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Table 3-1. Using Sequence Numbers to Order Notifications
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Notification
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n1.1 n1.2 n2
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Sequence Number
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1 2 3
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Any numbering scheme is possible, as long as it identifies the relative order between two arbitrary values. The scheme makes missing notifications obvious, but you still have a problem. When C receives e2 with sequence number 2, how does it know that a notification with a smaller sequence number (e1.2) is in transit Without adding something to your scheme, C can t know. An entirely different approach uses formal methods to prove or deduce causal order.32 Rather than trying to determine causal order a posteriori, when notifications arrive, it can be more advantageous to design the delivery mechanism so that notifications are sent in the right order to begin with, shifting the ordering emphasis from the receivers to the senders.
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Partial Order
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In a system implemented with multiple parts, many parts can exchange notifications. From the perspective of each part (be it an object or component), it usually isn t important to know the order of incoming and outgoing notifications with respect to the timing of events in the entire system. What counts is the partial order of the notifications, which considers only a subset of all the notifications in the system typically, those associated with a given part or its immediate neighborhoods. The concept of partial ordering was first introduced by Lamport33 and is described in Figure 3-36.
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32. Masoud Mansouri-Samani and Morris Sloman, GEM: A Generalized Event Monitoring Language for Distributed Systems, IEE/IOP/BCS Distributed Systems Engineering Journal, June 1997. Christian Toinard, Gerard Florin, and C. Carrez, A Formal Method to Prove Ordering Properties of Multicast Systems, ACM SIGOPS Operating Systems Review, October 1999. 33. Leslie Lamport, Time, Clocks, and the Ordering of Events in a Distributed System, Communications of the ACM, July 1978.
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CHAPTER 3 NOTIFICATION DELIVERY
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Partial System
e4 e5
e6 e7
e8 e9
Figure 3-36. Identifying a partial system on which a partial ordering of notifications can be applied From the point of view of D, the only ordering that matters regards notifications e3, e8, and e9. Using Lamport s notation, the partial order of two notifications p and q is described by an arrow, so p q indicates that p precedes q in the partial order. The notification p therefore has the potential to affect q. The partial order for notifications related to D might be e3 e8 e9. Partial order is much easier to deal with in a distributed system, because each system can maintain its own internal clock and order events according to this clock. Using partial ordering, you adopt a centrist view of notifications: The only ones you care about are those in a given subset of the overall system. The typical subset includes one part, with its direct neighbors. There are two broad ways notifications can be partially ordered: depth-first and breadth-first. The following sections describe them in some detail.
Depth-First
With depth-first delivery, an event source must block when sending a notification and remain blocked until the recipient is through processing the notification. The recipient s processing might entail sending notifications to other components or even back to the original sender. Figure 3-37 shows an example.
B n1
n2 n3 C
Figure 3-37. A system using depth-first notification delivery order Following the detection of event e1, A sends a notification to B. While B is processing n1, A is blocked. When B gets the notification, it reacts by sending a notification n2 to C. While C is processing n2, B is also blocked. Once C is through, B unblocks and returns control to A, which now can send the next notification n3. Figure 3-38 shows the system timing. The delivery order might appear to be stable and predictable, but you must take care if event handlers can make reentrant calls back to the event source. If, after receiving n2, C sends a notification to A, A might not be prepared to handle it, since it is blocked waiting for B to finish handling n1. If A locks a resource before sending n1 and needs access to that resource to handle the callback from C, then the system deadlocks. A simple solution is to adopt the policy to never lock a resource while sending a notification.
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