govcr

govcr

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Records and replays HTTP / HTTPS interactions for offline unit / behavioural / integration tests thereby acting as an HTTP mock. You can also use goovcr for API simulation.

This project was inspired by php-vcr which is a PHP port of VCR for ruby.

This project is an adaptation for Google's Go / Golang programming language.

Table of content

• Simple VCR example
• Install
• Glossary of Terms
• Concepts
  • VCRSettings
• Match a request to a cassette track
• Track mutators
• Cassette encryption
• Cookbook
  • Run the examples
  • Recipe: VCR with custom http.Client
  • Recipe: Remove Response TLS
  • Recipe: Change the playback mode of the VCR
  • Recipe: VCR with encrypted cassette
  • Recipe: VCR with encrypted cassette - custom nonce generator
  • Recipe: Cassette decryption
  • Recipe: Changing cassette encryption
  • Recipe: VCR with cassette storage on AWS S3
  • Recipe: VCR with a custom RequestMatcher
  • Recipe: VCR with a replaying Track Mutator
  • Recipe: VCR with a recording Track Mutator
  • More
• Stats
• Run the tests
• Bugs
• Improvements
• Limitations
• Contribute
• Community Support Appeal

Simple VCR example

// See TestExample1 in tests for fully working example.
func TestExample1() {
    vcr := govcr.NewVCR(
        govcr.NewCassetteLoader("MyCassette1.json"),
        govcr.WithRequestMatchers(govcr.NewMethodURLRequestMatchers()...), // use a "relaxed" request matcher
    )

    vcr.Client.Get("http://example.com/foo")
}

The first time you run this example, MyCassette1.json won't exist and TestExample1 will make a live HTTP call.

On subsequent executions (unless you delete the cassette file), the HTTP call will be played back from the cassette and no live HTTP call will occur.

Note:

We use a "relaxed" request matcher because example.com injects an "Age" header that varies per-request. Without a mutator, govcr's default strict matcher would not match the track on the cassette and keep sending live requests (and record them to the cassette).

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Install

go get github.com/seborama/govcr/v14@latest

For all available releases, please check the releases tab on github.

And your source code would use this import:

import "github.com/seborama/govcr/v14"

For versions of govcr before v5 (which don't use go.mod), use a dependency manager to lock the version you wish to use (perhaps v4)!

# download legacy version of govcr (without go.mod)
go get gopkg.in/seborama/govcr.v4

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Glossary of Terms

VCR: Video Cassette Recorder. In this context, a VCR refers to the engine and data that this project provides. A VCR is both an HTTP recorder and player. When you use a VCR, HTTP requests are replayed from previous recordings (tracks saved in cassette files, on the filesystem or in AWS S3, etc). When no previous recording exists for the request, it is performed live on the HTTP server, after what it is saved to a track on the cassette.

cassette: a sequential collection of tracks. This is in effect a JSON file.

Long Play cassette: a cassette compressed in gzip format. Such cassettes have a name that ends with '.gz'.

tracks: a record of an HTTP request. It contains the request data, the response data, if available, or the error that occurred.

ControlPanel: the creation of a VCR instantiates a ControlPanel for interacting with the VCR and conceal its internals.

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Concepts

govcr is a wrapper around the Go http.Client. It can record live HTTP traffic to files (called "cassettes") and later replay HTTP requests ("tracks") from them instead of live HTTP calls.

Cassette files can be stored on the filesystem or on a cloud storage service (AWS S3), etc.

The code documentation can be found on godoc.

When using govcr's http.Client, the request is matched against the tracks on the 'cassette':

• The track is played where a matching one exists on the cassette,
• otherwise the request is executed live to the HTTP server and then recorded on cassette for the next time.

Note on a govcr typical flow

The normal govcr flow is test-oriented. Traffic is recorded by default unless a track already existed on the cassette at the time it was loaded.

