We all know that any business and financial transaction has to strictly fulfill the ACID properties. Steadily, transactional applications are being deployed in cloud environments. Now, in the big data era, distributed computing is becoming the mainstream computing model. NoSQL databases are very prominent and dominant these days in order to do justice to big data. Increasingly, there is an assortment of segregated yet connected data sources as well as stores to perform high-performance data access, processing, and retrieval. In this case, strong transactional consistency is not being maintained. Rather, the application should go for eventual consistency. While these steps are being performed, the overall view of the system state may be inconsistent, but when the operation has completed and all of the steps have been executed, the system should become consistent again.
A significant challenge in the eventual consistency model is how to handle a step that has failed irrecoverably. In this case, it may be necessary to undo all of the work completed by the previous steps in the operation. However, the data cannot simply be rolled back because other concurrent instances of the application may have since changed it. Even in cases where the data has not been changed by a concurrent instance, undoing a step might not simply be a matter of restoring the original state. It may be necessary to apply various business-specific rules.
Compensation has been the typical response when a transaction fails. This pattern is mainly to undo the work performed by a series of steps, which together define an eventually consistent operation if one or more of the steps fail. A compensating transaction might not be able to simply replace the current state with the state the system was in at the start of the operation because this approach could overwrite changes made by other concurrent instances of an application. Rather, it must be an intelligent process that takes into account any work done by concurrent instances. This process will usually be application-specific, driven by the nature of the work performed by the original operation.
A common approach to implementing an eventually consistent operation that requires compensation is to use a workflow. As the original operation proceeds, the system records information about each step and how the work performed by that step can be undone. If the operation fails at any point, the workflow rewinds back through the steps it has completed and performs the work that reverses each step.