What's the best method for handling SDK errors?

[rnhurt]

I'm very new to Rust and am having a bit of trouble with error processing. I'm trying to write event data to CloudWatchLogs and one of the first things to do is create a log stream. This code will create the log stream without problems if it doesn't already exist.

    match client
        .create_log_stream()
        .log_group_name("/my/group/name")
        .log_stream_name("my_test_stream")
        .send()
        .await
    {
        Ok(_) => println!("Created new log stream."),
        Err(e) => match e {
            Error => println!("Log stream already exists"),
            _ => panic!("Error creating log stream: {}", e),
        }
    }

If the log stream exists I get a ResourceAlreadyExistsException, which is fine, but if I get another error I would like to panic!(). The code above doesn't really work because the Errro match matches ALL errors and the panic!() call never happens.

I guess my question is, how do you match on a particular Error? Am I missing a module somewhere or something?

[Velfi]

Errors in the Rust SDKs have a kind field that contains this information and we provide methods for checking this. Here's the docs for CreateLogStreamError.

With this in mind, your code could look like this:

    match client
        .create_log_stream()
        .log_group_name("/my/group/name")
        .log_stream_name("my_test_stream")
        .send()
        .await
    {
        Ok(_) => println!("Created new log stream."),
        Err(e) => {
            if e.is_resource_already_exists_exception() { println!("Log stream already exists") }
            else { panic!("Error creating log stream: {}", e) };
        }
    }

[rnhurt]

Thank you for the docs for "CreateLogStreamError".

I updated my code but it's still not happy; I'm getting the following error. Am I missing something else?

error[E0599]: no method named `is_resource_already_exists_exception` found for enum `SdkError` in the current scope
  --> src/main.rs:75:18
   |
75 |             if e.is_resource_already_exists_exception() {
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ method not found in `SdkError<CreateLogStreamError>`

[Velfi]

My apologies, I neglected to show how to get to the error inside the SdkError.

match client
    .create_log_stream()
    .log_group_name("/my/group/name")
    .log_stream_name("my_test_stream")
    .send()
    .await {
        Ok(_) => println!("Created new log stream."),
        Err(aws_sdk_cloudwatchlogs::types::SdkError::ServiceError { err, .. }) if err.is_resource_already_exists_exception() => {
            println!("Log stream already exists");
        }
        Err(e) =>  panic!("Error creating log stream: {}", e),
    };

[rnhurt]

Oh ... well ... that's an interesting pattern. I'm not sure I would have ever figured that out on my own.
It would be great if an error handling example or two were added to the docs somewhere.

[ivan-kiselev]

I'd say AWS SDK generally lacks good examples of errors handling. Errors structure is somewhat complex and it takes A WHILE to figure it out on one's own.

[github-actions[bot]]

Hello! Reopening this discussion to make it searchable.

[michaelrommel]

Hi, I have a lot of trouble, too, to understand how one can parse an - as it seems arbitrary - unknown depth number of nested errors like this one:

Ideally I would get to the bottom of this and make a more user friendly error message "Unauthorized", if that's the underlying reason.

Are there any good pointers, how to do that?

[Velfi]

To output a more detailed error context message, use DisplayErrorContext.

use aws_sdk_s3::error::DisplayErrorContext;

fn example(err: &E) 
where
    E: std::error::Error
{
    println!("There was an unhandled error: {}", DisplayErrorContext(err));
}

In this example I'm importing it from the S3 crate but it's available at the same path in all SDK crates.