Building transactions¶
Introduction¶
Understanding and implementing transactions in Corda is key to building and implementing real world smart contracts. It is only through construction of valid Corda transactions containing appropriate data that nodes on the ledger can map real world business objects into a shared digital view of the data in the Corda ledger. More importantly as the developer of new smart contracts it is the code which determines what data is well formed and what data should be rejected as mistakes, or to prevent malicious activity. This document details some of the considerations and APIs used to when constructing transactions as part of a flow.
The Basic Lifecycle Of Transactions¶
Transactions in Corda are constructed in stages and contain a number of
elements. In particular a transaction’s core data structure is the
net.corda.core.transactions.WireTransaction
, which is usually
manipulated via a
net.corda.core.contracts.General.TransactionBuilder
and contains:
1. A set of Input state references that will be consumed by the final accepted transaction.
2. A set of Output states to create/replace the consumed states and thus become the new latest versions of data on the ledger.
3. A set of Attachment
items which can contain legal documents, contract
code, or private encrypted sections as an extension beyond the native
contract states.
4. A set of Command
items which give a context to the type of ledger
transition that is encoded in the transaction. Also each command has an
associated set of signer keys, which will be required to sign the
transaction.
5. A signers list, which is populated by the TransactionBuilder
to
be the union of the signers on the individual Command objects.
6. A notary identity to specify the Notary node which is tracking the
state consumption. (If the input states are registered with different
notary nodes the flow will have to insert additional NotaryChange
transactions to migrate the states across to a consistent notary node,
before being allowed to mutate any states.)
7. Optionally a timestamp that can used in the Notary to time bound the period in which the proposed transaction stays valid.
Typically, the WireTransaction
should be regarded as a proposal and
may need to be exchanged back and forth between parties before it can be
fully populated. This is an immediate consequence of the Corda privacy
model, which means that the input states are likely to be unknown to the
other node.
Once the proposed data is fully populated the flow code should freeze
the WireTransaction
and form a SignedTransaction
. This is key to
the ledger agreement process, as once a flow has attached a node’s
signature it has stated that all details of the transaction are
acceptable to it. A flow should take care not to attach signatures to
intermediate data, which might be maliciously used to construct a
different SignedTransaction
. For instance in a foreign exchange
scenario we shouldn’t send a SignedTransaction
with only our sell
side populated as that could be used to take the money without the
expected return of the other currency. Also, it is best practice for
flows to receive back the DigitalSignature.WithKey
of other parties
rather than a full SignedTransaction
objects, because otherwise we
have to separately check that this is still the same
SignedTransaction
and not a malicious substitute.
The final stage of committing the transaction to the ledger is to
notarise the SignedTransaction
, distribute this to all appropriate
parties and record the data into the ledger. These actions are best
delegated to the FinalityFlow
, rather than calling the individual
steps manually. However, do note that the final broadcast to the other
nodes is asynchronous, so care must be used in unit testing to
correctly await the Vault updates.
Gathering Inputs¶
One of the first steps to forming a transaction is gathering the set of
input references. This process will clearly vary according to the nature
of the business process being captured by the smart contract and the
parameterised details of the request. However, it will generally involve
searching the Vault via the VaultService
interface on the
ServiceHub
to locate the input states.
To give a few more specific details consider two simplified real world
scenarios. First, a basic foreign exchange Cash transaction. This
transaction needs to locate a set of funds to exchange. A flow
modelling this is implemented in FxTransactionBuildTutorial.kt
.
Second, a simple business model in which parties manually accept, or
reject each other’s trade proposals which is implemented in
WorkflowTransactionBuildTutorial.kt
. To run and explore these
examples using the IntelliJ IDE one can run/step the respective unit
tests in FxTransactionBuildTutorialTest.kt
and
WorkflowTransactionBuildTutorialTest.kt
, which drive the flows as
part of a simulated in-memory network of nodes. When creating the
IntelliJ run configuration for these unit test set the workspace
points to the root r3prototyping
folder and add
-javaagent:lib/quasar.jar
to the VM options, so that the Quasar
instrumentation is correctly configured.
For the Cash transaction let’s assume the cash resources are using the
standard CashState
in the :financial
Gradle module. The Cash
contract uses FungibleAsset
states to model holdings of
interchangeable assets and allow the split/merge and summing of
states to meet a contractual obligation. We would normally use the
generateSpend
method on the VaultService
to gather the required
amount of cash into a TransactionBuilder
, set the outputs and move
command. However, to elucidate more clearly example flow code is shown
here that will manually carry out the inputs queries using the lower
level VaultService
.
