Since our business objects are within the center tier, information is passing via them on its method between the consumer interface and the host database. It would be good, then, if our enterprise objects supplied some assist for this information movement together with their purely enterprise performance, such as an invoice class's CalculateOverdueCharge methodology. Support for Disconnected Operations The first area in which a business object can provide assist is in data validation. Again, who better to specify the acceptable sum of money for a withdrawal than the BankAccount class itself? The enterprise object, not the UI tier, is the better place for validation logic. The second area of enterprise object benefit associated to data movement is change monitoring. We have already seen this capability in ADO.NET's DataRow class. Each row tracks whether or not it has been added, modified, or deleted for the explanation that information desk was final synchronized with the supply database. It could be good if our enterprise objects offered this same state tracking. I n some circumstances, APCs can symbolize a lightweight interthread communication mechanism. If you understand the HAN D L E of a thread you wish to sign, and that thread has carried out an alertable wait, then queueing an APC is usually significantly faster than waking the target thread by using kernel objects . It does require kernel tran sitions on the caller and callee, however direct thread-to-thread communication is faster than the final objective kernel objects that must deal with a wide range of different difficult conditions. They introduce a form of reentrancy, which may cause reliability problems in both native and in managed code alike. The thread performing the alertable wait has no control over what the APC truly does. This means, as an example, that the APC might await issues alertably, dispatching more APCs on the thread if these are alertable waits too. This can result in messy situa tions as a outcome of you might end up with a single stack that might be a hodgepodge of a quantity of logical actions. If the APC waits for a mutex object that the thread already owns, then the APC will be granted access to it even though information protected by the mutex may be in an inconsistent state as a end result of recur sion. This is seldom feasible because reentrancy is unpredictable. If you P / Invoke to Qu e u e U s e rAPC, the APC may be subsequently dispatched when managed code cannot be run, corresponding to whereas certain critical regions of code in the CLR are executing. This could result in deadlocks in cases where nonrecursive locks are used .
And it'd even occur in the middle of a garbage col lection, whereas the GC is blocked . Finally, this can introduce safety vulnerabilities into your code as a end result of, unlike correct mechanisms of queuing asynchronously work, the CLR will not have a chance to seize and restore a safety context. We'll take a glance at how to correctly write spin-waits in Chapter 1 4, Performance and Scalability. Starving excessive priority work is an actual downside, particularly in actual time or mission crucial systems, the place some background processing interferes with a more necessary time sensitive operation. This is one reason that changing thread priorities should be averted, until you might have a really compelling reason to take action. Windows has a system thread referred to as the balance set supervisor, whose job primarily centers around management of digital reminiscence tables. But one other one of its duties includes rudimentary starvation management. Priority decays at every quantum, until the thread reaches its authentic priority again. This nearly guarantees that the thread will get a chance to run soon and, within the case of priority inversion, lengthy sufficient to release its lock. But then once more, 4 seconds is a very lengthy time to attend for the starvation to kick in, so even with this assist, priority inversion and hunger are problems. Many different solutions to starvation are potential.
The kernel uses IRQLs to prevent interrupts, including context switches, throughout some important areas. You may construct such a scheme in user mode, however lack of help for precedence inheritance is one of a number of often cited the purpose why NT is generally inadequate as a real-time or embedded OS. The code in Ma i n that initiated the shutdown could have orphaned s_loc k by calling Ex it while it was held. The similar would have occurred if we hooked up an event handler to AppDoma i n . Proce s s E x i t that tried to acquire s_loc k, for example. This similar coverage applies to any synchronization objects including man aged reader/ author locks, events and condition variables, and any other kind of interthread communication. You would possibly count on that mutexes would behave in managed code as they do in Win32 during process exit, provided that Mutex is a thin wrapper over the OS mutex APIs. Wa it O n e on an orphaned mutex to throw a Mutex Ab a n do ne d E x c e p t i o n . If that occurred, the unhandled exception would in all probability crash the finalizer thread and, hence, the whole process throughout shutdown. Because shutdown-oriented managed code runs before E x i t P roc e s s is called, threads that own abandoned mutexes are simply suspended ; thus, the mutexes aren't aban doned, and makes an attempt to acquire them will hang.
The manifestation of these kinds of hangs is commonly not horrible. Many finalizers are supposed to clear up intra course of state anyway, and because HAND L E lifetime is tied to the process lifetime, Windows will close them mechanically during process exit. But a hang implies that extra library and application logic won' t run, like flushing F i l e S t r e a m write buffers. And for any cross-process state, you need to all the time have a fail protected plan in place, such as detecting corrupt machine-wide state and repairing it upon the subsequent program restart. This is similar to what must be accomplished with native code, provided that the method will terminate should you try to acquire an orphaned lock. Finally, a 2 second pause would not look like much, but it's long sufficient that the majority users will notice it. Avoid ing cross-thread coordination throughout shutdown is considered a best practice, and it could help to improve the consumer experience for shutdowns. Orchestration capabilities for fine-grained intraprocess work, but i s limited in that true concurrency just isn't used within the ensuing packages . Message Passing Interface is a common programming mannequin utilized in distributed HPC situations. In message based parallelism systems, concurrency is driven by sending and receiving messages. To the extreme, the one way to generate concur rency is by creating separate agents with enforced isolation, and the one method to carry out synchronization is through messages. Specialized languages corresponding to Erlang take this strategy . In addition to the basic capability to send and receive messages, these sys tems usually offer refined pattern matching capabilities, much like these obtainable in functional programming languages corresponding to F#. The CCR additionally supports comparable capa bilities by way of library calls. Other programming fashions exhibit much of the same fashion of pro gramming of message based mostly parallelism but with out the sophisticated capabilities. For example, GUI programming-as we'll focus on extra in Chapter 1 6-is primarily based on sending messages from worker threads to the GUI thread .
