In RESTful API design, a singleton resource is a resource that represents a single, unique entity or object. Unlike collections, which represent a group of resources that can be retrieved and manipulated as a set, singleton resources represent a single resource that can be retrieved and manipulated individually.
A common example of a singleton resource is a user resource. Each user in a system is a unique entity that can be represented as a singleton resource. The user resource might have its own unique identifier, such as a user ID, and can be retrieved and manipulated using HTTP methods such as GET, PUT, POST, and DELETE.
Singleton resources are typically used for resources that are logically singular and do not have a meaningful collection representation. For example, a system configuration resource or a system status resource might be modeled as a singleton resource.
It is worth noting that there is some debate in the RESTful API community about the use of singleton resources. Some argue that they violate the uniform interface constraint of RESTful API design, as they do not have a standard collection representation. Others argue that they are a useful design pattern for modeling certain types of resources, and can be implemented in a RESTful way if designed carefully.