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Creating a proposal

A block consists of a header, transactions, votes (the commit), and a list of evidence of malfeasance (eg. signing conflicting votes). Outstanding evidence items get priority over outstanding transactions in the mempool. All in all, the block MUST NOT exceed ConsensusParams.Block.MaxBytes, or 100MB if ConsensusParams.Block.MaxBytes == -1.

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Reaping transactions from the mempool

When we reap transactions from the mempool, we calculate maximum data size by subtracting maximum header size (MaxHeaderBytes), the maximum protobuf overhead for a block (MaxOverheadForBlock), the size of the last commit (if present) and evidence (if present). While reaping we account for protobuf overhead for each transaction. 
func MaxDataBytes(maxBytes, evidenceBytes int64, valsCount int) int64 {
  return maxBytes -
  MaxOverheadForBlock -
  MaxHeaderBytes -
  MaxCommitBytes(valsCount) -
  evidenceBytes
}
If ConsensusParams.Block.MaxBytes == -1, we reap all outstanding transactions from the mempool

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Preparing the proposal

Once the transactions have been reaped from the mempool according to the rules described above, CometBFT calls PrepareProposal to the application with the transaction list that has just been reaped. As part of this call the application can remove, add, or reorder transactions in the transaction list. The RequestPrepareProposal contains two important fields:
  • MaxTxBytes, which contains the value returned by MaxDataBytes described above. The application MUST NOT return a list of transactions whose size exceeds this number.
  • Txs, which contains the list of reaped transactions.
For more details on PrepareProposal, please see the relevant part of the spec

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Validating transactions in the mempool

Before we accept a transaction in the mempool, we check if its size is no more than . is calculated using the same formula as above, except we assume there is no evidence. 
func MaxDataBytesNoEvidence(maxBytes int64, valsCount int) int64 {
  return maxBytes -
    MaxOverheadForBlock -
    MaxHeaderBytes -
    MaxCommitBytes(valsCount)
}