at:tutorial:distribution
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at:tutorial:distribution [2007/05/04 08:32] – * elisag | at:tutorial:distribution [2008/09/16 17:29] – * tvcutsem | ||
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- | < | + | ====== Distributed Programming ====== |
- | This tutorial is under heavy construction! | + | |
- | </ | + | |
- | ====== Distributed Programming ====== | + | Building on the actor-based concurrency model explained in the [[actors|previous chapter]], this chapter discusses the distribution provisions of AmbientTalk. For actors to communicate across the boundaries of a single device, actors need to be capable of discovering one another' |
- | This tutorial chapter discusses how AmbientTalk virtual machines can discover and communicate with each other over the network. | + | These requirements correspond to the cornerstones of the Ambient-Oriented Programming paradigm. The seamless |
- | The integration of distribution was one of the main concerns in the design of AmbientTalk | + | |
- | More specifically, as a distributed programming language that adheres | + | Before delving in these topics, we illustrate how to activate |
- | ===== Starting the Network.. | + | ===== Going Online |
AmbientTalk provides an unique native object, named '' | AmbientTalk provides an unique native object, named '' | ||
- | When the virtual machine goes online, | + | When the virtual machine goes online, the built-in |
+ | |||
+ | Taking a virtual machine | ||
< | < | ||
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AmbientTalk provides language support to make some objects available to other objects residing in remote actors by means of the '' | AmbientTalk provides language support to make some objects available to other objects residing in remote actors by means of the '' | ||
< | < | ||
- | defstripe | + | deftype |
def service := object: { | def service := object: { | ||
- | def print(aDoc) { | + | |
- | system.println(" | + | system.println(" |
- | } | + | } |
}; | }; | ||
export: service as: Printer; | export: service as: Printer; | ||
</ | </ | ||
- | When an object its exported by its actor, it becomes discoverable by other actors by means of the service type. Internally, this means that the object is placed in the export table of its actor. As shown in the example, a service type is represented by a [[actors# | + | When an object its exported by its actor, it becomes discoverable by other actors by means of the service type. Internally, this means that the object is placed in the export table of its actor. As shown in the example, a service type is represented by a type tag. This means that services types are not associated with a set of methods, but they denote an abstract publication topic that objects exports. As a type tag, a service type can thus be a subtype of one or more other service types. For example, an object could offer a color printing services by exporting the following |
< | < | ||
- | defstripe | + | deftype |
</ | </ | ||
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< | < | ||
when: InstantMessenger discovered: { |messenger| | when: InstantMessenger discovered: { |messenger| | ||
- | when: (messenger< | + | |
- | buddyList.put(name, | + | buddyList.put(name, |
- | system.println(" | + | system.println(" |
- | | + | }; |
}; | }; | ||
</ | </ | ||
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< | < | ||
- | We are using a future to get the return value of the '' | + | We are using a future to get the return value of the '' |
</ | </ | ||
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As '' | As '' | ||
- | |||
===== Partial Failure Handling ===== | ===== Partial Failure Handling ===== | ||
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Let us consider again the example instant messenger application described in previous section to further explain the semantics of AmbientTalk' | Let us consider again the example instant messenger application described in previous section to further explain the semantics of AmbientTalk' | ||
- | When an object discovers a service type, the '' | + | When an object discovers a service type, the '' |
- | - Objects are always passed by far reference, except for isolate objects which are passed by copy. | + | - Objects are always passed |
- Native data types are always passed by copy. | - Native data types are always passed by copy. | ||
- | |||
When a remote far reference receives a messages, it flushes the message to the remote object providing that it is connected. If the remote far reference is disconnected, | When a remote far reference receives a messages, it flushes the message to the remote object providing that it is connected. If the remote far reference is disconnected, | ||
- | Therefore, a remote far reference abstracts a client object from the actual network connection state. However, it is often useful for an application to be informed when a connection to a remote object is lost or reconnected. To this end, AmbientTalk offers language constructs to install observers on a far reference which are triggered | + | Therefore, a remote far reference abstracts a client object from the actual network connection state. However, it is often useful for an application to be informed when a connection to a remote object is lost or reconnected. To this end, AmbientTalk offers language constructs to install observers on a far reference which are triggered |
< | < | ||
when: InstantMessenger discovered: { |messenger| | when: InstantMessenger discovered: { |messenger| | ||
- | ... | + | |
- | when: messenger disconnected: | + | |
- | system.println(" | + | system.println(" |
- | }; | + | }; |
- | when: messenger reconnected: | + | |
- | system.println(" | + | system.println(" |
- | }; | + | }; |
}; | }; | ||
</ | </ | ||
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This code illustrate how the instant messenger application notifies when a buddy goes online or offline. In the above code, '' | This code illustrate how the instant messenger application notifies when a buddy goes online or offline. In the above code, '' | ||
+ | In order to cope with partial failures, AmbientTalk also allows developers to retract all currently unsent messages from the far reference outbox by means of the '' | ||
+ | |||
+ | The '' | ||
+ | |||
+ | < | ||
+ | when: Service discovered: { | reference | | ||
+ | when: reference disconnected: | ||
+ | messages := retract: reference; | ||
+ | } | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | The construct returns a table containing copies of all messenges that were sent to this far reference, but not yet transmitted by the far reference to the remote object pointed to. Note that this has the side effect that the returned messages will not be sent automatically anymore; the programmer is thus responsible to explicitly resend all messages that were retracted but still need to be sent. | ||
+ | |||
+ | The function '' | ||
===== Garbage collecting remote references ===== | ===== Garbage collecting remote references ===== | ||
+ | |||
+ | < | ||
+ | This is an advanced topic and probably does not belong in the tutorial. Moreover, it discusses features to be used only by implementors of reflective language constructs. It would better be replaced by a thorough explanation of leased object references. | ||
+ | </ | ||
As explained in the previous section, AmbientTalk' | As explained in the previous section, AmbientTalk' | ||
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< | < | ||
- | As you may have noticed, the '' | + | As you may have noticed, the '' |
</ | </ | ||
- | On the client side, taking offline an object results in a permanent disconnection of the remote references pointing to it. In other words, despite having network connection, unexporting an object renders remote far references permanently disconnected. This implies that client have to deal explicitly with unexported objects. To this end, '' | + | On the client side, taking offline an object results in a permanent disconnection of the remote references pointing to it. In other words, despite having network connection, unexporting an object renders remote far references permanently disconnected. This implies that client have to deal explicitly with unexported objects. To this end, '' |
< | < | ||
when: messenger takenOffline: | when: messenger takenOffline: | ||
- | system.println(" | + | |
- | | + | //clean certain resources associated to the buddy |
}; | }; | ||
</ | </ | ||
- | 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 | + | 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 |
Note that disconnection, | Note that disconnection, | ||
< | < | ||
- | The complete implementation of the instant messenger application explained along this section | + | The complete implementation of the instant messenger application explained along this chapter |
</ | </ |
at/tutorial/distribution.txt · Last modified: 2009/01/30 16:13 by tvcutsem