Introduction to vcr

Scott Chamberlain

2024-10-25

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 and record 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, httr and httr2 HTTP request packages.

Check out the HTTP testing book for a lot more documentation on vcr, webmockr, and crul, and other packages.

Elevator pitch

• Setup vcr for your package with vcr::use_vcr()
• Tweak the configuration to protect your secrets
• Sprinkle your tests with vcr::use_cassette() to save HTTP interactions to disk in “cassettes” files
• If you want to test for package behavior when the API returns e.g. a 404 or 503 code, edit the cassettes, or use webmockr

Now your tests can work without any internet connection!

Demo of adding vcr testing to an R package, corresponding narrative.

Installation

CRAN

install.packages("vcr")

Development version

remotes::install_github("ropensci/vcr")

library("vcr")

Getting Started

The docs assume you are using testthat for your unit tests.

use_vcr

You can then set up your package to use vcr with:

vcr::use_vcr()

This will:

• put vcr into the DESCRIPTION
• check that testthat is setup
• setup testthat if not
• set the recorded cassettes to be saved in and sourced from tests/fixtures
• setup a config file for vcr
• add an example test file for vcr
• make a .gitattributes file with settings for vcr
• make a ./tests/testthat/helper-vcr.R file

What you will see in the R console:

◉ Using package: vcr.example  
◉ assuming fixtures at: tests/fixtures  
✓ Adding vcr to Suggests field in DESCRIPTION  
✓ Creating directory: ./tests/testthat  
◉ Looking for testthat.R file or similar  
✓ tests/testthat.R: added  
✓ Adding vcr config to tests/testthat/helper-vcr.example.R  
✓ Adding example test file tests/testthat/test-vcr_example.R  
✓ .gitattributes: added  
◉ Learn more about `vcr`: https://books.ropensci.org/http-testing/

Protecting secrets

Secrets often turn up in API work. A common example is an API key. vcr saves responses from APIs as YAML files, and this will include your secrets unless you indicate to vcr what they are and how to protect them. The vcr_configure function has the filter_sensitive_data argument function for just this situation. The filter_sensitive_data argument takes a named list where the name of the list is the string that will be used in the recorded cassettes instead of the secret, which is the list item. vcr will manage the replacement of that for you, so all you need to do is to edit your helper-vcr.R file like this:

library("vcr") # *Required* as vcr is set up on loading
invisible(vcr::vcr_configure(
  dir = "../fixtures"
))
vcr::check_cassette_names()

Use the filter_sensitive_data argument in the vcr_configure function to show vcr how to keep your secret. The best way to store secret information is to have it in a .Renviron file. Assuming that that is already in place, supply a named list to the filter_sensitive_data argument.

library("vcr")
invisible(vcr::vcr_configure(
  filter_sensitive_data = list("<<<my_api_key>>>" = Sys.getenv('APIKEY')),  # add this
  dir = "../fixtures"
))
vcr::check_cassette_names()

Notice we wrote Sys.getenv('APIKEY') and not the API key directly, otherwise you’d have written your API key to a file that might end up in a public repo.

The will get your secret information from the environment, and make sure that whenever vcr records a new cassette, it will replace the secret information with <<<my_api_key>>>. You can find out more about this in the HTTP testing book chapter on security.

The addition of the line above will instruct vcr to replace any string in cassettes it records that are equivalent to your string which is stored as the APIKEY environmental variable with the masking string <<<my_api_key>>>. In practice, you might get a YAML that looks a little like this:

http_interactions:
- request:
    method: post
    ...
    headers:
      Accept: application/json, text/xml, application/xml, */*
      Content-Type: application/json
      api-key: <<<my_api_key>>>
    ...

Here, my APIKEY environmental variable would have been stored as the api-key value, but vcr has realised this and recorded the string <<<my_api_key>>> instead.

Once the cassette is recorded, vcr no longer needs the API key as no real requests will be made. Furthermore, as by default requests matching does not include the API key, things will work.

Now, how to ensure tests work in the absence of a real API key?

E.g. to have tests pass on continuous integration for external pull requests to your code repository.

• vcr does not need an actual API key for requests once the cassettes are created, as no real requests will be made.
• you still need to fool your package into believing there is an API key as it will construct requests with it. So add the following lines to a testthat setup file (e.g. tests/testthat/helper-vcr.R)

if (!nzchar(Sys.getenv("APIKEY"))) {
  Sys.setenv("APIKEY" = "foobar")
}

Using an .Renviron

A simple way to manage local environmental variables is to use an .Renviron file. Your .Renviron file might look like this:

APIKEY="mytotallysecretkey"

You can have this set at a project or user level, and usethis has the usethis::edit_r_environ() function to help edit the file.

Basic usage

In tests

In your tests, for whichever tests you want to use vcr, wrap them in a vcr::use_cassette() call like:

library(testthat)
vcr::use_cassette("rl_citation", {
  test_that("my test", {
    aa <- rl_citation()

    expect_is(aa, "character")
    expect_match(aa, "IUCN")
    expect_match(aa, "www.iucnredlist.org")
  })
})

OR put the vcr::use_cassette() block on the inside, but put testthat expectations outside of the vcr::use_cassette() block:

library(testthat)
test_that("my test", {
  vcr::use_cassette("rl_citation", {
    aa <- rl_citation()
  })

  expect_is(aa, "character")
  expect_match(aa, "IUCN")
  expect_match(aa, "www.iucnredlist.org")
})

Don’t wrap the use_cassette() block inside your test_that() block with testthat expectations inside the use_cassette() block, as you’ll only get the line number that the use_cassette() block starts on on failures.

