Page 139 - ExtendSim User Guide
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Tutorial 113
Adding complexity
Explicit routing
In the model so far, the number exiting from one wash bay is probably larger than the number exiting from the other. (You can see this in the Exit block’s dialog.) Since you have not speci- fied any rules concerning how the cars are routed to a wash bay, a car will go to the first avail- able bay. However, if both bays are free, the car will go to the bay that was first connected in the model. This implicit routing is not obvious and is rarely what you want.
Unless it is completely unimportant in the model, you should always explicitly state the routing of items using the Select Item In and Select Item Out blocks. Otherwise, the order in which their connections were made will dictate the routing, as discussed in “Implicit routing” on page 164.
To explicitly specify the order in which items go to free inputs, use the Select Item Out block.
Insert a Select Item Out block (Item library) between the Queue and the two Activity blocks and label it “Select Route”.
Connect the item inputs of the blocks.
This section of the model should look like the
screenshot to the right.
The Select Item Output block has many options.
You can specify that the item is routed to a random
output, to a specific output based on a value at the
“select” input connector or based on the item’s attri-
bute or priority, or sequentially. Using the various
options to route items is described fully in “Select Item Out dialog” on page 162.
In the dialog of the Select Item Out block:
Choose the option Select output based on: sequential Select If output is blocked: item will try unblocked outputs
This causes the items to be sequentially routed between the two bays. If the selected bay is blocked, the item won’t wait for it to be free but will instead be routed to the other bay if it is available.
When you run the model, the same number of cars will have been washed as in the previous model, but each wash bay will have been used equally.
☞ The preceding examples have two wash bays and purposefully don’t take into consideration the model assumption that 25% of the cars want wax in addition to a wash. You could specify the second bay as also providing waxing, and use the option Select output based on: ran- dom in the Select Item Out block to route 25% of the cars to that bay. But this tutorial will explore a more powerful method for accomplishing this in the section “Item attributes” on page 114.
Requiring resources
Up to this point, the Car Wash model assumes that you drive the car through the wash. How- ever, many car washes require that an attendant do this. This situation can be easily modeled as a number of resources (the attendants) which are required by and made available to the model. As is discussed in “Modeling resources” on page 223, there are two methods to model resources:
Explicit routing
Discrete Event

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