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Discrete Event Simulation (DES) Using AnyLogic 6

 Anylogic supports Discrete Event Simulation. Anylogic comes with a fairly robust library of discrete event elements that can be used to rapidly develop a discrete event models by dragging and dropping these library elements. The following graphic shows some of the typical library elements used in a model:

These elements can be used to rapidly develop a model using a Flowchart or a Layout, or a Network-based modeling approach. This approach can be used to model processes, sequences of activities, and utilization of resources.

Anylogic enables drag and drop modeling and naturally interoperates with all other AnyLogic modeling constructs. Models are scalable and extensible. All blocks are AnyLogic active objects made of standard components (ports, statecharts, timers). Anylogic supports sophisticated web based animation capabilities:

Discrete Event Simulation with Anylogic 6

The world around us appears to be "continuous": the greater majority of processes we observe consist of continuous changes. However, when we analyze those processes, in many cases it makes sense to abstract from their continuous nature and consider only some "important moments", "events" in the system lifetime. The modeling paradigm that suggests to approximate the real-world processes with such events is called Discrete Event Modeling. (The term Discrete Event is also used in the narrower sense to denote "process-centric" modeling where the system is described as a process flowchart.) Discrete Event modeling is fully supported by AnyLogic as well as System Dynamics and Agent Based Modeling.

Here are some examples of events: a customer arrives at a shop, a truck finishes unloading, a conveyor stops, a new product is launched, the inventory level reaches a certain threshold, etc. In discrete event modeling the movement of e.g. a train from point A to point B would be modeled with two events: departure and arrival, and the actual movement will become a time delay (interval) between them. (This does not mean however that you cannot animate the train as moving, in fact in AnyLogic you can produce visually continuous animations for logically discrete models).

Event in AnyLogic model (which as you know is an abstraction) takes zero time to execute, is atomic (will not interfere with any other event execution), may change the model when it is executed, in particular may schedule other events. If AnyLogic engine is executing a purely discrete model, the time is essentially a sequence of events, and the engine just jumps from one event to another. If several events are scheduled to occur at the same time (are simultaneous), they are serialized, i.e. executed one after another in some internal (not guaranteed) order. If the order is important for you, you should take care of it when developing the model so that the simulation results do not depend on the engine implementation.

At the low level (at the level of AnyLogic modeling primitives) events may be scheduled by two types of objects: Events and Statecharts. There is however, a library of higher-level objects that help you to create discrete event patterns frequently used in process-centric modeling (queuing, resource usage, entity generation, etc.) - it is called Enterprise Library.

Events and Dynamic Events

The simplest way to generate events in AnyLogic is to use model elements of the same name – events. There are two types of events: event and dynamic event. Both of them are responsible for scheduling some user-defined actions in the model. They are commonly used to model delays and timeouts.

The difference between the event and the dynamic event is that the dynamic event deletes itself after its action is executed, whereas an event survives and can be restarted. However, there can be multiple instances of the same dynamic event scheduled concurrently in the model. 

Static and dynamic events

Events

To schedule some action using AnyLogic event, you should add the event on the active object diagram, and then configure it by specifying, when it should occur and what action should be performed on its occurrence.

Adding an Event

 To add an event

1.       Choose the Event  element from the Model page of the Palette view.

2.       Click in the graphical editor where you want to place the event.

Scheduling an Event

To schedule the event occurrence, you should define the event trigger. There are three event trigger types:

·             Timeout

·             Boolean condition

·             Rate

Timeout triggered event

The event will occur exactly in the defined timeout time after it was started. Optionally, the event may be made cyclic and set to occur at startup.

 To define a timeout triggered event

1.       Choose Timeout from the Trigger Type drop-down list.

2.       Specify the timeout in the Timeout edit box below. It might be an expression that is evaluated to obtain the timeout.

3.       Specify the timeout event mode. From the Mode drop-down list, choose, whether this event should be

o   Occurs once – event will occur only once exactly in the specified Occurrence time. You can define the ocurrence time as a calendar date or as a number of model time units to be passed from the model start. Please note that here you define the absolute time and in the case the event will be created after the specified time, it will never occur.

o   Cyclic – event will occur cyclically. You define the First occurrence time and the Recurrence time for an event. The first ocurrence time can be defined either as a calendar date, or as a number of model time units to be passed from the model start. 

