[[Property:title|Single-element producer-consumer]]
[[Property:weight|-15]]
[[Property:uuid|25d3e585-0eb6-efa8-ada9-8ee596df5ada]]
=Description=

The single-element producer-consumer is a simpler variant of the classic [http://en.wikipedia.org/wiki/Producer-consumer_problem producer-consumer] problem. A producer produces products, in this case integers, into a single-element inventory. The products are then consumed from inventory by a consumer. The producer, consumer, and inventory are managed by separate [[Concurrent programming with SCOOP#Processors|processors]], so any access they have to one another must be synchronized through scoop mechanisms.

=Highlights=

In the single-element producer-consumer only a single producer and single consumer are created, and there is only storage allowing for a single instance of the product. So, effectively in this example, the bounded buffer of the classic producer-consumer problem has a size of one. 

The classes modeling the different actors have obvious names: <code>PRODUCER</code>, <code>CONSUMER</code>, and <code>INVENTORY</code>. The root class of the example creates one <code>separate</code> instance of each of these, and then brings the producer and consumer to life.

The <code>PRODUCER</code> class supports a procedure <code>produce</code> in which a product is produced and stored in the single-element <code>INVENTORY</code>. The producer can only produce an element if the inventory is currently empty. Class <code>INVENTORY</code> exports a boolean query <code>has_item</code> which is the indicator of whether a product has been produced and is available for consumption. So <code>{PRODUCER}.produce</code> has a precondition that depends upon <code>{INVENTORY}.has_item</code> being false. Because the inventory is handled by a separate processor, this precondition is [[Concurrent programming with SCOOP#Preconditions|uncontrolled]] and will cause the producer to wait until the condition is true to proceed. 

The <code>CONSUMER</code> class works in a way that is largely the symmetrical opposite of <code>PRODUCER</code>. The consumer tries to <code>consume</code> the item from the <code>INVENTORY</code>. But this only possible if an item has been produced and is available. So <code>{CONSUMER}.consume</code> has a "wait" precondition based on the query <code>{INVENTORY}.has_item</code>.

So the heart of the problem is the synchronization between producer and consumer sharing a single inventory. If there's already a product in inventory, the producer cannot produce more and must wait. Only when the consumer consumes the current product can the producer produce again. For the consumer's part, if there's a product currently in inventory, then the consumer can consume that product. Otherwise, the consumer must wait until the producer produces a new product. The synchronization is handled by the SCOOP mechanism of uncontrolled (wait) preconditions on the inventory.