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Resources and Shifts 229
Modeling resources
For this model it is essential to be able to track information about the airplane’s flight time. Therefore the ability to assign attributes to the airplane resource is critical.
Stripping attributes from resource items
As described in “Properties when items are unbatched” on page 218, an item returning to a Resource Item block after batching may have many attributes that are irrelevant to the return- ing item. The Resource Item block provides the option of stripping attributes or keeping them with resources that are recycled; the default is to strip them. When tracking resource informa- tion using attributes (as in the above model), you will not want to strip the attributes, so the block is unchecked. However, in cases where you are not concerned with attribute values after the item has been recycled, you may want to strip the attributes so that the item will be “clean” when it comes out of the Resource Item block again.
☞ The Queue Matching block (Item library) holds different types of items until the requirements for each type have been met. This can be useful when modeling the release of resources and items into a Batch block. For more information, see “Matching items using the Queue Match- ing block” on page 150.
Other methods for modeling resources
The Resource Pool and Resource Item methods described earlier in this chapter use specialized blocks to explicitly represent resources. The ExtendSim architecture provides many additional methods for modeling resources. For example, it might be simpler or provide more control to imply a lack of resources by limiting capacity in some blocks or to model resources just like any other item in the model.
Implicit resources
A resource can be implied in a model by restricting or scheduling the capacity of residence type blocks like the Activity and Queue. These blocks are useful for implicitly modeling resources.
For instance, the “Simple Resource Pool model” on page 225 illustrates how to model resources using the resource pool blocks. A simpler method would be to use the Activity block to represent a limited resource, without using explicit resource blocks. In this case, you would remove the Resource Pool and Resource Pool Release blocks from the model, set the Queue as a FIFO sorted queue, and set the maximum items in the Activity block to three. The Activity’s capacity limitation would have the same constraining effect as the Resource Pool block in the original Simple Resource Pool model.
Another advantage of modeling resources implicitly is that the Activity block can be shutdown and brought back online using the Shutdown block, as shown in “Shutting down” on page 193. This is common when modeling random failure.
Conceptual resources
The concept of what is a resource is not limited to the explicit (resource pool and resource item) or implicit (capacity-constrained) methods of representing resources. Theoretically, a resource is anything where its availability can restrict items flowing from point A to point B. Some examples are:
• Any item can conceptually represent a resource. For example, batching a “bus” item with “people” items, where the bus is required before the batched bus/people item can be released from a Queue (see “Delaying kits” on page 215). Note that the bus is created as any other item, not as a resource item from the Resource Item block.
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