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# Consensus Reactor
Consensus Reactor defines a reactor for the consensus service. It contains the ConsensusState service that
manages the state of the Tendermint consensus internal state machine.
When Consensus Reactor is started, it starts Broadcast Routine which starts ConsensusState service.
Furthermore, for each peer that is added to the Consensus Reactor, it creates (and manages) the known peer state
(that is used extensively in gossip routines) and starts the following three routines for the peer p:
Gossip Data Routine, Gossip Votes Routine and QueryMaj23Routine. Finally, Consensus Reactor is responsible
for decoding messages received from a peer and for adequate processing of the message depending on its type and content.
The processing normally consists of updating the known peer state and for some messages
(`ProposalMessage`, `BlockPartMessage` and `VoteMessage`) also forwarding message to ConsensusState module
for further processing. In the following text we specify the core functionality of those separate unit of executions
that are part of the Consensus Reactor.
## ConsensusState service
Consensus State handles execution of the Tendermint BFT consensus algorithm. It processes votes and proposals,
and upon reaching agreement, commits blocks to the chain and executes them against the application.
The internal state machine receives input from peers, the internal validator and from a timer.
Inside Consensus State we have the following units of execution: Timeout Ticker and Receive Routine.
Timeout Ticker is a timer that schedules timeouts conditional on the height/round/step that are processed
by the Receive Routine.
### Receive Routine of the ConsensusState service
Receive Routine of the ConsensusState handles messages which may cause internal consensus state transitions.
It is the only routine that updates RoundState that contains internal consensus state.
Updates (state transitions) happen on timeouts, complete proposals, and 2/3 majorities.
It receives messages from peers, internal validators and from Timeout Ticker
and invokes the corresponding handlers, potentially updating the RoundState.
The details of the protocol (together with formal proofs of correctness) implemented by the Receive Routine are
discussed in separate document. For understanding of this document
it is sufficient to understand that the Receive Routine manages and updates RoundState data structure that is
then extensively used by the gossip routines to determine what information should be sent to peer processes.
## Round State
RoundState defines the internal consensus state. It contains height, round, round step, a current validator set,
a proposal and proposal block for the current round, locked round and block (if some block is being locked), set of
received votes and last commit and last validators set.
```golang
type RoundState struct {
Height int64
Round int
Step RoundStepType
Validators ValidatorSet
Proposal Proposal
ProposalBlock Block
ProposalBlockParts PartSet
LockedRound int
LockedBlock Block
LockedBlockParts PartSet
Votes HeightVoteSet
LastCommit VoteSet
LastValidators ValidatorSet
}
```
Internally, consensus will run as a state machine with the following states:
- RoundStepNewHeight
- RoundStepNewRound
- RoundStepPropose
- RoundStepProposeWait
- RoundStepPrevote
- RoundStepPrevoteWait
- RoundStepPrecommit
- RoundStepPrecommitWait
- RoundStepCommit
## Peer Round State
Peer round state contains the known state of a peer. It is being updated by the Receive routine of
Consensus Reactor and by the gossip routines upon sending a message to the peer.
```golang
type PeerRoundState struct {
Height int64 // Height peer is at
Round int // Round peer is at, -1 if unknown.
Step RoundStepType // Step peer is at
Proposal bool // True if peer has proposal for this round
ProposalBlockPartsHeader PartSetHeader
ProposalBlockParts BitArray
ProposalPOLRound int // Proposal's POL round. -1 if none.
ProposalPOL BitArray // nil until ProposalPOLMessage received.
Prevotes BitArray // All votes peer has for this round
Precommits BitArray // All precommits peer has for this round
LastCommitRound int // Round of commit for last height. -1 if none.
LastCommit BitArray // All commit precommits of commit for last height.
CatchupCommitRound int // Round that we have commit for. Not necessarily unique. -1 if none.
CatchupCommit BitArray // All commit precommits peer has for this height & CatchupCommitRound
}
```
## Receive method of Consensus reactor
The entry point of the Consensus reactor is a receive method. When a message is received from a peer p,
normally the peer round state is updated correspondingly, and some messages
are passed for further processing, for example to ConsensusState service. We now specify the processing of messages
in the receive method of Consensus reactor for each message type. In the following message handler, `rs` and `prs` denote
`RoundState` and `PeerRoundState`, respectively.
