A self-contained implementation of AWS Signature V4 for signing HTTP requests. It handles canonical request construction, header signing, and credential scope. This component is extracted from the AWS SDK and can be reused in any Node.js project that needs to sign requests to AWS services (e.g., SES, S3). It depends only on the crypto utility and v4_credentials module, both also in the repo, but those are small and could be extracted together.

Implements a recursive fallback strategy for multi-provider setups. Given an array of providers, it attempts each in order, logging warnings on failure, and returns the first successful result. If all providers fail, it throws the last error. This pattern is generic and can be reused for any list of async functions that share the same interface, not just notification providers.

Implements a stateful round-robin strategy that rotates through providers on each call, with fallback to the next provider on failure. It mutates an array to maintain rotation state, which is acceptable for a singleton strategy. This pattern is reusable for any load-balancing scenario where providers are interchangeable.

A simple registry for lazy singleton instantiation. It stores instances by key and creates them on first access via a factory function. This pattern is useful for managing shared resources (e.g., database connections, logger instances) and can be extracted as a standalone utility.