[request]: Ability to mock *::Client

[NickLarsenNZ]

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Tell us about your request
Some way of being able to mock the Clients

Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?
In a contrived piece of code, I'm trying to list EC2 instances - but I'd like to test this offline by mocking the aws_sdk_ec2::Client.

Are you currently working around this issue?
I'm not (just using a real account, and weakening my test assertions).

Additional context
I wondered if there should be some traits to make mocking portions of the Clients easier.

[rcoh]

hello! is it just one request you're making? It's possible to swap out the entire HTTP connection used by the client for one that returns canned responses.

Here's an example where we do this to test the KMS client: https://github.com/awslabs/aws-sdk-rust/blob/main/sdk/kms/tests/integration.rs#L24-L49

Note that assuming you don't care about the request being made, you don't need to fill out the request in test connection, it just gets ignored.

To grab the response body, you can turn on trace logging:

// in your main function:
tracing_subscriber::fmt::init();

then:

RUST_LOG='smithy_http=trace' cargo run

I should also add that you can build your own more complex test connections (eg. to pass requests through except for the request you care about). The key piece is the tower service implementation: https://github.com/awslabs/aws-sdk-rust/blob/main/sdk/smithy-client/src/test_connection.rs#L117-L141

Just make sure that your type implements clone.

[rcoh]

I'll keep this issue open as a place to track work on the general topic of testing code using the SDK

[phyber]

I'm using something along these lines to successfully mock client calls, it's not the best example but hopefully you get the idea.
I call this at the beginning of a test to get a client with the responses I want, then I call the client as usual.

    // S3Config and S3Client are aliases to the AWS SDK since I already
    // had structs called Client and Config
    fn mock_client(data_files: Vec<&str>) -> Client {
        let creds = Credentials::from_keys(
            "ATESTCLIENT",
            "atestsecretkey",
            Some("atestsessiontoken".to_string()),
        );

        let conf = S3Config::builder()
            .credentials_provider(creds)
            .region(aws_sdk_s3::Region::new("eu-west-1"))
            .build();

        // Get a vec of events based on the given data_files
        let events = data_files
            .iter()
            .map(|d| {
                let path = Path::new("test-data").join(d);
                let data = fs::read_to_string(path).unwrap();

                // Events
                (
                    // Request
                    http::Request::builder()
                        .body(SdkBody::from("request body"))
                        .unwrap(),

                    // Response
                    http::Response::builder()
                        .status(200)
                        .body(SdkBody::from(data))
                        .unwrap(),
                )
            })
            .collect();

        let conn = TestConnection::new(events);
        let conn = DynConnector::new(conn);
        let client = S3Client::from_conf_conn(conf, conn);

        // My client is wrapped with other things for various reasons.
        Client { client }
    }

The data files here are XML files with API responses that I want to test against, they live in a test-data directory at the root of the crate.

[Fishrock123]

@phyber How do you access DynConnection and what is TestConnection?

Would be curious to see what that code needs for 0.2

[rcoh]

TestConnection is in aws-smithy-client gated behind the test-util feature, DynConnector is also in aws-smithy-client

[phyber]

@phyber How do you access DynConnection and what is TestConnection?

Would be curious to see what that code needs for 0.2

Sorry for not including the use statements with that. As rcoh says, they're in aws_smithy_client.

    use aws_smithy_client::erase::DynConnector;
    use aws_smithy_client::test_connection::TestConnection;

My test code didn't change much after an upgrade to 0.2, just a few minor changes related to Credentials::from_keys (enabling the feature so I could keep the same test code, static keys used only in tests, not in the real code), and the various DateTime stuff so I could convert to/from chrono types.

[rcoh]

This is now well supported by aws-smithy-mocks-experimental please give it a try!

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