= Threading and the !SystemQueue =

Ariba makes it easy to avoid threading issues like synchronization and upcoming problems like deadlocks, race conditions, or access violations. The threading model in Ariba is completely serial, parallelism is avoided whenever possible. If you are using Ariba to write a service or application, two simple guidelines apply:

 * Code event driven
 * Don't block or wait

When you comply with these two points, you will never get into any threading problems!

== Overview of Threading in Ariba == 

Ariba runs one main thread, called the !SystemQueue. All events in Ariba originate the !SystemQueue, that means that all functionality is triggered by events that are placed into the !SystemQueue. As all functionality is triggerd through the !SystemQueue, only one thread exists in Ariba and you can be sure that all code always runs in sync. The concept of one main queue exists in several software projects, and in gernal is called [http://en.wikipedia.org/wiki/Event_loop Event loop]:

 * MS Windows uses one main thread in GUI applications (also called the ''main loop'') to schedule and execute GUI changes (details on this are available [http://en.wikipedia.org/wiki/Message_loop_in_Microsoft_Windows here]. The {{{WinMain}}} function in Windows applications that are linked against the Windows Subsystem is such an event loop. It uses the Win32 API {{{GetMessage}}}, {{{TranslateMessage}}}, and {{{DispatchMessage}}} to implement serial message processing in Windows.
 * OMNeT++ has an ''event queue'' that completely handles execution of simulations (called [http://www.omnetpp.org/doc/api/classcScheduler.html cScheduler]. If you compile Ariba with simulation support, the !SystemQueue gets synchronized with the OMNeT++ event queue for seamless integration.
 * The X Windows System uses event queues in XLib and GLib.

== !SystemQueue Internals == 

The !SystemQueue class has one main thread that uses {{{boost::condition}}} with the {{{wait}}} and {{{signal}}} functionality to implement a queue. The thread blocks and waits until a new event is inserted into the queue. It then executes this event and calls the event handler that is assigned to this event. The insertion and execution of an event therefore are decoupled in terms of threading and run asynchronous. 

One example of events are imcoming network messages. When a packet comes in from the network a new event is generated and the packet attacked to this event. The event is inserted into the !SystemQueue and gets processed asynchronous from the !SystemQueue thread. 

== Using the !SystemQueue == 

Every mechanisms that gets triggered by some external event must get synchronized with the main !SystemQueue thread. If you e.g. observe the network card and want to execute some stuff on network changes, you must synchronized with the !SystemQueue. Why? Because a network packet might get processed and call some code that you would maybe call, too. Therefore producing parallel access to some code and causing threading problems. 

Synchronizing with the main Ariba thread is easy, thanks to the !SystemQueue. We will not explain step-by-step how to use the !SystemQueue in your code:

 1. In you {{{.h}}} file integrate the !SystemQueue header and some {{{using declerations}}};

{{{
#!cpp
#include "spovnet/common/system/SystemQueue.h"
using spovnet::common::SystemQueue;
using spovnet::common::SystemEvent;
using spovnet::common::SystemEventType;
using spovnet::common::SystemEventListener;
}}}

 2. Derive your class from from a {{{SystemEventListener}}} to be able to receive events:

{{{
#!cpp
class MyClass : public SystemEventListener
}}}

 3. Redefine the {{{SystemEventListener}}} interface:

{{{
#!cpp
protected:
    virtual void handleSystemEvent( const SystemEvent& event );
}}}


== Using Blocking Code ==