corda/docs/source/tut-two-party-flow.rst

Updating the flow

To update the flow, we'll need to do two things:

• Update the lender's side of the flow to request the borrower's signature
• Create a flow for the borrower to run in response to a signature request from the lender

Updating the lender's flow

In the original CorDapp, we automated the process of notarising a transaction and recording it in every party's vault by invoking a built-in flow called FinalityFlow as a subflow. We're going to use another pre-defined flow, called CollectSignaturesFlow, to gather the borrower's signature.

We also need to add the borrower's public key to the transaction's command, making the borrower one of the required signers on the transaction.

In IOUFlow.java/IOUFlow.kt, update IOUFlow.call as follows:

...

// We create the transaction components.
val outputState = IOUState(iouValue, ourIdentity, otherParty)
val outputContract = IOUContract::class.jvmName
val outputContractAndState = StateAndContract(outputState, outputContract)
val cmd = Command(IOUContract.Create(), listOf(ourIdentity.owningKey, otherParty.owningKey))

// We add the items to the builder.
txBuilder.withItems(outputContractAndState, cmd)

// Verifying the transaction.
txBuilder.verify(serviceHub)

// Signing the transaction.
val signedTx = serviceHub.signInitialTransaction(txBuilder)

// Creating a session with the other party.
val otherpartySession = initiateFlow(otherParty)

// Obtaining the counterparty's signature.
val fullySignedTx = subFlow(CollectSignaturesFlow(signedTx, listOf(otherpartySession), CollectSignaturesFlow.tracker()))

// Finalising the transaction.
subFlow(FinalityFlow(fullySignedTx))

...

import com.google.common.collect.ImmutableList;
import java.security.PublicKey;
import java.util.Collections;
import java.util.List;

...

// We create the transaction components.
IOUState outputState = new IOUState(iouValue, getOurIdentity(), otherParty);
String outputContract = IOUContract.class.getName();
StateAndContract outputContractAndState = new StateAndContract(outputState, outputContract);
List<PublicKey> requiredSigners = ImmutableList.of(getOurIdentity().getOwningKey(), otherParty.getOwningKey());
Command cmd = new Command<>(new IOUContract.Create(), requiredSigners);

// We add the items to the builder.
txBuilder.withItems(outputContractAndState, cmd);

// Verifying the transaction.
txBuilder.verify(getServiceHub());

// Signing the transaction.
final SignedTransaction signedTx = getServiceHub().signInitialTransaction(txBuilder);

// Creating a session with the other party.
FlowSession otherpartySession = initiateFlow(otherParty);

// Obtaining the counterparty's signature.
SignedTransaction fullySignedTx = subFlow(new CollectSignaturesFlow(
        signedTx, ImmutableList.of(otherpartySession), CollectSignaturesFlow.tracker()));

// Finalising the transaction.
subFlow(new FinalityFlow(signedTx));

return null;

To make the borrower a required signer, we simply add the borrower's public key to the list of signers on the command.

We now need to communicate with the borrower to request their signature. Whenever you want to communicate with another party in the context of a flow, you first need to establish a flow session with them. If the counterparty has a FlowLogic registered to respond to the FlowLogic initiating the session, a session will be established. All communication between the two FlowLogic instances will then place as part of this session.

Once we have a session with the borrower, we gather the borrower's signature using CollectSignaturesFlow, which takes:

• A transaction signed by the flow initiator
• A list of flow-sessions between the flow initiator and the required signers

And returns a transaction signed by all the required signers.

We then pass this fully-signed transaction into FinalityFlow.

Creating the borrower's flow

We're now ready to write the lender's flow, which will respond to the borrower's attempt to gather our signature. In a new IOUFlowResponder.java file in Java, or within the App.kt file in Kotlin, add the following class:

...

import net.corda.core.transactions.SignedTransaction

...

