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# 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
| **future**
`multisession` | 📶
♻️
| parallelly via background R sessions on current machine
| **future**
`cluster` | 📶
♻️*
| parallelly in external R sessions on current, local, and/or remote machines
| **future**
`multicore` | 📶
♻️
| (not recommended) parallelly via forked R processes on current machine; not with GUIs like RStudio; not on Windows
| **[future.callr]**
`callr` | 📶(next)
♻️(next)
| parallelly via transient **[callr]** background R sessions on current machine; all memory is returned when as each future is resolved
| **[future.mirai]**
`mirai_multisession` | 📶(next)
♻️(next)
| parallelly via **[mirai]** background R sessions on current machine; low latency
| **[future.mirai]**
`mirai_cluster` |♻️(next)
| parallelly via **[mirai]** daemons running locally or remotely
| **[future.batchtools]**
`batchtools_lsf`
`batchtools_openlava`
`batchtools_sge`
`batchtools_slurm`
`batchtools_torque` | 📶(soon)
♻️(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; * disabled by default
(next): next release; (soon): in a near-future release
[batchtools]: https://cran.r-project.org/package=batchtools
[callr]: https://cran.r-project.org/package=callr
[mirai]: https://cran.r-project.org/package=mirai
[future]: https://future.futureverse.org
[future.apply]: https://future.apply.futureverse.org
[future.batchtools]: https://future.batchtools.futureverse.org
[future.callr]: https://future.callr.futureverse.org
[future.mirai]: https://future.mirai.futureverse.org
[future.tests]: https://future.tests.futureverse.org
[furrr]: https://furrr.futureverse.org
[doFuture]: https://doFuture.futureverse.org
[progressr]: https://progressr.futureverse.org