### NewRoundStepMessage handler
```
handleMessage(msg):
if msg is from smaller height/round/step then return
// Just remember these values.
prsHeight = prs.Height
prsRound = prs.Round
prsCatchupCommitRound = prs.CatchupCommitRound
prsCatchupCommit = prs.CatchupCommit
Update prs with values from msg
if prs.Height or prs.Round has been updated then
reset Proposal related fields of the peer state
if prs.Round has been updated and msg.Round == prsCatchupCommitRound then
prs.Precommits = psCatchupCommit
if prs.Height has been updated then
if prsHeight+1 == msg.Height && prsRound == msg.LastCommitRound then
prs.LastCommitRound = msg.LastCommitRound
prs.LastCommit = prs.Precommits
} else {
prs.LastCommitRound = msg.LastCommitRound
prs.LastCommit = nil
}
Reset prs.CatchupCommitRound and prs.CatchupCommit
```
### CommitStepMessage handler
```
handleMessage(msg):
if prs.Height == msg.Height then
prs.ProposalBlockPartsHeader = msg.BlockPartsHeader
prs.ProposalBlockParts = msg.BlockParts
```
### HasVoteMessage handler
```
handleMessage(msg):
if prs.Height == msg.Height then
prs.setHasVote(msg.Height, msg.Round, msg.Type, msg.Index)
```
### VoteSetMaj23Message handler
```
handleMessage(msg):
if prs.Height == msg.Height then
Record in rs that a peer claim to have ⅔ majority for msg.BlockID
Send VoteSetBitsMessage showing votes node has for that BlockId
```
### ProposalMessage handler
```
handleMessage(msg):
if prs.Height != msg.Height || prs.Round != msg.Round || prs.Proposal then return
prs.Proposal = true
prs.ProposalBlockPartsHeader = msg.BlockPartsHeader
prs.ProposalBlockParts = empty set
prs.ProposalPOLRound = msg.POLRound
prs.ProposalPOL = nil
Send msg through internal peerMsgQueue to ConsensusState service
```
### ProposalPOLMessage handler
```
handleMessage(msg):
if prs.Height != msg.Height or prs.ProposalPOLRound != msg.ProposalPOLRound then return
prs.ProposalPOL = msg.ProposalPOL
```
### BlockPartMessage handler
```
handleMessage(msg):
if prs.Height != msg.Height || prs.Round != msg.Round then return
Record in prs that peer has block part msg.Part.Index
Send msg trough internal peerMsgQueue to ConsensusState service
```
### VoteMessage handler
```
handleMessage(msg):
Record in prs that a peer knows vote with index msg.vote.ValidatorIndex for particular height and round
Send msg trough internal peerMsgQueue to ConsensusState service
```
### VoteSetBitsMessage handler
```
handleMessage(msg):
Update prs for the bit-array of votes peer claims to have for the msg.BlockID
```
## Gossip Data Routine
It is used to send the following messages to the peer: `BlockPartMessage`, `ProposalMessage` and
`ProposalPOLMessage` on the DataChannel. The gossip data routine is based on the local RoundState (`rs`)
and the known PeerRoundState (`prs`). The routine repeats forever the logic shown below:
```
1a) if rs.ProposalBlockPartsHeader == prs.ProposalBlockPartsHeader and the peer does not have all the proposal parts then
Part = pick a random proposal block part the peer does not have
Send BlockPartMessage(rs.Height, rs.Round, Part) to the peer on the DataChannel
if send returns true, record that the peer knows the corresponding block Part
Continue
1b) if (0 < prs.Height) and (prs.Height < rs.Height) then
help peer catch up using gossipDataForCatchup function
Continue
1c) if (rs.Height != prs.Height) or (rs.Round != prs.Round) then
Sleep PeerGossipSleepDuration
Continue
// at this point rs.Height == prs.Height and rs.Round == prs.Round
1d) if (rs.Proposal != nil and !prs.Proposal) then
Send ProposalMessage(rs.Proposal) to the peer
if send returns true, record that the peer knows Proposal
if 0 <= rs.Proposal.POLRound then
polRound = rs.Proposal.POLRound
prevotesBitArray = rs.Votes.Prevotes(polRound).BitArray()
Send ProposalPOLMessage(rs.Height, polRound, prevotesBitArray)
Continue
2) Sleep PeerGossipSleepDuration
```
### Gossip Data For Catchup
This function is responsible for helping peer catch up if it is at the smaller height (prs.Height < rs.Height).
The function executes the following logic:
if peer does not have all block parts for prs.ProposalBlockPart then
blockMeta = Load Block Metadata for height prs.Height from blockStore
if (!blockMeta.BlockID.PartsHeader == prs.ProposalBlockPartsHeader) then
Sleep PeerGossipSleepDuration
return
Part = pick a random proposal block part the peer does not have
Send BlockPartMessage(prs.Height, prs.Round, Part) to the peer on the DataChannel
if send returns true, record that the peer knows the corresponding block Part
return
else Sleep PeerGossipSleepDuration
## Gossip Votes Routine
It is used to send the following message: `VoteMessage` on the VoteChannel.
