rOpenSci | HTTP Testing With the Newest Release of vcr

HTTP Testing With the Newest Release of vcr

A new version of vcr was just released. See the release notes for all the details. I want to highlight a few of the more notable changes.

🔗 New contributor

vcr has a new author: Maëlle Salmon. She has been contributing to the package quite a lot lately, including many documentation improvements and big updates to the HTTP Testing in R which covers vcr in addition to some other packages.

🔗 New serializer

vcr now has two serializer options: YAML (which we had before) and JSON (the new one). Serializing here refers to what format the HTTP request and response are put in before being written to a file. If you’re a keen observer of these two data formats, you’ll know these are not that different, with YAML a superset of JSON. The clear advantage of JSON straight away is that it avoids the common problem with vcr enabled tests on Windows platform where using YAML leads to errors due to something about the yaml package. The jsonlite package, which does the heavy lifting when using JSON, does not suffer from the same problems on Windows.

Keep in mind that by default the JSON serializer does not pretty print the json to disk to save disk space by eliminating the newlines; you can turn pretty printing on by setting json_pretty=TRUE.

With json_pretty=FALSE (the default) all the JSON is on one line:

{"http_interactions":[{"request":{"method":"get","uri":"","headers":{"User-Agent":"r-curl/4.3 crul/1.0.0 rOpenSci(rcol/0.1.0)"}},"response":{"status":{"status_code":"200","message":"OK","explanation":"Request fulfilled, document follows"},"body":{"encoding":"UTF-8","file":false,"string":"{\"result\":[{\"datasetKey\":1000,\"id\":\"2\",\"verbatimKey\":856}"}},"recorded_at":"2020-12-09 02:22:12 GMT","recorded_with":"vcr/, webmockr/0.7.4"}]}

When json_pretty=TRUE the JSON is on many lines and easier to read, but resulting files will be larger:

  "http_interactions": [
      "request": {
        "method": "get",
        "uri": "",
        "headers": {
          "User-Agent": "r-curl/4.3 crul/1.0.0 rOpenSci(rcol/0.1.0)"
      "response": {
        "status": {
          "status_code": "200",
          "message": "OK",
          "explanation": "Request fulfilled, document follows"
        "body": {
          "encoding": "UTF-8",
          "file": false,
          "string": "{\"result\":[{\"datasetKey\":1000,\"id\":\"2\",\"verbatimKey\":856}"
      "recorded_at": "2020-12-09 02:22:12 GMT",
      "recorded_with": "vcr/, webmockr/0.7.4"

To see the JSON serializer in action, the rcol package (not on CRAN) uses the JSON serializer - an example JSON fixture.

If you are currently using the YAML serializer and want to use the JSON serializer, delete all of your YAML based cassettes and update your vcr configuration globally:

vcr::vcr_configure(dir = "../fixtures", serialize_with = "json")

Or, set per cassette:

vcr::use_cassette("some_neat_code", {
}, serialize_with = "json")

🔗 Secrets

vcr has two new options for managing secrets: filter_request_headers and filter_response_headers. They both can be set globally with vcr_configure() or per cassette. They both allow for completely removing specific headers as well as replacing the value of a specific header with a value you specify.

These two new filters work differently than filter_sensitive_data; the two new ones only remove headers or replace values of headers, whereas filter_sensitive_data uses regex to replace values anywhere in the request or response. The three filters can be used in combination or alone.

See the Security section of the HTTP Testing in R book for more on vcr secrets.

🔗 Request matching

Two new request matchers now work: host and path.

Matching on host means that you are matching on just the hostname, e.g., in, the hostname or host is That is, if you match on host you ignore the scheme (so requests could have http or https), and you ignore anything else in the url after the hostname. So if you do requests to and they will match even though their paths components do not match.

Matching on path means that you are matching on any path components of a URL. For example, if a URL is, the path is foo/bar. So if you do requests to and they will match even though their hostnames do not match.

See the Configure vcr request matching for more on request matching.

🔗 The insert/eject workflow

Previous to this version the only way to use vcr was through use_cassette(). Now you have another option.

The combination of insert_cassette() and eject_cassette() now allow more flexibility in vcr usage. Whereas use_cassette() requires you to pass code that uses vcr into a code block within the function call, insert_cassette/eject_cassette allow you to turn on vcr, run any code you want to use vcr without embedding in a code block, then turn it off when you’re done. That looks like:

con <- crul::HttpClient$new(url = "")
con$get("get") # does use vcr
con$get("get") # does NOT use vcr

Alternatively, you can assign the output of insert_cassette() to an object and call $eject() on that object.

x <- insert_cassette("leothelion")
con <- crul::HttpClient$new(url = "")
con$get("get") # does use vcr
con$get("get") # does NOT use vcr

This inject/eject workflow has already been used for caching HTTP requests in vignettes.

🔗 Better debugging experience

There is a new vcr article debugging that helps guide you through debugging tests that use vcr. Paths work sightly different when running tests hitting a button in RStudio or running devtools::test() vs. running tests line by line in an interactive R session. The article includes discussion of a new function vcr::vcr_test_path() - based on testthat::test_path() - which will set the correct path to vcr files whether you are in an interactive R session or not. The article also includes guidance on logging to help in your debugging process.

🔗 Documentation

Many documentation improvements were made, much of it by Maëlle. Repeated parts of documentation were put into man/rmdhunks (1, 2) to be used in various places of the packages documentation. A new debugging vignette was created - see above section. In addition, a separate vignette was parsed out covering internals.

🔗 Get in touch

For an in depth comparison of the various tools for HTTP testing see HTTP Testing in R, which includes coverage of vcr. Make sure to also explore the vcr docs which have new and updated vignettes.

Get in touch if you have any questions/comments/feature requests.

🔗 Footnotes

  1. ↩︎

  2. ↩︎