req_perform_promise: Perform request asynchronously using the promises package

In r-lib/httr2: Perform HTTP Requests and Process the Responses

Perform request asynchronously using the promises package

Description

This variation on req_perform() returns a promises::promise() object immediately and then performs the request in the background, returning program control before the request is finished. See the promises package documentation for more details on how to work with the resulting promise object.

If using together with later::with_temp_loop() or other private event loops, a new curl pool made by curl::new_pool() should be created for requests made within the loop to ensure that only these requests are being polled by the loop.

Like with req_perform_parallel(), exercise caution when using this function; it's easy to pummel a server with many simultaneous requests. Also, not all servers can handle more than 1 request at a time, so the responses may still return sequentially.

req_perform_promise() also has similar limitations to the req_perform_parallel() function, it:

• Will not retrieve a new OAuth token if it expires after the promised request is created but before it is actually requested.

• Does not perform throttling with req_throttle().

• Does not attempt retries as described by req_retry().

• Only consults the cache set by req_cache() when the request is promised.

Usage

req_perform_promise(req, path = NULL, pool = NULL)

Arguments

req	A httr2 request object.
path	Optionally, path to save body of the response. This is useful for large responses since it avoids storing the response in memory.
pool	Optionally, a curl pool made by curl::new_pool(). Supply this if you want to override the defaults for total concurrent connections (100) or concurrent connections per host (6).

Value

a promises::promise() object which resolves to a response if successful or rejects on the same errors thrown by req_perform().

Examples

## Not run: 
library(promises)
request_base <- request(example_url()) |> req_url_path_append("delay")

p <- request_base |> req_url_path_append(2) |> req_perform_promise()

# A promise object, not particularly useful on its own
p

# Use promise chaining functions to access results
p %...>%
  resp_body_json() %...>%
  print()


# Can run two requests at the same time
p1 <- request_base |> req_url_path_append(2) |> req_perform_promise()
p2 <- request_base |> req_url_path_append(1) |> req_perform_promise()

p1 %...>%
  resp_url_path %...>%
  paste0(., " finished") %...>%
  print()

p2 %...>%
  resp_url_path %...>%
  paste0(., " finished") %...>%
  print()

# See the [promises package documentation](https://rstudio.github.io/promises/)
# for more information on working with promises

## End(Not run)

r-lib/httr2 documentation built on Jan. 11, 2025, 10:21 a.m.