Multiglom Media, vendor of archiving solutions, wants to add hashing to their external messaging system after a couple of inexplainable data changes were discovered.
It shall be possible to automatically compute a hash (method configurable) over each message.
The purpose of the hasher is computing a hash over given message content. The hash algorithm shall be chosen based on the system context.
Main Features
- Compute hash (String) over each message (String).
- Make the hash algorithm configurable, so it can be selected from the context at runtime.
Table 1. Quality Goals
| No. | Quality | Motivation |
|---|---|---|
| 1 | Performance | The performance impact shall be minimal, for certain scenarios it shall be possible to define a dummy algorithm not impacting processing speed at all. |
| 2 | Extendability | It shall be easy to add more algorithms. |
| 3 | Testability | The design shall encourage testing decoupled from the rest of the application. |
In this scenario we want to apply the Strategy Pattern to define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it (GoF).
We have identified the hash algorithm to be encapsulated in a strategy.
Initially, we provide two concrete strategies: Crc32HashStrategy and Sha1HashStrategy.
The only method computeHash() of the HashStrategy interface takes a String and returns a String, so a dummy implementation (without performance impact) would be easy to implement.
Open StrategyTest.java to start playing with this pattern. By setting the log-level for this pattern to DEBUG in logback.xml you can watch the pattern working step by step.
- It is worth mentioning that a Strategy by definition only provides a single operation (method). If you intend to encapsulate multiple operations that typically belong together, you should call that Policy rather than Strategy as described by (Shalloway-Trott).
- (GoF) Gamma, E., Helm, R., Johnson, R., Vlissides, J.: Design Patterns – Elements of Reusable Object-Oriented Software. Addison-Wesley (1995).
- (Shalloway-Trott) Shalloway, A., Trott, J.R.: Design Patterns Explained – A New Perspective on Object-Oriented Design. Addision-Wesley (2002). Additional information at www.netobjectives.com.