tendermint/docs/app-dev/abci-spec.md

ABCI Specification

Message Types

ABCI requests/responses are defined as simple Protobuf messages in this schema file. TendermintCore sends the requests, and the ABCI application sends the responses. Here, we provide an overview of the messages types and how they are used by Tendermint. Then we describe each request-response pair as a function with arguments and return values, and add some notes on usage.

Some messages (Echo, Info, InitChain, BeginBlock, EndBlock, Commit), don't return errors because an error would indicate a critical failure in the application and there's nothing Tendermint can do. The problem should be addressed and both Tendermint and the application restarted. All other messages (SetOption, Query, CheckTx, DeliverTx) return an application-specific response Code uint32, where only 0 is reserved for OK.

Some messages (SetOption, Query, CheckTx, DeliverTx) return non-deterministic data in the form of Info and Log. The Log is intended for the literal output from the application's logger, while the Info is any additional info that should be returned.

The first time a new blockchain is started, Tendermint calls InitChain. From then on, the Block Execution Sequence that causes the committed state to be updated is as follows:

BeginBlock, [DeliverTx], EndBlock, Commit

where one DeliverTx is called for each transaction in the block. Cryptographic commitments to the results of DeliverTx, EndBlock, and Commit are included in the header of the next block.

Tendermint opens three connections to the application to handle the different message types:

• Consensus Connection - InitChain, BeginBlock, DeliverTx, EndBlock, Commit
• Mempool Connection - CheckTx
• Info Connection - Info, SetOption, Query

The Flush message is used on every connection, and the Echo message is only used for debugging.

Note that messages may be sent concurrently across all connections -a typical application will thus maintain a distinct state for each connection. They may be referred to as the DeliverTx state, the CheckTx state, and the Commit state respectively.

See below for more details on the message types and how they are used.

Request/Response Messages

Echo

• Request:
  • Message (string): A string to echo back
• Response:
  • Message (string): The input string
• Usage:
  • Echo a string to test an abci client/server implementation

Flush

• Usage:
  • Signals that messages queued on the client should be flushed to the server. It is called periodically by the client implementation to ensure asynchronous requests are actually sent, and is called immediately to make a synchronous request, which returns when the Flush response comes back.

Info

• Request:
  • Version (string): The Tendermint version
• Response:
  • Data (string): Some arbitrary information
  • Version (Version): Version information
  • LastBlockHeight (int64): Latest block for which the app has called Commit
  • LastBlockAppHash ([]byte): Latest result of Commit
• Usage:
  • Return information about the application state.
  • Used to sync Tendermint with the application during a handshake that happens on startup.
  • Tendermint expects LastBlockAppHash and LastBlockHeight to be updated during Commit, ensuring that Commit is never called twice for the same block height.

SetOption

• Request:
  • Key (string): Key to set
  • Value (string): Value to set for key
• Response:
  • Code (uint32): Response code
  • Log (string): The output of the application's logger. May be non-deterministic.
  • Info (string): Additional information. May be non-deterministic.
• Usage:
  • Set non-consensus critical application specific options.
  • e.g. Key="min-fee", Value="100fermion" could set the minimum fee required for CheckTx (but not DeliverTx - that would be consensus critical).

InitChain

• Request:
  • Validators ([]Validator): Initial genesis validators
  • AppStateBytes ([]byte): Serialized initial application state
• Response:
  • ConsensusParams (ConsensusParams): Initial consensus-critical parameters.
  • Validators ([]Validator): Initial validator set.
• Usage:
  • Called once upon genesis.

Query

• Request:
  • Data ([]byte): Raw query bytes. Can be used with or in lieu of Path.
  • Path (string): Path of request, like an HTTP GET path. Can be used with or in liue of Data.
  • Apps MUST interpret '/store' as a query by key on the underlying store. The key SHOULD be specified in the Data field.
  • Apps SHOULD allow queries over specific types like '/accounts/...' or '/votes/...'
  • Height (int64): The block height for which you want the query (default=0 returns data for the latest committed block). Note that this is the height of the block containing the application's Merkle root hash, which represents the state as it was after committing the block at Height-1
  • Prove (bool): Return Merkle proof with response if possible
• Response:
  • Code (uint32): Response code.
  • Log (string): The output of the application's logger. May be non-deterministic.
  • Info (string): Additional information. May be non-deterministic.
  • Index (int64): The index of the key in the tree.
  • Key ([]byte): The key of the matching data.
  • Value ([]byte): The value of the matching data.
  • Proof ([]byte): Proof for the data, if requested.
  • Height (int64): The block height from which data was derived. Note that this is the height of the block containing the application's Merkle root hash, which represents the state as it was after committing the block at Height-1
• Usage:
  • Query for data from the application at current or past height.
  • Optionally return Merkle proof.

BeginBlock

• Request:
  • Hash ([]byte): The block's hash. This can be derived from the block header.
  • Header (struct{}): The block header
  • Validators ([]SigningValidator): List of validators in the current validator set and whether or not they signed a vote in the LastCommit
  • ByzantineValidators ([]Evidence): List of evidence of validators that acted maliciously
• Response:
  • Tags ([]cmn.KVPair): Key-Value tags for filtering and indexing
• Usage:
  • Signals the beginning of a new block. Called prior to any DeliverTxs.
  • The header is expected to at least contain the Height.
  • The Validators and ByzantineValidators can be used to determine rewards and punishments for the validators.

