Streamlined Blockchains: A Simple and Elegant Approach (A Tutorial and Survey)

Abstract

A blockchain protocol (also called state machine replication) allows a set of nodes to agree on an ever-growing, linearly ordered log of transactions. The classical consensus literature suggests two approaches for constructing a blockchain protocol: (1) through composition of single-shot consensus instances often called Byzantine Agreement; and (2) through direct construction of a blockchain where there is no clear-cut boundary between single-shot consensus instances. While conceptually simple, the former approach precludes cross-instance optimizations in a practical implementation. This perhaps explains why the latter approach has gained more traction in practice: specifically, well-known protocols such as Paxos and PBFT all follow the direct-construction approach.

In this tutorial, we present a new paradigm called “streamlined blockchains” for directly constructing blockchain protocols. This paradigm enables a new family of protocols that are extremely simple and natural: every epoch, a proposer proposes a block extending from a notarized parent chain, and nodes vote if the proposal’s parent chain is not . Whenever a block gains votes, it becomes notarized. Whenever a node observes a notarized chain with blocks of consecutive epochs at the end, then the entire chain chopping off blocks at the end is final.

By varying the parameters highlighted in , we illustrate two variants for the partially synchronous and synchronous settings respectively. We present very simple proofs of consistency and liveness. We hope that this tutorial provides a compelling argument why this new family of protocols should be used in lieu of classical candidates (e.g., PBFT, Paxos, and their variants), both in practical implementation and for pedagogical purposes.

A Notations

Variable	Meaning
${{\mathsf{chain}}}$	Blockchain
${{\mathsf{chain}}} [:\ell ]$	Prefix of ${{\mathsf{chain}}}$ upto and including the $\ell$-th block
${{\mathsf{chain}}} [:-\ell ]$	Prefix of ${{\mathsf{chain}}}$ after removing the trailing $\ell$ blocks
$\Delta$	maximum network delay between honest nodes (during a period of synchrony)
e	Epoch number, i.e., a collection of $2\Delta$ rounds
n	Number of nodes
$h_{-1}$	Parent hash encoded in a block
${\mathsf{TXs}}$	Application-specific payload in a block, e.g., a set of transactions to confirm
H	Collision-resistant hash function for hashing blockchains
$H^*$	A random oracle for proposer election