Imagine you are writing an application that consumes an HTTP API. You make calls, receive JSON/XML/protobufs/etc, and use that data for something else. You like and use TDD to make sure that your app works properly, and you add tests for as much as you possibly can. However, when it comes to the code that actually makes those HTTP calls, you have to make some adjustments, because you don't actually want to be making those HTTP calls during your tests. In languages that have the ability to dynamically change things at runtime, this would be where you would add mocks. In Rust, it can be a little more challenging, and require a little more thought up-front1,2.

In this article we will make a small Rust library that uses the reqwest http client library, and see what we can do to adequately test the business logic. We assume you have the Rust toolchain installed, and are at least passingly familiar with programming in Rust.

Project setup

First, let's make our new Rust library, and add some dependencies to it:

$ cargo new --lib foo-client $ cd foo-client $ $EDITOR Cargo.toml

Make sure your Cargo.toml dependencies section looks like this:

[dependencies] # There is new stuff on `master` that we need for this reqwest = { git = "https://github.com/seanmonstar/reqwest" } url = "1.7" serde_json = "1.0"

Creating the client

Now, let's get our new library actually doing something. First, we'll create the FooClient :

// src/lib.rs pub struct FooClient { client: reqwest::Client, } impl FooClient { pub fn new() -> FooClient { FooClient { client: reqwest::Client::new(), } } }

(I'm using edition = 2018 here, hence why you don't see extern crate reqwest anywhere)

Okay, so we have our client, now let's give it something to do.

// src/lib.rs use serde_json::Value; pub struct FooClient { client: reqwest::Client, } impl FooClient { pub fn new() -> FooClient { FooClient { client: reqwest::Client::new(), } } pub fn get_widget(&self, id: &str) -> Result<Value, Box<Error>> { let url = Url::parse("https://example.com/widget/")? .join(id)?; let value: Value = self.client.get(url) .send()? .json()?; Ok(value) } }

Great! Now we can retrieve a widget by it's "id" . Usually we'd want to have better error management, but hey, Box<Error> is better than .unwrap() everywhere, right?

Now that our client can retrieve the widget resources it needs, we need to test this (yes, I know that technically we should have written the tests first, just go with me here). In order to avoid actually making HTTP calls during our tests, we're going to restructure our code just a bit. Essentially, we need abstract away the process of taking a reqwest::Request , and turning it into a request::Response . This way, we can do one thing in our production code, and something else in our tests.

So, let's add a trait for this:

// src/lib.rs impl FooClient { // ... } pub trait HttpSend { fn send(&self, request: reqwest::RequestBuilder) -> Result<reqwest::Response, Box<Error>>; }

and now we parameterize our client using this trait:

// src/lib.rs pub struct FooClient<S: HttpSend> { client: reqwest::Client, sender: S, } impl<S: HttpSend> FooClient<S> { // we'll take care of `pub fn new` in a minute... pub fn get_widget(&self, id: &str) -> Result<Value, Box<Error>> { let url = Url::parse("https://example.com/widget/")? .join(id)?; let value: Value = self.sender .send(self.client.get(url))? .json()?; Ok(value) } }

(Did you notice the slight change to the get_widget method?)

Before we can actually get this to compile & work correctly, we need two more pieces: first, the actual implementation of HttpSend for our client:

pub trait HttpSend { fn send(&self, request: reqwest::RequestBuilder) -> Result<reqwest::Response, Box<Error>>; } pub struct Sender; impl HttpSend for Sender { fn send(&self, request: reqwest::RequestBuilder) -> Result<reqwest::Response, Box<Error>> { Ok(request.send()?) } }

Easy enough, yeah? Lastly, let's make our client use this implementation:

pub struct FooClient<S: HttpSend=Sender> { client: reqwest::Client, sender: S, } impl FooClient<Sender> { pub fn new() -> FooClient<Sender> { FooClient { client: reqwest::Client::new(), sender: Sender, } } } impl<S: HttpSend> FooClient<S> { pub fn with_sender(sender: S) -> FooClient<S> { FooClient { client: reqwest::Client::new(), sender: sender, } } pub fn get_widget(..) // etc }

Nice! Now our client can send HTTP requests like normal, the user of the library doesn't have to deal with the HttpSend implementation at all (thanks to the default we set — <S: HttpSend=Sender> ), and we can swap out the “Sender” at compile time with our own implementation!