A typical usage:

• run your test once to produce the cassette
• from this point forward, when the test runs again, it will use the cassette

During live recording, the same request can be repeated and recorded many times. Playback occurs in the order the requests were saved on the cassette. See the tests for an example (TestConcurrencySafety).

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VCRSettings

This structure contains parameters for configuring your govcr recorder.

Settings are populated via With* options:

• Use WithClient to provide a custom http.Client otherwise the default Go http.Client will be used.
• See vcrsettings.go for more options such as WithRequestMatchers, WithTrackRecordingMutators, WithTrackReplayingMutators, ...
• TODO: WithLogging enables logging to help understand what govcr is doing internally.

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Match a request to a cassette track

By default, govcr uses a strict RequestMatcher function that compares the request's headers, method, full URL, body, and trailers.

Another RequestMatcher (obtained with NewMethodURLRequestMatcher) provides a more relaxed comparison based on just the method and the full URL.

In some scenarios, it may not possible to match tracks exactly as they were recorded.

This may be the case when the request contains a timestamp or a dynamically changing identifier, etc.

You can create your own matcher on any part of the request and in any manner (like ignoring or modifying some headers, etc).

The input parameters received by a RequestMatcher are scoped to the RequestMatchers. This affects the other RequestMatcher's. But it does not permeate throughout the VCR to the original incoming HTTP request or the tracks read from or written to the cassette.

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Track mutators

The live HTTP request and response traffic is protected against modifications. While govcr could easily support in-place mutation of the live traffic, this is not a goal.

Nonetheless, govcr supports mutating tracks, either at recording time or at playback time.

In either case, this is achieved with track Mutators.

A Mutator can be combined with one or more On conditions. All On conditions attached to a mutator must be true for the mutator to apply - in other words, they are logically "and-ed".

To help construct more complex yet readable predicates easily, govcr provides these pre-defined functions for use with On:

• Any achieves a logical "or" of the provided predicates.
• All achieves a logical "and" of the provided predicates.
• Not achieves a logical "not" of the provided predicates.
• None is synonymous of "Not Any".

Examples:

myMutator.
    On(Any(...)). // proceeds if any of the "`...`" predicates is true
    On(Not(Any(...)))  // proceeds if none of the "`...`" predicates is true (i.e. all predicates are false)
    On(Not(All(...))).  // proceeds if not every (including none) of the "`...`" predicates is true (i.e. at least one predicate is false, possibly all of them).

A track recording mutator can change both the request and the response that will be persisted to the cassette.

A track replaying mutator transforms the track after it was matched and retrieved from the cassette. It does not change the cassette file.

While a track replaying mutator could change the request, it serves no purpose since the request has already been made and matched to a track by the time the replaying mutator is invoked. The reason for supplying the request in the replaying mutator is for information. In some situations, the request details are needed to transform the response.

The track replaying mutator additionally receives an informational copy of the current HTTP request in the track's Response under the Request field i.e. Track.Response.Request. This is useful for tailoring track replays with current request information. See TestExample3 for illustration.

Refer to the tests for examples (search for WithTrackRecordingMutators and WithTrackReplayingMutators).

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Cassette encryption

Your cassettes are likely to contain sensitive information in practice. You can choose to not persist it to the cassette with a recording track mutator. However, in some situations, this information is needed. Enters cassette encryption.

Cassettes can be encrypted with two Go-supported ciphers:

• AES-GCM (12-byte nonce, 16 or 32-byte key)
• ChaCha20Poly1305 (24-byte nonce, 32-byte key)

You will need to provide a secret key to a "Crypter" that will take care of encrypting to file and decrypting from file the cassette contents transparently.

The cryptographic "nonce" is stored with the cassette, in its header. The default strategy to generate a n-byte random nonce.

It is possible to provide a custom nonce generator.

Cassettes are expected to be of somewhat reasonable size (at the very most a few MiB). They are fully loaded in memory. Under these circumstances, chunking is not needed and not supported.

