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CHAPTER 9 EVENT-BASED INTERACTION PATTERNS
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has higher priority than the original message. Using a separate abort message also makes it clear who has the power to interrupt the interaction.
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Caller
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Command Feedback Abort Execute Command
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Figure 9-17. An interruptible transparent interaction
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Example 20 An office worker wishes to have lunch. She goes to the company cafeteria, finds a short
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waiting line, and gets in line. The line progresses along nicely until the drink dispenser machine breaks. The line stops moving. After waiting a few minutes to see if the problem gets resolved, she decides to get in a different line. In this situation, the woman is monitoring the progress of the waiting line. She can see that a problem developed before reaching the head of the line. Being in a position to monitor the situation, the person is able to make an informed decision to stop waiting and choose a different course of action.
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Example 21 A machine is designed to measure a person s blood pressure automatically. It works by
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having patients insert their left arm into a circular strap and pressing a Start button. The machine internally uses a control component (SysCom) to monitor the Start button. When the button is pressed, SysCom issues a start command to the strap control component (StrapCom), which begins pressurizing the strap to tighten it around the patient s arm. Figure 9-18 shows the interactions between SysCom and StrapCom.
SysCom
StrapCom
Start
[ || ]
Tighten Strap Get Pressure
Pressure
Max Pressure Reached Without Detecting Heartbeat.
Abort
Strap Is Pressurized
Figure 9-18. A blood-pressure-measurement machine using an interruptible transparent interaction As pressurization is in progress, SysCom makes frequent status requests to obtain the instantaneous blood-pressure reading. The strap pressure needs to be increased until StrapCom can read a
CHAPTER 9 EVENT-BASED INTERACTION PATTERNS
heartbeat. If the pressure reaches a limit value before a heartbeat is detected, SysCom concludes that the strap wasn t placed around the arm correctly and issues an abort command to release the strap.
Handshaking
Context A process P1 wishes to transfer a large amount of information to a process P2. The information can be broken down into a series of messages. It may be necessary to stop the message flow before all the intended information is sent. P1, P2, or both may have control over when messages should stop being sent. Forces
Perhaps the P1 caller gains access to this information one piece at a time, from a third party, or perhaps the communication channel between P1 and P2 can only handle messages of a certain size.
When lengthy or complex operations can be carried out by the repeated application of simpler ones, the interaction consists of an iteration: The caller issues a command, and the callee acknowledges it. The caller continues to repeat the command (possibly with different data) until the overall operation is complete, or until one of the two parties stops the process. I ll call this type of interaction a handshaking interaction, because of its similarity with hardware handshaking, used frequently in data communications, where the receiver tells the sender when it is ready for the next command or message. Figure 9-19 shows the pattern.
Caller
Callee
Cycle Repeats Until All Commands Are Processed or One of the Parties Ends It.
Command Handshake
Figure 9-19. The basic handshaking interaction pattern Each handshake tells the caller to send more information, if available. The callee can return a handshake immediately upon receipt of each command, or after each command has been processed. An important decision to make when using a handshaking interaction is this: Who has control over termination of the iteration the caller or the callee The answer depends on which party knows when enough is enough. If the caller knows there are no further commands available, the caller will control termination. If the callee can arbitrarily decide when to bail out of the interaction, the callee will control termination. It s also possible for both parties to control termination. When the callee controls termination, the handshake might contain a reserved field or value to indicate a termination request to make the caller stop issuing commands or sending data.
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