On to testing!

Ok, so, now that we have all of that in place, we're finally ready to write some tests! Well, okay, we have to make something in our test module to build from first, but then we'll get to write some tests!

First, our test module:

#[cfg(test)] mod tests { use std::error::Error; use super::{FooClient, HttpSend}; }

Now, we need to make an HttpSend er that can be used in our tests. In order to do this, we'll need to add one more dependency. In your Cargo.toml , add this section:

[dev-dependencies] http = "0.1.13"

This pulls in the http crate, which is used by reqwest , hyper , and other crates, and contains many standard types needed to work with http . We are going to be using it for it's http::response::Builder and http::Response types. Why is that? Because for our mock HttpSend er, we will need to manually create reqwest::Response objects. The only problem is, there is no way to manually construct reqwest::Response objects! Well, at least, there didn't used to be. Recently, reqwest gained the ability to construct reqwest::Response objects from http::Response objects. And since http::Response objects can be manually constructed, that's what we'll do!

#[cfg(test)] mod tests { use std::{ cell::RefCell, error::Error }; use super::{FooClient, HttpSend}; use http::response; pub struct MockSender( RefCell<response::Builder>, &'static str ); impl HttpSend for MockSender { fn send(&self, _: reqwest::RequestBuilder) -> Result<reqwest::Response, Box<Error>> { let mut builder = self.0.borrow_mut(); let response = builder.body(self.1)?; Ok(response.into()) } } }

Whew! Okay, so what are we doing here? Since our HttpSend trait takes &self , but our response::Builder methods take &mut self , we need to wrap the Builder in a RefCell so that we can get a mutable reference to it. Normally we might wrap this in a Mutex or something else so that we don't try to mutably borrow it more than once, but in this case we know we will only be using it for the single test case that the MockSender gets created for.

But why even put a Builder in the MockSender ? Why not just store the http::Response directly? Unfortunately, we cannot because response.into() consumes the response, which we couldn't do if the response was part of MockSender , since MockSender is borrowed inside .send() and we can't move out of it. So instead, we store both the builder, and the request body, and then we can use those two pieces to create the http::Response within the .send() method, which means we can move it, consume it, or whatever else we might want to do with it.

With this mock sender, we can write a test for the get_widget method:

// src/lib.rs #![macro_use] extern crate serde_json; // ... #[cfg(test)] mod tests { use std::{ cell::RefCell, error::Error }; use super::{FooClient, HttpSend}; use http::response; pub struct MockSender(RefCell<response::Builder>, &'static str); impl HttpSend for MockSender { fn send(&self, _: reqwest::RequestBuilder) -> Result<reqwest::Response, Box<Error>> { let mut builder = self.0.borrow_mut(); let response = builder.body(self.1)?; Ok(response.into()) } } #[test] fn get_widget() { let mut builder = response::Builder::new(); builder.status(200); let body = r#"{ "id": 42, "foo": "bar", "baz": "quux" }"#; let sender = MockSender(RefCell::new(builder), body); let client = FooClient::with_sender(sender); let result = client.get_widget("42") .expect("get_widget() call did not succeed"); assert_eq!( result, json!({ "id": 42, "foo": "bar", "baz": "quux" }) ); } }

(Note that we had to add #![macro_use] extern crate serde_json to the top so we could get the json!() macro)

Nice! Though it seems like a lot of setup for a single call, maybe we could abstract some of that setup away?

fn client_with_response(status: u16, body: &'static str) -> FooClient<MockSender> { let mut builder = response::Builder::new(); builder.status(status); let sender = MockSender(RefCell::new(builder), body); FooClient::with_sender(sender) } #[test] fn get_widget() { let client = client_with_response(200, r#"{ "id": 42, "foo": "bar", "baz": "quux" }"#); let result = client.get_widget("42") .expect("get_widget() call did not succeed"); assert_eq!( result, json!({ "id": 42, "foo": "bar", "baz": "quux" }) ); } }

There we go, that looks a little better.