As a reminder, you should never use a nonce value more than once with the same private key. It would compromise the encryption.

Please refer to the Cookbook for decryption and changes to encryption (such as cipher & key rotation).

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Cookbook

Run the examples

Please refer to the examples directory for examples of code and uses.

Observe the output of the examples between the 1st run and the 2nd run of each example.

The first time they run, they perform a live HTTP call (Executing request to live server).

However, on second execution (and subsequent executions as long as the cassette is not deleted) govcr retrieves the previously recorded request and plays it back without live HTTP call (Found a matching track). You can disconnect from the internet and still playback HTTP requests endlessly!

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Recipe: VCR with custom http.Client

Sometimes, your application will create its own http.Client wrapper (for observation, etc) or will initialise the http.Client's Transport (for instance when using https).

In such cases, you can pass the http.Client object of your application to VCR.

VCR will wrap your http.Client. You should use vcr.HTTPClient() in your tests when making HTTP calls.

// See TestExample2 in tests for fully working example.
func TestExample2() {
    // Create a custom http.Transport for our app.
    tr := http.DefaultTransport.(*http.Transport)
    tr.TLSClientConfig = &tls.Config{
        InsecureSkipVerify: true, // just an example, not recommended
    }

    // Create an instance of myApp.
    // It uses the custom Transport created above and a custom Timeout.
    app := &myApp{
        httpClient: &http.Client{
            Transport: tr,
            Timeout:   15 * time.Second,
        },
    }

    // Instantiate VCR.
    vcr := govcr.NewVCR(
        govcr.NewCassetteLoader(exampleCassetteName2),
        govcr.WithClient(app.httpClient),
    )

    // Inject VCR's http.Client wrapper.
    // The original transport has been preserved, only just wrapped into VCR's.
    app.httpClient = vcr.HTTPClient()

    // Run request and display stats.
    app.Get("https://example.com/foo")
}

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Recipe: Remove Response TLS

Use the provided mutator track.ResponseDeleteTLS.

Remove Response.TLS from the cassette recording:

vcr := govcr.NewVCR(
    govcr.NewCassetteLoader(exampleCassetteName2),
    govcr.WithTrackRecordingMutators(track.ResponseDeleteTLS()),
    //             ^^^^^^^^^
)
// or, similarly:
vcr.AddRecordingMutators(track.ResponseDeleteTLS())
//     ^^^^^^^^^

Remove Response.TLS from the track at playback time:

vcr := govcr.NewVCR(
    govcr.NewCassetteLoader(exampleCassetteName2),
    govcr.WithTrackReplayingMutators(track.ResponseDeleteTLS()),
    //             ^^^^^^^^^
)
// or, similarly:
vcr.AddReplayingMutators(track.ResponseDeleteTLS())
//     ^^^^^^^^^

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Recipe: Change the playback mode of the VCR

govcr support operation modes:

• Normal HTTP mode: replay from the cassette if a track matches otherwise place a live call.
• Live only: never replay from the cassette.
• Offline: playback from cassette only, return a transport error if no track matches.
• Read only: normal behaviour except that recording to cassette is disabled.

Normal HTTP mode

vcr := govcr.NewVCR(
    govcr.NewCassetteLoader(exampleCassetteName2),
    // Normal mode is default, no special option required :)
)
// or equally:
vcr.SetNormalMode()

Live only HTTP mode

vcr := govcr.NewVCR(
    govcr.NewCassetteLoader(exampleCassetteName2),
    govcr.WithLiveOnlyMode(),
)
// or equally:
vcr.SetLiveOnlyMode()

Read only cassette mode

vcr := govcr.NewVCR(
    govcr.NewCassetteLoader(exampleCassetteName2),
    govcr.WithReadOnlyMode(),
)
// or equally:
vcr.SetReadOnlyMode(true) // `false` to disable option

