Saturday, December 15, 2012

On decision latency

James Taylor wrote in his Blog an article on decision latency, the illustration above is taken from his article. 
The latency for getting a decision consists of three time intervals:

The Capture latency is the time taken from the event occurrence in the universe until it is detected (captured) by the processing system
The Analysis latency is the time taken from the time that the event is detected by the system, until the time it is  processed and creates the derived event/data that triggers an action, and the Decision latency has two parts - which I think should be distinct,  the time to make the decision how to react, and the time it takes to react.

These four type of latency are actually orthogonal.  For the Capture latency what is required is an infrastructure of instrumentation, sensors and communication; for the Analysis latency -- the event processing system latency, and possibly additional analysis required by querying historical databases,  for the decision latency --  the decision tools (either a decision management system or optimization system), and for the action latency -- the speed in which it can be applied. 

Why the decision latency is important? -- here we are getting back to the real-time issue.  In some cases the time is critical for decisions,  an example that we saw a few weeks ago is in the Israeli anti-rockets systems
known as "Iron Dome" (Kipat Barzel).  The system detects that a rocket was launched, and then it needs to analyze its course and determine whether it is going to hit a populated area, in order to decide whether it is cost-effective to send a missile to hit the rocket (this decision is needed since a missile is very expensive), and if yes,  determine when and where the missile will intercept the rocket and launch the missile at the right time to the right direction.  Latency in all the four phases is critical., and the success of this system saved many lives.    
Decision latency is not only related to defense systems,  it exists in different other areas - healthcare is certainly one of them, but also business.  The time scale does not have to be seconds,   decision latency can span minutes or hours,  

We can look at the value of decision as function of its latency,  and can see that there are soft real-time cases which the value of decision goes gradually to zero, Firm real-time where it goes to zero at a certain deadline.
Hard essential in which it goes to a certain penalty (such as missing SLA), and hard critical where it is going to a minus infinity (a disaster case), the Iron Dome case is of that type.  

Of course, there are no miracles, and the cost of a system grows when bounded latency need to be guaranteed (for example cost of extremely robust communication).  For each type of event-based system there is a need to calculate the trade-off between the value of latency and the cost. 

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