Published on January 23, 2014
Architectural Patterns [PART 4] (Concurrency Pattern) Based on Pattern-Oriented Software Architecture, Patterns for Concurrent and Networked Objects, Volume 2 by Douglas Schmidt, Michael Stal, Hans Rohnert and Frank Buschmann
Active Object It decouples method execution from method invocation to enhance concurrency and simplify synchronized access to objects that reside in their own thread of control. Components – Proxy – Resides in client thread . It provides an interface that allows clients to invoke publicly accessible methods on an active object. Client – It invokes public methods provided by proxy which in turn creates method request object . when client invokes a method on a proxy, it receives a future. Each future reserves space for the invoked method to store its result. The future allows the client to obtain the result of the method invocation after the servant finishes executing the method. Method Request object - It provides an interface for executing the methods of an active object. This interface also contains guard methods that can be used to determine when a method request can be executed Activation list – proxy updated the list to keep track of pending method requests created by the proxy and which method requests can execute. Scheduler - It runs in the active object's thread. It decides which method request to execute next on an active object. It scheduler uses the activation list to manage method requests that are pending execution. Servant – It defines methods correspond to the interface of the proxy and method requests the proxy creates. It may also contain other predicate methods that method requests can use to implement their guards. The servant method is executed on the active object.
Active Object Implimentation –
Half-Sync/Half-Async This pattern decouples asynchronous and synchronous service processing in concurrent systems without reducing efficiency. Decompose services in the system into two layers (synchronous and asynchronous) and add a queuing layer between them to mediate the communication between services in the asynchronous and synchronous layers. Components: Synchronous service layer - Services in this layer run in separate threads or processes that can block while performing operations.(e.g. ftp, telnet) Asynchronous service layer - Services in this layer performs lower-level processing services and cannot block while performing operations. (e.g. Hardware interrupts) Queuing layer - It is responsible for notifying services in one layer when messages are passed to them from the other layer.
Half-Sync/Half-Async External event sources - Generate events that are received and processed by the asynchronous service layer (e.g. end-user terminal, disk controllers) Asynchronous interrupts strategy - when an event occurs on an external event source, an interrupt notifies the handler associated with the event, which then processes the event to completion. Proactive I/O strategy - I/O operations are executed by an asynchronous operation processor. When an asynchronous operation finishes, the asynchronous operation processor generates a completion event. This event is then dispatched to the handler associated with the event, which processes the event to completion. Services in the asynchronous and synchronous layers can exchange messages via queuing layer. This queuing layer buffers messages so that synchronous and asynchronous services can run concurrently. Notification strategy - It may be required to notify a service in one layer when messages addressed to it arrive from another layer.
Leader/Followers This pattern provides an efficient concurrency model where multiple threads take turns sharing a set of event sources in order to detect, demultiplex, dispatch, and process service requests that occur on the event sources A pool of threads shares a set of event sources efficiently by taking turns, demultiplexing events that arrive on these event sources and synchronously dispatching the events to application services that process them. Components Handle set - It is a collection of handles that can be used to wait for one or more events to occur on handles in the set. A handle set returns to its caller when it is possible to initiate an operation on a handle in the set without the operation blocking. event handlers - They implement the hook method(s) responsible for processing events received from a handle
Leader/Followers Thread pool – Threads that take turns playing three roles. Leader role - The leader thread waits for an event to occur on any handle in the handle set. If there is no current leader thread available, the underlying operating system can queue events internally until a leader thread is available. Follower role - After the leader thread has detected a new event, it uses the thread pool to choose a follower thread to become the new leader. Processing role - This thread concurrently demultiplexes the event it detected to the event's associated handler and then dispatches the handler's hook method to process the event. After the processing thread has run its event handling to completion, it can rejoin the thread pool.
Leader/Followers Implementation – Leader Thread Follower Thread Thread Set Concrete Event Handler Handle Set Join() Become new leader Join() wait_for_event() wait Event promote_new_leader() Become new leader handle_event() wait_for_event() select
Thread-Specific Storage This pattern allows multiple threads to use one 'logically global' access point to retrieve an object that is local to a thread—called a 'threadspecific object'—without incurring locking overhead for each access to the object. Components thread-specific object - It is an instance of an object that can be accessed only by a particular thread (e.g. errno) key factory - Keys generated by the key factory are assigned from a single range of values to ensure that each thread-specific object is 'logically' global. thread-specific object set – it contains the collection of thread-specific objects that are associated with a particular thread. (e.g. OS threads library implements the thread-specific object set that contains the errno data) thread-specific object proxy - It can be defined to enable clients to access a specific type of thread-specific object.(e.g. errno is implemented as a preprocessor macro that plays the role of the proxy ) Application threads are clients that use thread-specific object proxies to access particular thread-specific objects that reside in thread-specific storage
Thread-Specific Storage Implementation ThreadSpecific Object Set Thread – Specific Object Proxy Application Thread Method() Key get() TSObject Result Result ThreadSpecific Object Method()
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