Multiglom Crapjuls, a vendor of compostable jewels, has decided to rework its legacy home-grown CRM-platform BABBLER rather than replacing it.
Team Gogo is in charge to rework the way the different modules deal with properties. Currently, it is a mix of property file access, database calls and startup parameters. After discussing different approaches the decision was made to provide a central facility where properties can be queried (read-access).
Property management (storage/maintenance/change tracking) is a different concern (out-of-scope here). However, there is rumor that team Custos will implement anything based on git, but this is a different story. 😏
The purpose of the Property Access Facitily (PAF) is to provide access to properties from various modules of BABBLER.
Main Features
- Read-only access to properties.
- Provide a method to query a property by name, return an empty string if not found.
- Provide a method to query a property by name and to specify a default value to be returned if the property was not found.
Table 1. Quality Goals
| No. | Quality | Motivation |
|---|---|---|
| 1 | Availability | It shall be easy to use PAF from any code location. |
| 2 | Robustness | PAF is supplementary and shall handle problems gracefully if possible. |
| 3 | Testability | It shall be easy for developers to mock PAF for integration testing. |
In this scenario we want to apply the Registry Pattern to provide a well-known object that other objects can use to find common objects and services (Fowler).
In the scenario above we realize PAF as a PropertyRegistry.
This PropertyRegistry will be made available throughout the application code of BABBLER.
Independent from the progress of the Custos team a local PropertyRegistry can be provided (based on a property file). For integration testing developers can even configure a Mock-PropertyRegistry providing just the values required for the test.
Open RegistryTest.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.
- As mentioned in the book implementing the Registry pattern and Dependency Injection is rather complementary than contradictory. In the given example code the access to the PropertyRegistry (a singleton) is static. In other environments a reference to the registry might be injected in the using component.
- A registry can easily refresh values on a regular basis (for the same key, next time the updated value/reference will be returned). Updating references and values set via Dependency Injection is quite complicated, especially in conjunction with multi-threading.
- (Fowler) Fowler, M.: Patterns of Enterprise Application Architecture. Addison-Wesley (2002)