Offline HTTP mode

vcr := govcr.NewVCR(
    govcr.NewCassetteLoader(exampleCassetteName2),
    govcr.WithOfflineMode(),
)
// or equally:
vcr.SetOfflineMode()

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Recipe: VCR with encrypted cassette

At time of creating a new VCR with govcr:

// See TestExample4 in tests for fully working example.
vcr := govcr.NewVCR(
    govcr.NewCassetteLoader(exampleCassetteName4).
        WithCipher(
            encryption.NewChaCha20Poly1305WithRandomNonceGenerator,
            "test-fixtures/TestExample4.unsafe.key"),
)

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Recipe: VCR with encrypted cassette - custom nonce generator

This is nearly identical to the recipe "VCR with encrypted cassette", except we pass our custom nonce generator.

Example (this can also be achieved in the same way with the ControlPanel):

type myNonceGenerator struct{}

func (ng myNonceGenerator) Generate() ([]byte, error) {
    nonce := make([]byte, 12)
    if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
        return nil, err
    }
    return nonce, nil
}

vcr := govcr.NewVCR(
    govcr.NewCassetteLoader(exampleCassetteName4).
        WithCipherCustomNonce(
            encryption.NewChaCha20Poly1305,
            "test-fixtures/TestExample4.unsafe.key",
            nonceGenerator),
)

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Recipe: Cassette decryption

govcr provides a CLI utility to decrypt existing cassette files, should this be wanted.

The command is located in the cmd/govcr folder, to install it:

go install github.com/seborama/govcr/v14/cmd/govcr@latest

Example usage:

govcr decrypt -cassette-file my.cassette.json -key-file my.key

decrypt writes to the standard output to avoid errors or lingering decrypted files.

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Recipe: Changing cassette encryption

The cassette cipher can be changed for another with SetCipher.

For safety reasons, you cannot use SetCipher to remove encryption and decrypt the cassette. See the cassette decryption recipe for that.

vcr := govcr.NewVCR(...)
err := vcr.SetCipher(
    encryption.NewChaCha20Poly1305WithRandomNonceGenerator,
    "my_secret.key",
)

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Recipe: VCR with cassette storage on AWS S3

At time of creating a new VCR with govcr, provide an initialised S3 client:

// See TestExample5 in tests for fully working example.
s3Client := /* ... */
s3f := fileio.NewAWS(s3Client)

vcr := govcr.NewVCR(govcr.
    NewCassetteLoader(exampleCassetteName5).
    WithStore(s3f),
)

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Recipe: VCR with a custom RequestMatcher

This example shows how to handle situations where a header in the request needs to be ignored, in this case header X-Custom-Timestamp (or the track would not match and hence would not be replayed).

This could be necessary because the header value is not predictable or changes for each request.

vcr.SetRequestMatchers(
    govcr.DefaultMethodMatcher,
    govcr.DefaultURLMatcher,
    func(httpRequest, trackRequest *track.Request) bool {
        // we can safely mutate our inputs:
        // mutations affect other RequestMatcher's but _not_ the
        // original HTTP request or the cassette Tracks.
        httpRequest.Header.Del("X-Custom-Timestamp")
        trackRequest.Header.Del("X-Custom-Timestamp")

        return govcr.DefaultHeaderMatcher(httpRequest, trackRequest)
    },
)

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Recipe: VCR with a replaying Track Mutator

In this scenario, the API requires a "X-Transaction-Id" header to be present. Since the header changes per-request, as needed, replaying a track poses two concerns:

• the request won't match the previously recorded track because the value in "X-Transaction-Id" has changed since the track was recorded
• the response track contains the original values of "X-Transaction-Id" which is also a mis-match for the new request.

One of different solutions to address both concerns consists in:

• providing a custom request matcher that ignores "X-Transaction-Id"
• using the help of a replaying track mutator to inject the correct value for "X-Transaction-Id" from the current HTTP request.

How you specifically tackle this in practice really depends on how the API you are using behaves.

