Systems and methods for using plug-in modules to process transactions in a proxy, wherein portions of the transaction information such as client requests and corresponding server responses may be transformed before delivery to their destinations. One embodiment comprises an application programming interface (API) implemented in a proxy that enables the use of individual plug-in modules to perform transaction processing, including transformations on requests and/or responses handled by the proxy. In one embodiment, transactions are broken down into stages comprising initializing the transaction, sending an opening statement, getting a request, transforming the request, logging in and obtaining authorization, generating a response, transforming the response, sending the response, and terminating the transaction. Because the proxy uses plug-in modules for the stages, functionality and protocol support can be easily modified, and code re-use is increased.