While blockchain is fundamentally a storage solution, it principally differs from a database [5,6]. A database is an organized collection of data representing the current system state. The main functionality of a database is to allow efficient data retrieval, fusion, and aggregation triggered by user queries. In contrast, most blockchain implementations represent a ledger in which a history of transactions (or, more generally, of changes to the system state) is recorded. For example, there is simply no concept of a user balance in Bitcoin! While Ethereum keeps track of a contract state, it only provides limited means to retrieve and process state data, as explicitly defined by the contract. It does not support abstractions of a flexible query language, data view, schema, join, etc. Besides, Ethereum records a history of all changes to the state, which results in blockchain space being more expensive and the storage less efficient compared to a database.
As a result, only specific data elements (such as short transactions or indices) are stored on a blockchain. Most blockchain-based applications combine blockchain with offchain storage (databases or dedicated file systems). There are many implementations that support such a combination, e.g., StorJ [2], Filecoin [3], BigchainDB [4], etc. IPFS [1] is often mentioned as a solution for off-chain storage that lends itself to integration with blockchain storage in such combined systems.
The goal of this project is to investigate and compare how off-chain storage is combined with blockchain storage in such systems. What is the typical ratio between them in terms of the amount space consumed? What else (apart from short transaction and indices) can be stored on blockchain and how it affects the performance? The student will need both to study the design of these systems based on the white papers and documentation available, and actually run these system to get a first-hand experience.
References:
[1] IPFS - Content Addressed, Versioned, P2P File System (DRAFT 3)
https://ipfs.io/ipfs/.../ipfs.draft3.pdf
[2] Storj - whitepaper
https://storj.io/storj.pdf
[3] Filecoin - whitepaper
https://filecoin.io/filecoin.pdf
[4] BigchainDB - whitepaper
https://www.bigchaindb.com/whitepaper/
[5] Debunking blockchain myths by Roman Vitenberg
heim.ifi.uio.no/~romanvi/debunking-bc-myths.pdf
[6] Deconstructing Blockchains: Concepts, Systems, and Insights by Kaiwen Zhang, Roman Vitenberg, Hans-Arno Jacobsen
http://folk.uio.no/romanvi/Papers/bc-tutorial-debs-master.pdf