Which cloud storage type requires special consideration on the part of the cloud customer to ensure they do not program themselves into a vendor…

Structured data storage, particularly Database-as-a-Service (DBaaS) offerings, poses the highest risk of vendor lock-in. Customers often write application code that relies on the provider's proprietary APIs, unique database extensions, and specific management features. Migrating such an application to a different cloud provider requires not only moving the data but also significant and costly refactoring of the application code to work with a new database system. This high switching cost is the essence of vendor lock-in.

A. Unstructured: This is a data classification, not a storage type. While moving large volumes can be difficult, the storage APIs (e.g., S3) are often standardized.

B. Object: Object storage has a de facto standard API (Amazon S3 API) that many providers support, which helps mitigate vendor lock-in compared to proprietary database services.

C. Volume: Volume (block) storage presents a standard block device to the OS. While snapshots can be proprietary, the filesystems and data on them are generally portable.

1. National Institute of Standards and Technology. (2011). NIST Cloud Computing Reference Architecture (NIST Special Publication 500-292). (Section 5.3.3, "Portability and Interoperability," discusses how a lack of standardized interfaces can lead to vendor lock-in, a problem most acute in complex PaaS/DBaaS offerings).

2. Weinman, J. (2011). The Other Side of the Cloud: The Realities of Cloud-based Services. IEEE Cloud Computing. (This article discusses the "Hotel California" effect, or vendor lock-in, highlighting data and application-level dependencies, which are most pronounced with structured database services). DOI: https://doi.org/10.1109/MCC.2011.82 (Page 3, "Data Lock-in")

3. Stanford University. (CS 142 Courseware). Web Applications, Lecture 18: Cloud Computing. (Lecture notes often discuss the trade-offs of cloud services, including how PaaS offerings like managed databases can increase development speed at the cost of portability and potential vendor lock-in).