datamatrix net documentation REACTIVE, ASYNCHRONOUS, AND PARALLEL PROGRAMMING in Visual Basic .NET

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CHAPTER 13 REACTIVE, ASYNCHRONOUS, AND PARALLEL PROGRAMMING
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Caution If you use locks inside data structures, then do so only in a simple way that uses them to enforce the
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concurrency properties you ve documented. Don t lock just for the sake of it, and don t hold locks longer than necessary. In particular, beware of making indirect calls to externally supplied function values, interfaces, or abstract members while a lock is held. The code providing the implementation may not be expecting to be called when a lock is held and may attempt to acquire further locks in an inconsistent fashion.
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Using ReaderWriterLock
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It s common for mutable data structures to be read more than they re written. Indeed, mutation is often used only to initialize a mutable data structure. In this case, you can use a .NET ReaderWriterLock to protect access to a resource. For example, consider the following two functions: open System.Threading let readLock (rwlock : ReaderWriterLock) f = rwlock.AcquireReaderLock(Timeout.Infinite) try f() finally rwlock.ReleaseReaderLock() let writeLock (rwlock : ReaderWriterLock) f = rwlock.AcquireWriterLock(Timeout.Infinite) try f(); Thread.MemoryBarrier() finally rwlock.ReleaseWriterLock() Listing 13-15 shows how to use these functions to protect the MutablePair class. Listing 13-15. Shared-Memory Code with a Race Condition type MutablePair<'T,'U>(x:'T,y:'U) = let mutable currentX = x let mutable currentY = y let rwlock = new ReaderWriterLock() member p.Value = readLock rwlock (fun () -> (currentX,currentY)) member p.Update(x,y) = writeLock rwlock (fun () -> currentX <- x; currentY <- y)
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CHAPTER 13 REACTIVE, ASYNCHRONOUS, AND PARALLEL PROGRAMMING
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Some Other Concurrency Primitives
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Table 13-7 shows some of the other shared-memory concurrency primitives available in the .NET Framework. Table 13-7. .NET Shared-Memory Concurrency Primitives
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System.Threading.WaitHandle System.Threading.AutoResetEvent
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Description
A synchronization object for signaling the control of threads. A two-state (on/off) WaitHandle that resets itself to off automatically after the signal is read. Similar to a two-state traffic light. A two-state (on/off) WaitHandle that requires a call to ManualResetEvent.Reset() to set it off. A lock-like object that can be shared between operating system processes. Used to limit the number of threads simultaneously accessing a resource. However, use a mutex or lock if at most one thread can access a resource at a time. Atomic operations on memory locations. Especially useful for atomic operations on F# reference cells.
System.Threading.ManualResetEvent
System.Threading.Mutex
System.Threading.Semaphore
System.Threading.Interlocked
Summary
This chapter covered concurrent, reactive, and asynchronous programming, which is a set of topics of growing importance in modern programming because of the widespread adoption of multicore microprocessors, network-aware applications, and asynchronous I/O channels. It discussed in depth background processing and a powerful F# construct called asynchronous workflows. Finally, the chapter covered applications of asynchronous workflows to message-processing agents and web crawling, and examined covered some of the shared-memory primitives for concurrent programming on the .NET platform. The next chapter looks at web programming, from serving web pages to delivering applications via web browsers.
C H A P T E R 14
Building Smart Web Applications
Delivering content and applications via web browsers is one of the most important aspects of modern software development. This chapter examines how you can build web applications using F#. The topics covered are as follows: Serving static files and dynamic content by directly responding to HTTP requests Using the ASP.NET framework to develop page-based, server-side web applications that use input controls and read data from a database using F# LINQ A walk-through of the ASP.NET essentials: organizing web forms and sites, understanding the various types of server controls (web and HTML), user creating controls, and configuring and debugging web applications The fundamentals of the ASP.NET event model: page and control events, the page life cycle, posting information from a page (postback and cross-page posting), and maintaining state A look at techniques to build client-based web applications, including the use of the WebSharper project Consuming web services to deliver programmatic services via web connections
Serving Static Web Content
When you point your browser at a web page or call a web service from your application, you re effectively issuing one or more requests (commands) to a web (HTTP) server. HTTP commands are simple text-based instructions that are automatically generated by your web browser. For instance, when your browser goes to a particular URL, it does the following: Requests the page from the web server and waits for the response Analyzes the contents of the page in the response for further content to be fetched (images, for example), and issues the appropriate requests if necessary Displays the results, and executes any dynamic scripts and content contained in the page
A response can be a verbatim copy of a resource found on the web server (most often a static file such as an image, a style sheet, or a media file) or can be generated on the fly. This section shows how you can use F# to serve content directly. Listing 14-1 shows a simple web server written directly in F#.
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