# A Future for R: Available Future Backends

The future package comes with built-in future backends that leverage the parallel package part of R itself. In addition to these backends, others exist in package extensions, e.g. future.callr, future.mirai, and future.batchtools. Below is an overview of the most common backends that you as an end-user can chose from.

Package / Backend | Features | How futures are evaluated
--- | --- | ---
future `sequential` | 📶 ♻️ | sequentially and in the current R process; default Example: `plan(sequential)`
future `multisession` | 📶 ♻️ | parallelly via background R sessions on current machine Examples: `plan(multisession)` and `plan(multisession, workers = 2)`
future `cluster` | 📶 ♻️* | parallelly in external R sessions on current, local, and/or remote machines Examples: `plan(cluster, workers = "raspberry-pi")`, `plan(cluster, workers = c("localhost", "n1", "n1", "pi.example.org"))`
future `multicore` | 📶 ♻️ | (not recommended) parallelly via forked R processes on current machine; not with GUIs like RStudio; not on Windows Examples: `plan(multicore)` and `plan(multicore, workers = 2)`
future.callr `callr` | 📶 ♻️ | parallelly via transient callr background R sessions on current machine; all memory is returned when as each future is resolved Examples: `plan(callr)` and `plan(callr, workers = 2)`
future.mirai `mirai_multisession` | 📶 ♻️ | parallelly via mirai background R sessions on current machine; low latency Examples: `plan(mirai_multisession)` and `plan(mirai_multisession, workers = 2)`
future.mirai `mirai_cluster` | ♻️ | parallelly via mirai daemons running locally or remotely Example: `plan(mirai_cluster)`
future.batchtools `batchtools_lsf` `batchtools_openlava` `batchtools_sge` `batchtools_slurm` `batchtools_torque` | 📶(soon) ♻️ | parallelly on HPC job schedulers (Load Sharing Facility [LSF], OpenLava, TORQUE/PBS, Son/Sun/Oracle/Univa Grid Engine [SGE], Slurm) via batchtools; for long-running tasks; high latency

📶: futures relay progress updates in real-time, e.g. progressr
♻️: futures are interruptible and restartable; * interrupts are disabled by default
(next): next release; (soon): in a near-future release