getURIAsynchronous: Download multiple URIs concurrently, with inter-leaved downloads

Description

This function allows the caller to specify multiple URIs to download at the same time. All the requests are submitted and then the replies are processed as data becomes available on each connection. In this way, the responses are processed in an inter-leaved fashion, with a chunk from one response from one request being processed and then followed by a chunk from a different request.

Downloading documents asynchronously involves some trade-offs. The switching between different streams, detecting when input is available on any of them involves a little more processing and so increases the consumption of CPU cycles. On the other hand, there is a potentially large saving of time when one considers total time to download. See http://www.omegahat.net/RCurl/concurrent.xml for more details. This is a common trade-off that arises in concurrent/parallel/asynchronous computing.

getURI calls this function if more than one URI is specified and async is TRUE, the default in this case. One can also download the (contents of the) multiple URIs serially, i.e. one after the other using getURI with a value of FALSE for async.

Usage

getURIAsynchronous(url, ..., .opts = list(), write = NULL, curl = getCurlHandle(), multiHandle = getCurlMultiHandle(), perform = Inf, .encoding = integer(), binary = rep(NA, length(url)))

Arguments

url

a character vector identifying the URIs to download.

...

named arguments to be passed to curlSetOpt when creating each of the different curlHandle objects.

.opts

a named list or CURLOptions object identifying the curl options for the handle. This is merged with the values of ... to create the actual options for the curl handle in the request.

write

an object giving the functions or routines that are to be called when input is waiting on the different HTTP response streams. By default, a separate callback function is associated with each input stream. This is necessary for the results to be meaningful as if we use a single reader, it will be called for all streams in a haphazard order and the content interleaved. One can do interesting things however using a single object.

curl

the prototypical curlHandle that is duplicated and used in in

multiHandle

this is a curl handle for performing asynchronous requests.

perform

a number which specifies the maximum number of calls to curlMultiPerform that are to be made in this function call. This is typically either 0 for no calls or Inf meaning process the requests until completion. One may find alternative values useful, such as 1 to ensure that the requests are dispatched.

.encoding

an integer or a string that explicitly identifies the encoding of the content that is returned by the HTTP server in its response to our query. The possible strings are ‘UTF-8’ or ‘ISO-8859-1’ and the integers should be specified symbolically as CE_UTF8 and CE_LATIN1. Note that, by default, the package attempts to process the header of the HTTP response to determine the encoding. This argument is used when such information is erroneous and the caller knows the correct encoding.

binary

a logical vector identifying whether each URI has binary content or simple text.

Value

multiHandle: the curl multi-handle, of class MultiCURLHandle-class. This can be used in further calls to curlMultiPerform
write: the write argument (after it was potentially expanded to a list). This can then be used to fetch the results of the requests when the requests are completed in the future.

Details

This uses curlMultiPerform and the multi/asynchronous interface for libcurl.

References

Curl homepage http://curl.haxx.se

Examples

Run this code

  uris = c("http://www.omegahat.net/RCurl/index.html",
           "http://www.omegahat.net/RCurl/philosophy.xml")
  txt = getURIAsynchronous(uris)
  names(txt)
  nchar(txt)

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