vcr introduction

vcr introduction

vcr is an R port of the Ruby gem VCR (i.e., a translation, there’s no Ruby here :))

vcr helps you stub HTTP requests so you don’t have to repeat HTTP requests.

The main use case is for unit tests, but you can use it outside of the unit test use case.

vcr works with the crul and httr HTTP request packages; we’re working on support for curl.

Check out the http testing book for a lot more documentation on vcr, webmockr, and crul

Installation

CRAN

install.packages("vcr")

Development version

remotes::install_github("ropensci/vcr")

library("vcr")

Basic usage

library(vcr)
library(crul)

cli <- crul::HttpClient$new(url = "https://eu.httpbin.org")
system.time(
  use_cassette(name = "helloworld", {
    cli$get("get")
  })
)

The request gets recorded, and all subsequent requests of the same form used the cached HTTP response, and so are much faster

system.time(
  use_cassette(name = "helloworld", {
    cli$get("get")
  })
)

Importantly, your unit test deals with the same inputs and the same outputs - but behind the scenes you use a cached HTTP response - thus, your tests run faster.

The cached response looks something like (condensed for brevity):

http_interactions:
- request:
    method: get
    uri: https://eu.httpbin.org/get
    body:
      encoding: ''
      string: ''
    headers:
      User-Agent: libcurl/7.54.0 r-curl/3.2 crul/0.5.2
  response:
    status:
      status_code: '200'
      message: OK
      explanation: Request fulfilled, document follows
    headers:
      status: HTTP/1.1 200 OK
      connection: keep-alive
    body:
      encoding: UTF-8
      string: "{\n  \"args\": {}, \n  \"headers\": {\n    \"Accept\": \"application/json,
        text/xml, application/xml, */*\", \n    \"Accept-Encoding\": \"gzip, deflate\",
        \n    \"Connection\": \"close\", \n    \"Host\": \"httpbin.org\", \n    \"User-Agent\":
        \"libcurl/7.54.0 r-curl/3.2 crul/0.5.2\"\n  }, \n  \"origin\": \"136.152.208.176\",
        \n  \"url\": \"https://httpbin.org/get\"\n}\n"
  recorded_at: 2018-04-03 22:55:02 GMT
  recorded_with: vcr/0.0.7.9324

All components of both the request and response are preserved, so that the HTTP client (in this case crul) can reconstruct its own response just as it would if it wasn’t using vcr.