Superseded: configure exponential backoff while polling for tasks during the pipeline.

## Arguments

- min
Positive numeric of length 1, minimum polling interval in seconds. Must be at least

`sqrt(.Machine$double.eps)`

.- max
Positive numeric of length 1, maximum polling interval in seconds. Must be at least

`sqrt(.Machine$double.eps)`

.- rate
Positive numeric of length 1, greater than or equal to 1. Multiplicative rate parameter that allows the exponential backoff minimum polling interval to increase from

`min`

to`max`

. Actual polling intervals are sampled uniformly from the current minimum to`max`

.

## Details

This function is superseded and is now only relevant to other
superseded functions `tar_make_clustermq()`

and `tar_make_future()`

.
`tar_make()`

uses `crew`

in an efficient non-polling way, making
exponential backoff unnecessary.

## Backoff

In high-performance computing it can be expensive to repeatedly poll the
priority queue if no targets are ready to process. The number of seconds
between polls is `runif(1, min, max(max, min * rate ^ index))`

,
where `index`

is the number of consecutive polls so far that found
no targets ready to skip or run, and `min`

, `max`

, and `rate`

are arguments to `tar_backoff()`

.
(If no target is ready, `index`

goes up by 1. If a target is ready,
`index`

resets to 0. For more information on exponential,
backoff, visit https://en.wikipedia.org/wiki/Exponential_backoff).
Raising `min`

or `max`

is kinder to the CPU etc. but may incur delays
in some instances.

## See also

Other utilities:
`tar_active()`

,
`tar_call()`

,
`tar_cancel()`

,
`tar_definition()`

,
`tar_described_as()`

,
`tar_envir()`

,
`tar_group()`

,
`tar_name()`

,
`tar_path()`

,
`tar_path_script()`

,
`tar_path_script_support()`

,
`tar_path_store()`

,
`tar_path_target()`

,
`tar_source()`

,
`tar_store()`

## Examples

```
if (identical(Sys.getenv("TAR_EXAMPLES"), "true")) { # for CRAN
tar_dir({ # tar_dir() runs code from a temp dir for CRAN.
tar_option_set(backoff = tar_backoff(min = 0.001, max = 0.1, rate = 1.5))
})
}
```