govcr
A Word Of Warning
I'm in the process of partly rewriting govcr to offer better support for cassette mutations. This is necessary because when I first designed govcr, I wanted cassettes to be immutable as much as golang can achieve this. Since then, I have received requests to permit cassette mutations at recording time.
The next release of govcr will bring breaking changes for those who are using govcr v4 or older. In exchange for the inconvenience, it will bring new features and a refreshed code base for future enhancements.
If you're happy with govcr as it is, use a dependency manager to lock the version of govcr you wish to use!
End Of: A Word Of Warning
Records and replays HTTP / HTTPS interactions for offline unit / behavioural / integration tests thereby acting as an HTTP mock.
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.
Install
go get github.com/seborama/govcr
For all available releases, please check the releases tab on github.
You can pick a specific major release for compatibility. For example, to use a v4.x release, use this command:
go get gopkg.in/seborama/govcr.v4
And your source code would use this import:
import "gopkg.in/seborama/govcr.v4"
Glossary of Terms
VCR: Video Cassette Recorder. In this context, a VCR refers to the overall 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). When no previous recording exists for the request, it is performed live on the HTTP server the request is intended to, after what it is saved to a track on the cassette.
cassette: a sequential collection of tracks. This is in effect a JSON file saved under directory ./govcr-fixtures (default). The cassette is given a name when creating the VCR which becomes the filename (with an extension of .cassette).
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.
PCB: Printed Circuit Board. This is an analogy that refers to the ability to supply customisations to some aspects of the behaviour of the VCR (for instance, disable recordings or ignore certain HTTP headers in the request when looking for a previously recorded track).
Documentation
govcr is a wrapper around the Go http.Client which offers the ability to replay pre-recorded HTTP requests ('tracks') instead of live HTTP calls.
govcr can replay both successful and failed HTTP transactions.
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,
- or the request is executed live to the HTTP server and then recorded on cassette for the next time (unless option
DisableRecordingis used).
When multiple matching tracks exist for the same request on the cassette (this can be crafted manually inside the cassette or can be simulated when using RequestFilters), the tracks will be replayed in the same order as they were recorded. See the tests for an example (TestPlaybackOrder).
When the last request matching track has been replayed, govcr cycles back to the first track again and so on.
Cassette recordings are saved under ./govcr-fixtures (by default) as *.cassette files in JSON format.
You can enable Long Play mode that will compress the cassette content. This is enabled by using the cassette name suffix .gz. The compression used is standard gzip.
It should be noted that the cassette name will be of the form 'MyCassette.gz" in your code but it will appear as "MyCassette.cassette.gz" on the file system. You can use gzip to compress and de-compress cassettes at will. See TestLongPlay() in govcr_test.go for an example usage. After running this test, notice the presence of the file govcr-fixtures/TestLongPlay.cassette.gz. You can view its contents in various ways such as with the gzcat command.
VCRConfig
This structure contains parameters for configuring your govcr recorder.
VCRConfig.CassettePath - change the location of cassette files
Example:
vcr := govcr.NewVCR("MyCassette",
&govcr.VCRConfig{
CassettePath: "./govcr-fixtures",
})
VCRConfig.DisableRecording - playback or execute live without recording
Example:
vcr := govcr.NewVCR("MyCassette",
&govcr.VCRConfig{
DisableRecording: true,
})
In this configuration, govcr will still playback from cassette when a previously recorded track (HTTP interaction) exists or execute the request live if not. But in the latter case, it won't record a new track as per default behaviour.
VCRConfig.Logging - disable logging
Example:
vcr := govcr.NewVCR("MyCassette",
&govcr.VCRConfig{
Logging: false,
})
This simply redirects all govcr logging to the OS's standard Null device (e.g. nul on Windows, or /dev/null on UN*X, etc).
VCRConfig.RemoveTLS - disable TLS recording
Example:
vcr := govcr.NewVCR("MyCassette",
&govcr.VCRConfig{
RemoveTLS: true,
})
As RemoveTLS is enabled, govcr will not record the TLS data from the HTTP response on the cassette track.
Features
-
Record extensive details about the request, response or error (network error, timeout, etc) to provide as accurate a playback as possible compared to the live HTTP request.
-
Recordings are JSON files and can be read in an editor.
-
Custom Go
http.Client's can be supplied. -
Custom Go
http.Transport/http.RoundTrippers. -
http / https supported and any other protocol implemented by the supplied
http.Client'shttp.RoundTripper. -
Hook to define HTTP headers that should be ignored from the HTTP request when attempting to retrieve a track for playback. This is useful to deal with non-static HTTP headers (for example, containing a timestamp).
-
Hook to transform the Header / Body of an HTTP request to deal with non-static data. The purpose is similar to the hook for headers described above but with the ability to modify the data.
-
Hook to transform the Header / Body of the HTTP response to deal with non-static data. This is similar to the request hook however, the header / body of the request are also supplied (read-only) to help match data in the response with data in the request (such as a transaction Id).
-
Ability to switch off automatic recordings. This allows to play back existing records or make a live HTTP call without recording it to the cassette.
