.NET Mobile Application Development Concurrency in Distributed Clients

Introduction  Concurrency occurs in all distributed systems  Controlling concurrency is essentially to developing scalable, correct distributed applications  In this session and the next we will consider >Multi-threading and concurrency >Concurrency in distributed clients >Concurrency control mechanisms

Concurrency in Distributed Applications  Distributed applications are inherently concurrent >Services have to deal with multiple simultaneous clients in a scalable manner >Clients may need to perform multiple simultaneous activities ­Accept user input and use services  Concurrency needs to be used and managed correctly to avoid >Performance penalties >Incorrect operation and sequencing of activities

Concurrency in Distributed Clients  Concurrency is mainly used in distributed clients to overcome latencies resulting from distributed interactions  This has several benefits >Increased client performance ­Client can perform other activities / processing whilst remote interactions proceed >Increased client responsiveness and improved user experience ­User interface does not freeze when remote interactions or lengthy computations occur ­Application responds to user input in a timely manner

Multithreading  Thread >An independent unit of program execution  Multithreaded program >Multiple threads execute simultaneously ­Threads multiplexed onto available CPU’s using timeslicing ­Context switch incurs (minor) overhead >Threads exist in the same process and share common address space  Multithreading >can improve performance only when there is an operation with some latency ­the longer the delay, the greater the potential performance benefit from using threads >will not improve performance if threads are competing for finite (e.g. CPU) resources; multithreading will actually result in worse performance

Threading Issues  Multithreading can improve performance  Multithreading will introduce >additional complexity >unusual errors that are difficult to detect, replicate and debug >additional overhead; OS must manage all the threads  Getting threading right is difficult; need to >use synchronization and locking to ensure correct interleaving of thread operations and interactions >balance potential improvements from threading against additional overheads (number of threads) >avoid deadlock, livelock, race conditions, etc caused by incorrect / insufficient / too much synchronizationdeadlocklivelockrace conditions

Threading in.NET  Can use threads in.NET by >Using.NET types with built-in threading support ­Types that support asynchronous operation (i.e. has Begin? and End? methods) ­Windows forms types through the Invoke() method >Manually creating and manipulating threads using the System.Threading.Thread type

Using Asynchronous Delegates  A delegate is >a type-safe pointer to a method (function pointer) >an instance of the Delegate type  Delegate type has two important methods >BeginInvoke() ­invokes the method referred to by the delegate on a separate thread ­immediately returns an IAsyncResult instance and allows calling thread to continue >EndInvoke() ­allows retrieval of results from a method invoked earlier using BeginInvoke() ­needs to be given IAsyncResult instance returned by BeginInvoke()

Asynchronous Delegate Example delegate int MyDelegate(int i, int j); class Program { public static void Main() { Maths m = new Maths(); // Create the delegate - note that BeginInvoke always takes 2 extra parameters MyDelegate dlgt = new MyDelegate(m.Add); // Asynchronosly invoke it IAsyncResult iar = dlgt.BeginInvoke(2, 2, null, null); // Do something else whilst call progresses // Wait for async method to complete and retrieve the results int result = dlgt.EndInvoke(iar); } class Maths { public int Add(int a, int b) { return a+b; } Criticisms EndInvoke() call blocks if method invoked by delegate not yet completed Can poll for completion using the IAsyncResult instance Neither option is good

Using Callbacks  Callback >method called when an asynchronously invoked delegate completes its operation  Callbacks >eliminate inefficient polling >simplify coding  Callback method >signature must match System.AsyncCallback delegate type >AsyncCallback instance referencing desired callback method must be passed to BeginInvoke() >callback is passed an IAsyncResult object when it is invoked ­use this to call EndInvoke() and retrieve results

Asynchronous Callback Example delegate int MyDelegate(int i, int j); class Program{ static MyDelegate dlgt; public static void Main() { Maths m = new Maths(); AsyncCallback cdlgt = new AsyncCallback(MyCallback); dlgt = new MyDelegate(m.Add); // Penultimate BeginInvoke() parameter is callback delegate IAsyncResult iar = dlgt.BeginInvoke(2, 2, cdlgt, null); Console.ReadLine(); // Do something else } public static void MyCallback(IAsyncResult iar) { int result = dlgt.EndInvoke(iar); // Use IAsyncResult instance to retrieve the results } class Maths { public int Add(int a, int b){ return a+b; } } NOTE: Callback method executes on yet another thread

