Context
The presentation-tier request handling mechanism receives many different types of requests, which require varied types of processing. Some requests are simply forwarded to the appropriate handler component, while other requests must be modified, audited, or uncompressed before being further processed.
Problem
Preprocessing and post-processing of a client Web request and response are required.
When a request enters a Web application, it often must pass several entrance tests prior to the main processing stage. For example,
- Has the client been authenticated?
- Does the client have a valid session?
- Is the client’s IP address from a trusted network?
- Does the request path violate any constraints?
- What encoding does the client use to send the data?
- Do we support the browser type of the client?
Some of these checks are tests, resulting in a yes or no answer that determines whether processing will continue. Other checks manipulate the incoming data stream into a form suitable for processing.
Forces
- Common processing, such as checking the data-encoding scheme or logging information about each request, completes per request.
- Centralization of common logic is desired.
- Services should be easy to add or remove unobtrusively without affecting existing components, so that they can be used in a variety of combinations, such as:
-Logging and authentication
-Debugging and transformation of output for a specific client
-Uncompressing and converting encoding scheme of input
Solution
Create pluggable filters to process common services in a standard manner without requiring changes to core request processing code. The filters intercept incoming requests and outgoing responses, allowing preprocessing and post-processing. We are able to add and remove these filters unobtrusively, without requiring changes to our existing code.
