Tuple Spaces: Queues or Databases?

Dan Creswell asks: Are tuple spaces more like queues or more like databases? They fit a broad set of niches between message queues and relational or object databases.

Queues can implement variations of one shared data structure: namely, queues.

Spaces can implement variations of shared queues, but also variations of shared arrays, shared dictionaries, and other "shared memory" (i.e. "random access") structures.

A space is typically centralized, where a queue product typically supports distributing the queue's end points around a network.

A space is typically better at heterogenous random access, where a more traditional database (e.g. a relational database) is typically better at homogenous sequential (and relational) access.