Introduction to ratelimitr

Tarak Shah

2017-06-01

The basics

Use ratelimitr to limit the rate at which functions are called. A rate-limited function that allows n calls per period will never have a window of time of length period that includes more than n calls.

library(ratelimitr)
f <- function() NULL

# create a version of f that can only be called 10 times per second
f_lim <- limit_rate(f, rate(n = 10, period = 1))

# time without limiting
system.time(replicate(11, f()))
##    user  system elapsed 
##    0.01    0.00    0.02
# time with limiting
system.time(replicate(11, f_lim()))
##    user  system elapsed 
##    0.01    0.00    1.06

Multiple rates

Published rate limits often have multiple types of limits. Here is an example of limiting a function so that it never evaluates more than 10 times per .1 seconds, but additionally never evaluates more than 50 times per 1 second.

f_lim <- limit_rate(
    f, 
    rate(n = 10, period = .1), 
    rate(n = 50, period = 1)
)

# 10 calls do not trigger the rate limit
system.time( replicate(10, f_lim()) )
##    user  system elapsed 
##       0       0       0
# note that reset does not modify its argument, but returns a new
# rate-limited function with a fresh timer
f_lim <- reset(f_lim)
system.time( replicate(11, f_lim()) )
##    user  system elapsed 
##    0.00    0.00    0.14
# similarly, 50 calls don't trigger the second rate limit
f_lim <- reset(f_lim)
system.time( replicate(50, f_lim()) )
##    user  system elapsed 
##    0.02    0.00    0.55
# but 51 calls do:
f_lim <- reset(f_lim)
system.time( replicate(51, f_lim()) )
##    user  system elapsed 
##    0.00    0.00    1.04

Multiple functions sharing one (or more) rate limit(s)

To limit a group of functions together, just pass limit_rate a list of functions instead of a single function. Make sure the list is named, the names will be how you access the rate-limited versions of the functions:

f <- function() "f"
g <- function() "g"
h <- function() "h"

# passing a named list to limit_rate
limited <- limit_rate(
    list(f = f, g = g, h = h), 
    rate(n = 3, period = 1)
)

# now limited is a list of functions that share a rate limit. examples:
limited$f()
## [1] "f"
limited$g()
## [1] "g"

The new functions are subject to a single rate limit, regardless of which ones are called or in what order they are called.

# the first three function calls should not trigger a delay
system.time(
    {limited$f(); limited$g(); limited$h()}
)
##    user  system elapsed 
##       0       0       0
limited <- reset(limited)

# but to evaluate a fourth function call, there will be a delay
system.time({
    limited$f()
    limited$g() 
    limited$h() 
    limited$f()
})
##    user  system elapsed 
##    0.01    0.00    1.03