# Tendermint Consensus Reactor

Tendermint Consensus is a distributed protocol executed by validator processes to agree on the next block to be added to the Tendermint blockchain. The protocol proceeds in rounds, where each round is a try to reach agreement on the next block. A round starts by having a dedicated process (called proposer) suggesting to other processes what should be the next block with the ProposalMessage. The processes respond by voting for a block with VoteMessage (there are two kinds of vote messages, prevote and precommit votes). Note that a proposal message is just a suggestion what the next block should be; a validator might vote with a VoteMessage for a different block. If in some round, enough number of processes vote for the same block, then this block is committed and later added to the blockchain. ProposalMessage and VoteMessage are signed by the private key of the validator. The internals of the protocol and how it ensures safety and liveness properties are explained in a forthcoming document.

For efficiency reasons, validators in Tendermint consensus protocol do not agree directly on the block as the block size is big, i.e., they don't embed the block inside Proposal and VoteMessage. Instead, they reach agreement on the BlockID (see BlockID definition in Blockchain section) that uniquely identifies each block. The block itself is disseminated to validator processes using peer-to-peer gossiping protocol. It starts by having a proposer first splitting a block into a number of block parts, that are then gossiped between processes using BlockPartMessage.

Validators in Tendermint communicate by peer-to-peer gossiping protocol. Each validator is connected only to a subset of processes called peers. By the gossiping protocol, a validator send to its peers all needed information (ProposalMessage, VoteMessage and BlockPartMessage) so they can reach agreement on some block, and also obtain the content of the chosen block (block parts). As part of the gossiping protocol, processes also send auxiliary messages that inform peers about the executed steps of the core consensus algorithm (NewRoundStepMessage and NewValidBlockMessage), and also messages that inform peers what votes the process has seen (HasVoteMessage, VoteSetMaj23Message and VoteSetBitsMessage). These messages are then used in the gossiping protocol to determine what messages a process should send to its peers.

We now describe the content of each message exchanged during Tendermint consensus protocol.

# ProposalMessage

ProposalMessage is sent when a new block is proposed. It is a suggestion of what the next block in the blockchain should be.

type ProposalMessage struct {
    Proposal Proposal
}

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# Proposal

Proposal contains height and round for which this proposal is made, BlockID as a unique identifier of proposed block, timestamp, and POLRound (a so-called Proof-of-Lock (POL) round) that is needed for termination of the consensus. If POLRound >= 0, then BlockID corresponds to the block that is locked in POLRound. The message is signed by the validator private key.

type Proposal struct {
    Height           int64
    Round            int
    POLRound         int
    BlockID          BlockID
    Timestamp        Time
    Signature        Signature
}

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# VoteMessage

VoteMessage is sent to vote for some block (or to inform others that a process does not vote in the current round). Vote is defined in the Blockchain section and contains validator's information (validator address and index), height and round for which the vote is sent, vote type, blockID if process vote for some block (nil otherwise) and a timestamp when the vote is sent. The message is signed by the validator private key.

type VoteMessage struct {
    Vote Vote
}

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# BlockPartMessage

BlockPartMessage is sent when gossipping a piece of the proposed block. It contains height, round and the block part.

type BlockPartMessage struct {
    Height int64
    Round  int
    Part   Part
}

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# NewRoundStepMessage

NewRoundStepMessage is sent for every step transition during the core consensus algorithm execution. It is used in the gossip part of the Tendermint protocol to inform peers about a current height/round/step a process is in.

type NewRoundStepMessage struct {
    Height                int64
    Round                 int
    Step                  RoundStepType
    SecondsSinceStartTime int
    LastCommitRound       int
}

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# NewValidBlockMessage

NewValidBlockMessage is sent when a validator observes a valid block B in some round r, i.e., there is a Proposal for block B and 2/3+ prevotes for the block B in the round r. It contains height and round in which valid block is observed, block parts header that describes the valid block and is used to obtain all block parts, and a bit array of the block parts a process currently has, so its peers can know what parts it is missing so they can send them. In case the block is also committed, then IsCommit flag is set to true.

type NewValidBlockMessage struct {
    Height           int64
    Round            int    
    BlockPartsHeader PartSetHeader
    BlockParts       BitArray
    IsCommit         bool
}

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# ProposalPOLMessage

ProposalPOLMessage is sent when a previous block is re-proposed. It is used to inform peers in what round the process learned for this block (ProposalPOLRound), and what prevotes for the re-proposed block the process has.

type ProposalPOLMessage struct {
    Height           int64
    ProposalPOLRound int
    ProposalPOL      BitArray
}

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# HasVoteMessage

HasVoteMessage is sent to indicate that a particular vote has been received. It contains height, round, vote type and the index of the validator that is the originator of the corresponding vote.

type HasVoteMessage struct {
    Height int64
    Round  int
    Type   byte
    Index  int
}

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# VoteSetMaj23Message

VoteSetMaj23Message is sent to indicate that a process has seen +2/3 votes for some BlockID. It contains height, round, vote type and the BlockID.

type VoteSetMaj23Message struct {
    Height  int64
    Round   int
    Type    byte
    BlockID BlockID
}

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# VoteSetBitsMessage

VoteSetBitsMessage is sent to communicate the bit-array of votes a process has seen for a given BlockID. It contains height, round, vote type, BlockID and a bit array of the votes a process has.

type VoteSetBitsMessage struct {
    Height  int64
    Round   int
    Type    byte
    BlockID BlockID
    Votes   BitArray
}

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