Intercepting Filter Design Pattern


Graphical Representation:



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.

The classic solution consists of a series of conditional checks, with any failed check aborting the request. Nested if/else statements are a standard strategy, but this solution leads to code fragility and a copy-and-paste style of programming, because the flow of the filtering and the action of the filters is compiled into the application.

The key to solving this problem in a flexible and unobtrusive manner is to have a simple mechanism for adding and removing processing components, in which each component completes a specific filtering action.



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

o Logging and authentication

o Debugging and transformation of output for a specific client

o 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.

We are able, in effect, to decorate our main processing with a variety of common services, such as security, logging, debugging, and so forth. These filters are components that are independent of the main application code, and they may be added or removed declaratively. For example, a deployment configuration file may be modified to set up a chain of filters. The same configuration file might include a mapping of specific URLs to this filter chain. When a client requests a resource that matches this configured URL mapping, the filters in the chain are each processed in order before the requested target resource is invoked.



Participants and Responsibilities

FilterManager

The FilterManager manages filter processing. It creates the FilterChain with the appropriate filters, in the correct order, and initiates processing.

FilterChain

The FilterChain is an ordered collection of independent filters.

FilterOne, FilterTwo, FilterThree

These are the individual filters that are mapped to a target. The FilterChain coordinates their processing.

Target

The Target is the resource requested by the client.



Consequences

- Centralizes Control with Loosely Coupled Handlers
Filters provide a central place for handling processing across multiple requests, as does a controller. Filters are better suited to massaging requests and responses for ultimate handling by a target resource, such as a controller. Additionally, a controller often ties together the management of numerous unrelated common services, such as authentication, logging, encryption, and so forth, while filtering allows for much more loosely coupled handlers, which can be combined in various combinations.

- Improves Reusability
Filters promote cleaner application partitioning and encourages reuse. These pluggable interceptors are transparently added or removed from existing code, and due to their standard interface, they work in any combination and are reusable for varying presentations.

- Declarative and Flexible Configuration
Numerous services are combined in varying permutations without a single recompile of the core code base.

Information Sharing is Inefficient
Sharing information between filters can be inefficient, since by definition each filter is loosely coupled. If large amounts of information must be shared between filters, then this approach may prove to be costly.