Which of the following is NOT considered a storage backend example?

In the context of HashiCorp Vault, storage backends are the components that store the data for Vault, including secrets and metadata. They provide the underlying infrastructure for data persistence and retrieval.

Google Spanner, MySQL, and Postgres are database systems that can be utilized as storage backends for Vault. These databases directly support transactional capabilities and provide a structured way to handle the data Vault needs to store securely.

Amazon S3, on the other hand, is primarily an object storage service. While it can be used in conjunction with Vault for certain functionalities, it is not a traditional database that offers structured data storage in the same way as relational databases like MySQL or Postgres. Therefore, it is not typically categorized under the standard list of Vault storage backends, which focus on systems that manage structured data and support complex queries.

Thus, identifying Amazon S3 as not considered a storage backend in the context of Vault is accurate, as it does not align with the traditional definitions of what constitutes a storage backend for managing secrets and encrypted data in Vault.