An example application of design patterns in the software industry


A common type of mechanism used to solve problems in the software industry is the workflow mechanism. at least in a very broad manner of description. A service has to do a particular job, but to do that particular job it has to query a couple of other services, and then process all the information, persist it and then update a downstream service with the processed information. I'll give an example from the advertising domain.

Any advertising platform must find out a way to filter out bad ads, could be because of vulgar test, photo not following resolution guidelines etc. The seller interface registers the new ads in the main database. The ad quality service periodically gets the new ads from the registration service which are marked as under processing. The ad quality service has to make the ad go through multiple local checks with local persistence, and then finally it has to update the ad server with the final verdict on whether the ad follows all guidelines and is good to go, or update the ad server with the reason the ad was rejected. This final update itself can be a combination of many service calls, like one service update that the ad text is good, the other to update ad photo does not follow policy guidelines, and another to update which regions this ad can be shown and so on. If any of these steps fail, it can lead to an inconsistent state of the complete ad information.

This scenario requires reverting all the changes done by the ad quality service if any of the steps failed. This is solved by using the undo pattern, where all steps are implemented with their inverse/reverse also implemented in the class/service and when any step fails all the previous steps are reverted. As patterns are named differently in different sources, I am using Gang of Four as a reference and the patterns from this book used are 'Chain of Responsibility' and 'Command' patterns in combination. I like to call it 'Chain of Command'.

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