Performance Modeling and Benchmarking of Event-Based ... - DVS

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54 CHAPTER 4. PERFORMANCE ENGINEERING OF EVENT-BASED SYSTEMS Pattern 4: Time Controlled Pull I Timer 1‘Event 1‘Event 1‘Event 1‘Event Producer T1 Broker T2 Consumer T2-I: Pull Event T2-II: Go back to sleep Broker Event Timer Trigger 1 1 1 1 Consumer Event Timer Trigger Timer Trigger 1 1 Timer Trigger Figure 4.12: Time-Controlled Pull Pattern Characteristics • Pull-based communication • Time controlled behavior: the consumer connects frequently to the broker to pull one event after a certain time interval. Example An event consumer connects frequently to a broker to check whether notifications are waiting. If yes, he downloads exactly one of them to process it. Afterwards, the consumer closes the connection and waits a specified period of time before reconnecting. A real world scenario represented by this pattern are Wireless-Sensor Networks (WSN). To save battery, the nodes are mostly disconnected. However, they connect frequently for a short moment to other nodes to exchange events and data. Description In modeling techniques such as QPNs, events are pushed by transitions from one place to another, e.g., after an event was served at one place, it is pushed to the next place. However, in scenarios like our example we are facing pull-based communication behavior and an approach is needed to reflect this in our models. Therefore we introduce a methodology how pull-based communication can be modeled using, e.g., QPNs. To model all aspects of our scenario, we presents additionally a way to implement time-controlled behavior. In our scenario the Consumer connects frequently to the Broker, tries to pull, if available, exactly one event and disconnect. To model this behavior, we need a timer which is responsible for establishing the connection. As shown in Figure 4.12 we implement the timer by adding a queuing place Timer (scheduling strategy: infinite server) and a new token color named Trigger, which triggers the establishment of the connections and the pull attempt after a certain time
4.2. PERFORMANCE MODELING PATTERN 55 interval. The interval between two connections is controlled by the service demand of the Trigger token on the Timer place (S T rigger,T imer ). Since the consumer disconnects immediately after the pull attempt there is no need to model connection times. We model the scenario as an endless loop composed of four phases. In the first phase, the consumer is disconnected and the Timer is processing the Trigger token. When the Timer has processed the Trigger token, the second phase starts and the connection is established. In the next phase, the consumer tries to pull an Event. Finally, the consumer closes the connection and we re-enter the first phase. In the following we illustrate our modeling approach in more detail: 1. System Initialization In the beginning, the connections is closed and one Trigger token is stored in the depository of the Timer place (end of the first phase). 2. Open and Close Connections When the Trigger token is available in the depository of the Timer, transition T2 is fired. Transition T2 implements the establishment of a connection (II. phase), the pull attempt (III. phase) and the disconnection (IV. phase). Two possibilities exist (see Figure 4.12): (a) The connection is established and the consumer pulls one Event from the Broker (T2-I ). (b) The connection is established, but no Event is available (T2-II ). In both cases, the connection has to be closed afterwards. This is implemented by adding the Trigger token to the Timer queue. When the connection is closed, we are back in the first phase. After t=S T rigger,T imer 2 , the Timer has processed the Trigger token and moves it to its depository. The next step is to enter the second phase and to open a connection to the broker again. Note T2-II should only be fired if no Event is available in the depository of the Broker. If an Event is available, T2-I has to be fired. Again, we have to face the limitation of standard QPNs that transition priorities are not supported. In this scenario, the easiest way is to define Firing Weight of T2-II the firing weights of the transitions so that Firing Weight of T2-I is close to zero, i.e. the possibility that T2-II is fired although a Event token exists is close to zero. QPN Definition Places: Place Type Description Producer S Publishes events. Broker Q Broker for all incoming events. Timer Q Timer place (scheduling strategy: Infinite Server). Consumer S Pulls events from broker. Colors: Color Event Trigger Description Represents the published event. Triggers pull commands. Init No. of Colors: Color Place Count Trigger Trigger Store 1 2 Since the Timer is using InfiniteServer as scheduling strategy, the response time of the queue for a token is equal to its service demand (and waiting time is zero).
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4.3.4 Tool Extension . . . . . . .
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5.7 Proportions of the Interactions
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List of Acronyms Acronym λ t,k j A
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Acronym SPEC-OSG SPE SPN SQL ST SUT
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Chapter 1 Introduction 1.1 Motivati
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Bibliography [1] J. Abbott, K. B. M
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