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Transaction Lifecycle

This document describes the lifecycle of a transaction from creation to the committed state changes. The majority of this page is copied from the Cosmos-sdk Docs. Transaction definition is described in a different doc. The transaction will be referred to as Tx.

Creation​

Transaction Creation​

There are various ways to create transactions. While the command-line is a straightforward method to interact with an application, transactions can also be generated through a gRPC or REST interface or another entry point specified by the application developer. The interaction between the user and the application varies based on the web interface or wallet they use. For example, users can create Tx through platforms like Lunie.io and sign them using a Ledger Nano S.

Transaction Creation via Command Line​

One of the main application interfaces is the command-line interface. The transaction Tx can be created by the user inputting a command in the following format from the command-line, providing the type of transaction in [command], arguments in [args], and configurations such as gas prices in [flags]:

gnfd tx [command] [args] [flags]

This command will automatically create the transaction, sign it using the account's private key, and broadcast it to the specified peer node.

There are several required and optional flags for transaction creation. The --from flag specifies which account the transaction is originating from. For example, if the transaction is sending coins, the funds will be drawn from the specified from address.

Gas and Fees​

Additionally, there are several flags users can use to indicate how much they are willing to pay in fees:

  • --gas refers to how much gas. Different from other cosmos blockchain where gas represents computational resources, on greenfield blockchain, the gas of a transaction is predefined. It is suggested to be estimated by providing auto as the value for --gas.
  • --gas-prices specifies how much the user is willing to pay per unit of gas, which can be one or multiple denominations of tokens. For example, --gas-prices=5000000000BNB means the user is willing to pay 5Gwei BNB per unit of gas.
  • --fees specifies how much in fees the user is willing to pay in total.
  • --timeout-height specifies a block timeout height to prevent the tx from being committed past a certain height.

The ultimate value of the fees paid is equal to the gas multiplied by the gas prices. In other words, fees = ceil(gas * gasPrices). Thus, since fees can be calculated using gas prices and vice versa, the users specify only one of the two.

Later, validators decide whether to include the transaction in their block by comparing the given or calculated gas-prices to their local min-gas-prices. Tx will be rejected if its gas-prices is not high enough, so users are incentive to pay more.

CLI Example​

Users of the application app can enter the following command into their CLI to generate a transaction to send 1000wei BNB from a senderAddress to a recipientAddress. 

gnfd tx send <recipientAddress> 1000BNB --from <senderAddress> --gas auto

Add transactions to the Mempool​

Each full-node (running Tendermint) that receives a Tx sends an ABCI message, CheckTx, to the application layer to check for validity, and receives an abci.ResponseCheckTx. If the Tx passes the checks, it will be added to the Mempool of each node. The Mempool is an in-memory pool of transactions that is unique to each node, and the transaction will remain pending until it is included in a block. honest nodes will discard Tx if it is found to be invalid. Prior to consensus, nodes continuously check incoming transactions and gossip them to their peers.

In this section, we will provide you with details on the functions that are used to add a transaction to the Mempool and how they work.

Types of Checks​

The full-nodes perform stateless, then stateful checks on Tx during the CheckTx ABCI function, with the goal to identify and reject an invalid transaction as early on as possible to avoid wasted computation.

Stateless checks do not require nodes to access state - light clients or offline nodes can do them - and are thus less computationally expensive. Stateless checks include making sure addresses are not empty, enforcing non-negative numbers, and other logic specified in the definitions.

Stateful checks validated transactions and messages based on a committed state. Some examples of the checks that a transaction goes through include verifying that the necessary values are present and can be transacted, checking if the sender is authorized to transact or has the correct ownership, and ensuring that the address has sufficient funds. At any given moment, full-nodes typically have multiple versions of the application's internal state for different purposes. For example, nodes will execute state changes while in the process of verifying transactions, but still need a copy of the last committed state in order to answer queries - they should not respond using state with uncommitted changes.

In order to verify a Tx, full-nodes call the CheckTx ABCI function, which includes both stateless and stateful checks. Further validation happens later in the DeliverTx stage. CheckTx goes through several steps, beginning with decoding Tx.

