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Of all the ways in which notifications can be ordered, the most important one is causal order, which indicates that notifications reach their destinations ordered in the same way as the events that triggered them. When dealing with causality, it is important to understand what I mean by event order. Events in the real world can occur at any time. If a software system is waiting for certain conditions to occur, there is going to be a difference between the time a condition occurs and the time it is detected as an event. For example, assume three conditions occur in the order e1, e2, e3. If the system uses a loop to check for each event, it might detect e3 first, then e1 and e2, as shown in Figure 3-34.
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CHAPTER 3 NOTIFICATION DELIVERY
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Check for e2 Repeat Continuously Check for e3
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Figure 3-34. Detecting events in the wrong order After X checks for e1, that event occurs, so it is missed. The same thing occurs for e2, so X misses detecting the first two events, because they occur after their presence is checked in the polling loop. Object X does catch the third one, though. A bit later, the loop repeats, and the first two events are detected. To avoid detecting events out of order, the system could avoid polling and rely on real-time hardware detection. Each time a condition occurs, it generates an interrupt, causing the appropriate interrupt service routine to be called to trigger the event detection. But you re still not out of the woods. While servicing the interrupt for the first event, other events can occur, so you must take care to ensure the capture of events that occur while another event is being handled. The details are beyond the scope of this book. Notifications sent from a sender might trigger events in the receivers. If notifications are delivered using procedure calls, each notification could directly invoke the appropriate event-handler method, ensuring that the various handlers are invoked in the same order as the incoming notifications. But here again there is a potential problem regarding how to handle the incoming notifications that are concurrent, produced from callers in different processes or threads. For the purposes of this discussion, I ll consider the event order to be the order in which the events are detected, without worrying if the detected order reflects the real order. A common ordering problem occurs with multicast event notifications. Consider the example in Figure 3-35.
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Figure 3-35. A system using multicast notifications Object A detects event e1 and consequently sends a multicast notification, first to B and then to C. Object B treats the arrival of n1.1 as an event (which you can call e2), producing the notification n2 sent to C. An important issue is this: Does n1.2 reach C before or after n2 The diagram doesn t tell us. The order of arrival of n1.2 and n2 depends entirely on the implementation. The order is causal if n1.2 reaches C before n2, because the event e1 occurred before event e2, which triggers n2. Problems might occur if notifications arrive in noncausal order, because the receiver might not be in the proper state to accommodate the notifications. Much work has been done regarding the
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CHAPTER 3 NOTIFICATION DELIVERY
preservation of causal order in notification delivery. One approach is to add timestamps to each notification in a system. The timestamps are obtained from a centralized timeserver. During debugging, timestamps allow you to see the relative ordering of notifications captured using a diagnostic tool. At run time, however, timestamps don t help as much as you might think. Using the example in Figure 3-35, assume Object C receives a timestamped notification n2 before n1.2. In all likelihood, C has no way of knowing that n1.2 is on its way, unless it waits for a certain amount of time, postponing the handling of n2. Systems aren t usually designed to linger after an event occurs, so the fact that n1.2 arrives with a timestamp doesn t help the system process n1.2 and n2 in causal order. Another approach for ensuring the preservation of causal order is to add a number, taken from a monotonically increasing number sequence, to each notification sent in the system. A centralized number server would have to generate the numbers, so all notifications in a given system would use numbers obtained from the same server. Assume the notifications in Figure 3-35 were numbered as in Table 3-1.
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