Systems and methods for processing network transactions by breaking down the transactions into several stages and performing the processing corresponding to each stage using a different plug-in module. One embodiment of the invention comprises an application programming interface (API) that enables the use of individual plug-in modules to perform different stages of the processing of a transaction. The API defines a series of interfaces to corresponding plug-in processing modules, where each interface corresponds to a stage of processing of a transaction and the API conveys transaction information from one plug-in module to the next, until all of the processing stages have been completed. The API may be embodied in a network transaction processing application that is adaptable to different functionalities and different protocols by plugging different modules into the interface provided by the API.