Two ways to read aws access key/secret. Different CredentialProvider

1. instance can have instance role. When an application run in instance, it can InstanceProfileCredentialsProvider() to retrieve instance role and have the access. For example, instance_profile from EMR cluster is the role for EMR instance.
2. StsAssumeRoleSessionCredentialsProvider
3. AWSStaticCredentialsProvider
4. AWSCredentialsProviderChain. It will test different credential one by one, until it finds one.

Here is a code example