The objective of this project is to implement your own decentralized Blockchain key-value store.

This project is a requirement to pass the course. It must be completed and submitted with all deliverables to the Montefiore submission system by the hard deadline of December 21, 2018 23:59.

You are allowed to implement the project by yourself, or in groups with a maximum of 3 students.

You are tasked with the implementation of a distributed key-value store. This data system should provide a service to store key-value pairs while guaranteeing their persistence and integrity, i.e. such that the historical content of the key-value store cannot be altered by any third party.

Your architecture should be designed around three main components:

• a top-level store component that interfaces with the user-level application. The top-level API should provide the following methods:
  • put(key, value): Stores the value with the associated key.
    • If a value for key already exists in the store, then a new version of the pair is created with the value value.
    • Please make note of the fact that the put operation doesn't necessarily delivers after it has been completed. It may therefore be a good idea to add a callback procedure (or a different mechanism) that is called whenever the key has been added to the system, or if a failure occurred.
  • retrieve(key): Searches the store for the latest value with the specified key.
  • retrieve_all(key): Retrieves all values historically recorded in the store for the specified key.
• a blockchain component that takes care of the decentralized and persistent storage.
• a broadcast component for communication between nodes.

Additional requirements:

• Your system should run in a distributed and decentralized fashion, on multiple machines.
• The store should be resilient to (one or multiple) faults.
• Your implementation must be able to handle conflicts.
• In your blockchain implementation, proof of work should have a parameterizable difficulty level.
• Your broadcast implementation should be sufficiently reliable while remaining efficient (pick wisely).
• You must provide a bootstrapping procedure for new nodes joining the network.

Beyond this coarse structure, you are free to design and implement this project in any way you consider appropriate.

• Code should be in Python, with a proper modular implementation.
• Style, documentation and unit tests will be evaluated.
• You can bootstrap your project from the stub provided in /code/. You are free to modify, remove or extend any part of it.
• We recommend network communication to be implemented through a REST API, in order to simplify development.
• Provide instructions and code for running your implementation of the key-value store, as well as for reproducing all experiments and results (including plots, if any).
• Do not over-engineer. We expect a working proof-of-concept, not a full fledged professional solution. Prefer simplicity over complexity.

Your project must comes with a written report (in PDF) that includes:

• A detailed description of your general architecture, its individual components and their interactions. Specify clearly your set of assumptions.
• A discussion of the consequences and scalability of your broadcast implementation.
• A discussion about the applicability of a blockchain to this problem.
• An experiment demonstrating the resistance of the key-value store against faults.
• An experiment showing the transaction throughput with respect to the difficulty level.
• An experiment demonstrating a 51% attack by rewriting an older version of a key.

Experimental results should all come with a critical discussion.