Omitting many many details:
- ExecutorService = incoming queue + worker threads
- CompletionService - incoming queue + worker threads + output queue
A service that decouples the production of new asynchronous tasks from the consumption of the results of completed tasks. Producers submit tasks for execution. Consumers take completed tasks and process their results in the order they complete. A CompletionService can for example be used to manage asynchronous IO, in which tasks that perform reads are submitted in one part of a program or system, and then acted upon in a different part of the program when the reads complete, possibly in a different order than they were requested. --- From CompletionService Javadoc.