// This is equivalent to the VaultService.generateSpend
// Which is brought here to make the filtering logic more visible in the example
private fun gatherOurInputs(serviceHub: ServiceHub,
amountRequired: Amount<Issued<Currency>>,
notary: Party?): Pair<List<StateAndRef<Cash.State>>, Long> {
// Collect cash type inputs
val cashStates = serviceHub.vaultService.currentVault.statesOfType<Cash.State>()
// extract our key identity for convenience
val ourKey = serviceHub.myInfo.legalIdentity.owningKey
// Filter down to our own cash states with right currency and issuer
val suitableCashStates = cashStates.filter {
val state = it.state.data
(state.owner == ourKey)
&& (state.amount.token == amountRequired.token)
}
require(!suitableCashStates.isEmpty()) { "Insufficient funds" }
var remaining = amountRequired.quantity
// We will need all of the inputs to be on the same notary.
// For simplicity we just filter on the first notary encountered
// A production quality flow would need to migrate notary if the
// the amounts were not sufficient in any one notary
val sourceNotary: Party = notary ?: suitableCashStates.first().state.notary
val inputsList = mutableListOf<StateAndRef<Cash.State>>()
// Iterate over filtered cash states to gather enough to pay
for (cash in suitableCashStates.filter { it.state.notary == sourceNotary }) {
inputsList += cash
if (remaining <= cash.state.data.amount.quantity) {
return Pair(inputsList, cash.state.data.amount.quantity - remaining)
}
remaining -= cash.state.data.amount.quantity
}
throw IllegalStateException("Insufficient funds")
}
As a foreign exchange transaction we expect an exchange of two
currencies, so we will also require a set of input states from the other
counterparty. However, the Corda privacy model means we do not know the
other node’s states. Our flow must therefore negotiate with the other
node for them to carry out a similar query and populate the inputs (See
the ForeignExchangeFlow
for more details of the exchange). Having
identified a set of Input StateRef
items we can then create the
output as discussed below.
For the trade approval flow we need to implement a simple workflow
pattern. We start by recording the unconfirmed trade details in a state
object implementing the LinearState
interface. One field of this
record is used to map the business workflow to an enumerated state.
Initially the initiator creates a new state object which receives a new
UniqueIdentifier
in its linearId
property and a starting
workflow state of NEW
. The Contract.verify
method is written to
allow the initiator to sign this initial transaction and send it to the
other party. This pattern ensures that a permanent copy is recorded on
both ledgers for audit purposes, but the state is prevented from being
maliciously put in an approved state. The subsequent workflow steps then
follow with transactions that consume the state as inputs on one side
and output a new version with whatever state updates, or amendments
match to the business process, the linearId
being preserved across
the changes. Attached Command
objects help the verify method
restrict changes to appropriate fields and signers at each step in the
workflow. In this it is typical to have both parties sign the change
transactions, but it can be valid to allow unilateral signing, if for instance
one side could block a rejection. Commonly the manual initiator of these
workflows will query the Vault for states of the right contract type and
in the right workflow state over the RPC interface. The RPC will then
initiate the relevant flow using StateRef
, or linearId
values as
parameters to the flow to identify the states being operated upon. Thus
code to gather the latest input state would be:
// Helper method to locate the latest Vault version of a LinearState from a possibly out of date StateRef
inline fun <reified T : LinearState> ServiceHub.latest(ref: StateRef): StateAndRef<T> {
val linearHeads = vaultService.linearHeadsOfType<T>()
val original = toStateAndRef<T>(ref)
return linearHeads.get(original.state.data.linearId)!!
}
// Pull in the latest Vault version of the StateRef as a full StateAndRef
val latestRecord = serviceHub.latest<TradeApprovalContract.State>(ref)
Generating Commands¶
For the commands that will be added to the transaction, these will need
to correctly reflect the task at hand. These must match because inside
the Contract.verify
method the command will be used to select the
validation code path. The Contract.verify
method will then restrict
the allowed contents of the transaction to reflect this context. Typical
restrictions might include that the input cash amount must equal the
output cash amount, or that a workflow step is only allowed to change
the status field. Sometimes, the command may capture some data too e.g.
the foreign exchange rate, or the identity of one party, or the StateRef
of the specific input that originates the command in a bulk operation.
