Conflict Sets in Java

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332 Conflict Sets
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Classes that track the execution state of underlying operations can use this information to decide what to do about new incoming requests One of the main applications is construction of custom-made exclusion policies that provide more fine-grained exclusion control than seen in 2 To illustrate, consider an Inventory class with methods to store and retrieve objects, each of which has a unique description Suppose that these operations are somewhat time-consuming, but are implemented in a way that does not necessarily require low-level synchronization In this case, we can allow those operations that do not semantically conflict with each other to execute at the same time, thus permitting more concurrency than possible in a fully synchronized version of the class In the classic context of this form of policy control, basic functionality is arranged via database transactions, but we will illustrate using javautilHashtable Even though the fully synchronized Hashtable class allows an Inventory class to be defined without worrying about some low-level synchronization details, we still want to place some semantic constraints on the store and retrieve operations One policy choice is:
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A retrieve operation should not run concurrently with a store operation since the store might be in the process of adding exactly the item requested, in which case you don't want to return a failure indication Two or more retrieve operations should not execute at the same time, since one may be in the process of removing the item requested by the others
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We could have made other decisions here, for example even allowing all operations to operate concurrently, thus allowing failures Also, we could have based the policies on the internal implementation details of the operations For example, the above choices would also hold here if the retrieve method were programmed in a way that required exclusion, but store did not Several formal and semiformal notations have been devised to help represent this kind of information The most widely used method, which suffices for most concurrency control problems of this kind, is based on conflict sets sets of pairs of actions that cannot co-occur For example, here the conflict set is merely:
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{ (store, retrieve), (retrieve, retrieve) }
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This information can serve both as documentation of class semantics and as a guide for implementing these semantics via execution-state tracking 3321 Implementation Classes based on conflict sets can employ before/after designs (see 14) in which ground actions are surrounded by code that maintains the intended exclusion relations The following mechanics can be implemented via any before/after pattern:
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For each method, declare a counter field representing whether or not the method is in progress Isolate each ground action in a non-public method Write public versions of the methods that surround the ground action with before/after control: o Each synchronized before-action first waits until all non-conflicting methods have terminated, as indicated by counters It then increments the counter associated with the method o Each synchronized after-action decrements the method counter and performs notifications to wake up other waiting methods
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Applying these steps directly to the methods of an Inventory class leads to:
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class Inventory { protected final Hashtable items = new Hashtable(); protected final Hashtable suppliers = new Hashtable(); // execution state tracking variables: protected int storing = 0; // number of in-progress stores protected int retrieving = 0; // number of retrieves
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// ground actions: protected void doStore(String description, Object item, String supplier) { itemsput(description, item); suppliersput(supplier, description); } protected Object doRetrieve(String description) { Object x = itemsget(description); if (x != null) itemsremove(description); return x; } public void store(String description, Object item, String supplier) throws InterruptedException { synchronized(this) { // before-action while (retrieving != 0) // don't overlap with retrieves wait(); ++storing; // record exec state } try { doStore(description, item, supplier); // ground action } finally { // after-action synchronized(this) { // signal retrieves if (--storing == 0) // only necessary when hit zero notifyAll(); } } } public Object retrieve(String description) throws InterruptedException { synchronized(this) { // before-action // wait until no stores or retrieves while (storing != 0 || retrieving != 0) wait(); ++retrieving; } try { return doRetrieve(description); // ground action
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