-
Record SSL certificates.
Filter functions
In some scenarios, it may not possible to match tracks the way they were recorded.
For instance, the request contains a timestamp or a dynamically changing identifier, etc.
In other situations, the response needs a transformation to be receivable.
Building on from the above example, the response may need to provide the same identifier that the request sent.
Filters help you deal with this kind of practical aspects of dynamic exchanges.
Refer to examples/example6.go for advanced examples.
Influencing request comparison programatically at runtime.
RequestFilters receives the request Header / Body to allow their transformation. Both the live request and the recorded request on track are filtered in order to influence their comparison (e.g. remove an HTTP header to ignore it completely when matching).
Transformations are not persisted and only for the purpose of influencing comparison.
Runtime transforming of the response before sending it back to the client.
ResponseFilters is the flip side of RequestFilters. It receives the response Header / Body to allow their transformation. Unlike RequestFilters, this influences the response returned from the request to the client. The request header is also passed to ResponseFilter but read-only and solely for the purpose of extracting request data for situations where it is needed to transform the Response (such as to retrieve an identifier that must be the same in the request and the response).
Examples
Example 1 - Simple VCR
When no special HTTP Transport is required by your http.Client, you can use VCR with the default transport:
package main
import (
"fmt"
"github.com/seborama/govcr"
)
const example1CassetteName = "MyCassette1"
// Example1 is an example use of govcr.
func Example1() {
vcr := govcr.NewVCR(example1CassetteName, nil)
vcr.Client.Get("http://example.com/foo")
fmt.Printf("%+v\n", vcr.Stats())
}
If the cassette exists and a track matches the request, it will be played back without any real HTTP call to the live server.
Otherwise, a real live HTTP call will be made and recorded in a new track added to the cassette.
Tip:
To experiment with this example, run it at least twice: the first time (when the cassette does not exist), it will make a live call. Subsequent executions will use the track on cassette to retrieve the recorded response instead of making a live call.
Example 2 - Custom VCR Transport
Sometimes, your application will create its own http.Client wrapper 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 with its own which you can inject back into your application.
package main
import (
"crypto/tls"
"fmt"
"net/http"
"time"
"github.com/seborama/govcr"
)
const example2CassetteName = "MyCassette2"
// myApp is an application container.
type myApp struct {
httpClient *http.Client
}
func (app myApp) Get(url string) {
app.httpClient.Get(url)
}
// Example2 is an example use of govcr.
// It shows the use of a VCR with a custom Client.
// Here, the app executes a GET request.
func Example2() {
// Create a custom http.Transport.
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.
myapp := &myApp{
httpClient: &http.Client{
Transport: tr,
Timeout: 15 * time.Second,
},
}
// Instantiate VCR.
vcr := govcr.NewVCR(example2CassetteName,
&govcr.VCRConfig{
Client: myapp.httpClient,
})
// Inject VCR's http.Client wrapper.
// The original transport has been preserved, only just wrapped into VCR's.
myapp.httpClient = vcr.Client
// Run request and display stats.
myapp.Get("https://example.com/foo")
fmt.Printf("%+v\n", vcr.Stats())
}
Example 3 - Custom VCR, POST method
Please refer to the source file in the examples directory.
This example is identical to Example 2 but with a POST request rather than a GET.
Example 4 - Custom VCR with a RequestFilters
This example shows how to handle situations where a header in the request needs to be ignored (or the track would not match and hence would not be replayed).
For this example, logging is switched on. This is achieved with Logging: true in VCRConfig when calling NewVCR.
package main
import (
"fmt"
"strings"
"time"
"net/http"
"github.com/seborama/govcr"
)
const example4CassetteName = "MyCassette4"
// Example4 is an example use of govcr.
// The request contains a custom header 'X-Custom-My-Date' which varies with every request.
// This example shows how to exclude a particular header from the request to facilitate
// matching a previous recording.
// Without the RequestFilters, the headers would not match and hence the playback would not
// happen!
func Example4() {
vcr := govcr.NewVCR(example4CassetteName,
&govcr.VCRConfig{
RequestFilters: govcr.RequestFilters{
govcr.RequestDeleteHeaderKeys("X-Custom-My-Date"),
},
Logging: true,
})
// create a request with our custom header
req, err := http.NewRequest("POST", "http://example.com/foo", nil)
if err != nil {
fmt.Println(err)
}
req.Header.Add("X-Custom-My-Date", time.Now().String())
// run the request
vcr.Client.Do(req)
fmt.Printf("%+v\n", vcr.Stats())
}
Tip:
Remove the RequestFilters from the VCRConfig and re-run the example. Check the stats: notice how the tracks no longer replay.
Example 5 - Custom VCR with a RequestFilters and ResponseFilters
This example shows how to handle situations where a transaction Id in the header needs to be present in the response. This could be as part of a contract validation between server and client.
Note: This is useful when some of the data in the request Header / Body needs to be transformed before it can be evaluated for comparison for playback.
package main
import (
"fmt"
"strings"
"time"
"net/http"
"github.com/seborama/govcr"
)
const example5CassetteName = "MyCassette5"
// Example5 is an example use of govcr.
