SmartShards: Churn-Tolerant Continuously Available Distributed Ledger

TL;DR

SmartShards tackles cross-shard coordination and churn in shard-based blockchains by introducing overlapping shards, where every peer belongs to two shards and interacts via a shared overlap of size $x$. Cross-shard transactions are validated first in the source shard and then, upon receiving sufficient $Transfer$ notifications from the overlap, in the target shard, using PBFT within each shard and ensuring consistency across the ledger with properties $LValidity$, $LAgreement$, and $LLiveness$. The authors prove fault tolerance under per-shard faults $f$ with $f < x/2$, extend the design to weak SMS-based churn handling, and validate performance through QUANTAS simulations showing favorable throughput and resilience to churn compared with non-overlapping sharding. They also discuss extensions, including multi-source/target transactions via a consolidation phase and defense against adaptive adversaries, highlighting practical considerations for implementation such as block confirmations and SMS integration. Overall, SmartShards provides a scalable, churn-tolerant approach to distributed ledgers with improved cross-shard efficiency and robustness in dynamic networks.

Abstract

We present SmartShards: a new sharding algorithm for improving Byzantine tolerance and churn resistance in blockchains. Our algorithm places a peer in multiple shards to create an overlap. This simplifies cross-shard communication and shard membership management. We describe SmartShards, prove it correct and evaluate its performance. We propose several SmartShards extensions: defense against a slowly adaptive adversary, combining transactions into blocks, fortification against the join/leave attack.

Figures (6)

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• Figure : $SmartShards$, Variables
• Figure : $SmartShards$, Actions

Theorems & Definitions (10)

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