The GUI thread has a top-level event loop where its sole function is to receive and dispatch messages by way of event handlers. 1.1.2 The .NET Compact Framework Microsoft first created the .NET Framework for desktop and server techniques, after which built the .NET Compact Framework as a smaller, client-oriented framework. The two frameworks have lots in common, together with the supported knowledge types as well as a common set of names for namespaces, classes, properties, methods, and occasions. These frequent components were meant to ease the transition for programmers from desktop and server programming to gadget programming. The frequent architecture additionally allows code sharing between desktop and device programming. In reality, it's quite straightforward to write down Compact Framework code that may run on a desktop or server system. Several components make this potential, together with source-level compatibility and binary compatibility. At the level of the source code, the Compact Framework has an identical naming scheme as the full .NET Framework. Going the opposite way—taking a full .NET Framework and running on the .NET Compact Framework—is more difficult because the gadget framework supports a subset of the complete framework. To develop for each desktop and device, create separate Visual Studio tasks.
The Visual Studio tool set offers minimal help for the simple reason that few builders have expressed the need for this sort of growth support. We expect the necessity to develop, particularly as builders construct a typical code base to run on gadgets, on desktops, on server systems, and, eventually, on the Microsoft Azure cloud operating system. At the level of binary compatibility, each frameworks share a typical file format and a standard machine-level instruction set . This means, for example, that a single managed-code binary file can be deployed to each desktop methods (e.g., Windows Vista) as nicely as Windows Mobile units. The operative word right here is that it may be deployed, however there are obstacles to this support for binary compatibility. Using a thread pool as a substitute of express thread ing will get you away from thread management minutia and back to fixing your corporation or area problems. Most programmers can be very suc cessful at concurrent programming without ever having to create a sin gle thread by hand, due to carefully engineered Windows and CLR thread pool implementations. Identifying patterns that emerge, abstracting them away, and hiding the use of threads and thread swimming pools are additionally other useful strategies. It's com mon to layer techniques so that many of the threading work is hidden inside concrete elements. A server program, for instance, normally does not have any thread based code in callbacks; instead, there's a top-level pro cessing loop that's answerable for moving work to run on threads. No mat ter what mechanisms you employ, however, synchronization necessities are at all times pervasive unless different state administration strategies are employed . Nevertheless, threads are a primary ingredient of life. And maybe you' ll end up one day constructing such a layer of abstraction. Deciding precisely when it's a good idea to intro duce further threads isn't as easy as you might think. At the same time, introducing too few will result in underutilized hardware and wasted alternative. Mutex The mutex-also referred to as the mutant in the Windows kernel-is a ker nel object that is meant solely for synchronization functions. A mutex's pur pose is to facilitate building the mu tually unique (hence the abbreviated name mut-ex) critical regions of the kind that had been launched in Chapter 2, Synchronization and Time.
The mutual exclusion property is completed by the mutex object transitioning between the nonsignaled and signaled states atomically. When a mutex is in the signaled state, it is available for acquisition; that's, there isn't a current proprietor. A subsequent wait will atom ically switch the mutex right into a non signaled state. When a mutex is nonsignaled, there's a single thread that currently owns the mutex. Mutex ownership is predicated on the physical OS thread used to wait on the mutex in each native and managed code. This allows Windows to offer errors in cases the place a thread erroneously tries to release a mutex when it isn't the current owner. In different synchronization primitives, similar to events, this situation isn' t caught though it represents an error in the program. For techniques during which logical work might migrate between separate threads, or the place multiple pieces of logical work might share the same bodily thread, this can pose issues. Such is the case for fibers, as described in Chapter 9, Fibers, because a quantity of fibers could be mul tiplexed onto the identical OS thread and may even migrate between them over time. The CLR denotes the acquisition and launch of affinity through the use of the Th r e a d . Done by the interrupt handler queuing a DPC to execute the hard work, which is guaranteed to run earlier than the thread returns back to user-mode. In truth, that is how preemption based context switches happen. An APC is sim ilar, but can execute user-mode callbacks and solely run when the thread has no different useful work to do, indicated by the thread coming into something called an alertable wait. When, particularly, the thread will carry out an alertable wait is unknowable, and it may never occur. Therefore, APCs are usually used for less crucial and less time delicate work, or for instances in which performing an alertable wait is a needed part of the programming mannequin that customers program towards.