The first time you run the tests, a “cassette” i.e. a file with recorded HTTP interactions, is created at tests/fixtures/rl_citation.yml. The times after that, the cassette will be used. If you change your code and more HTTP interactions are needed in the code wrapped by vcr::use_cassette("rl_citation", delete tests/fixtures/rl_citation.yml and run the tests again for re-recording the cassette.

Outside of tests

If you want to get a feel for how vcr works, although you don’t need too.

library(vcr)
library(crul)

cli <- crul::HttpClient$new(url = "https://eu.httpbin.org")
system.time(
  use_cassette(name = "helloworld", {
    cli$get("get")
  })
)
#>    user  system elapsed 
#>   0.109   0.000   0.626

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")
  })
)
#>    user  system elapsed 
#>   0.085   0.004   0.089

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\": \"111.222.333.444\",
        \n  \"url\": \"https://eu.httpbin.org/get\"\n}\n"
  recorded_at: 2018-04-03 22:55:02 GMT
  recorded_with: vcr/0.1.0, webmockr/0.2.4, crul/0.5.2

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.

Less basic usage

For tweaking things to your needs, make sure to read the docs about configuration (e.g., where are the fixtures saved? can they be re-recorded automatically regulary?) and request matching (how does vcr match a request to a recorded interaction?)

Terminology

• vcr: the name comes from the idea that we want to record something and play it back later, like a vcr
• cassette: A thing to record HTTP interactions to. Right now the only option is the file system (writing to files), but in the future could be other things, e.g. a key-value store like Redis
• fixture: A fixture is something used to consistently test a piece of software. In this case, a cassette (just defined above) is a fixture - used in unit tests. If you use our setup function vcr_setup() the default directory created to hold cassettes is called fixtures/ as a signal as to what the folder contains.
• Persisters: how to save requests - currently only option is the file system
• serialize: translating data into a format that can be stored; here, translate HTTP request and response data into a representation on disk to read back later
• Serializers: how to serialize the HTTP response - currently only option is YAML; other options in the future could include e.g. JSON
• insert cassette: create a cassette (all HTTP interactions will be recorded to this cassette)
• eject cassette: eject the cassette (no longer recording to that cassette)
• replay: refers to using a cached result of an http request that was recorded earlier

Workflows

vcr for tests

• See usage section

• When running tests or checks of your whole package, note that some users have found different results with devtools::check() vs. devtools::test(). It’s not clear why this would make a difference. Do let us know if you run into this problem.

vcr in your R project

You can use vcr in any R project as well.

• Load vcr in your project
• Similar to the above example, use use_cassette to run code that does HTTP requests.
• The first time a real request is done, and after that the cached response will be used.

Configuration

See also the configuration vignette.

We set the following defaults:

• dir = "."
• record = "once"
• match_requests_on = "c("method", "uri")"
• allow_unused_http_interactions = TRUE
• serialize_with = "yaml"
• json_pretty = FALSE
• persist_with = "FileSystem"
• ignore_hosts = NULL
• ignore_localhost = FALSE
• ignore_request = NULL
• uri_parser = "crul::url_parse"
• preserve_exact_body_bytes = FALSE
• turned_off = FALSE
• re_record_interval = NULL
• clean_outdated_http_interactions = FALSE
• allow_http_connections_when_no_cassette = FALSE
• cassettes = list()
• linked_context = NULL
• log = FALSE
• log_opts = list(file = "vcr.log", log_prefix = "Cassette", date = TRUE)
• filter_sensitive_data = NULL
• filter_sensitive_data_regex = NULL
• filter_request_headers = NULL
• filter_response_headers = NULL
• filter_query_parameters = NULL
• write_disk_path = NULL
• verbose_errors = FALSE
• quiet = TRUE
• warn_on_empty_cassette = TRUE

You can get the defaults programmatically with

vcr_config_defaults()

You can change all the above defaults with vcr_configure():

vcr_configure()

Calling vcr_configuration() gives you some of the more important configuration parameters in a nice tidy print out

vcr_configuration()
#> <vcr configuration>
#>   Cassette Dir: /tmp/RtmpM4JXwm
#>   Record: once
#>   Serialize with: yaml
#>   URI Parser: crul::url_parse
#>   Match Requests on: method, uri
#>   Preserve Bytes?: FALSE
#>   Logging?: FALSE
#>   ignored hosts: 
#>   ignore localhost?: FALSE
#>   Write disk path:

For more details refer to the configuration vignette

Matching/Matchers

vcr looks for similarity in your HTTP requests to cached requests. You can set what is examined about the request with one or more of the following options:

• body
• headers
• host
• method
• path
• query
• uri

By default, we use method (HTTP method, e.g., GET) and uri (test for exact match against URI, e.g., http://foo.com).

You can set your own options by tweaking the match_requests_on parameter:

use_cassette(name = "one", {
    cli$post("post", body = list(a = 5))
  },
  match_requests_on = c('method', 'headers', 'body')
)

For more details refer to the request matching vignette.

Note about missing features

There’s a number of features in this package that are not yet supported, but for which their parameters are found in the package.

We’ve tried to make sure the parameters that are ignored are marked as such. Keep an eye out for package updates for changes in these parameters, and/or let us know you want it and we can move it up in the priority list.