The gossip votes routine is based on the local RoundState (`rs`)
and the known PeerRoundState (`prs`). The routine repeats forever the logic shown below:
```
1a) if rs.Height == prs.Height then
if prs.Step == RoundStepNewHeight then
vote = random vote from rs.LastCommit the peer does not have
Send VoteMessage(vote) to the peer
if send returns true, continue
if prs.Step <= RoundStepPrevote and prs.Round != -1 and prs.Round <= rs.Round then
Prevotes = rs.Votes.Prevotes(prs.Round)
vote = random vote from Prevotes the peer does not have
Send VoteMessage(vote) to the peer
if send returns true, continue
if prs.Step <= RoundStepPrecommit and prs.Round != -1 and prs.Round <= rs.Round then
Precommits = rs.Votes.Precommits(prs.Round)
vote = random vote from Precommits the peer does not have
Send VoteMessage(vote) to the peer
if send returns true, continue
if prs.ProposalPOLRound != -1 then
PolPrevotes = rs.Votes.Prevotes(prs.ProposalPOLRound)
vote = random vote from PolPrevotes the peer does not have
Send VoteMessage(vote) to the peer
if send returns true, continue
1b) if prs.Height != 0 and rs.Height == prs.Height+1 then
vote = random vote from rs.LastCommit peer does not have
Send VoteMessage(vote) to the peer
if send returns true, continue
1c) if prs.Height != 0 and rs.Height >= prs.Height+2 then
Commit = get commit from BlockStore for prs.Height
vote = random vote from Commit the peer does not have
Send VoteMessage(vote) to the peer
if send returns true, continue
2) Sleep PeerGossipSleepDuration
```
## QueryMaj23Routine
It is used to send the following message: `VoteSetMaj23Message`. `VoteSetMaj23Message` is sent to indicate that a given
BlockID has seen +2/3 votes. This routine is based on the local RoundState (`rs`) and the known PeerRoundState
(`prs`). The routine repeats forever the logic shown below.
```
1a) if rs.Height == prs.Height then
Prevotes = rs.Votes.Prevotes(prs.Round)
if there is a ⅔ majority for some blockId in Prevotes then
m = VoteSetMaj23Message(prs.Height, prs.Round, Prevote, blockId)
Send m to peer
Sleep PeerQueryMaj23SleepDuration
1b) if rs.Height == prs.Height then
Precommits = rs.Votes.Precommits(prs.Round)
if there is a ⅔ majority for some blockId in Precommits then
m = VoteSetMaj23Message(prs.Height,prs.Round,Precommit,blockId)
Send m to peer
Sleep PeerQueryMaj23SleepDuration
1c) if rs.Height == prs.Height and prs.ProposalPOLRound >= 0 then
Prevotes = rs.Votes.Prevotes(prs.ProposalPOLRound)
if there is a ⅔ majority for some blockId in Prevotes then
m = VoteSetMaj23Message(prs.Height,prs.ProposalPOLRound,Prevotes,blockId)
Send m to peer
Sleep PeerQueryMaj23SleepDuration
1d) if prs.CatchupCommitRound != -1 and 0 < prs.Height and
prs.Height <= blockStore.Height() then
Commit = LoadCommit(prs.Height)
m = VoteSetMaj23Message(prs.Height,Commit.Round,Precommit,Commit.blockId)
Send m to peer
Sleep PeerQueryMaj23SleepDuration
2) Sleep PeerQueryMaj23SleepDuration
```
## Broadcast routine
The Broadcast routine subscribes to an internal event bus to receive new round steps, votes messages and proposal
heartbeat messages, and broadcasts messages to peers upon receiving those events.
It broadcasts `NewRoundStepMessage` or `CommitStepMessage` upon new round state event. Note that
broadcasting these messages does not depend on the PeerRoundState; it is sent on the StateChannel.
Upon receiving VoteMessage it broadcasts `HasVoteMessage` message to its peers on the StateChannel.
`ProposalHeartbeatMessage` is sent the same way on the StateChannel.

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# 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](blockchain.md) 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 `CommitStepMessage`), 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.
```go
type ProposalMessage struct {
Proposal Proposal
}
```
### Proposal
Proposal contains height and round for which this proposal is made, BlockID as a unique identifier
of proposed block, timestamp, and two fields (POLRound and POLBlockID) that are needed for
termination of the consensus. The message is signed by the validator private key.
```go
type Proposal struct {
Height int64
Round int
Timestamp Time
BlockID BlockID
POLRound int
POLBlockID BlockID
Signature Signature
}
```
NOTE: In the current version of the Tendermint, the consensus value in proposal is represented with
PartSetHeader, and with BlockID in vote message. It should be aligned as suggested in this spec as
BlockID contains PartSetHeader.
