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INTERVIEW QUESTION:

What is the difference between a Pool and a Swimlane?

Posted by Chris Adams

Article Rating // 36040 Views // 2 Additional Answers & Comments

Categories: Business Analysis, Systems Analysis, Unified Modeling Language (UML), Business Process Modeling Notation (BPMN)

ANSWER

A “swimlane” is a generic industry term used to describe the grouping of common activities in a process diagram into labeled rectangular area.

Two of the more common standards for process diagrams are the UML Activity Diagram and the BPMN Business Process Diagram.

In UML, the generic “swimlane” is represented by a concept that is called an Activity Partition.  The UML 2.0 specification describes an activity partition as a “kind of activity group for identifying actions that have some characteristic in common.”  Activity Partitions can be further divided into Activity Sub-partitions. There is really no difference between an Activity Partition and an Activity Sub-Partition other than the level at which the group activities based on a common set of characteristics.

In BPMN, the generic term “swimlane” is represented by either a Pool or a Lane.  A Pool is a graphical representation of a participant within the business process diagram.  Pools can represent different businesses, a generic business partner (like a supplier, manufacturer, or buyer), or be more granular if modeling a single business without external dependencies.  Additionally, a Pool may reference a process and show the internal details (white box), or it may have no internal details (black box). A Pool acts as a container for the activities and sequence flows between activities. 

Lanes are mentioned in the BPMN specification as being used to organize and categorize activities within a pool.  However, the precise meaning of a lane or its appropriate scope is left up to the modeler, as the specification is silent in this area.

So, besides the fact that a Lane is used to subdivide a Pool, what is ultimately the difference between the two?  Sequence flows can cross Lanes between activities but they cannot cross Pools. Only messages can pass between Pools.  This is by design and is a major benefit of the BPMN notation. Messages indicate transfer of information within a conversation between two parties.  These are points of risk within the business process and deserve extra attention.  Both minimizing points of communication as well as ensuring clear communications take place help reduce process risk.

Additionally, participants of one pull have no control over the completion of activities that may occur in another pool. Whereas, activities divided between lanes can sometimes be performed by the same person, even if they that person is performing a different role (wearing multiple hats).

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ADDITIONAL ANSWERS / COMMENTS

slowlearner posted on Thursday, May 12, 2011 2:14 PM
"Sequence flows can cross Lanes between activities but they cannot cross Pools. Only messages can pass between Pools. This is by design and is a major benefit of the BPMN notation."

Agreed. Another way of putting it is that a process is always contained within a pool. Process flow does not cross pool boundaries. Sequence flows denote the actual flow of control *within* a process. Messages are just that, our process can send them to and receive them from an external participant (another pool). This may or may not influence (trigger, change, terminate) an "other" process in that other pool, but always remember that this "other" process is not a continuation of your process.

"Messages indicate transfer of information within a conversation between two parties. These are points of risk within the business process and deserve extra attention."

This is true if your process depends on a response from the external participant (pool). A fire-and-forget scenario doesn't pose any risk to your process (unless your send-message activity fails *and* your process *has to* send it).

Also, i would argue that similar points of risk exist *within* your local process when flow of control passes to another role (i.e. lane) in the process. Isn't it a local "handoff" with attendant risks?
nebedumw posted on Friday, September 9, 2011 10:24 AM
great article
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