Advantages of Asynchronous Invocation  Implicitly using threads by using asynchronous invocation has several advantages >environment takes care of creating and managing all threads for us >no need for explicit concurrency control operations >improves performance by enabling long-running tasks to be started without stalling application  Asynchronous invocation not suitable for >code which needs to communicate between threads >applications which need to prioritize threads >this requires manual thread creation

Creating.NET Threads  Threads are created by >declaring a method which will be the body of the thread ­must be of return type void and accept no parameters ­can call any other methods >creating a ThreadStart delegate instance which refers to this method >creating a new Thread object, passing it the delegate >calling Thread.Start()  System.Threading.Thread >is the primary type used for managing threads >allows threads to be created, started, stopped, suspended, resumed, joined, delayed, etc

Custom Threading Example class Program { public static void Main() { // Create the threads Thread tA = new Thread(new ThreadStart(TaskA)); Thread tB = new Thread(new ThreadStart(TaskB)); tA.Start(); // Start the threads tB.Start(); Console.ReadLine(); // Pause until user presses key } private static void TaskA() { for (int i = 0; i < 10; i++) { Console.WriteLine("Task A at: "+i); WasteTime(1000); } } private static void TaskB() { for (int i = 0; i < 10; i++) { Console.WriteLine("Task B at: "+i); WasteTime(1000); } }

Thread Priorities  Threads are selected for execution based on their priority >highest priority execute first, lowest priority last  All threads created equally with default normal priority >priority can be adjusted via Thread.Priority property >five relative priority levels supported ­Lowest, Below Normal, Normal, Above Normal, Highest  Setting thread priorities correctly is important >giving a thread too high a priority can lead to starvation of other threads

Thread Priority Example class Program { public static void Main() { Thread tA = new Thread(new ThreadStart(TaskA)); Thread tB = new Thread(new ThreadStart(TaskB)); tA.Priority = ThreadPriority.AboveNormal; tB.Priority = ThreadPriority.Normal; tA.Start(); tB.Start(); // Start the threads Console.ReadLine(); // Pause until user presses key } private static void TaskA() { for (int i = 0; i < 10; i++) { Console.WriteLine("Task A at: "+i); WasteTime(1); } } private static void TaskB() { for (int i = 0; i < 10; i++) { Console.WriteLine("Task B at: "+i); WasteTime(1); } }

Threaded Clients  Any operation which will take a long time to complete or adversely affect application responsiveness is a good candidate for executing on a separate thread  e.g. Windows Media Player 9 >Info Center View takes time to retrieve data from the Internet >executing this on separate thread from user interface allows user to control playback in the meantime >when data is returned from Internet, UI can be appropriately updated

User Interfaces in.NET Threaded Clients  Windows Forms controls in a.NET application >are not thread safe >can only be safely operated upon by the thread which owns them  Consider the Windows Media Player example with a thread which fetches data from the Internet. This thread >is separate from the UI thread >should not update the application’s UI as it is not the owner of the UI controls

Win Forms and Threading  Windows.Forms.Control base class provides an Invoke() method which is passed a delegate to another method >Delegate must be of type MethodInvoker  Delegated method acts as a callback used to update the relevant control  Calling Invoke() causes the delegated update method to be safely invoked on the User Interface thread which owns the control

WinForms Threading Example  Simple Form application with two threads >UI thread created by Application.Run() >separate thread used to periodically increments counter >button controls operation of counter thread  UIUpdater() displays latest count value in lblCount Label  Counter thread >creates delegate to UIUpdater() >calls lblCount.Invoke() with this delegate each time it wishes to update the count value in the UI

Concurrency Control  Two key aspects to concurrency control  Mutual exclusion >Preventing object / resource from being accessed by more than one thread at once >Needed to prevent corruption of state in presence of multiple client updates >Implemented through locking  Synchronization >Imposes order on interleaving of operations of multiple threads (usually for correctness) >Implemented using semaphores, condition variables or monitors

Condition Synchronization  Needed when a thread wishes to perform an operation which can only sensibly(safely) be performed if another thread has itself taken some other action or is in some defined state  Example : A bounded buffer has 2 condition synchronization >producer thread must not attempt to put data in buffer if it is full >consumer thread must not attempt to get data from buffer if it is empty >Is mutual exclusion necessary?