## 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 [Blockchain](blockchain.md) 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.
```go
type VoteMessage struct {
Vote Vote
}
```
## BlockPartMessage
BlockPartMessage is sent when gossipping a piece of the proposed block. It contains height, round
and the block part.
```go
type BlockPartMessage struct {
Height int64
Round int
Part Part
}
```
## ProposalHeartbeatMessage
ProposalHeartbeatMessage is sent to signal that a node is alive and waiting for transactions
to be able to create a next block proposal.
```go
type ProposalHeartbeatMessage struct {
Heartbeat Heartbeat
}
```
### Heartbeat
Heartbeat contains validator information (address and index),
height, round and sequence number. It is signed by the private key of the validator.
```go
type Heartbeat struct {
ValidatorAddress []byte
ValidatorIndex int
Height int64
Round int
Sequence int
Signature Signature
}
```
## 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.
```go
type NewRoundStepMessage struct {
Height int64
Round int
Step RoundStepType
SecondsSinceStartTime int
LastCommitRound int
}
```
## CommitStepMessage
CommitStepMessage is sent when an agreement on some block is reached. It contains height for which
agreement is reached, block parts header that describes the decided 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.
```go
type CommitStepMessage struct {
Height int64
BlockID BlockID
BlockParts BitArray
}
```
TODO: We use BlockID instead of BlockPartsHeader (in current implementation) for symmetry.
## 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.
```go
type ProposalPOLMessage struct {
Height int64
ProposalPOLRound int
ProposalPOL BitArray
}
```
## 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.
```go
type HasVoteMessage struct {
Height int64
Round int
Type byte
Index int
}
```
## 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.
```go
type VoteSetMaj23Message struct {
Height int64
Round int
Type byte
BlockID BlockID
}
```
## 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.
```go
type VoteSetBitsMessage struct {
Height int64
Round int
Type byte
BlockID BlockID
Votes BitArray
}
```

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# Proposer selection procedure in Tendermint
This document specifies the Proposer Selection Procedure that is used in Tendermint to choose a round proposer.
As Tendermint is “leader-based protocol”, the proposer selection is critical for its correct functioning.
Let denote with `proposer_p(h,r)` a process returned by the Proposer Selection Procedure at the process p, at height h
and round r. Then the Proposer Selection procedure should fulfill the following properties:
`Agreement`: Given a validator set V, and two honest validators,
p and q, for each height h, and each round r,
proposer_p(h,r) = proposer_q(h,r)
`Liveness`: In every consecutive sequence of rounds of size K (K is system parameter), at least a
single round has an honest proposer.
`Fairness`: The proposer selection is proportional to the validator voting power, i.e., a validator with more
voting power is selected more frequently, proportional to its power. More precisely, given a set of processes
with the total voting power N, during a sequence of rounds of size N, every process is proposer in a number of rounds
equal to its voting power.
We now look at a few particular cases to understand better how fairness should be implemented.
If we have 4 processes with the following voting power distribution (p0,4), (p1, 2), (p2, 2), (p3, 2) at some round r,
we have the following sequence of proposer selections in the following rounds:
`p0, p1, p2, p3, p0, p0, p1, p2, p3, p0, p0, p1, p2, p3, p0, p0, p1, p2, p3, p0, etc`
Let consider now the following scenario where a total voting power of faulty processes is aggregated in a single process
p0: (p0,3), (p1, 1), (p2, 1), (p3, 1), (p4, 1), (p5, 1), (p6, 1), (p7, 1).
In this case the sequence of proposer selections looks like this:
`p0, p1, p2, p3, p0, p4, p5, p6, p7, p0, p0, p1, p2, p3, p0, p4, p5, p6, p7, p0, etc`
In this case, we see that a number of rounds coordinated by a faulty process is proportional to its voting power.
We consider also the case where we have voting power uniformly distributed among processes, i.e., we have 10 processes
each with voting power of 1. And let consider that there are 3 faulty processes with consecutive addresses,
for example the first 3 processes are faulty. Then the sequence looks like this:
`p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, etc`
In this case, we have 3 consecutive rounds with a faulty proposer.
One special case we consider is the case where a single honest process p0 has most of the voting power, for example:
(p0,100), (p1, 2), (p2, 3), (p3, 4). Then the sequence of proposer selection looks like this:
p0, p0, p0, p0, p0, p0, p0, p0, p0, p0, p0, p0, p0, p1, p0, p0, p0, p0, p0, etc
This basically means that almost all rounds have the same proposer. But in this case, the process p0 has anyway enough
voting power to decide whatever he wants, so the fact that he coordinates almost all rounds seems correct.