imit_model / AnyLogic / UsersManual(AnyLogic)

Chapter 7. Message passing

Similarly, define the peekMessage() method to access the message without removing it from a queue:

Object peekMessage(){

return messages.get(0);

}

Note that the extracted message is the Object class instance and you may need to cast it explicitly to the original message class.

For instance, we send messages of the Message type. Then we need to cast the extracted message instance to this class in the following way:

Message message = (Message)getMessage();

Generally, you will need to check the port queue size before accessing the messages from the queue:

if (messages.size() > 0)

Message message = (Message)getMessage();

If needed, you can implement a limited queue. First, specify the capacity of the queue by defining capacity member variable in the Additional class code of the port:

int capacity = 44;

Queue would not accept an incoming message if the capacity is reached.

Substitute the code in the On receive action with the following code to perform the current queue size checking and discard messages if the queue is full.

if ( messages.size() < capacity )

messages.add( 0, msg );

return false;

You can further change the queue behavior. For instance, you may want to store the latest messages in your queue. So when the message arrives to the port with a full queue you can remove the oldest message stored. Therefore, you should substitute the code in the On receive action with the following:

messages.add( 0, msg );

Chapter 7. Message passing

Extract the message with the highest priority from the priority queue by calling the getMessage() method, specified in the Additional class code property of the port:

PriorityMsg getMessage(){

return (PriorityMsg)messages.remove(0);

}

Similarly, define the peekMessage() method to access the message without removing it from a queue:

PriorityMsg peekMessage(){

return (PriorityMsg)messages.get(0);

}

Generally, you will need to check the port queue size before accessing messages from the queue:

if (messages.size() > 0)

PriorityMessage message = getMessage();

7.4.12Connecting ports at runtime

You can connect ports programmatically at runtime by calling methods connect() and map(). To disconnect ports, you call the methods disconnect() and unmap(). Thus you can easily create complex systems with sophisticated topologies. You can change port connections at runtime to model systems with dynamically changing connections, in particular, systems with mobile objects. The example of a system with dynamically changing connections is given in section 7.4.13, “Modeling a system with dynamically changing structure”.

Use the connect()/disconnect() methods to connect or disconnect ports, located on the same level of containment hierarchy and the map()/unmap() methods otherwise.

In other words, use the connect()/disconnect() methods to connect or disconnect:

•Ports of encapsulated objects

•A private port with a port of an encapsulated object

Chapter 7. Message passing

Use Port.disconnect( portA, portB ); statement, or

portA.disconnect( portB );

Note that it does not really matter in what order the ports are specified. You can write the following line as well:

portB.disconnect( portA );

You can write this code anywhere you like in the parent active object class (objectC in the first case and objectA in the second one).

Using the map()/unmap() methods

The method map() is used for connecting ports located on the different levels of the containment hierarchy, namely a public port with a private port or a public port with a port of an encapsulated object. Ports are disconnected by calling the unmap() method.

Figure 98. A public port and a port of an encapsulated object

objectA

portA

portB

Figure 99. A public port and a private port

Figure 98 and Figure 99 show the situations when the map()/unmap() methods are used.

Chapter 7. Message passing

port.unmap( statechart );

7.4.13Modeling a system with dynamically changing structure

AnyLogic enables you to model not only systems with static structure, but also systems where object interconnections change dynamically. Thus you can easily create complex models with flexible structures. AnyLogic is the only visual tool that supports creation of truly dynamic models – the ones with dynamically evolving structure and component interconnection.

You can model systems with dynamically changing structure by changing port connections at runtime. To connect ports at runtime use the port methods connect() and map(). To disconnect ports, use the methods disconnect() and unmap(). See section 7.4.12, “Connecting ports at runtime” for more details on using these methods.

Suppose you are modeling an object moving in 1D space along the array of sensor stations. The object emits signals, which are accepted by the nearest station.

The model is shown in Figure 102. The mobile object is represented by wanderer object. Sensor stations are represented by the replicated object sensors.

Figure 102. A wandering object

The active object wanderer sends messages through its output port out. These messages are accepted by the sensor that is currently connected to the wanderer. At random time moments, the class System moves the wanderer.

Chapter 7. Message passing

The data feedback technique is the following. Since the data is represented by a Java object, and the recipient receives the reference to this object, when the recipient modifies this data object, it affects the original data object stored at the sender as well. Thus you can instantaneously modify the sender’s data sent within a message without sending any response message, carrying modified data back to the sender.

For instance, you send the message with the information in the number member variable.

DataMsg dm = new DataMsg();

dm.number = 38;

traceln(“Original number ” + dm.number );

port.send(msg);

traceln(“Modified number ” + dm.number );

You can instantaneously change the value of the sent data object by changing the value of the passed object in the recipient port’s On receive action property:

DataMsg data = (DataMsg)msg;

data.number=35;

return true;

The dm.number value stored at the sender object will be changed immediately (it is evidenced with the original and modified values, printed in the AnyLogic Log window). You do not need to send a supplementary message, carrying the new value to the message sender.

7.4.15Implementing instantaneous message exchange

You can implement instantaneous message exchange between active objects by sending a response message on a message reception. It is important that at the time the initial message sending is finished all the responses from the recipients are already received. Thus, you can perform some helper operations, e.g. pass messages with some information about the current state of recipient or implement object polling.

Instantaneous message exchange is implemented by specifying response message sending back to the message sender in the port’s On receive action property. You can perform several message exchanging iterations between objects. The message exchange is performed by a