Experiments in Open Source: Exploring vcr-sharp for Http record and playback

Dev Tips


I’ve always said that reading source code is as important as write it – especially as part of the learning process. You learn a ton about how other coders think and solve problems, plus you learn about lots of new libraries and methods you may not be familiar with.

Last week I noticed this tweet from Brendan Forster about an experiment he’s working on. He is interesting in your feedback on his experiment and if you would use it.

He’s created a new library for .NET called vcr-sharp that is inspired by the vcr Ruby gem and the scotch .NET library.

Again, he’s made it clear he’s just experimenting but I think this has some interesting potential.

Vcr-sharp lets you record and playback HTTP requests! In this example, WithCassette is an extension method on HttpClientFactory. That extension method sets up a DelgatingHandler to a ReplayingHandler. That ReplayingHandler “loads the cassette” and returns it as a cached response.

using (var httpClient = HttpClientFactory.WithCassette("my-test-scenario"))
{
var request = new HttpRequestMessage(HttpMethod.Get, "http://www.iana.org/domains/reserved");
var response = await httpClient.SendAsync(request);
var body = await response.Content.ReadAsStringAsync();
body.ShouldContain("Example domains");
}

Also worth noting is that within the VCR-Sharp library Brendan uses an assertion library for .NET called “Shouldly.” Shouldly has some interesting extension methods that let you express how you Assert within your Tests.

They say – this is the old Assert way:

Assert.That(contestant.Points, Is.EqualTo(1337));

For your troubles, you get this message, when it fails:

Expected 1337 but was 0

They say – this is how it Should be:

contestant.Points.ShouldBe(1337);

Which is just syntax, so far, but check out the message when it fails:

contestant.Points should be 1337 but was 0

Another example:

Assert.That(map.IndexOfValue("boo"), Is.EqualTo(2));    // -> Expected 2 but was 1
map.IndexOfValue("boo").ShouldBe(2);                    // -> map.IndexOfValue("boo") should be 2 but was 1

It makes tests very easy to read. A nice bit of syntactic sugar:

[Fact]
public async Task AppendsNewRequestToCache()
{
Environment.SetEnvironmentVariable("VCR_MODE", "Cache");
var session = "append-second-request";

using (var httpClient = HttpClientFactory.WithCassette(session))
{
var request = new HttpRequestMessage(HttpMethod.Get, "https://www.iana.org/performance/ietf-statistics");
var response = await httpClient.SendAsync(request);
}

var cassette = await ReadCassetteFile(session);
cassette.http_interactions.Length.ShouldBe(2);
}

It also uses BenchmarkDotNet, which you may be familiar with. It allows you to mark methods as [Benchmark] methods and you’ll get smart warming up, running, teardowns and statistics like this;

[Benchmark]
public async Task ReadFromCache()
{

using (var httpClient = HttpClientFactory.WithCassette("example-test"))
{
var request = new HttpRequestMessage(HttpMethod.Get, "http://www.iana.org/domains/reserved");
var response = await httpClient.SendAsync(request);
}
} Output:

        Method |     Mean |    Error |   StdDev |
-------------- |---------:|---------:|---------:|
ReadFromCache | 684.1 us | 3.154 us | 2.796 us |

I’d encourage you to check vcr-sharp out over at https://github.com/shiftkey/vcr-sharp, read the source code, and think about how you’d use it. I am sure Brendan would appreciate your thoughts and feedback in the GitHub Issues! Also check out how he uses Tests, Shouldly, and BenchmarkDotNet in his project and consider how you’d use them in yours!


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