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Design of vcr

Source: vignettes/design.Rmd
design.Rmd

This section explains vcr’s internal design and architecture.

How vcr works

The steps:

  1. Use either vcr::use_cassette() or vcr::local_cassette()
  1. If you use vcr::local_cassette(), make sure to run vcr::eject_cassette() when you’re done to stop recording
  1. When you first run a request with vcr there’s no cached data to use, so we allow HTTP requests until your request is done.
  2. The real HTTP request is made and we record a response in memory (not written to disk).
  3. We then disallow HTTP requests so that if the request is done again we use the cached response.
  4. The last thing we do is write the HTTP interaction to disk in a mostly human readable form.

When you run that request again using vcr::use_cassette() or vcr::local_cassette():

  • We attempt to match the request to cached requests, and since we stubbed the request the first time we used the cached response.

Of course if you do a different request, even slightly (but depending on which matching format you decided to use), then the request will have no matching stub and no cached response, and then a real HTTP request is done - we then cache it, then subsequent requests will pull from that cached response.

You can use any of the three HTTP R clients: crul, httr and httr2.

The main use case we are going for in vcr is to deal with real HTTP requests and responses, so we allow real HTTP requests when we need to, and turn it off when we don’t.

Where vcr comes from

vcr was “ported” from the Ruby gem (aka, library) of the same name1. Because it was ported from Ruby, an object-oriented programming language I thought it would be easier to use an object system in R that most closely resemble that used in Ruby (at least in my opinion). This thinking lead to choosing R6. Since v2 we’ve mostly removed use of R6 throughout the package.

Principles

An easy to use interface hides complexity

As described above, vcr uses some R6 internally, but users interact with normal R functions.

Hooks into HTTP clients

Perhaps the most fundamental thing about that this package work is how it knows what HTTP requests are being made. This stumped me for quite a long time. When looking at Ruby vcr, at first I thought it must be “listening” for HTTP requests somehow. Then I found out about monkey patching; that’s how it’s achieved in Ruby. That is, the Ruby vcr package literally overrides certain methods in Ruby HTTP clients, hijacking internals of the HTTP clients.

However, monkey patching is not allowed in R. Thus, in R we have to somehow have “hooks” into HTTP clients in R. Fortunately, Scott is the maintainer of one of the HTTP clients, crul, so was able to quickly create a hook. Fortunately, there was already a hook mechanism in the httr and httr2 packages.

Internal classes

An overview of some of the more important aspects of vcr.

Configuration

An internal object (the) is created when vcr is loaded with the default vcr configuration options in a list. This class keeps track of default and user specified configuration options. You can access the using triple namespace :::, though it is not intended for general use. Whenever you make calls to vcr_configure() or other configuration functions, the is affected.

Cassette class

Cassette is an R6 class that handles internals/state for each cassette. Each time you run use_cassette()/local_cassette() this class is used. The class has quite a few methods in it, so there’s a lot going on in the class. Ideally the class would be separated into subclasses to handle similar sets of logic, but there’s not an easy way to do that with R6.

How HTTP requests are handled

RequestHandler and it’s child classes RequestHandlerCrul, RequestHandlerHttr and RequestHandlerHttr2, handle the logic for for crul, httr and httr2, respectively. These classes determine what to do with each HTTP request. The options for each HTTP request include:

  • Ignored You can ignore HTTP requests under certain rules using the configuration options ignore_hosts and ignore_localhost
  • Stubbed by vcr This is an HTTP request for which a match is found in the cassette defined in the use_cassette()/insert_cassette() call. In this case the matching request/response from the cassette is returned with no real HTTP request allowed.
  • Recordable This is an HTTP request for which no match is found in the cassette defined in the use_cassette()/insert_cassette() call. In this case a real HTTP request is allowed, and the request/response is recorded to the cassette.
  • Unhandled If none of the above are triggered, you’ll get a hopefully helpful error message.

Serializers

Serializers handle in what format cassettes are written to files on disk. The current options are YAML (default), JSON, and QS2. YAML was implemented first in vcr because that’s the default option in Ruby vcr.

An R6 class Serializer is the parent class for all serializer types; YAML, JSON, and QS2 are all R6 classes that inherit from Serializer. All YAML, JSON, and QS2 define just two methods: serialize() and deserialize() for converting R structures to yaml, json, or qs2, and converting yaml, json, or qs2 back to R structures, respectively.