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at:tutorial:distribution [2007/05/04 08:32] – * elisagat:tutorial:distribution [2007/05/04 08:34] – * elisag
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 Let us consider again the example instant messenger application described in previous section to further explain the semantics of AmbientTalk's remote object references and how they deal with transient disconnections.  Let us consider again the example instant messenger application described in previous section to further explain the semantics of AmbientTalk's remote object references and how they deal with transient disconnections. 
  
-When an object discovers a service type, the ''when'' observers are triggered receiving as parameter a remote far reference to the remote object discovered. As explained in previous sections, far references operates asynchronously. When a client object sends a message via a remote reference, the message is buffered in the remote far reference and the client does not even wait for the message to be delivered. This is crucial in distributed computing in order to prevent race conditions. The parameter passing in the context of distribution works similar to the inter-actor message sending semantics:+When an object discovers a service type, the ''when'' observers are triggered receiving as parameter a remote far reference to the remote object discovered. As explained in previous sections, far references operates asynchronously. When a client object sends a message via a remote reference, the message is buffered in the remote far reference and the client does not even wait for the message to be delivered. This is crucial in distributed computing in order to prevent race conditions. The parameter passing semantics for messages sent to remote objects works similar to the inter-actor message sending semantics:
  
   - Objects are always passed by far reference, except for isolate objects which are passed by copy.   - Objects are always passed by far reference, except for isolate objects which are passed by copy.
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 This code illustrate how the instant messenger application notifies when a buddy goes online or offline. In the above code, ''messenger'' is a remote reference to another remote buddy discovered. Note that installing disconnected observers also allows developers to clean certain resources when a remote reference becomes disconnected. However, when an instant messengers disconnects, the remote object referred to by ''messenger'' remains exported. This implies that remote objects remains pointed by a disconnected remote reference which prevents them from being garbage collected. In fact, ''messenger'' are never garbage unlesss explicit cancelation of the subscription. But other types of objects which are only relevant within the context of an interaction should become eventually candidates for garbage collection. In the next section, we detail how AmbientTalk deals with distributed memory management.  This code illustrate how the instant messenger application notifies when a buddy goes online or offline. In the above code, ''messenger'' is a remote reference to another remote buddy discovered. Note that installing disconnected observers also allows developers to clean certain resources when a remote reference becomes disconnected. However, when an instant messengers disconnects, the remote object referred to by ''messenger'' remains exported. This implies that remote objects remains pointed by a disconnected remote reference which prevents them from being garbage collected. In fact, ''messenger'' are never garbage unlesss explicit cancelation of the subscription. But other types of objects which are only relevant within the context of an interaction should become eventually candidates for garbage collection. In the next section, we detail how AmbientTalk deals with distributed memory management. 
- 
  
 ===== Garbage collecting remote references ===== ===== Garbage collecting remote references =====
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 </code> </code>
  
-Be aware that unexporting a object will not only trigger the takenOffline observers but also the disconnected observers since the taking offline event is also considered as a logical disconnection between two devices. Unlike disconnection observers, the block of code takenOffline observer will be triggered only once. The remote object becomes then subject to be eventually reclaimed by the garbage collector. As a result, the disconnected and reconnected observers will be no longer trigger for such remote reference. +Be aware that unexporting a object will not only trigger the takenOffline observers but also the disconnected observers since the taking offline event is also considered as a logical disconnection between two devices. Unlike the network observers, the block of code of a takenOffline observer will be triggered only once. The remote object becomes then subject to be eventually reclaimed by the garbage collector. As a result, the disconnected and reconnected observers will be no longer trigger for such remote reference. 
  
 Note that disconnection, reconnection and takenOffline observers can be also installed for the inter-actor far references. Such inter-actor far references are local to a virtual machine and as such, network failures cannot happen. In that case, the disconnection and takenOffline observers are triggered when the object pointed to by the far reference is taken offline. But the reconnected observers won't be never triggered since an unexported object cannot be exported again.  Note that disconnection, reconnection and takenOffline observers can be also installed for the inter-actor far references. Such inter-actor far references are local to a virtual machine and as such, network failures cannot happen. In that case, the disconnection and takenOffline observers are triggered when the object pointed to by the far reference is taken offline. But the reconnected observers won't be never triggered since an unexported object cannot be exported again. 
  
 <note> <note>
-The complete implementation of the instant messenger application explained along this section can be found in the file at/demo/InstantMessenger.at.+The complete implementation of the instant messenger application explained along this chapter can be found in the file at/demo/InstantMessenger.at.
 </note> </note>
at/tutorial/distribution.txt · Last modified: 2009/01/30 16:13 by tvcutsem