As more and more blockchain systems get deployed to production, two problems are frequently encountered:
One system which addresses these two problems is Cosmos. Cosmos uses Tendermint, a high-performance BFT consensus algorithm, and the Cosmos SDK, a toolkit which enables developers to launch their own proof-of-stake blockchain on top of Tendermint.
However, Tendermint was conceived years ago, and researchers have made great strides on BFT consensus since then.
Many of us at Paradigm have been excited about the latest developments on high-throughput & low-latency consensus using directed acyclic graphs (DAGs), in particular the Narwhal mempool and the Tusk and Bullshark consensus algorithms, and beyond. (Check out these blog posts for a more gentle introduction to DAG-based consensus protocols.)
Narwhal/Bullshark (N/B) promises higher transaction throughput, and responsiveness (meaning confirmation latency of N/B is a function of the actual network delay, rather than of the network delay upper bound
Given the above advantages, we thought to ourselves: Can we replace Tendermint with Narwhal & Bullshark (N/B), while targeting compatibility with the Cosmos SDK stack?
At a recent two-day internal hackathon, we built a proof-of-concept which achieved that! For this purpose, we attached to the N/B codebase a small shim that allows it to “speak the language” (APIs) of Cosmos clients on the one hand and Cosmos applications on the other. That was enough to port over some of the simple decentralized application code examples given in the Cosmos documentation to use N/B instead of Tendermint.
You can find the proof-of-concept code here: https://github.com/gakonst/narwhal-abci-evm
To understand what exactly we did, let’s take a look at the anatomy of a Cosmos node:
A Cosmos node consists of an instance of Tendermint Core (TC) for everything related to consensus, and an instance of the application consisting of the useful logic whose execution we want to decentralize. TC offers RPC endpoints to end-user clients (e.g., for submitting transactions, or for querying the application’s state).
TC talks to the local instance of the application state machine via the Application Blockchain Interface (ABCI). For the purposes of ABCI, TC acts as a client which initiates requests, and the application acts as a server which replies with responses. A simple request/response pair is “Query”. TC uses “Query” to forward to the application any end-user queries about the application state received via the RPC.
Other important request/response pairs “deliver” the ledger of transactions that consensus has been reached upon to the application, where they are used as inputs to drive the application’s state machine. In particular, when a new block is confirmed in consensus, “BeginBlock” is called with block metadata, followed by “DeliverTx” for each transaction in the block, “EndBlock” again with block metadata, and “Commit” to persist the resulting state. Note that since all Tendermint instances reach consensus on the transaction ledger and thereby on the sequence of ABCI calls to the application, the application state machine gets replicated in lockstep across all nodes of the network.
A natural point to hook into this stack was thus to remove TC, and replace it with N/B, augmented with a shim that both provides an RPC endpoint to clients, and delivers the consensus ledger via ABCI to the application:
Indeed, after about two days of hacking, we are able to run a simple ABCI app consisting of an EVM execution environment on top of N/B, where we could issue transactions and query their outcome via TC RPC:
The demo consensus network is run by four nodes (each running on localhost), whose RPC endpoints are reachable on TCP ports 3002, 3009, 3016, and 3023, respectively. There are three accounts, Alice (initially 1.5 ETH), Bob (initially 0 ETH), and Charlie (initially 0 ETH). Alice performs a double spend, sending 1 ETH each to Bob and Charlie in two different transactions that get input to the nodes at ports 3009 and 3016, respectively. Note that only one transaction can make it. Eventually, nodes reach consensus on which transaction gets executed in Foundry's EVM, and the application state is updated in lockstep across all nodes. The update is reflected in subsequent balance queries.
We built a prototype Cosmos/ABCI application that used Narwhal/Bullshark as the consensus algorithm instead of Tendermint.
In that process, we learned that ABCI is quite Tendermint-specific, despite its aspiration to be more generic. For instance, it assumes a simple blockchain structure, with certain metadata present in the block headers (e.g., the previous block’s state root). The latest consensus protocols, however, whether they are based on multiple parallel chains or DAGs, do not fit this simple corset anymore.
To move beyond the proof-of-concept stage, more would need to be done:
Acknowledgements: Special thanks to Zaki Manian, Lefteris Kokoris-Kogias, Matt Huang, and Dan Robinson for fruitful discussions and comments on an earlier draft of this post, and to Achal Srinivasan for the beautiful diagrams.
Disclaimer: This post is for general information purposes only. It does not constitute investment advice or a recommendation or solicitation to buy or sell any investment and should not be used in the evaluation of the merits of making any investment decision. It should not be relied upon for accounting, legal or tax advice or investment recommendations. This post reflects the current opinions of the authors and is not made on behalf of Paradigm or its affiliates and does not necessarily reflect the opinions of Paradigm, its affiliates or individuals associated with Paradigm. The opinions reflected herein are subject to change without being updated.