Please note that you define absolute times. If event will be created after the specified first occurrence time, the model will schedule the first occurrence time in the past and then will successively increment it with Recurrence time. The first time that will exceed the current model time will be used as the actual first occurrence time. The next occurrence times will be counted as it should be by adding the defined Recurrence time. 

If you want to use relative times (i.e. count the first occurrence time from the moment this event will be created), use time() function in the First occurrence time property. time() will refer to the moment this event was created at.

o   User control – the event will work in manual mode. It will occur if and only of user calls the event’s restart method passing the timeout as the argument, for instance mytimer.restart(15). If the timeout is fixed, you can define it once in the Timeout property and simply call mytimer.restart() without specifying the timeout as a method parameter.

Condition triggered event

You may define a Boolean condition as the trigger for the event. In this case the event will occur when the defined condition will become true.

 To define a condition triggered event 

1.       Choose On Condition from the Trigger Type drop-down list.

2.       Specify the Boolean condition in the Condition edit box below.

If the active object has continuously changing variables, the numeric engine constantly monitors the condition. In purely discrete models the condition is tested when something changes in the active obejct, i.e. when onChange() is called.

Rate triggered event

A rate is a form of updateable exponential timeout. If you define a rate triggered event, the event will occur periodically with time intervals distributed exponentially with the parameter rate, i.e. if the rate is 5, the event will occur on average 5 times per time unit.

 To define a rate triggered event

1.       Choose Rate from the Trigger Type drop-down list.

2.       Specify the rate in the Rate edit box below.

If the rate changes dynamically, the event occurrence gets re-scheduled; such changes may only be noticed by the event if onChange() is called for the active object.

Specifying an Event Action

Event’s action is specified in the Action property of the event. There you can write Java code to be executed on the event occurrence. Or may be more convenient is defining this code as a function and just placing this function call in the Action field.

Manual Controlling an Event

A static event can expire any number of times. To schedule a static event, you call its method restart(), specifying the timeout value as a parameter. When the event occurs, it calls the method action(). By calling the method reset(), you deactivate the event until the next call to restart().

Dynamic Event

Dynamic events are used to schedule any number of concurrent and independent events. For example, a communication channel which is able to transmit an arbitrary number of messages concurrently can be modeled with the help of dynamic events that are created for each message. Another example is a server with infinite capacity.

Defining a Dynamic Event

 To define a new dynamic event

1.       Choose the Dynamic Event  element from the Model page of the Palette view.

2.       Click in the graphical editor where you want to place the dynamic event.

Configuring a Dynamic Event

Event action

The event’s action is specified in the Action property of the event. There you can write Java code to be executed on the event occurrence. Or may be more convenient is defining this code as a function and just placing this function call in the Action field.

Dynamic event parameters

You may define some optional parameters. Dynamic event parameters can be accessed as member variables of the dynamic event object.
Parameters are defined in the Parameters table on the Parameters page of the Properties view. Each row of the table specifies one particular parameter.

Every parameter should be declared in form:

·             Name - the name of the parameter

·             Type - the type of the parameter

·             Default Value - the optional default value of the parameter. When creating an instance of the dynamic event class, you may specify actual parameter values or leave the default ones.

Since the order of parameters in the table is significant, AnyLogic enables changing the parameter order just moving them up or down using the buttons    and   .

To remove parameter, select the parameter row in the table and click the Remove button.

Specifying an Event Action

Event’s action is specified in the Action property of the event. There you can write Java code to be executed on the event occurrence. Or may be more convenient is defining this code as a function and just placing this function call in the Action field.

Scheduling Dynamic Events

To create new dynamic event instance, use the method that is generated automatically by AnyLogic for every dynamic event declared on the active object diagram. 

void create_<dynamic_event_name>( double timeout, <1st_Parameter_Type> param1, <2nd_Parameter_Type> param2, …)

You pass the timeout to the function as the first parameter, then you specify the parameter values in the order they were defined in the Parameters table for the dynamic event. 

Time counting begins at the moment of creating the instance of a dynamic event. When the event timeout expires, AnyLogic executes the event’s action and then deletes this instance of dynamic event. If the method reset() of the dynamic event is called before expiry, this instance of event is deleted and its action is never performed.