Decoding the transactions​

When a Tx is received by the application from the underlying consensus engine (e.g. Tendermint), it is still in its encoded []byte form and needs to be unmarshalled in order to be processed. Then, the runTx function is executed in the runTxModeCheck mode. This mode runs all checks but exit before executing messages and writing state changes.

Confirm the transactions messages (ValidateBasic)​

Messages sdk.Msg are extracted from Tx. The ValidateBasic method of the sdk.Msg interface implemented by the module developer is run for each transaction. To discard obviously invalid messages, the BaseApp type calls the ValidateBasic method very early in the processing of the message in the CheckTx and DeliverTx transactions. ValidateBasic can include only stateless checks (checks that do not require access to the state).

AnteHandler​

After the ValidateBasic method have done the checking, the AnteHandlers are run. The AnteHandler is a set of modules that check transactions validity. Technically, they are optional, but in practice, they are very often present to perform signature verification, gas calculation, fee deduction and other core operations related to blockchain transactions.

A copy of the cached context is provided to the AnteHandler, which performs limited checks specified for the transaction type. Using a copy allows the AnteHandler to do stateful checks for Tx without modifying the last committed state, and revert to the original if the execution fails.

Gas Payment Process​

The Context, which keeps a GasMeter that will track how much gas has been used during the execution of Tx, is initialized. The user-provided amount of gas for the Tx is known as GasWanted. If the GasConsumed parameter, the amount of gas consumed during the execution, ever exceeds the GasWanted amount, the execution will stop and the changes made to the cached copy of the state won't be committed. Otherwise, the CheckTx ABCI function sets the amount of GasUsed at the same value as GasConsumed and returns it in the result. After calculating the gas and fee values, validator-nodes check that the user-specified gas-prices is greater than their locally defined min-gas-prices.

Discard or Addition of transactions to Mempool​

If the Tx fails at any point during CheckTx, it is discarded, and the transaction lifecycle ends at that point. Otherwise, if it passes CheckTx successfully, the default protocol is to relay it to peer nodes and add it to the Mempool so that the Tx becomes a candidate to be included in the next block.

The mempool serves the purpose of keeping track of transactions seen by all full-nodes. Full-nodes keep a mempool cache of the last mempool.cache_size transactions they have seen, as a first line of defense to prevent replay attacks. Ideally, the mempool.cache_size is large enough to encompass all the transactions in the full mempool. If the mempool cache is too small to keep track of all the transactions, CheckTx is responsible for identifying and rejecting replayed transactions.

Currently existing preventative measures include fees and a sequence (nonce) counter to distinguish replayed transactions from identical but valid ones. If an attacker tries to spam nodes with many copies of a Tx, full-nodes keeping a mempool cache will reject identical copies instead of running the CheckTx function on all of them. Even if the copies have incremented sequence numbers, attackers are disincentivized by the need to pay fees.

Validator nodes keep a mempool to prevent replay attacks, just as full-nodes do, but also use it as a pool of unconfirmed transactions in preparation of block inclusion. Note that even if a Tx passes all checks at this stage, it is still possible to be found invalid later on, because CheckTx does not fully validate the transaction (i.e. it does not actually execute the messages).

Inclusion in a Block​

Consensus occurs in rounds, during which validator nodes agree on which transactions to accept. Each round begins with a proposer creating a block of the most recent transactions and ends with validators, special full-nodes with voting power responsible for consensus, agreeing to accept the block or go with a nil block instead. Validator nodes execute the consensus algorithm, the Tendermint BFT, confirming the transactions using ABCI requests to the application, in order to come to this agreement.

The first step of consensus is the block proposal. One proposer amongst the validators is chosen by the consensus algorithm to create and propose a block - in order for a Tx to be included, it must be in this proposer's mempool.

State Changes​

The next step of the consensus is to execute the transactions to fully validate them. All full-nodes that receive a block proposal from the correct proposer execute the transactions by calling the ABCI functions BeginBlock, DeliverTx for each transaction, and EndBlock. While each full-node runs everything locally, this process yields a single, unambiguous result, since the messages' state transitions are deterministic and transactions are explicitly ordered in the block proposal.

