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--- at:tutorial:distribution [2007/05/04 08:34] – * elisag
+++ at:tutorial:distribution [2007/05/04 09:06] – * elisag
@@ Line 92: / Line 92: @@
 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.
+In other to cope with partial failures, AmbientTalk also allows developers to retract all currently unsent messages from the far reference outbox by means of the ''retract'' language construct. This is specially useful in the context of distribution, since developers can have explicit control on the messages that are buffered but have not been sent while the remote far reference is disconnected. Any undelivered messages accumulated by the remote reference can be then for example forwarded to another remote object or simply cancelled.
+The ''retract'' language construct takes as argument the far reference of which to retract outgoing message send. One can store the unsent messages upon disconnection of a service type ''Service'' as follows:
+<code>
+when: Service discovered: { | reference |
+    when: reference disconnected: {
+         messages := retract: reference;
+    }
+}
+</code>
+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.
 ===== Garbage collecting remote references =====
@@ Line 104: / Line 118: @@
 <note>
-As you may have noticed, the ''takeOffline'' language construct is reminiscent to the so-called ''delete'' operation provided by some languages without built-in local garbage collection. It is indeed a design decision providing the minimal set of language constructs for distributed memory management. Rather than defining one specific distributed garbage collector directly in the interpreter, we have opted to enable the experimentation of distributed garbage collection techniques from within AmbientTalk itself. Thanks to AmbientTalk reflective infrastructure, it has been possible to build more sophisticated distributed garbage collection techniques to avoid the extra burden that takeOffline places on developers. More specifically, AmbientTalk integrates leasing techniques with the notion of a remote object reference, and provides the resulting ''leased object references''. The system library shipped with AmbientTalk contains a reflective implementation of leased object references together with a set of language constructs to manipulate them.
+As you may have noticed, the ''takeOffline'' language construct is reminiscent to the so-called //delete// operation provided by some languages without built-in local garbage collection. It is indeed a design decision providing the minimal set of language constructs for distributed memory management. Rather than defining one specific distributed garbage collector directly in the interpreter, we have opted to enable the experimentation of distributed garbage collection techniques from within AmbientTalk itself. Thanks to AmbientTalk reflective infrastructure, it has been possible to build more sophisticated distributed garbage collection techniques to avoid the extra burden that takeOffline places on developers. More specifically, AmbientTalk integrates leasing techniques with the notion of a remote object reference, and provides the resulting //leased object references//. The system library shipped with AmbientTalk contains a reflective implementation of leased object references together with a set of language constructs to manipulate them.
 </note>
-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:takenOffline'' was implemented in order for developers to be able to install observers on an far reference which are notified when the object pointed to is taken offline. The construct takes as parameter a far reference and a block of code that is executed when the taken offline event is notified. As an example, the instant messenger application can clean certain resources when a buddy shuts down its instant messenger application as follows:
+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:takenOffline'' has been implemented in order for developers to be able to install observers on an far reference which are notified when the object pointed to is taken offline. The construct takes as parameter a far reference and a block of code that is executed when the taken offline event is notified. As an example, the instant messenger application can clean certain resources when a buddy shuts down its instant messenger application as follows:
 <code>
@@ Line 121: / Line 135: @@
 <note>
-The complete implementation of the instant messenger application explained along this chapter 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>