Limit how often a function can be called

Decorator that allows at most calls calls in any window of period seconds. A call that would exceed the limit sleeps until the oldest call in the window has aged out. Calls are counted when they start, so a call that errors still uses up its slot, as it does for an API.

Usage

rate_limit(f, calls, period = 1)

Arguments

f

The function to decorate. Usually supplied by %@% rather than directly.

calls

Maximum number of calls in one window. A whole number of at least 1.

period

Length of the window in seconds. Defaults to 1.

Value

A decorator.

Details

Call rate_limit(calls, period) to build a decorator, then apply it with %@% or by calling it on a function. Each decorated function keeps its own record of calls, so two functions decorated separately have separate limits.

Stack it inside retry() so that every attempt is throttled, including retries. Stacking it outside would only throttle whole calls, and retries could still exceed the limit.

Examples

f <- rate_limit(calls = 3, period = 1) %@% function(x) x
vapply(1:3, f, numeric(1)) # immediate
#> [1] 1 2 3
system.time(f(4)) # waits for the window to clear
#>    user  system elapsed 
#>   0.001   0.000   0.897 

# Throttle every attempt, retries included
fetch <- retry(stop = stop_after_attempt(3)) %@%
  rate_limit(calls = 5, period = 1) %@%
  function(url) url