scripts such that they run in parallel, e.g.

\$ Rscript analysis.R --parallel=4

This does, of course, require that the script uses futures and the future package.

If --parallel=1 is specified, or equivalently -p 1, then futures are resolved using a single process.

Specifying these command-line options override any other startup settings.