# Mock fastly::http::CandidateResponse

**URL:** <https://community.fastly.com/t/mock-fastly-candidateresponse/3975>\
**Category:** Rust SDK\
**Created:** [April 8, 2025, 6:26pm UTC](https://community.fastly.com/t/mock-fastly-candidateresponse/3975 "2025-04-08T18:26:02Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![arayaryoma](https://sea1.discourse-cdn.com/flex019/user_avatar/community.fastly.com/arayaryoma/32/811_2.png) [@arayaryoma](https://community.fastly.com/u/arayaryoma)\
**Post date:** [April 8, 2025, 6:26pm UTC](https://community.fastly.com/t/mock-fastly-candidateresponse/3975/1 "2025-04-08T18:26:02Z")

</div>

# Request for CandidateResponse testing/mocking support in Fastly Rust SDK

## Motivation

I’m working on a Compute application where I need to thoroughly test functions that manipulate cache behavior via `CandidateResponse`. For example, I have a function that applies specific cache control rules:

```rust
fn apply_cache_control_rules(resp: &mut CandidateResponse) {
    if let Some(surrogate_control) = resp.get_header_str("surrogate-control") {
        if surrogate_control.contains("no-cache") {
            resp.set_ttl(Duration::from_secs(0));
        }
        if surrogate_control.contains("no-store") {
            resp.set_header("surrogate-control", "no-store, private");
            resp.set_uncacheable(true);
        }
    }
}

fn main() -> Result<(), Error> {
    let mut req = Request::from_client();

    let beresp = req.send("backend")?;

    req.set_after_send(|resp: &mut CandidateResponse| {
        apply_cache_control_rules(resp);
    });

    Ok(beresp.send_to_client())
}

```

### Current Testing Options and Their Limitations

I’m currently facing significant challenges when trying to test this code:

1. **Integration tests with Viceroy:**  
While Viceroy is excellent for testing end-to-end functionality, it doesn’t allow access to `Request::set_after_send`.

2. **Unit tests:**  
For unit testing, I need to create a mock implementation of `CandidateResponse`. However, the current SDK design makes this exceptionally difficult:

## Feature Request

Could the Fastly Rust SDK team consider adding mocking capabilities for `CandidateResponse` and similar structures? This would greatly improve the testability of Compute applications. Some potential solutions might include:

1. Providing mock implementations of the required dependencies for `CandidateResponse::new()`
2. Adding a dedicated test utility for creating stub/mock instances of `CandidateResponse`

---

<div class="post-metadata">

**Author:** ![aspires](https://sea1.discourse-cdn.com/flex019/user_avatar/community.fastly.com/aspires/32/464_2.png) [@aspires](https://community.fastly.com/u/aspires)\
**Post date:** [April 9, 2025, 11:04pm UTC](https://community.fastly.com/t/mock-fastly-candidateresponse/3975/2 "2025-04-09T23:04:58Z")

</div>

This is a really solid example stub and write up @arayaryoma. I really appreciate the time you took to build this out. I’ll pass this on to to the team as a feature request.