// See TestExample3 in tests for fully working example.
vcr := govcr.NewVCR(
    govcr.NewCassetteLoader(exampleCassetteName3),
    govcr.WithRequestMatchers(
        func(httpRequest, trackRequest *track.Request) bool {
            // Remove the header from comparison.
            // Note: this removal is only scoped to the request matcher, it does not affect the original HTTP request
            httpRequest.Header.Del("X-Transaction-Id")
            trackRequest.Header.Del("X-Transaction-Id")

            return govcr.DefaultHeaderMatcher(httpRequest, trackRequest)
        },
    ),
    govcr.WithTrackReplayingMutators(
        // Note: although we deleted the headers in the request matcher, this was limited to the scope of
        // the request matcher. The replaying mutator's scope is past request matching.
        track.ResponseDeleteHeaderKeys("X-Transaction-Id"), // do not append to existing values
        track.ResponseTransferHTTPHeaderKeys("X-Transaction-Id"),
    ),
)

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Recipe: VCR with a recording Track Mutator

Recording and replaying track mutators are the same. The only difference is when the mutator is invoked.

To set recording mutators, use govcr.WithTrackRecordingMutators when creating a new VCR, or use the SetRecordingMutators or AddRecordingMutators methods of the ControlPanel that is returned by NewVCR.

See the recipe "VCR with a replaying Track Mutator" for the general approach on creating a track mutator. You can also take a look at the recipe "Remove Response TLS".

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More

TODO: add example that includes the use of .On* predicates

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Stats

VCR provides some statistics.

To access the stats, call vcr.Stats() where vcr is the ControlPanel instance obtained from NewVCR(...).

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Run the tests

make test

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Bugs

• The recording of TLS data for PublicKey's is not reliable owing to a limitation in Go's json package and a non-deterministic and opaque use of a blank interface in Go's certificate structures. Some improvements are possible with gob.

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Improvements

• When unmarshaling the cassette fails, rather than fail altogether, it may be preferable to revert to live HTTP call.

• The code has a number of TODO's which should either be taken action upon or removed!

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Limitations

Go empty interfaces (interface{} / any)

Some properties / objects in http.Response are defined as interface{} (or any).

This can cause json.Unmarshal to fail (example: when the original type was big.Int with a big integer indeed - json.Unmarshal attempts to convert to float64 and fails).

Currently, this is dealt with by removing known untyped fields from the tracks. This is the case for PublicKey in certificate chains of the TLS data structure.

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Support for multiple values in HTTP headers

Repeat HTTP headers may not be properly handled. A long standing TODO in the code exists but so far no one has complained :-)

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HTTP transport errors

govcr also records http.Client errors (network down, blocking firewall, timeout, etc) in the track for future playback.

Since errors is an interface, when it is unmarshalled into JSON, the Go type of the error is lost.

To circumvent this, govcr serialises the object type (ErrType) and the error message (ErrMsg) in the track record.

Objects cannot be created by name at runtime in Go. Rather than re-create the original error object, govcr creates a standard error object with an error string made of both the ErrType and ErrMsg.

In practice, the implications for you depend on how much you care about the error type. If all you need to know is that an error occurred, you won't mind this limitation.

Mitigation: Support for common errors (network down) has been implemented. Support for more error types can be implemented, if there is appetite for it.

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Contribute

You are welcome to submit a PR to contribute.

Please try and follow a TDD workflow: tests must be present and as much as is practical to you, avoid toxic DDT (development driven testing).

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Community Support Appeal

⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️
⭐️
⭐️ 📣 Community Support Appeal 📣
⭐️
⭐️ Please show your love by giving a star to this project.
⭐️
⭐️ It takes a lot of personal time and effort to maintain and expand features.
⭐️
⭐️ If you are using govcr, show me it is worth my continuous effort by giving it a star.
⭐️
⭐️ 🙏 You'll be my star 😊 🙏
⭐️
⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️⭐️

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