This data will be used to further aid the Contract.verify
, because
to ensure consistent, secure and reproducible behaviour in a distributed
environment the Contract.verify
, transaction is the only allowed to
use the content of the transaction to decide validity.
Another essential requirement for commands is that the correct set of
CompositeKeys
are added to the Command on the builder, which will be
used to form the set of required signers on the final validated
transaction. These must correctly align with the expectations of the
Contract.verify
method, which should be written to defensively check
this. In particular, it is expected that at minimum the owner of an
asset would have to be signing to permission transfer of that asset. In
addition, other signatories will often be required e.g. an Oracle
identity for an Oracle command, or both parties when there is an
exchange of assets.
Generating Outputs¶
Having located a set of StateAndRefs
as the transaction inputs, the
flow has to generate the output states. Typically, this is a simple call
to the Kotlin copy
method to modify the few fields that will
transitioned in the transaction. The contract code may provide a
generateXXX
method to help with this process if the task is more
complicated. With a workflow state a slightly modified copy state is
usually sufficient, especially as it is expected that we wish to preserve
the linearId
between state revisions, so that Vault queries can find
the latest revision.
For fungible contract states such as Cash
it is common to distribute
and split the total amount e.g. to produce a remaining balance output
state for the original owner when breaking up a large amount input
state. Remember that the result of a successful transaction is always to
fully consume/spend the input states, so this is required to conserve
the total cash. For example from the demo code:
// Gather our inputs. We would normally use VaultService.generateSpend
// to carry out the build in a single step. To be more explicit
// we will use query manually in the helper function below.
// Putting this into a non-suspendable function also prevents issues when
// the flow is suspended.
val (inputs, residual) = gatherOurInputs(serviceHub, sellAmount, request.notary)
// Build and an output state for the counterparty
val transferedFundsOutput = Cash.State(sellAmount, request.counterparty.owningKey)
if (residual > 0L) {
// Build an output state for the residual change back to us
val residualAmount = Amount(residual, sellAmount.token)
val residualOutput = Cash.State(residualAmount, serviceHub.myInfo.legalIdentity.owningKey)
return FxResponse(inputs, listOf(transferedFundsOutput, residualOutput))
} else {
return FxResponse(inputs, listOf(transferedFundsOutput))
}
Building the WireTransaction¶
Having gathered all the ingredients for the transaction we now need to
use a TransactionBuilder
to construct the full WireTransaction
.
The initial TransactionBuilder
should be created by calling the
TransactionType.General.Builder
method. (The other
TransactionBuilder
implementation is only used for the NotaryChange
flow where
ContractStates
need moving to a different Notary.) At this point the
Notary to associate with the states should be recorded. Then we keep
adding inputs, outputs, commands and attachments to fill the
transaction. Examples of this process are:
// Modify the state field for new output. We use copy, to ensure no other modifications.
// It is especially important for a LinearState that the linearId is copied across,
// not accidentally assigned a new random id.
val newState = latestRecord.state.data.copy(state = verdict)
// We have to use the original notary for the new transaction
val notary = latestRecord.state.notary
// Get and populate the new TransactionBuilder
// To destroy the old proposal state and replace with the new completion state.
// Also add the Completed command with keys of all parties to signal the Tx purpose
// to the Contract verify method.
val tx = TransactionType.
General.
Builder(notary).
withItems(
latestRecord,
newState,
Command(TradeApprovalContract.Commands.Completed(),
listOf(serviceHub.myInfo.legalIdentity.owningKey, latestRecord.state.data.source.owningKey)))
tx.setTime(serviceHub.clock.instant(), Duration.ofSeconds(60))
// We can sign this transaction immediately as we have already checked all the fields and the decision
// is ultimately a manual one from the caller.
tx.signWith(serviceHub.legalIdentityKey)
// Convert to SignedTransaction we can pass around certain that it cannot be modified.
val selfSignedTx = tx.toSignedTransaction(false)
private fun buildTradeProposal(ourStates: FxResponse, theirStates: FxResponse): SignedTransaction {
// This is the correct way to create a TransactionBuilder,
// do not construct directly.
