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Attributes for Routing model
To specify whether or not an item must have a process performed on it, set an item’s attribute to a yes-or-no value using the Random Number block (Value library) as shown in the Attri- butes for Routing model, below:
Attributes for Routing model
The Empirical distribution in the Random Number block specifies that 75% of the items do not require checking (0 value for the CheckItem attribute) and 25% do (1 for attribute value). The Select Item Out block, set to Select output based on: property, reads the attribute value to determine which of two routes the item will take, one through the checking line (value = 1) and the other around it (value = 0). The Select Item In block is used to combine both lines into one stream, exiting the simulation.
This method is especially useful if the checking process takes more than one step. For instance, you may need to transport the item to the checking station using transportation blocks, but only if checking is needed. All those steps would be between the Select Item Out block and the Select Item In block.
With this model, an item that needs to be checked can be pulled into the Select Item Out block but not be able to advance because there is already an item in the Activity. To prevent this, you can cause the block to predict the path of an item before it enters, as discussed page 163.
☞ An item that requires checking that is blocked in the Select Item Out block will also block other items that arrive after it, even if they do not need to be checked. If this is not how your process works, insert a Queue before the Activity to hold items that need checking.
The example “Machines that can only process certain types of items” on page 174 is another instance of using attributes to route items. For a very different approach, the DB Job Shop model located in the folder Examples\Discrete Event\Routing uses information from the ExtendSim database to route items.
Throw and Catch Attributes model
As described in “Throw Item and Catch Item blocks for merging item streams” on page 160, the Throw Item block can be used to route items to a specific Catch Item block that is identi- fied by its label. Throw Item blocks can also be used to route items to different Catch Item blocks depending on the value of an item’s attribute or priority. A modification of the Attri-
Routing 169
Items going to several paths
Discrete Event

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