Chaining providers

You can chain credential providers by using the Aws\Credentials\CredentialProvider::chain() function. This function accepts a variadic number of arguments, each of which are credential provider functions. This function then returns a new function that’s the composition of the provided functions, such that they are invoked one after the other until one of the providers returns a promise that is fulfilled successfully.

The defaultProvider uses this composition to check multiple providers before failing. The source of the defaultProvider demonstrates the use of the chain function.

// This function returns a provider
public static function defaultProvider(array $config = [])
{
    // This function is the provider, which is actually the composition
    // of multiple providers. Notice that we are also memoizing the result by
    // default.
    return self::memoize(
        self::chain(
            self::env(),
            self::ini(),
            self::instanceProfile($config)
        )
    );
}