Locks and Mutual Exclusion  Critical Section - a sequence of statements that appear to execute indivisibly  Mutual Exclusion - synchronization required to protect a critical section  Locks - implement mutual exclusion and guard critical sections >Code construct which marks a critical section of code that must be executed under mutual exclusion

Locking  In an OO environment, every object has an associated lock  Lock construct specifies the object on which the lock should be obtained  First thread to execute the lock statement will be granted the lock and can execute protected section of code >Whilst first thread has the lock, any other threads executing the lock statement will block until lock is released. Waiting threads will then compete for the lock.

Example: Locking in C# int Withdraw(int amount) { lock (this) { if (balance >= amount) { Console.WriteLine("Balance before Withdrawal : " + balance); Console.WriteLine("Amount to Withdraw : -" + amount); balance = balance - amount; Console.WriteLine("Balance after Withdrawal : " + balance); return amount; } else { return 0; // transaction rejected }

Locking - Disadvantages  Locking is essential to preventing conflicting updates but can adversely affect performance  If scope of lock is too great >forces some threads to wait >decreases potential concurrency and throughput / performance  If scope of lock is too small >forces reacquisition of locks, leading increased lock contention and reduced throughput / performance >can result in difficult-to-debug race conditions

Advanced Locking  Mutual exclusion >not needed if all threads are reading values >needed if at least one thread is writing values  ReaderWriter lock >allows concurrent read access for multiple threads, or >write access for a single thread >uses separate locks for readers and writers; clients must specify which lock they want

Advanced Locking Example ReaderWriterLock rwl = new ReaderWriterLock(); void ReadFromResource(int timeOut) { try { rwl.AcquireReaderLock(timeOut); try { / Safe for this thread to read from // the shared resource. } finally { // Ensure that the lock is released. rwl.ReleaseReaderLock(); } catch (ApplicationException) { // The reader request timed out. } void WriteToResource(int timeOut) { try { rwl.AcquireWriterLock(timeOut); try { // It is safe for this thread to read // or write from the shared resource. } finally { // Ensure that the lock is released. rwl.ReleaseWriterLock(); } catch (ApplicationException) { // The writer lock request timed out. }

Monitors  Monitor >code construct that provides both mutual exclusion and condition synchronization >acts like a ‘smart’ lock  Thread enters the monitor and attempts to acquire lock >if successful, thread executes under mutual exclusion; thread frees lock when finished >if unsuccessful, thread blocks until monitor is available

Monitors: Condition Synchronisation  When inside a monitor, condition variables can be used >Thread can Wait() on condition variable ­thread blocks and releases lock, freeing monitor to other threads ­when condition becomes true, thread unblocks, reacquires lock and executes under mutual exclusion >Thread can Signal() a condition variable ­notifies waiting threads that condition is true ­teleases lock, freeing monitor to other threads

.NET Monitors Example public class MyClass { private object x; public void DoSomething() { Monitor.Enter(x); try { // Code that needs protected by the monitor. // Monitor.Wait and Monitor.Pulse can be // used in here } finally { // Ensure that you exit Monitor. Monitor.Exit(x); }

Common Concurrency Problems  Two common problems  Deadlock >A situation where two or more threads are unable to proceed because each is waiting for the others to do something >Avoid by ­always acquiring / releasing locks in the same order ­ensuring that all Wait()’s have a matching Signal()  Race Conditions >Anomalous behaviour due to unexpected critical dependence on the relative ordering of operations >Can occur is scope of locks is too small and some code left outside critical sections  These problems are >Easy to unwittingly introduce >Difficult to detect and remove

 In this session we have discussed >The role of concurrency in distributed clients >Multi-threading and thread management in.NET >Asynchronous invocation and callback >Locks, monitors and performance >Alternative thread coordination mechanisms Summary

Reading and Resources Reading  Coulouris, Dollimore & Kindberg, Distributed Systems: Concepts and Design, 3 rd Edition, Addison-Wesley, 2001 Chapter 12, pp 465 – 514  MacDonald, Microsoft.NET Distributed Applications: Integrating XML Web Services and.NET Remoting, Microsoft Press, 2003 Chapter 6, pp 175 – 220