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CometBFT Spec

This is a markdown specification of CometBFT. It defines the base data structures, how they are validated, and how they are communicated over the network.

If you find discrepancies between the spec and the code that do not have an associated issue or pull request on github, please submit them to our bug bounty!

Contents

Data Structures

Consensus Protocol

P2P and Network Protocols

  • The Base P2P Layer: multiplex the protocols ("reactors") on authenticated and encrypted TCP connections
  • Peer Exchange (PEX): gossip known peer addresses so peers can find each other
  • Block Sync: gossip blocks so peers can catch up quickly
  • Consensus: gossip votes and block parts so new blocks can be committed
  • Mempool: gossip transactions so they get included in blocks
  • Evidence: sending invalid evidence will stop the peer

RPC

  • RPC SPEC: Specification of the CometBFT remote procedure call interface.

Software

  • ABCI: Details about interactions between the application and consensus engine over ABCI
  • Write-Ahead Log: Details about how the consensus engine preserves data and recovers from crash failures

Overview

CometBFT provides Byzantine Fault Tolerant State Machine Replication using hash-linked batches of transactions. Such transaction batches are called "blocks". Hence, CometBFT defines a "blockchain".

Each block in CometBFT has a unique index - its Height. Heights in the blockchain are monotonic. Each block is committed by a known set of weighted Validators. Membership and weighting within this validator set may change over time. CometBFT guarantees the safety and liveness of the blockchain as long as less than 1/3 of the total weight of the Validator set is malicious or faulty.

A commit in CometBFT is a set of signed messages from more than 2/3 of the total weight of the current Validator set. Validators take turns proposing blocks and voting on them. Once enough votes are received, the block is considered committed. These votes are included in the next block as proof that the previous block was committed - they cannot be included in the current block, as that block has already been created.

Once a block is committed, it can be executed against an application. The application returns results for each of the transactions in the block. The application can also return changes to be made to the validator set, as well as a cryptographic digest of its latest state.

CometBFT is designed to enable efficient verification and authentication of the latest state of the blockchain. To achieve this, it embeds cryptographic commitments to certain information in the block "header". This information includes the contents of the block (eg. the transactions), the validator set committing the block, as well as the various results returned by the application. Note, however, that block execution only occurs after a block is committed. Thus, application results can only be included in the next block.

Also note that information like the transaction results and the validator set are never directly included in the block - only their cryptographic digests (Merkle roots) are. Hence, verification of a block requires a separate data structure to store this information. We call this the State. Block verification also requires access to the previous block.