Ambient-Oriented Programming

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at:tutorial:distribution [2009/01/29 15:21] – elisagat:tutorial:distribution [2009/01/29 15:39] – elisag
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 </note> </note>
  
-===== Exporting and discovering objects =====+===== Exporting and Discovering Objects =====
  
 AmbientTalk provides language support to make some objects available to other objects residing in remote actors by means of the ''export:as:'' construct.  The ''export:as:'' construct takes as argument an object that is made remotely accessible and a service type under which the object can be discovered. For example, one exports a printing service as follows: AmbientTalk provides language support to make some objects available to other objects residing in remote actors by means of the ''export:as:'' construct.  The ''export:as:'' construct takes as argument an object that is made remotely accessible and a service type under which the object can be discovered. For example, one exports a printing service as follows:
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 More information pertaining to the API of the leased references language module can be found in the appendix. More information pertaining to the API of the leased references language module can be found in the appendix.
  
-===== Taking offline remote objects =====+===== Taking Offline Remote Objects =====
  
 AmbientTalk distributed memory management scheme has been based on [[http://en.wikipedia.org/wiki/Reference_counting#Variants_of_reference_counting|reference listing]] and network objects. Similar to these techniques, remote far references are implemented by means of a proxy at client-side, which encapsulates a wire representation of the exported object and whose methods are stubs that transform local message sends into distributed message sends. Messages sent to the server include method invocation information, as well as the wire representation of the receiver. Each actor maintains an export object table which maps wire representations onto local references to exported objects. This table is used by the interpreter to resolve remote far references into local object references. Objects in the export table are considered as root objects for the [[http://en.wikipedia.org/wiki/Garbage_collection_(computer_science)|local garbage collector]]. When an object is removed from the export table, it is no longer accessible remotely and, importantly, it no longer belongs to the set of root objects and as such, it becomes subject to garbage collection once it is no longer locally referenced. AmbientTalk distributed memory management scheme has been based on [[http://en.wikipedia.org/wiki/Reference_counting#Variants_of_reference_counting|reference listing]] and network objects. Similar to these techniques, remote far references are implemented by means of a proxy at client-side, which encapsulates a wire representation of the exported object and whose methods are stubs that transform local message sends into distributed message sends. Messages sent to the server include method invocation information, as well as the wire representation of the receiver. Each actor maintains an export object table which maps wire representations onto local references to exported objects. This table is used by the interpreter to resolve remote far references into local object references. Objects in the export table are considered as root objects for the [[http://en.wikipedia.org/wiki/Garbage_collection_(computer_science)|local garbage collector]]. When an object is removed from the export table, it is no longer accessible remotely and, importantly, it no longer belongs to the set of root objects and as such, it becomes subject to garbage collection once it is no longer locally referenced.
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 <note> <note>
-Leased object references make use of the ''takeOffline:'' primitive to terminate the access to a remote object once the lease time elapses. Note that the ''takeOffline'' primitive can be considered the equivalent to the so-called [[http://en.wikipedia.org/wiki/Manual_memory_management|''delete'' operation]] provided by some sequential languages without built-in local garbage collection.  Rather than using this primitive for garbage collection purposes, regular AmbientTalk developers are encouraged to make use of the high-level language constructs for leasing explained in the previous section which aids them to deal with both transient and permanent disconnections and properly reclaim their remote objects.+[[distribution#dealing_with_permanent_failures|Leased object references]] make use of the ''takeOffline:'' primitive to terminate the access to a remote object once the lease time elapses. Note that the ''takeOffline'' primitive can be considered the equivalent to the so-called [[http://en.wikipedia.org/wiki/Manual_memory_management|''delete'' operation]] provided by some sequential languages without built-in local garbage collection.  Rather than using this primitive for garbage collection purposes, regular AmbientTalk developers are encouraged to make use of the high-level language constructs for leasing explained in the previous section which aids them to deal with both transient and permanent disconnections and properly reclaim their remote objects.
 </note> </note>
  
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 </code> </code>
  
-The construct takes as parameter a far reference and a block of code that is executed when the taken offline event is notified to the virtual machine. ''when:takenOffline'' can be considered as a special kind of ''when:disconnected'' observers which are triggered only in case of "logical" disconnection, but not in cases of network disconnections.+The construct takes as parameter a far reference and a block of code that is executed when the taken offline event is notified to the virtual machine. ''when:takenOffline'' observers can be considered as a special kind of ''when:disconnected'' observers which are triggered only in case of "logical" disconnection, but not in cases of network disconnections.
  
 <note> <note>
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 </note> </note>
  
-====Distributed unit testing and ''takeOffline''====+====Distributed unit testing and takeOffline====
  
 As previously mentioned, the ''takeOffline'' primitive notifies connected virtual machines when taking an object offline. Since AmbientTalk 2.12, the primitive not only notifies remote virtual machines, but it also notifies the local virtual machine where the ''takeOffline'' was executed so that if any inner actor has local far references to that object, it will also get notified. In other words, if an object is taken offline, all actors holding far references to it (independent of their location) get notified.  As previously mentioned, the ''takeOffline'' primitive notifies connected virtual machines when taking an object offline. Since AmbientTalk 2.12, the primitive not only notifies remote virtual machines, but it also notifies the local virtual machine where the ''takeOffline'' was executed so that if any inner actor has local far references to that object, it will also get notified. In other words, if an object is taken offline, all actors holding far references to it (independent of their location) get notified. 
-This semantics are useful for unit test purposes. The unit testing framework shipped with AmbinentTalk has support to perform asynchronous invocations which can be used to perform concurrent or distributed unit tests. However, distributed unit tests couldn't simulate network disconnections as ''network.offline'' works at actor level. In other words, ''network.offline'' simulates a disconnection of the complete virtual machine, but it cannot simulate the disconnection of a particular actor inside the VM. By means of ''takeOffline'' primitive, you can cut the access of an object contained in any actor (within the same VM or a remote VM),  enabling the simulation of the disconnection of an actor inside a virtual machine. 
  
- +This semantics are useful for unit test purposes. The [[appendix#unit_testing_framework|unit testing framework]] shipped with AmbientTalk has support to perform asynchronous invocations which can be used to perform concurrent or distributed unit tests. However, distributed unit tests couldn't simulate network disconnections as ''network.offline'' works at actor level. In other words, ''network.offline'' simulates a disconnection of the complete virtual machine, but it cannot simulate the disconnection of a particular actor inside the VM. By means of ''takeOffline'' primitive, you can cut the access of an object contained in any actor (within the same VM or a remote VM),  enabling the simulation of the disconnection of an actor inside a virtual machine.
-<note> +
-The complete implementation of the instant messenger application explained along this chapter can be found in the file ''at/demo/InstantMessenger.at''. +
-</note>+
at/tutorial/distribution.txt · Last modified: 2009/01/30 16:13 by tvcutsem