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Design of vcr

Source: vignettes/design.Rmd
design.Rmd

This section explains vcr’s internal design and architecture.

Where vcr comes from and why R6

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. The exported functions users interact with are not R6 classes, but are rather normal R functions. However, most of the internal code in the package uses R6. Thus, familiarity with R6 is important for people that may want to contribute to vcr, but not required at all for vcr users.

Principles

An easy to use interface hides complexity

As described above, vcr uses R6 internally, but users interact with normal R functions. Internal functions that are quite complicated are largely R6 while exported, simpler functions users interact with are normal R functions.

Class/function names are inherited from Ruby vcr

Since R vcr was ported from Ruby, we kept most of the names of functions/classes and variables. So if you’re wondering about why a function, class, or variable has a particular name, its derivation can not be found out in this package, for the most part that is.

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.

The actual hooks are not in vcr, but in webmockr. vcr depends on webmockr for hooking into HTTP clients httr, httr2 and crul.

Internal classes

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

Configuration

An internal object (vcr_c) is created when vcr is loaded with the default vcr configuration options inside of an R6 class VCRConfig - see https://github.com/ropensci/vcr/blob/main/R/onLoad.R. This class is keeps track of default and user specified configuration options. You can access vcr_c using triple namespace :::, though it is not intended for general use. Whenever you make calls to vcr_configure() or other configuration functions, vcr_c is affected.

Cassette class

Cassette is an R6 class that handles internals/state for each cassette. Each time you run use_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.

Of note in Cassette is that when called, within the initialize() call webmockr is used to create webmockr stubs.

How HTTP requests are handled

Within webmockr, there are calls to the vcr class RequestHandler, which has child classes RequestHandlerCrul, RequestHandlerHttr and RequestHandlerHttr2 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 This is a group of cases, all of which cause an error to be thrown with a message trying to help the user figure out how to fix the problem.

If you use vcr logging you’ll see these categories in your logs.

Serializers

Serializers handle in what format cassettes are written to files on disk. The current options are YAML (default) and JSON. 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 and JSON are both R6 classes that inherit from Serializer. Both YAML and JSON define just two methods: serialize() and deserialize() for converting R structures to yaml or json, and converting yaml or json back to R structures, respectively.

Environments

Logging

An internal environment (vcr_log_env) is used when you use logging. At this point it only keeps track of one variable - file - to be able to refer to what file is used for logging across many classes/functions that need to write to the log file.

A bit of housekeeping

Another internal environment (vcr__env) is used to keep track of a few items, including the current cassette in use, and the last vcr error.

Lightswitch

Another internal environment (light_switch) is used to keep track of users turning on and off vcr. See ?lightswitch.