# Transaction Lifecycle
This document describes the lifecycle of a transaction from creation to committed state changes. Transaction definition is described in a different doc. The transaction will be referred to as Tx
.
# Pre-requisite Readings
# Creation
# Transaction Creation
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]
:
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, which represents computational resources,Tx
consumes. Gas is dependent on the transaction and is not precisely calculated until execution, but can be estimated by providingauto
as the value for--gas
.--gas-adjustment
(optional) can be used to scalegas
up in order to avoid underestimating. For example, users can specify their gas adjustment as 1.5 to use 1.5 times the estimated gas.--gas-prices
specifies how much the user is willing pay per unit of gas, which can be one or multiple denominations of tokens. For example,--gas-prices=0.025uatom, 0.025upho
means the user is willing to pay 0.025uatom AND 0.025upho 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 or not 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 incentivized to pay more.
# CLI Example
Users of application app
can enter the following command into their CLI to generate a transaction to send 1000uatom from a senderAddress
to a recipientAddress
. It specifies how much gas they are willing to pay: an automatic estimate scaled up by 1.5 times, with a gas price of 0.025uatom per unit gas.
# Other Transaction Creation Methods
The command-line is an easy way to interact with an application, but Tx
can also be created using a gRPC or REST interface or some other entrypoint defined by the application developer. From the user's perspective, the interaction depends on the web interface or wallet they are using (e.g. creating Tx
using Lunie.io (opens new window) and signing it with a Ledger Nano S).
# Addition to Mempool
Each full-node (running Tendermint) that receives a Tx
sends an ABCI message (opens new window),
CheckTx
, to the application layer to check for validity, and receives an abci.ResponseCheckTx
. If the Tx
passes the checks, it is held in the nodes'
Mempool (opens new window), an in-memory pool of transactions unique to each node) pending inclusion 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.
# Types of Checks
The full-nodes perform stateless, then stateful checks on Tx
during CheckTx
, 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 nonnegative numbers, and other logic specified in the definitions.
Stateful checks validate transactions and messages based on a committed state. Examples include checking that the relevant values exist and are able to be transacted with, the address has sufficient funds, and the sender is authorized or has the correct ownership to transact. 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 CheckTx
, 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
When 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 unmarshaled in order to be processed. Then, the runTx
function is called to run in runTxModeCheck
mode, meaning the function will run all checks but exit before executing messages and writing state changes.
# ValidateBasic
Messages (sdk.Msg
) are extracted from transactions (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 BaseApptype calls the
ValidateBasicmethod very early in the processing of the message in the [
CheckTx](../core/baseapp.md#checktx) and [
DeliverTx](../core/baseapp.md#delivertx)) transactions.
ValidateBasic` can include only stateless checks (the checks that do not require access to the state).
# Guideline
Gas is not charged when ValidateBasic
is executed so we recommend only performing all necessary stateless checks to enable middleware operations (for example, parsing the required signer accounts to validate a signature by a middleware) and stateless sanity checks not impacting performance of the CheckTx phase.
Other validation operations must be performed when handling a message in a module Msg Server.
Example, if the message is to send coins from one address to another, ValidateBasic
likely checks for non-empty addresses and a non-negative coin amount, but does not require knowledge of state such as the account balance of an address.
See also Msg Service Validation.
# AnteHandler
After the ValidateBasic checks, the AnteHandler
s are run. 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 back to the original if the execution fails.
For example, the auth
(opens new window) module AnteHandler
checks and increments sequence numbers, checks signatures and account numbers, and deducts fees from the first signer of the transaction - all state changes are made using the checkState
.
# Gas
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 Tx
is known as GasWanted
. If GasConsumed
, the amount of gas consumed so during execution, ever exceeds GasWanted
, the execution will stop and the changes made to the cached copy of the state won't be committed. Otherwise, CheckTx
sets GasUsed
equal to 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 to Mempool
If at any point during CheckTx
the Tx
fails, it is discarded and the transaction lifecycle ends
there. 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, mempool.cache_size
is large enough to encompass all
of the transactions in the full mempool. If the 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
CheckTx
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, the process through which validator nodes come to agreement on which transactions to
accept, happens in rounds. 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, such as Tendermint BFT (opens new window),
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 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.
# DeliverTx
The DeliverTx
ABCI function defined in BaseApp
does the bulk of the
state transitions: it is run for each transaction in the block in sequential order as committed
to during consensus. Under the hood, DeliverTx
is almost identical to CheckTx
but calls the
runTx
function in deliver mode instead of check mode.
Instead of using their checkState
, full-nodes use deliverState
:
Decoding: Since
DeliverTx
is an ABCI call,Tx
is received in the encoded[]byte
form. Nodes first unmarshal the transaction, using theTxConfig
defined in the app, then callrunTx
inrunTxModeDeliver
, which is very similar toCheckTx
but also executes and writes state changes.Checks: Full-nodes call
validateBasicMsgs
and theAnteHandler
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 theAnteHandler
will not comparegas-prices
to the node'smin-gas-prices
since that value is local to each node - differing values across nodes would yield nondeterministic results.MsgServiceRouter
: WhileCheckTx
would have exited,DeliverTx
continues to runrunMsgs
to fully execute eachMsg
within the transaction. Since the transaction may have messages from different modules,BaseApp
needs to know which module to find the appropriate handler. This is achieved usingBaseApp
'sMsgServiceRouter
so that it can be processed by the module's ProtobufMsg
service. ForLegacyMsg
routing, theRoute
function is called via the module manager to retrieve the route name and find the legacyHandler
within the module.Msg
service: a ProtobufMsg
service, a step up fromAnteHandler
, is responsible for executing each message in theTx
and causes state transitions to persist indeliverTxState
.Gas: While a
Tx
is being delivered, aGasMeter
is used to keep track of how much gas is being used; if execution completes,GasUsed
is set and returned in theabci.ResponseDeliverTx
. If execution halts becauseBlockGasMeter
orGasMeter
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 final step is for 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 or not 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.
Note that 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 also 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.
# Next
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