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Automatically retry a request on failure

Source: R/req-retries.R
req_retry.Rd

req_retry() allows req_perform() to automatically retry failing requests. It's particularly important for APIs with rate limiting, but can also be useful when dealing with flaky servers.

By default, req_perform() will retry if the response is a 429 ("too many requests", often used for rate limiting) or 503 ("service unavailable"). If the API you are wrapping has other transient status codes (or conveys transience with some other property of the response), you can override the default with is_transient. And if you set retry_on_failure = TRUE, the request will retry if either the HTTP request or HTTP response doesn't complete successfully, leading to an error from curl, the lower-level library that httr2 uses to perform HTTP requests. This occurs, for example, if your Wi-Fi is down.

Delay

It's a bad idea to immediately retry a request, so req_perform() will wait a little before trying again:

  • If the response contains the Retry-After header, httr2 will wait the amount of time it specifies. If the API you are wrapping conveys this information with a different header (or other property of the response), you can override the default behavior with retry_after.

  • Otherwise, httr2 will use "truncated exponential backoff with full jitter", i.e., it will wait a random amount of time between one second and 2 ^ tries seconds, capped at a maximum of 60 seconds. In other words, it waits runif(1, 1, 2) seconds after the first failure, runif(1, 1, 4) after the second, runif(1, 1, 8) after the third, and so on. If you'd prefer a different strategy, you can override the default with backoff.

Usage

req_retry(
  req,
  max_tries = NULL,
  max_seconds = NULL,
  retry_on_failure = FALSE,
  is_transient = NULL,
  backoff = NULL,
  after = NULL
)

Arguments

req

A httr2 request object.

max_tries, max_seconds

Cap the maximum number of attempts (max_tries), the total elapsed time from the first request (max_seconds), or both.

max_tries is the total number of attempts made, so this should always be greater than one.

retry_on_failure

Treat low-level failures as if they are transient errors that can be retried.

is_transient

A predicate function that takes a single argument (the response) and returns TRUE or FALSE specifying whether or not the response represents a transient error.

backoff

A function that takes a single argument (the number of failed attempts so far) and returns the number of seconds to wait.

after

A function that takes a single argument (the response) and returns either a number of seconds to wait or NA. NA indicates that a precise wait time is not available and that the backoff strategy should be used instead.

Value

A modified HTTP request.

See also

req_throttle() if the API has a rate-limit but doesn't expose the limits in the response.

Examples

# google APIs assume that a 500 is also a transient error
request("http://google.com") |>
  req_retry(is_transient = \(resp) resp_status(resp) %in% c(429, 500, 503))
#> Setting `max_tries = 2`.
#> <httr2_request>
#> GET http://google.com
#> Body: empty
#> Policies:
#> • retry_max_tries: 2
#> • retry_on_failure: FALSE
#> • retry_is_transient: a function

# use a constant 10s delay after every failure
request("http://example.com") |>
  req_retry(backoff = \(resp) 10)
#> Setting `max_tries = 2`.
#> <httr2_request>
#> GET http://example.com
#> Body: empty
#> Policies:
#> • retry_max_tries: 2
#> • retry_on_failure: FALSE
#> • retry_backoff: a function

# When rate-limited, GitHub's API returns a 403 with
# `X-RateLimit-Remaining: 0` and an Unix time stored in the
# `X-RateLimit-Reset` header. This takes a bit more work to handle:
github_is_transient <- function(resp) {
  resp_status(resp) == 403 &&
    identical(resp_header(resp, "X-RateLimit-Remaining"), "0")
}
github_after <- function(resp) {
  time <- as.numeric(resp_header(resp, "X-RateLimit-Reset"))
  time - unclass(Sys.time())
}
request("http://api.github.com") |>
  req_retry(
    is_transient = github_is_transient,
    after = github_after
  )
#> Setting `max_tries = 2`.
#> <httr2_request>
#> GET http://api.github.com
#> Body: empty
#> Policies:
#> • retry_max_tries: 2
#> • retry_on_failure: FALSE
#> • retry_is_transient: a function
#> • retry_after: a function