Replicated Key Value storage using Android

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This was my fourth project in Android. This project involves designing a simple distributed replicated-value storage based on Dynamo. It is a simplified version of Dynamo, which includes partitioning, replicating and handling failures. The Simple Dynamo is built on top of the android content provider.

 System Architecture

     Design Overview

The design is pretty simple as seen from the above diagram. The Dynamo (Content Provider) has a server thread and client threads which communicate by sending messages. Quorum manager at the client side will check whether reader quorum size and writer quorum size is satisfied in the respective cases. There are different types of message for Put, insert, query, recovery etc, which simplifies the application logic.

Handling Failures

Case: Coordinator fails
   
    If the coordinator node fails, then its first successor is contacted to see if the operation can be     continued with the first successor acting as a make shift coordinator; if at this point, even the first successor has failed, then we proceed to contact the second successor which will now take on the role of make shift coordinator. If this whole process fails, we retry after a reasonable delay.


Case: Successor fails




Since only one node failure is assumed in the system at any given point in time, the quorum for read/write will be satisfied by getting two votes, one from the coordinator and the other from a successor, and the operation will continue seamlessly.


Node Recovery

Case: Get values from first predecessor and first successor | second successor
Case: Get values from second predecessor | second predecessor and first successor

In either of the above case the system can recover all its values successfully. These are the ideal cases given our system constraints. 

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