Pattern 23 (Resource-Initiated Execution - Offered Work Item)

FLASH animation of Resource-Initiated Execution - Offered Work Item pattern

Description

The ability for a resource to select a work item offered to it and commence work on it immediately.

Example

The Courier Driver selects the next Delivery work item from those offered and commences work on it.

Motivation

In some cases it is preferable to view a resource as being committed to undertaking a work item only when the resource has actually indicated that it is working on it. This approach to work distribution effectively speeds throughput by eliminating the notion of work item allocation. Work items remain on offer to the widest range of appropriate resources until one of them actually indicates they can commence work on it. Only at this time is the work item removed from being on offer and allocated to a specific resource.

Overview

There are two variants of this pattern as illustrated by the bold arcs in Figure 5, depending on whether the work item has been offered to a single resource (R:start_s) or to multiple resources (R:start_m). In both cases, the work item has its status changed from offered to started. It remains in the work list of the resource which initiated the work item. In the latter case, the work item has been offered to multiple resources and it is therefore necessary to remove it from all other work lists in which it may have appeared as an offer. This ensures that only one resource can actually work on it.

Context

There are no specific context conditions associated with this pattern.

Implementation

This approach to work distribution is adopted by Staffware, WebSphere MQ and COSA for shared work queues (e.g. group queues). For these systems, a work item remains on the queue until a resource indicates that it has commenced it. At this point, its status changes and no other resource can execute it although it remains on the shared queue until it is completed. iPlanet and Oracle BPEL adopt a similar approach for all work items and effectively presents each resource with a single amalgamated queue of work items allocated directly to it and also those offered to a range of resources. The resource must indicate when it wishes to commence a work item. This results in the status of the work item changing and it being removed from any other work queues on which it might have existed.

Issues

None identified.

Solutions

N/A.

Evaluation Criteria

An offering achieves full support if it satisfies the description for the pattern.

Product Evaluation

To achieve a + rating (direct support) or a +/- rating (partial support) the product should satisfy the corresponding evaluation criterion of the pattern. Otherwise a - rating (no support) is assigned.

Summary of Evaluation