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%\VignetteIndexEntry{A Future for R: Controlling Default Future Strategy}
%\VignetteAuthor{Henrik Bengtsson}
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%\VignetteKeyword{future}
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<%
library(R.utils)
`%<-%` <- future::`%<-%`
options("withCapture/newline" = FALSE)
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# <%@meta name="title"%>

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
```r
options(future.plan = "multisession")
```
to our `~/.Rprofile` startup script, the future package will resolve futures in parallel (asynchronously using all available cores), i.e.
```sh
$ Rscript -e "class(future::plan())"
[1] "multisession" "future"       "function"
```

Option `future.plan` is ignored if command-line option `--parallel` (`-p`) is specified.


## Environment variables

An alternative to using `options()` for setting option `future.plan` is to specify system environment variable `R_FUTURE_PLAN`.  If set, then the future package will set `future.plan` accordingly _when loaded_.  For example,
```sh
$ export R_FUTURE_PLAN=multisession
$ 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
```sh
$ 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
```r
> 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.
```sh
$ 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.


[future]: https://cran.r-project.org/package=future
