Implementing a Multithreaded Server in Software

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In this section, we explain how to deal with server application concurrency issues by making our climate control system capable of running in a multithreaded ORB environment Because difficult concurrency problems can crop up when you manage servant life cycles with respect to the objects they incarnate, our example is based on adding thread safety to the CCS object creation and removal operations introduced in 12 Specifically, we explore multithreading issues for the device creation and removal functions shown in Section 1263 for the servant locator version of the Evictor pattern We expect that several methods on our servants may be called simultaneously by different threads, so we use the fine-grained multithreading strategy Figure 211 illustrates the participants in a locator-based evictor implementation The evictor queue keeps a list of servants in LRU order Our servant locator adds servants to the queue as it creates them, but it first evicts the LRU servant if the queue has reached its maximum capacity You may want to revisit Section 126 to refresh your understanding of how we implemented the Evictor pattern using a servant locator for the single-threaded case
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Figure 211 Implementing the Evictor pattern using a servant locator
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For synchronization primitives, all examples in this section use the multithreading C++ wrappers that are freely available as part of the ACE toolkit[1] Even if your ORB implementation supplies its own multithreading wrappers, they are likely to be similar in form and function to the ACE wrappers
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[1] You can obtain directions for downloading the source code for the ACE C++ wrappers from http://wwwcswustl edu/~schmidt/ACE-obtainhtml
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2161 Review of CCS Life Cycle Operations In Section 123 we show how to add factory operations for creating thermometer and thermostat objects to the CCS::Controller interface
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#pragma prefix "acmecom" module CCS { // interface Controller { exception DuplicateAsset {}; Thermometer create_thermometer( in AssetType anum, in LocType loc ) raises(DuplicateAsset); Thermostat create_thermostat( in AssetType anum, in LocType loc, in TempType temp ) raises(DuplicateAsset, BadTemp);
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// Other operations
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To create a Thermometer, you invoke create_thermometer on the Controller object, passing it the asset number and the location of the new
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thermometer You create Thermostat objects similarly, but their factory operation also requires a value for the initial thermostat temperature setting After it is created, each Thermometer and Thermostat object is incarnated by a different servant The Thermometer interface supplies the remove operation For the reasons mentioned in Section 1255, we do not inherit remove from the CosLifeCycle::LifeCy-cleObject interface When a client invokes remove, the target Thermometer or Thermostat object is destroyed Any further invocations using the same object reference will cause the OBJECT_NOT_EXIST exception to be raised 2162 General Application Issues Our multithreaded evictor implementation example does not show POA creation You can assume that we have two POAs: the first one, for the singleton Controller object, is a child of the Root POA, and the other one, for the Thermometer and Thermostat objects, is a child of the first POA We explicitly activate the Controller object, and its POA has the ORB_CTRL_MODEL, PERSISTENT, RETAIN, UNIQUE_ID, and USE_ACTIVE_OBJECT_MAP_ONLY policy values We register the servant locator in the POA that supports the Thermometer and Thermostat objects This second POA has the ORB_CTRL_MODEL, PERSISTENT, NON_RETAIN, UNIQUE_ID, and USE_SERVANT_MANAGER policy values Note that putting the Controller object in its own separate POA in this example differs from the single-threaded example in Section 126 Because an ORB can concurrently dispatch requests to multiple POAs even if they each have the SINGLE_THREAD_MODEL policy value, the single-threaded example registers all servants under a single POA to prevent concurrent operations on the servants One result of that design is that the servant locator must recognize and handle the Controller object ID as a special case In our multithreaded evictor implementation, we handle concurrency explicitly, so we can safely use multiple POAs as described earlier Any Thermometer and Thermostat operations that access or modify the state of a device do so by sending messages over the ICP network We assume that these device operations are atomic, so we do not serialize our access to the ICP network Another difference between the single-threaded evictor implementation and the multithreaded version is the data structure that the Controller_impl servant uses to keep track of all the devices The single-threaded implementation uses an STL map to allow the Controller_impl to associate each servant with the asset number of the device it incarnates This arrangement allows the Controller_impl to directly delete servants when they are no longer needed In the multithreaded version, however, servants invoke delete on themselves, as Sections 2167 and 2168 describe Therefore, the multithreaded Controller_impl keeps only an STL set of device
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