Introduction
Blockchain is a technology that drives all the cryptocurrencies. In every one of them, a set of validator nodes are responsible for validating all the transactions. The validators are assumed to be rational and self-interested, i.e. they are only interested in making as much money possible for themselves. Under such assumptions, it is generally assumed that a required majority of the validators would agree on the sequence of transactions that have ever happened on the blockchain.
However, such blockchain validator nodes are generally expensive in terms of the size of the disk space they need. The oldest and most popular cryptocurrency, Bitcoin, for example, needs about 200 GB of disk space to store the entire transaction log. This makes it necessary to have a high-speed connection and a lot of time to even get started on mining or validation. This is a problem that prompted researchers to suggest sharding as a solution, i.e. storing only part of the log in each node, but storing the entire log as a whole. Sharding comes with its own challenges when it comes to validating transactions.