tendermint/spec/abci++/v4.md

# Tendermint v4 Markdown pseudocode

This is a multi-threaded implementation of ABCI++,
where ProcessProposal starts when the proposal is received, but ends before precommitting.

### Initialization

```go
h_p ← 0
round_p ← 0
step_p is one of {propose, prevote, precommit}
decision_p ← Vector()
lockedValue_p ← nil
validValue_p ← nil
validRound_p ← -1
```

### StartRound(round)

```go
function startRound(round) {
    round_p ← round
    step_p ← propose
    if proposer(h_p, round_p) = p {
        if validValue_p != nil {
            proposal ← validValue_p
        } else {
            txdata ← mempool.GetBlock()
            // getUnpreparedBlockProposal fills in header
            unpreparedProposal ← getUnpreparedBlockProposal(txdata)
            proposal ← ABCI.PrepareProposal(unpreparedProposal)
        }
        broadcast ⟨PROPOSAL, h_p, round_p, proposal, validRound_p⟩
    } else {
        schedule OnTimeoutPropose(h_p,round_p) to be executed after timeoutPropose(round_p)
    }
}
```

### ReceiveProposal

In the case where the local node is not locked on any round, the following is ran:

```go
upon ⟨PROPOSAL, h_p, round_p, v, −1) from proposer(h_p, round_p) while step_p = propose do {
    if valid(v) ∧ ABCI.VerifyHeader(h_p, v.header) ∧ (lockedRound_p = −1 ∨ lockedValue_p = v) {
        // We fork process proposal into a parallel process
        Fork ABCI.ProcessProposal(h_p, v)
        broadcast ⟨PREVOTE, h_p, round_p, id(v)⟩ 
    } else {
        broadcast ⟨PREVOTE, h_p, round_p, nil⟩ 
    }
    step_p ← prevote
}
```

In the case where the node is locked on a round, the following is ran:

```go
upon ⟨PROPOSAL, h_p, round_p, v, vr⟩
  from proposer(h_p, round_p)
  AND 2f + 1 ⟨PREVOTE, h_p, vr, id(v)⟩ 
  while step_p = propose ∧ (vr ≥ 0 ∧ vr < round_p) do {
    if valid(v) ∧ ABCI.VerifyHeader(h_p, v.header) ∧ (lockedRound_p ≤ vr ∨ lockedValue_p = v) {
        // We fork process proposal into a parallel process
        Fork ABCI.ProcessProposal(h_p, v)
        broadcast ⟨PREVOTE, h_p, round_p, id(v)⟩
    } else {
        broadcast ⟨PREVOTE, h_p, round_p, nil⟩
    }
    step_p ← prevote
}
```

### Prevote timeout

Upon receiving 2f + 1 prevotes, setup a timeout.

```go
upon 2f + 1 ⟨PREVOTE, h_p, vr, -1⟩ 
  with step_p = prevote for the first time, do {
    schedule OnTimeoutPrevote(h_p, round_p) to be executed after timeoutPrevote(round_p)
}
```

with OnTimeoutPrevote defined as:

```go
def OnTimeoutPrevote(height, round) {
    if (height = h_p && round = round_p && step_p = prevote) {
        // Join the ProcessProposal, and output any evidence in case it has some.
        processProposalOutput ← Join ABCI.ProcessProposal(h_p, v)
        for evidence in processProposalOutput.evidence_list {
            broadcast ⟨EVIDENCE, evidence⟩ 
        }

        precommit_extension ← ABCI.ExtendVote(h_p, round_p, nil)
        broadcast ⟨PRECOMMIT, h_p, round_p, nil, precommit_extension⟩
        step_p ← precommit
    }
}
```

### Receiving enough prevotes to precommit

The following code is ran upon receiving 2f + 1 prevotes for the same block

```go
upon ⟨PROPOSAL, h_p, round_p, v, *⟩
  from proposer(h_p, round_p)
  AND 2f + 1 ⟨PREVOTE, h_p, vr, id(v)⟩ 
while valid(v) ∧ step_p >= prevote for the first time do {
    if (step_p = prevote) {
        lockedValue_p ← v
        lockedRound_p ← round_p
        processProposalOutput ← Join ABCI.ProcessProposal(h_p, v)
        // If the proposal is valid precommit as before.
        // If it was invalid, precommit nil.
        // Note that ABCI.ProcessProposal(h_p, v).accept is deterministic for all honest nodes.
        precommit_value ← nil
        if processProposalOutput.accept {
            precommit_value ← id(v)
        }
        precommit_extension ← ABCI.ExtendVote(h_p, round_p, precommit_value)
        broadcast ⟨PRECOMMIT, h_p, round_p, precommit_value, precommit_extension⟩
        for evidence in processProposalOutput.evidence_list {
            broadcast ⟨EVIDENCE, evidence⟩ 
        }

        step_p ← precommit
    }
    validValue_p ← v
    validRound_p ← round_p
}
```

And upon receiving 2f + 1 prevotes for nil:

```go
upon 2f + 1 ⟨PREVOTE, h_p, round_p, nil⟩ 
  while step_p = prevote do {
    // Join ABCI.ProcessProposal, and broadcast any evidence if it exists.
    processProposalOutput ← Join ABCI.ProcessProposal(h_p, v)
    for evidence in processProposalOutput.evidence_list {
        broadcast ⟨EVIDENCE, evidence⟩ 
    }

    precommit_extension ← ABCI.ExtendVote(h_p, round_p, nil)
    broadcast ⟨PRECOMMIT, h_p, round_p, nil, precommit_extension⟩
    step_p ← precommit
}
```

### Upon receiving a precommit

Upon receiving a precommit `precommit`, we ensure that `ABCI.VerifyVoteExtension(precommit.precommit_extension) = true`
before accepting the precommit. This is akin to how we check the signature on precommits normally, hence its not wrapped
in the syntax of methods from the paper.

### Precommit timeout

Upon receiving 2f + 1 precommits, setup a timeout.

```go
upon 2f + 1 ⟨PRECOMMIT, h_p, vr, *⟩ for the first time, do {
    schedule OnTimeoutPrecommit(h_p, round_p) to be executed after timeoutPrecommit(round_p)
}
```

with OnTimeoutPrecommit defined as:

```go
def OnTimeoutPrecommit(height, round) {
    if (height = h_p && round = round_p) {
        StartRound(round_p + 1)
    }
}
```

### Upon Receiving 2f + 1 precommits

The following code is ran upon receiving 2f + 1 precommits for the same block

```go
upon ⟨PROPOSAL, h_p, r, v, *⟩
  from proposer(h_p, r)
  AND 2f + 1 ⟨ PRECOMMIT, h_p, r, id(v)⟩ 
  while decision_p[h_p] = nil do {
    if (valid(v)) {
        decision_p[h_p] ← v
        h_p ← h_p + 1
        reset lockedRound_p, lockedValue_p,validRound_p and validValue_p to initial values
        ABCI.FinalizeBlock(id(v))
        StartRound(0)
    }
}
```

If we don't see 2f + 1 precommits for the same block, we wait until we get 2f + 1 precommits, and the timeout occurs.