Now, obviously, we aren't testing much here. All we know is that serde_json is taking that response and correctly converting it to a serde_json::Value , which we don't really need to test ourselves. However, we could change get_widget to return one of our own data structures, which would then test that our deserialization is working correctly. We could use these mocked clients to test higher-level methods, whose business logic combines the use of multiple HTTP calls and transforms the outputs somehow.

We would also want to make a better MockSender for our tests. The one we have here is nice for a few simple tests here and there, but we would probably also want it doing some kind of validation about the RequestBuilder coming in, as well as making it possible to return different responses depending on the incoming request. This would be necessary to test methods that make multiple HTTP calls. There are a lot of improvements we could make, but for this post I just wanted to show a simple example. Expanding it is left as an exercise to the reader ;–)3

Full source

Finally, here's the full source code to our little project:

// src/lib.rs #[cfg_attr(test, macro_use)] extern crate serde_json; use std::error::Error; use serde_json::Value; use url::Url; pub struct FooClient<S: HttpSend=Sender> { client: reqwest::Client, sender: S, } impl FooClient<Sender> { pub fn new() -> FooClient<Sender> { FooClient { client: reqwest::Client::new(), sender: Sender, } } } impl<S: HttpSend> FooClient<S> { pub fn with_sender(sender: S) -> FooClient<S> { FooClient { client: reqwest::Client::new(), sender: sender, } } pub fn get_widget(&self, id: &str) -> Result<Value, Box<Error>> { let url = Url::parse("https://example.com/widget/")? .join(id)?; let value: Value = self.sender .send(self.client.get(url))? .json()?; Ok(value) } } pub trait HttpSend { fn send(&self, request: reqwest::RequestBuilder) -> Result<reqwest::Response, Box<Error>>; } pub struct Sender; impl HttpSend for Sender { fn send(&self, request: reqwest::RequestBuilder) -> Result<reqwest::Response, Box<Error>> { Ok(request.send()?) } } #[cfg(test)] mod tests { use super::{FooClient, HttpSend}; use std::error::Error; use std::cell::RefCell; use http::response; pub struct MockSender( RefCell<response::Builder>, &'static str ); impl HttpSend for MockSender { fn send(&self, _: reqwest::RequestBuilder) -> Result<reqwest::Response, Box<Error>> { let mut builder = self.0.borrow_mut(); let response = builder.body(self.1)?; let response = response.into(); Ok(response) } } fn client_with_response(status: u16, body: &'static str) -> FooClient<MockSender> { let mut builder = response::Builder::new(); builder.status(status); let sender = MockSender(RefCell::new(builder), body); FooClient::with_sender(sender) } #[test] fn get_widget() { let id = "42"; let client = client_with_response(200, r#"{ "id":42, "foo":"bar", "baz":"quux" }"# ); let result = client.get_widget(id).expect("Call failed"); assert_eq!( result, json!({ "id": 42, "foo": "bar", "baz": "quux" }) ); } }

Thanks to @seanmonstar for all his help!

1 One strategy has been https://github.com/leoschwarz/reqwest_mock, which is a great library that works by duplicating reqwest's API. Unfortunately, it has not been able to keep up with the API changes to reqwest and therefore I was unable to use it, which spurred my effort to get the technique in this article working properly.

2 You could also abstract all possible calls to the remote service into a Service layer, defined by a trait, and write tests using a mocked Service. I wanted to test a little closer to the HTTP communication without actually testing the HTTP communication itself, hence this article.

3 Sorry, I know this is the lazy way out