The Twisted Framework

The Twisted framework uses such an event-driven system and uses the so-called ``deferred'' object for building function chains. It is possible to attach ``callbacks'' and ``errbacks'' to such deferreds, which will be executed when the deferred is ``called back'' (``fired'').

A sample program is shown in listing 36, which depicts a simple function that is added to a deferred and fired five seconds after the program started.

language=C

Deferreds have various additional capabilities such as the following:

Error Handling:

Apart from adding callback functions to a deferred, so-called ``errbacks'' can be added. These errbacks will be executed in case a callback raised an error. The errback may then either catch this error, in which case the next callback is executed or re-raise an error, which leads to the execution of the following errback.

Returning Deferreds:

The return values of a callback function will be used as function arguments of the next function in the callback chain. In case a callback function needs to wait for another event, it is possible to create another deferred, add some callbacks or errbacks and return it instead of the return values. The last function in the callback chain of this ``inner'' deferred will then return the arguments for the callback in the ``outer'' chain.

Waiting for Multiple Deferreds:

Twisted provides functions that enable waiting for multiple deferreds. For instance, this feature can be used if a function has to wait for multiple requests, such as depicted in figure 63.

Apart from solving concurrency issues, Twisted implements libraries for various Internet protocols, such as TCP, HTTP, SMTP, Secure Shell (SSH), XML-RPC and many more. These libraries are of much use for the OPC Server design as HTTP will be used for transporting SOAP messages and ESD messages is based on a simple line-based TCP protocol. More information about the Twisted framework can be found in [FET06].