        -----------------------
        |Receive Block Proposal|
        -----------------------
                  |
              v
        -----------------------
        | BeginBlock          |
        -----------------------
                  |
              v
        -----------------------
        | DeliverTx(tx0)      |
        | DeliverTx(tx1)      |
        | DeliverTx(tx2)      |
        | DeliverTx(tx3)      |
        |   .         |
        |   .         |
        |   .         |
        -----------------------
                  |
              v
        -----------------------
        | EndBlock        |
        -----------------------
                  |
              v
        -----------------------
        | Consensus       |
        -----------------------
                  |
              v
        -----------------------
        | Commit          |
        -----------------------

DeliverTx ABCI function​

The DeliverTx ABCI function, defined in the BaseApp type, does the bulk of the state transitions: the function is executed for each transaction in the block in sequential order as committed to during consensus. Under the hood, DeliverTx is almost identical to the CheckTx function but calls the runTx function in the deliver mode instead of using the check mode. Instead of using their checkState, full-nodes use deliverState:

  • Decoding: Since DeliverTx is an ABCI call, the Tx is received in the encoded []byte form. First, the nodes unmarshal the transaction using the TxConfig defined in the app. Then, they call runTx in runTxModeDeliver, which is similar to CheckTx but also executes and writes state changes.

  • Checks and AnteHandler: Full-nodes call the validateBasicMsgs and the AnteHandler again. This second check happens because they may not have seen the same transactions during the addition to Mempool stage and a malicious proposer may have included invalid ones. One difference here is that the AnteHandler will not compare the gas-prices variable to the node's min-gas-prices variable since that value is local to each node - differing values across nodes would yield nondeterministic results.

  • MsgServiceRouter: While CheckTx would have exited, DeliverTx continues to run the runMsgs function to fully execute each Msg service method within the transaction. Since the transaction may have messages from different modules, the BaseApp type needs to know which module to find the appropriate handler. This is achieved using the BaseApp's MsgServiceRouter router so that it can be processed by the module's Protobuf Msg service.

  • Msg service: Protobuf Msg service is responsible for executing each message in the Tx and causes state transitions to persist in deliverTxState.

  • PostHandlers: PostHandlers run after the execution of the message. If they fail, the state change of runMsgs, as well of PostHandlers are both reverted.

  • Gas: While a Tx is being delivered, a GasMeter is used to keep track of how much gas is being used; if execution completes, GasUsed is set and returned to the abci.ResponseDeliverTx. If execution halts because BlockGasMeter or GasMeter has run out or something else goes wrong, a deferred function at the end appropriately errors or panics.

If there are any failed state changes resulting from a Tx being invalid or GasMeter running out, the transaction processing terminates and any state changes are reverted. Invalid transactions in a block proposal cause validator nodes to reject the block and vote for a nil block instead.

Commit the Block​

The final step is for the nodes to commit the block and state changes. Validator nodes perform the previous step of executing state transitions in order to validate the transactions, then sign the block to confirm it. Full nodes that are not validators do not participate in consensus - i.e. they cannot vote - but listen for votes to understand whether they should commit the state changes.

When they receive enough validator votes (2/3+ precommits weighted by voting power), full nodes commit to a new block to be added to the blockchain and finalize the state transitions in the application layer. A new state root is generated to serve as a merkle proof for the state transitions. Applications use the Commit ABCI method inherited from Baseapp; it syncs all the state transitions by writing the deliverState into the application's internal state. As soon as the state changes are committed, checkState start afresh from the most recently committed state and deliverState resets to nil in order to be consistent and reflect the changes.

Notes:

  • Not all blocks have the same number of transactions, and it is possible for consensus to result in a nil block or one with none at all.
  • In a public blockchain network, it is possible for validators to be byzantine, or malicious, which may prevent a Tx from being committed in the blockchain. Possible malicious behaviors include the proposer deciding to censor a Tx by excluding it from the block or a validator voting against the block.

At this point, the transaction lifecycle of a Tx is over: nodes have verified its validity, delivered it by executing its state changes, and committed those changes. The Tx itself, in []byte form, is stored in a block and appended to the blockchain.