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--- at:tutorial:distribution [2007/05/04 08:34] – * elisag
+++ at:tutorial:distribution [2007/05/04 09:05] – * elisag
@@ Line 69: / Line 69: @@
 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.
@@ 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 107: / Line 121: @@
 </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>