Since APCs additionally can be queued pro grammatically from user-mode, we'll return to this topic in Chapter 5, Win dows Kernel Synchronization. Both OPCs and APCs may be scheduled throughout processors to run asynchronously and at all times run in the context of whatever the thread is doing at the time they execute. Threads have a plethora of other interesting elements that we'll look at all through this chapter and the relaxation of the e-book, corresponding to priorities, thread local storage, and a lot of API surface space. Before all of that, let's evaluation what makes a managed CLR thread different from a local thread . This code is the last LINQ code on this model of our application. As we wrote these statements, we receivedIntelliSensehelp,allowingustoselectouroperations,seetheavailable properties, and invoke .NET methods or our personal methods as we went. In all, we used LINQ to outline properties, bind knowledge, transform objects in preparation for XML serialization, and find unsynchronized information. What makes the writing of this additional code much more disconcerting is the reality that we all know this code has already been written by others. Whoever designed and authored the System.Data, and related namespace, courses wrote change monitoring and data synchronization logic, for we can see it within the DataRow and DataAdapter lessons. Whoever wrote the DataContext class for LINQ to SQL on the desktop wrote change tracking and information synchronization logic. The companion program that calls our DLL is named CallDeviceDll. Like the opposite programs on this chapter, CallDeviceDll uses our ActiveSync wrapper library, YaoDurant.Win32.Rapi.dll, to entry the service of rapi.dll. There are two basic challenges to calling the CeRapiInvoke function in block mode. One is getting the info formatted to ship all the way down to the gadget; the second is retrieving the information when we get it back from the gadget.
For each of those, we rely on the IntPtr kind and the members of the Marshal class to do the heavy lifting for us. We get back a pointer to the memory, which is the parameter we move as enter to the CeRapiInvoke perform. We should allocate from the method heap because the CeRapiInvoke perform frees the reminiscence after it's done with it. It is a bidirectional course of, allowing the changes made throughout the controls to propagate back to the info objects. In Chapter 6 we'll see how to complete the journey, moving the info between the info object and the database of selection. All are capable of simple knowledge binding via the Bindings collection derived from the Control base class. Additionally, the list-oriented controls, ListBox and ComboBox, can reap the advantages of complicated binding through the DataSource property. The management developed specifically for binding to a knowledge object, the DataGrid, is essentially the most capable and essentially the most advanced; it supplies for tabular displaying, styling, scrolling, and monitoring of the present cell. It is the only .NET Compact Framework control written in managed code. Table 1.three summarizes the possible interactions between the 2 kinds of code and information. Managedcodeallocatesmanageddatafromrestrictedheapscontrolled by the managed execution engine. When managed objects are allotted from this heap, pointers present the reference saved in a program's variables. But these pointers are implicit pointers and can't be accessed like pointers in C for pointer arithmetic. Managed knowledge could be moved by the reminiscence manager with out your code noticing that it has been moved. And when no longer in use, managed data is reclaimed by automatic garbage collection. When passing pointers from managed code into native code, you typically don't need to fret concerning the pointers being made invalidated by having the objects moved. One exception happens when the native code holds onto a pointer for an prolonged time period. To handle this case, make certain to pin the information into place in order that the reminiscence does not transfer and the pointer stays valid over an extended period.
(For particulars on pinning managed objects, discuss with the GCHandle11 class.) Native data could be allotted either from managed code or from native code. Such data is unmanaged because the managed execution engine does not handle it. This signifies that any programmer who allocates native objects must make certain to write code to launch these objects. When referring to what such code does, we use the term handbook memory management, to paralleltheautomaticgarbagecollectionprovidedbytheruntimeformanaged knowledge. As soon as the E nt e r C r i t i c a l S e c t i o n name returns, the current thread "owns" the crucial part. This possession is mirrored within the state of the important section object itself. If a name to E n t e r C r i t i c a l S e c t i o n is made whereas one other thread holds the section, the calling thread will anticipate the section to turn into out there. This wait could final for an indefinite period of time, depending on the period of time the proudly owning thread holds the sec tion. Once the own ing thread leaves the crucial part, the waiting thread will either purchase the lock or be woke up and try to acquire the lock as quickly because it has been scheduled . This is as a end result of the auto-reset event is often lazily allocated upon its first use , that's, the first time con tention occurs on the lock, which might fail if the machine is low on assets. We'll describe why failure isn' t attainable on new OSs together with some historic perspective in a bit. 9.2.3 Remote Access to Device Property Databases The third sort of entry in the Windows CE object retailer is the Windows CE property database. This can be a flat-file supervisor that shops data as records in a memory-efficient way—suitable for small units with small storage space. Neither the .NET Compact Framework nor the .NET Framework offers any support for accessing Windows CE property databases. Such databases usually are not the same as SQL Server CE databases or Pocket Access databases; in brief, such database files cannot be manipulated utilizing ADO.NET courses. The major benefits of using property databases are their very small dimension and their database format in widespread with non–.NET Compact Framework packages. – SQL Server offers for detailed user access security. The syntax is more full in SQL Server than in SQL Server CE. – Views, triggers, capabilities, and saved procedures are supported. – GRANT, REVOKE, and DENY statements are supported, together with system saved procedures for sustaining consumer access rights.