@InitiatedBy(IOUFlow::class)
class IOUFlowResponder(val otherPartySession: FlowSession) : FlowLogic<Unit>() {
    @Suspendable
    override fun call() {
        val signTransactionFlow = object : SignTransactionFlow(otherPartySession, SignTransactionFlow.tracker()) {
            override fun checkTransaction(stx: SignedTransaction) = requireThat {
                val output = stx.tx.outputs.single().data
                "This must be an IOU transaction." using (output is IOUState)
                val iou = output as IOUState
                "The IOU's value can't be too high." using (iou.value < 100)
            }
        }

        subFlow(signTransactionFlow)
    }
}

package com.template.flow;

import co.paralleluniverse.fibers.Suspendable;
import com.template.state.IOUState;
import net.corda.core.contracts.ContractState;
import net.corda.core.flows.FlowException;
import net.corda.core.flows.FlowLogic;
import net.corda.core.flows.FlowSession;
import net.corda.core.flows.InitiatedBy;
import net.corda.core.flows.SignTransactionFlow;
import net.corda.core.transactions.SignedTransaction;
import net.corda.core.utilities.ProgressTracker;

import static net.corda.core.contracts.ContractsDSL.requireThat;

@InitiatedBy(IOUFlow.class)
public class IOUFlowResponder extends FlowLogic<Void> {
    private final FlowSession otherPartySession;

    public IOUFlowResponder(FlowSession otherPartySession) {
        this.otherPartySession = otherPartySession;
    }

    @Suspendable
    @Override
    public Void call() throws FlowException {
        class SignTxFlow extends SignTransactionFlow {
            private signTxFlow(FlowSession otherPartySession, ProgressTracker progressTracker) {
                super(otherPartySession, progressTracker);
            }

            @Override
            protected void checkTransaction(SignedTransaction stx) {
                requireThat(require -> {
                    ContractState output = stx.getTx().getOutputs().get(0).getData();
                    require.using("This must be an IOU transaction.", output instanceof IOUState);
                    IOUState iou = (IOUState) output;
                    require.using("The IOU's value can't be too high.", iou.getValue() < 100);
                    return null;
                });
            }
        }

        subFlow(new SignTxFlow(otherPartySession, SignTransactionFlow.Companion.tracker()));

        return null;
    }
}

As with the IOUFlow, our IOUFlowResponder flow is a FlowLogic subclass where we've overridden FlowLogic.call.

The flow is annotated with InitiatedBy(IOUFlow.class), which means that your node will invoke IOUFlowResponder.call when it receives a message from a instance of Initiator running on another node. What will this message from the IOUFlow be? If we look at the definition of CollectSignaturesFlow, we can see that we'll be sent a SignedTransaction, and are expected to send back our signature over that transaction.

We could handle this manually. However, there is also a pre-defined flow called SignTransactionFlow that can handle this process for us automatically. SignTransactionFlow is an abstract class, and we must subclass it and override SignTransactionFlow.checkTransaction.

Once we've defined the subclass, we invoke it using FlowLogic.subFlow, and the communication with the borrower's and the lender's flow is conducted automatically.

CheckTransactions

SignTransactionFlow will automatically verify the transaction and its signatures before signing it. However, just because a transaction is valid doesn't mean we necessarily want to sign. What if we don't want to deal with the counterparty in question, or the value is too high, or we're not happy with the transaction's structure?

Overriding SignTransactionFlow.checkTransaction allows us to define these additional checks. In our case, we are checking that:

• The transaction involves an IOUState - this ensures that IOUContract will be run to verify the transaction
• The IOU's value is less than some amount (100 in this case)

If either of these conditions are not met, we will not sign the transaction - even if the transaction and its signatures are valid.

Conclusion

We have now updated our flow to gather the lender's signature as well, in line with the constraints in IOUContract. We can now run our updated CorDapp, using the instructions here <hello-world-running>.

Our CorDapp now requires agreement from both the lender and the borrower before an IOU can be created on the ledger. This prevents either the lender or the borrower from unilaterally updating the ledger in a way that only benefits themselves.