The TaskBag is a service whose functionality is to provide a repository for ‘task descriptions’. It enables clients running in several computers to carry out parts of a computation in parallel. A master process places descriptions of sub-tasks of a computation in the TaskBag, and worker processes select tasks from the TaskBag and carry them out, returning descriptions of results to the TaskBag. The master then collects the results and combines them to produce the final result.

Question

The TaskBag service provides the following operations:

setTask allows clients to add task descriptions to the bag;

takeTask allows clients to take task descriptions out of the bag.

A client makes the request takeTask, when a task is not available but may be available soon. Discuss the advantages and drawbacks of the following alternatives:

(i) the server can reply immediately, telling the client to try again later;
(ii) make the server operation (and therefore the client) wait until a task becomes available;
(iii) use callbacks.

Answer 1

This is a straight-forward application of the ideas on synchronising server operations. One of the projects in the Project Work section is based on the TaskBag service.