Intermittent “An error occurred while loading credentials” after AssumeRole (Rust SDK)

When experiencing intermittent "An error occurred while loading credentials" errors with AssumeRole in the AWS SDK for Rust, several potential causes could be at play:

1. Network connectivity issues: Temporary network disruptions between your EC2 instance and the AWS STS service could cause intermittent failures.

2. Rate limiting: If you're making many AssumeRole calls in a short period, you might be hitting AWS STS API throttling limits.

3. Role session expiration: The temporary credentials obtained through AssumeRole have a limited lifetime. If your code is reusing credentials that have expired, this could cause failures.

4. IAM permission changes: If the permissions associated with the role you're assuming are being modified, this could lead to intermittent failures.

5. Missing error handling: The Rust SDK might be encountering errors that aren't being properly captured or logged in your implementation.

To address these issues, consider the following approaches:

1. Implement retry logic: Add exponential backoff retry logic around your AssumeRole calls to handle transient failures.

2. Enable detailed logging: Configure the SDK's logging to capture more detailed error information:

use aws_config::BehaviorVersion;

let config = aws_config::from_env()
.behavior_version(BehaviorVersion::latest())
.configure_logging(true, true)
.load()
.await;

3. Credential caching: Implement credential caching to reduce the frequency of AssumeRole calls, while being mindful of credential expiration.

4. Check role configuration: Verify that the IAM role has the necessary trust relationships and permissions.

5. Validate region configuration: Ensure the region specified matches where your resources are located.

6. Use the full configuration pattern: Instead of directly using the provider, consider using the full configuration pattern:

let config = aws_config::from_env()
.credentials_provider(
aws_config::sts::AssumeRoleProvider::builder(role_arn)
.session_name("session-name")
.build()
)
.load()
.await;

let client = aws_sdk_secretsmanager::Client::new(&config);

This approach leverages the SDK's built-in retry and error handling mechanisms, which might help with intermittent credential loading issues.
Sources
Use AssumeRole with an AWS SDK or CLI - AWS Identity and Access Management
AWS STS examples using SDK for Rust - AWS SDK for Rust
AWS STS examples using SDK for Rust - AWS SDK Code Examples