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CHAPTER 10 FUNCTIONAL ROLES
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threading details, as seen in the previous sections, an additional reason is that a worker may expose a set of low-level operations that are only useful to the system when combined together. The Coordinator could then be given the responsibility of invoking the low-level operations at the right time and in the right order to achieve the desired result. The outside system would then be able to call a high-level method in the Coordinator, without needing to deal with all the low-level details related to the underlying Worker. Coordinators are often used to control multiple workers. Under the Coordinator s supervision, the workers can collaborate to carry out a nontrivial task. The Coordinator handles all thread-related logic, necessary in those cases in which the managed workers need to be run concurrently. The workers are implemented using synchronous methods, making them easy to test individually. It is the Coordinator that puts the ensemble together, like an orchestra conductor, making the magic happen.
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When using coordinators as front ends or fa ades for groups of workers, you might view each coordinator-worker group as a functional team of the system, much like a project team in a business. Each Coordinator is responsible for its own workers, and each team implements a nontrivial subset of the system s requirements. You then might view the complete system as a collection of teams, each headed by a Coordinator, as shown in Figure 10-13.
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Figure 10-13. Envisioning a system as a network of Coordinator teams In Figure 10-13, the dotted gray boxes are team boundaries, not packaging boundaries. Coordinators are typically deployed in the same component as their subordinate workers. The arrows between the teams represent event-notification traffic. The workers in one team are allowed to talk only to co-workers in the same team or to the local Coordinator. Workers communicate solely using event notifications, so workers are not coupled to other team members. Workers can t talk to other teams directly. Only coordinators are allowed to communicate with other teams, and they do so with event notifications. The use of notifications to tie everything together keeps the teams decoupled from each other. A separate Binder object, described later in the chapter, can handle the task of wiring the teams together. Using the Coordinator-team approach, you can develop and test each team separately, using a dedicated test fixture that emulates the features of the other teams. You can take the team approach
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CHAPTER 10 FUNCTIONAL ROLES
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further, building super-teams out of simple teams. Using the business metaphor, a super-team would be roughly equivalent to a division in a large company. Each super-team would again be testable using a dedicated test fixture. You can design some software systems as a hierarchy of teams, in which the highest-level team includes all the teams in the system.
Key Coordination Tasks
Coordinating software workers is, in some ways, a problem similar to coordinating people. Everyone is different from everyone else in this world, so how you manage subordinate employees depends on several factors, including their expertise, their willingness to cooperate with others, their communication skills, their ability to work independently, and so on. Software coordinators face similar management issues, and coordinators typically handle tasks that fall into the following categories: Managing threading issues Managing a Worker team Mediating communication between workers Mediating communication between Coordinator teams Managing state across a group of workers I ve already provided some description on the first two categories in the Worker examples shown previously, but I ll reiterate and deepen the discussion a bit in the following sections.
Managing Threading Issues
When workers are required to execute operations concurrently, the threading logic can introduce a substantial amount of complexity. In many cases, you can remove the threading code from the Worker and migrate it into a coordinator. By doing so, you can implement all the worker methods as synchronous, making it much easier to design and test the Worker. A coordinator that controls worker threading can also synchronize workers together, when necessary for specific operations. Each worker can then focus on its own mission, without regard to how and when its results will be used with results of other workers in the same team. Coordinators generally use two types of asynchronous interactions with their subordinates: blind and transparent. In both cases, the workers involved are oblivious to threading issues. In both cases, the Coordinator creates a thread for each worker to run on. In a blind interaction, no further action is required, since workers provide no feedback. In transparent interactions, workers do provide feedback. Notifications delivered using method calls are received on worker threads. While handling these notifications, the Coordinator might need to fire events to other parts of the system or to other workers in the same team. The Coordinator often needs logic to synchronize with other threads before sending notifications. The most common case is the requirement to synchronize with the UI thread. Figure 10-10, back in the example on concurrent workers, showed a way to accomplish this. Synchronizing concurrent workers is another fairly common requirement. Assume a coordinator needs to manage two workers as a team. Figure 10-14 shows the class diagram of the system. A BuilderBinder class is used to instantiate the classes and bind them together. The Coordinator and workers interact solely through event notifications, delivered as untyped object calls. As a result, the Coordinator and Worker classes are completely decoupled from each other, and can therefore be developed, tested, and deployed independently. Let s assume the team exposes a single DoSomething method at the Coordinator level. To implement this method, the Coordinator must run Worker1.Method1 and Worker2.Method1 concurrently. Once both methods complete, Worker2.Method2 must run synchronously. The diagram in Figure 10-15 shows the interactions.
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