// Supposing a fictional application where the request contains a custom header
// 'X-Transaction-Id' which must be matched in the response from the server.
// When replaying, the request will have a different Transaction Id than that which was recorded.
// Hence the protocol (of this fictional example) is broken.
// To circumvent that, we inject the new request's X-Transaction-Id into the recorded response.
// Without the ResponseFilters, the X-Transaction-Id in the header would not match that
// of the recorded response and our fictional application would reject the response on validation!
func Example5() {
vcr := govcr.NewVCR(example5CassetteName,
&govcr.VCRConfig{
RequestFilters: govcr.RequestFilters{
govcr.RequestDeleteHeaderKeys("X-Transaction-Id"),
},
ResponseFilters: govcr.ResponseFilters{
// overwrite X-Transaction-Id in the Response with that from the Request
govcr.ResponseTransferHeaderKeys("X-Transaction-Id"),
},
Logging: true,
})
// create a request with our custom header
req, err := http.NewRequest("POST", "http://example.com/foo5", nil)
if err != nil {
fmt.Println(err)
}
req.Header.Add("X-Transaction-Id", time.Now().String())
// run the request
resp, err := vcr.Client.Do(req)
if err != nil {
fmt.Println(err)
}
// verify outcome
if req.Header.Get("X-Transaction-Id") != resp.Header.Get("X-Transaction-Id") {
fmt.Println("Header transaction Id verification failed - this would be the live request!")
} else {
fmt.Println("Header transaction Id verification passed - this would be the replayed track!")
}
fmt.Printf("%+v\n", vcr.Stats())
}
More examples
Refer to examples/example6.go for advanced examples.
All examples are in the examples directory.
Stats
VCR provides some statistics.
To access the stats, call vcr.Stats() where vcr is the VCR instance obtained from NewVCR(...).
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 sub-sequent 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!
Make utility
make examples
Manually
cd examples
go run *.go
Output
First execution - notice the stats show that a track was recorded (from a live HTTP call).
Second execution - no track is recorded (no live HTTP call) but 1 track is loaded and played back.
Running Example1...
1st run =======================================================
{TracksLoaded:0 TracksRecorded:1 TracksPlayed:0}
2nd run =======================================================
{TracksLoaded:1 TracksRecorded:0 TracksPlayed:1}
Complete ======================================================
Running Example2...
1st run =======================================================
{TracksLoaded:0 TracksRecorded:1 TracksPlayed:0}
2nd run =======================================================
{TracksLoaded:1 TracksRecorded:0 TracksPlayed:1}
Complete ======================================================
Running Example3...
1st run =======================================================
{TracksLoaded:0 TracksRecorded:1 TracksPlayed:0}
2nd run =======================================================
{TracksLoaded:1 TracksRecorded:0 TracksPlayed:1}
Complete ======================================================
Running Example4...
1st run =======================================================
2018/10/25 00:12:56 INFO - Cassette 'MyCassette4' - Executing request to live server for POST http://www.example.com/foo
2018/10/25 00:12:56 INFO - Cassette 'MyCassette4' - Recording new track for POST http://www.example.com/foo as POST http://www.example.com/foo
{TracksLoaded:0 TracksRecorded:1 TracksPlayed:0}
2nd run =======================================================
2018/10/25 00:12:56 INFO - Cassette 'MyCassette4' - Found a matching track for POST http://www.example.com/foo
{TracksLoaded:1 TracksRecorded:0 TracksPlayed:1}
Complete ======================================================
Running Example5...
1st run =======================================================
2018/10/25 00:12:56 INFO - Cassette 'MyCassette5' - Executing request to live server for POST http://www.example.com/foo5
2018/10/25 00:12:56 INFO - Cassette 'MyCassette5' - Recording new track for POST http://www.example.com/foo5 as POST http://www.example.com/foo5
Header transaction Id verification failed - this would be the live request!
{TracksLoaded:0 TracksRecorded:1 TracksPlayed:0}
2nd run =======================================================
2018/10/25 00:12:56 INFO - Cassette 'MyCassette5' - Found a matching track for POST http://www.example.com/foo5
Header transaction Id verification passed - this would be the replayed track!
{TracksLoaded:1 TracksRecorded:0 TracksPlayed:1}
Complete ======================================================
Run the tests
make test
or
go test -race -cover
Bugs
- None known
Improvements
-
When unmarshaling the cassette fails, rather than fail altogether, it would 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!
Limitations
Go empty interfaces (interface{})
Some properties / objects in http.Response are defined as interface{}. This can cause json.Unmarshall to fail (example: when the original type was big.Int with a big interger indeed - json.Unmarshal attempts to convert to float64 and fails).
Currently, this is dealt with by converting the output of the JSON produced by json.Marshal (big.Int is changed to a string).
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 :-)
HTTP transport errors
govcr also records http.Client errors (network down, blocking firewall, timeout, etc) in the track for future play back.
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.
Contribute
You are welcome to submit a PR to contribute.
Please follow a TDD workflow: tests must be present and avoid toxic DDT (dev driven testing).
