Transactions Queued at Activities

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Transactions collect at an activity in a queue and wait due to any combination of the following conditions:

Input Collections – Specifies a method of collection, such as batch or join. For more information, see Collection Types and Other Information.
Task Capacity – The number of transactions that can be serviced at an activity at any one time. When this capacity is exceeded, transactions are queued until other transactions leave the activity. You define the task capacity through the Properties dialog box - Task page - Step tab (see Task page: Define the type of task the activity performs).
Available Output – Transactions waiting for an open gate at the following activity or activities.
Resource Availability – Transactions may queue at an activity until certain resources become available. In this case, the queuing rules for that resource apply (see Activity Resource Requirements).

When transactions wait due to input collections, limited capacity, or available output, they arrange in the queuing rule order specified on the Inputs page of the Properties dialog box.

When transactions wait due to resource availability, they arrange in the queuing method order specified in the swimlane the resource is allocated to in the Define Resources dialog box - Allocations tab.

Queuing Rules and Queuing Methods

Specify queuing rules for input collections of transactions at an activity in the Properties dialog box - Inputs page. For resources, specify the queuing method in the Define Resources dialog box - Allocations tab with allocations shown.

The table below shows some examples of queuing rules and queuing methods and their purpose.

Rule	Example
First in, first out	Cars stop at a traffic light. The first car to arrive is the first to proceed through the light.
Last in, first out	The last person to enter a crowded elevator is the first one to leave.
Minimum of attribute	A printer uses an attribute called NumberPages to check documents and print the document with the lowest number of pages first. A document with a NumberPages value of 3 prints before a document with a NumberPages value of 10.
Maximum of attribute	Using the example for Minimum of attribute, above, the document with a NumberPages value of 10 prints first.

Priority and preemption also affect queuing order of transactions at an activity.

Priority – Transactions can be assigned a priority. The highest priority transactions are at the head of the input queue (see Priority and Preempt Transaction Attributes).
Preemption – Preemptive transactions can seize the use of resources from transactions with lower priority. However, they cannot take over any capacity from an activity. In other words, a preemptive transaction must still wait in the queue until capacity becomes available (see Priority and Preempt Transaction Attributes).

The time an activity spends waiting in an input queue is collected as part of the process statistics. You can assign this waiting time as Blocked or Inactive. These times are tracked separately. Blocked time is counted as part of the Transaction Time statistic.