// We also set the notary to match the input notary
val builder = TransactionType.General.Builder(ourStates.inputs.first().state.notary)
// Add the move commands and key to indicate all the respective owners and need to sign
val ourSigners = ourStates.inputs.map { it.state.data.owner }.toSet()
val theirSigners = theirStates.inputs.map { it.state.data.owner }.toSet()
builder.addCommand(Cash.Commands.Move(), (ourSigners + theirSigners).toList())
// Build and add the inputs and outputs
builder.withItems(*ourStates.inputs.toTypedArray())
builder.withItems(*theirStates.inputs.toTypedArray())
builder.withItems(*ourStates.outputs.toTypedArray())
builder.withItems(*theirStates.outputs.toTypedArray())
// We have already validated their response and trust our own data
// so we can sign
builder.signWith(serviceHub.legalIdentityKey)
// create a signed transaction, but pass false as parameter, because we know it is not fully signed
val signedTransaction = builder.toSignedTransaction(checkSufficientSignatures = false)
return signedTransaction
}
Completing the SignedTransaction¶
Having created an initial WireTransaction
and converted this to an
initial SignedTransaction
the process of verifying and forming a
full SignedTransaction
begins and then completes with the
notarisation. In practice this is a relatively stereotypical process,
because assuming the WireTransaction
is correctly constructed the
verification should be immediate. However, it is also important to
recheck the business details of any data received back from an external
node, because a malicious party could always modify the contents before
returning the transaction. Each remote flow should therefore check as
much as possible of the initial SignedTransaction
inside the unwrap
of
the receive before agreeing to sign. Any issues should immediately throw
an exception to abort the flow. Similarly the originator, should always
apply any new signatures to its original proposal to ensure the contents
of the transaction has not been altered by the remote parties.
The typical code therefore checks the received SignedTransaction
using the verifySignatures
method, but excluding itself, the notary
and any other parties yet to apply their signature. The contents of the
WireTransaction
inside the SignedTransaction
should be fully
verified further by expanding with toLedgerTransaction
and calling
verify
. Further context specific and business checks should then be
made, because the Contract.verify
is not allowed to access external
context. For example the flow may need to check that the parties are the
right ones, or that the Command
present on the transaction is as
expected for this specific flow. An example of this from the demo code is:
// First we receive the verdict transaction signed by their single key
val completeTx = receive<SignedTransaction>(source).unwrap {
// Check the transaction is signed apart from our own key and the notary
val wtx = it.verifySignatures(serviceHub.myInfo.legalIdentity.owningKey, it.tx.notary!!.owningKey)
// Check the transaction data is correctly formed
wtx.toLedgerTransaction(serviceHub).verify()
// Confirm that this is the expected type of transaction
require(wtx.commands.single().value is TradeApprovalContract.Commands.Completed) {
"Transaction must represent a workflow completion"
}
// Check the context dependent parts of the transaction as the
// Contract verify method must not use serviceHub queries.
val state = wtx.outRef<TradeApprovalContract.State>(0)
require(state.state.data.source == serviceHub.myInfo.legalIdentity) {
"Proposal not one of our original proposals"
}
require(state.state.data.counterparty == source) {
"Proposal not for sent from correct source"
}
it
}
After verification the remote flow will return its signature to the
originator. The originator should apply that signature to the starting
SignedTransaction
and recheck the signatures match.
Committing the Transaction¶
Once all the party signatures are applied to the SignedTransaction the
final step is notarisation. This involves calling NotaryFlow.Client
to confirm the transaction, consume the inputs and return its confirming
signature. Then the flow should ensure that all nodes end with all
signatures and that they call ServiceHub.recordTransactions
. The
code for this is standardised in the FinalityFlow
, or more explicitly
an example is:
// Notarise and distribute the completed transaction.
subFlow(FinalityFlow(allPartySignedTx, setOf(latestRecord.state.data.source, latestRecord.state.data.counterparty)))
Partially Visible Transactions¶
The discussion so far has assumed that the parties need full visibility
of the transaction to sign. However, there may be situations where each
party needs to store private data for audit purposes, or for evidence to
a regulator, but does not wish to share that with the other trading
partner. The tear-off/Merkle tree support in Corda allows flows to send
portions of the full transaction to restrict visibility to remote
parties. To do this one can use the
WireTransaction.buildFilteredTransaction
extension method to produce
a FilteredTransaction
. The elements of the SignedTransaction
which we wish to be hide will be replaced with their secure hash. The
overall transaction txid is still provable from the
FilteredTransaction
preventing change of the private data, but we do
not expose that data to the other node directly. A full example of this
can be found in the NodeInterestRates
Oracle code from the
irs-demo
project which interacts with the RatesFixFlow
flow.
Also, refer to the Transaction tear-offs documentation.