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CHAPTER 9 EVENT-BASED INTERACTION PATTERNS
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Pull Interactions
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Context A process P1 needs to monitor the status of a process P2. P1 only needs to know the status of P2 at specific times, and it gets the status by issuing a request to P2. P2 does not provide any information unless requested by P1. Forces
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If P1 doesn t need status information except at specific times, it is time-consuming and wasteful for P2 to send notifications every time a status change occurs. P1 might ignore many of the notifications. If only P1 knows when status information is necessary or useful, it is natural for P1 to control when status information is exchanged.
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The purpose of a pull interaction is for one party to obtain information from another. I ll use the word interrogator to designate the party starting the interaction, and the word respondent for the party that answers with the data. In a pull interaction, the interrogator sends a request to the respondent, which replies with a response, as shown in Figure 9-2.
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1. Request
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Interrogator Respondent
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2. Response
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Figure 9-2. The pull interaction pattern Linguistically, a pull interaction is associated with an interrogative sentence type. Pull interactions are effective when the following two things apply: 1. The interrogator needs to control when information is retrieved. 2. The interrogator only needs information at a given time. The interrogator s request can convey the type of information needed. The respondent s response supplies the information. If the interrogator uses a method call to query for information, the method can define parameters to hold the response information. If the interrogator uses a message for the query, a separate interaction is required for the response. The response is not considered a push interaction, because the interrogator determined its timing. Pull interactions are particularly effective when the interrogator only needs information at specific and infrequent times. It would be wasteful for the respondent to push the information using change notifications, especially if changes occurred frequently. Pull interactions are very common for two reasons: 1. They are simple to implement, because both the interrogator and the respondent can be synchronous. A simple method call is sufficient. 2. They apply to the common situation in which status changes in the respondent don t require the interrogator to react immediately: The interrogator will discover the change the next time it requests status. There are numerous examples of the pull pattern in everyday life and in computing.
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CHAPTER 9 EVENT-BASED INTERACTION PATTERNS
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Example 4 When you want to know the outdoor temperature, you might look out the window at an outdoor thermometer hanging on a wall and see the temperature. You are the interrogator, and the thermometer is the respondent. You control when to read the outdoor temperature. Unless you take the action of requesting the temperature by looking at the thermometer, you ll never know what the thermometer reading is. The thermometer can t, by itself, give you its temperature reading. Example 5 A software system needs to monitor the state of keys on a keyboard. Consider a simple system in which a keyboard device contains an initially empty FIFO buffer that stores the keys typed by the user. A separate process issues Read commands to the keyboard device to get the keys typed. Each Read command returns one key. The reader process is the interrogator, the keyboard device the respondent. The reader will never receive typed keys unless it asks the keyboard device explicitly.
The Round-Robin Polling Pattern
A process P1 needs to continuously monitor the status of a number of other processes Pi. Status changes can only be handled at a certain rate, decided by P1. The status requests can be fulfilled by the Pi processes very quickly e.g., by returning the value of an internal state variable or performing a simple computation.
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