Paper 2024/1680

Sunfish: Reading Ledgers with Sparse Nodes

Abstract

Users who wish to interact with blockchains typically engage with only a small number of decentralized applications (dApps) whose state they need to monitor. However, securely and trustlessly tracking the state of even a single dApp currently requires running a full client, which independently downloads and verifies the entire blockchain, re-executes all transactions, and reconstructs the global state. Operating a full client contrasts sharply with the traditional client–server paradigm, where clients retrieve only the data they need, and becomes increasingly difficult to sustain as blockchains' throughput increases. Light clients do not offer a viable alternative: while they are more resource-efficient thanks to their use of succinct proofs, they can verify only limited information about the ledger and its state and rely on additional trust assumptions for this. As a result, a gap emerges in the blockchain client design space: enabling secure and verifiable monitoring of dApp state, isolating the workload of a given dApp from the workload of the entire chain. To bridge this gap, we introduce a sparse client, a new type of blockchain client that only downloads the transactions that modify the state of a specific dApp and only computes and stores the dApp state, isolating the dApp's workload from the one of the entire chain. We also present Sunfish, a secure sparse client protocol available in two variants: one off-the-shelf compatible with Ethereum Virtual Machine (EVM)-based blockchains and one virtually compatible with any chain. We also introduce an event client, a special case of sparse clients that only tracks a particular stream of events emitted by a dApp. We benchmark sparse and event clients against a full client by implementing prototypes for Ethereum. Our results show that our sparse and event clients respectively save 66% and 85% operating cost when compared to a full node.