CheckTx

• Request:

  • Tx ([]byte): The request transaction bytes

• Response:

  • Code (uint32): Response code
  • Data ([]byte): Result bytes, if any.
  • Log (string): The output of the application's logger. May be non-deterministic.
  • Info (string): Additional information. May be non-deterministic.
  • GasWanted (int64): Amount of gas request for transaction.
  • GasUsed (int64): Amount of gas consumed by transaction.
  • Tags ([]cmn.KVPair): Key-Value tags for filtering and indexing transactions (eg. by account).

• Usage: Validate a mempool transaction, prior to broadcasting or proposing. CheckTx should perform stateful but light-weight checks of the validity of the transaction (like checking signatures and account balances), but need not execute in full (like running a smart contract).

  Tendermint runs CheckTx and DeliverTx concurrently with eachother, though on distinct ABCI connections - the mempool connection and the consensus connection, respectively.

  The application should maintain a separate state to support CheckTx. This state can be reset to the latest committed state during Commit. Before calling Commit, Tendermint will lock and flush the mempool, ensuring that all existing CheckTx are responded to and no new ones can begin. After Commit, the mempool will rerun CheckTx for all remaining transactions, throwing out any that are no longer valid. Then the mempool will unlock and start sending CheckTx again.

  Keys and values in Tags must be UTF-8 encoded strings (e.g. "account.owner": "Bob", "balance": "100.0", "date": "2018-01-02")

DeliverTx

• Request:
  • Tx ([]byte): The request transaction bytes.
• Response:
  • Code (uint32): Response code.
  • Data ([]byte): Result bytes, if any.
  • Log (string): The output of the application's logger. May be non-deterministic.
  • Info (string): Additional information. May be non-deterministic.
  • GasWanted (int64): Amount of gas requested for transaction.
  • GasUsed (int64): Amount of gas consumed by transaction.
  • Tags ([]cmn.KVPair): Key-Value tags for filtering and indexing transactions (eg. by account).
• Usage:
  • Deliver a transaction to be executed in full by the application. If the transaction is valid, returns CodeType.OK.
  • Keys and values in Tags must be UTF-8 encoded strings (e.g. "account.owner": "Bob", "balance": "100.0", "time": "2018-01-02T12:30:00Z")

EndBlock

• Request:
  • Height (int64): Height of the block just executed.
• Response:
  • ValidatorUpdates ([]Validator): Changes to validator set (set voting power to 0 to remove).
  • ConsensusParamUpdates (ConsensusParams): Changes to consensus-critical time, size, and other parameters.
  • Tags ([]cmn.KVPair): Key-Value tags for filtering and indexing
• Usage:
  • Signals the end of a block.
  • Called prior to each Commit, after all transactions.
  • Validator set and consensus params are updated with the result.
  • Validator pubkeys are expected to be go-wire encoded.

Commit

• Response:
  • Data ([]byte): The Merkle root hash
• Usage:
  • Persist the application state.
  • Return a Merkle root hash of the application state.
  • It's critical that all application instances return the same hash. If not, they will not be able to agree on the next block, because the hash is included in the next block!

Data Messages

Header

• Fields:
  • ChainID (string): ID of the blockchain
  • Height (int64): Height of the block in the chain
  • Time (int64): Unix time of the block
  • NumTxs (int32): Number of transactions in the block
  • TotalTxs (int64): Total number of transactions in the blockchain until now
  • LastBlockHash ([]byte): Hash of the previous (parent) block
  • ValidatorsHash ([]byte): Hash of the validator set for this block
  • AppHash ([]byte): Data returned by the last call to Commit - typically the Merkle root of the application state after executing the previous block's transactions
  • Proposer (Validator): Original proposer for the block
• Usage:
  • Provided in RequestBeginBlock
  • Provides important context about the current state of the blockchain - especially height and time.
  • Provides the proposer of the current block, for use in proposer-based reward mechanisms.

Validator

• Fields:
  • Address ([]byte): Address of the validator (hash of the public key)
  • PubKey (PubKey): Public key of the validator
  • Power (int64): Voting power of the validator
• Usage:
  • Provides all identifying information about the validator

SigningValidator

• Fields:
  • Validator (Validator): A validator
  • SignedLastBlock (bool): Indicated whether or not the validator signed the last block
• Usage:
  • Indicates whether a validator signed the last block, allowing for rewards based on validator availability

PubKey

• Fields:
  • Type (string): Type of the public key. A simple string like "ed25519". In the future, may indicate a serialization algorithm to parse the Data, for instance "amino".
  • Data ([]byte): Public key data. For a simple public key, it's just the raw bytes. If the Type indicates an encoding algorithm, this is the encoded public key.
• Usage:
  • A generic and extensible typed public key

Evidence

• Fields:
  • Type (string): Type of the evidence. A hierarchical path like "duplicate/vote".
  • Validator (Validator: The offending validator
  • Height (int64): Height when the offense was committed
  • Time (int64): Unix time of the block at height Height
  • TotalVotingPower (int64): Total voting power of the validator set at height Height