Consensus Backend
This section gives a detailed description of how IBFT utilizes the IBFT backend component to successfully reach consensus for a new block to be added to the blockchain.
The truth is that every blockchain (in the blockchain system) starts with a genesis block identical for all participants. This is also the case in the Blade solution. Each subsequent block is added to the blockchain using a consensus algorithm, where a set of validators agrees or disagrees with the proposed block. Besides, the data used by selected validators for adding the block varies. To ensure that IBFT always has access to the necessary data needed for creation of a new block, the IBFTBackend component is leveraged.
As already described in IBFT 2.0 Initialization, IBFT is initiated by invoking the RunSequence() method and is implemented as a state machine. The initial state used for start is newRound. In the initial state, IBFT checks whether the current node is the block proposer for the given round and height. If the current node is chosen to be the proposer for the round and height, then it creates a new block. The creation of a new block is done by the IBFTBackend, and IBFT calls the BuildProposal()method from the IBFTBackend, which returns the created block.
To ensure that the created block is known to other validators, the chosen proposer creates a PrePrepare message through which the block is sent to other validators. The creation of the PrePrepare message is also done in the IBFTBackend (by BuildPrePrepareMessage() method).
After the PrePrepare message is created, the proposer marks the new block as an accepted proposal. Other validators participating in the consensus must perform certain validations on the block as well, before marking it as an accepted proposal. The first method called in the proposed block verification is IBFTBackend's IsProposer(). This method checks whether the sender of the block is indeed the chosen proposer for the current round and height. After that, the IsValidProposalHash() method is called, which verifies whether the hash in the message matches the hash of the proposed block. To validate that the proposed block is correct, i.e., that the transactions in the block are valid, the IsValidProposal() method is used. This method executes all transactions and decides whether the sent block is valid. In the case that the round is greater than zero, during the verification of RoundChange messages, the IsValidValidator() method is called from the IBFTBackend to check whether the sender of the Round Change Certificate (RCC) is valid. The last method called from the IBFTBackend before IBFT transitions to a new state is BuildPrepareMessage(). After sending the Prepare message, IBFT transitions to the prepare state.
In the prepare state, a validator retrieves all Prepare messages from its Message storage for the corresponding height and round. Subsequently, it performs validation of these messages. For each message, the IsValidProposalHash() method is called from the IBFTBackend to verify whether the hash in the Prepare message matches the Proposal hash. If a sufficient number of Prepare messages exist, satisfying the quorum requirement, a Commit message is sent. After sending the message, IBFT transitions to a new state called commit.
Similarly to how the IBFT in the prepare state collects all Prepare messages from the Message storage, it now does the same but specifically collects Commit messages for a given round and height. Each message must undergo validation, and only valid messages are considered when checking the quorum for Commit messages. For each message, the first step is to verify whether the hash in the Commit message matches the Proposal hash. This verification is done by calling the IBFTBackend's IsValidProposalHash()method. After this verification, the next step is to check whether the signature of the Proposal hash in the CommittedSeal is indeed signed by the validator. Once the quorum for Commit messages is reached, IBFT transitions to the final state.
In the final state, IBFT adds a new block to its local blockchain and removes all messages from its Message storage that were used for previous decisions on the correctness of the block. The block is added to the blockchain by invoking the InsertProposal() method from the IBFT Backend. After successfully adding the new block to the blockchain, IBFT sends a signal on the roundDone channel, thus halting the execution of the initiated sequence.
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