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

Updating the flow

We now need to update our flow to achieve three things:

• Verifying that the transaction proposal we build fulfills the IOUContract constraints
• Updating the lender's side of the flow to request the borrower's signature
• Creating a response flow for the borrower that responds to the signature request from the lender

We'll do this by modifying the flow we wrote in the previous tutorial.

Verifying the transaction

In IOUFlow.java/Flows.kt, change the imports block to the following:

example-code/src/main/kotlin/net/corda/docs/kotlin/tutorial/twoparty/IOUFlow.kt

example-code/src/main/java/net/corda/docs/java/tutorial/twoparty/IOUFlow.java

And update IOUFlow.call by changing the code following the retrieval of the notary's identity from the network as follows:

example-code/src/main/kotlin/net/corda/docs/kotlin/tutorial/twoparty/IOUFlow.kt

example-code/src/main/java/net/corda/docs/java/tutorial/twoparty/IOUFlow.java

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, CollectSignaturesFlow, to gather the borrower's signature.

First, we need to update the command. We are now using IOUContract.Create, rather than TemplateContract.Commands.Action. We also want to make the borrower a required signer, as per the contract constraints. This is as simple as adding the borrower's public key to the transaction's command.

We also need to add the output state to the transaction using a reference to the IOUContract, instead of to the old TemplateContract.

Now that our state is governed by a real contract, we'll want to check that our transaction proposal satisfies these requirements before kicking off the signing process. We do this by calling TransactionBuilder.verify on our transaction proposal before finalising it by adding our signature.

Requesting the borrower's signature

We now need to communicate with the borrower to request their signature over the transaction. 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 can then pass this fully-signed transaction into FinalityFlow.

Creating the borrower's flow

On the lender's side, we used CollectSignaturesFlow to automate the collection of signatures. To allow the lender to respond, we need to write a response flow as well. In a new IOUFlowResponder.java file in Java, or within the App.kt file in Kotlin, add the following class:

example-code/src/main/kotlin/net/corda/docs/kotlin/tutorial/twoparty/IOUFlowResponder.kt

example-code/src/main/java/net/corda/docs/java/tutorial/twoparty/IOUFlowResponder.java

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 write our own flow to handle this process. However, there is also a pre-defined flow called SignTransactionFlow that can handle the process automatically. The only catch is that SignTransactionFlow is an abstract class - we must subclass it and override SignTransactionFlow.checkTransaction.

CheckTransactions

SignTransactionFlow will automatically verify the transaction and its signatures before signing it. However, just because a transaction is contractually 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 contractually valid.

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

Conclusion

We have now updated our flow to verify the transaction and gather the lender's signature, in line with the constraints defined in IOUContract. We can now re-run our updated CorDapp, using the same instructions as before <hello-world-running>.

Our CorDapp now imposes restrictions on the issuance of IOUs. Most importantly, IOU issuance 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.

After completing this tutorial, your CorDapp should look like this:

• Java: https://github.com/corda/corda-tut2-solution-java
• Kotlin: https://github.com/corda/corda-tut2-solution-kotlin

You should now be ready to develop your own CorDapps. You can also find a list of sample CorDapps here. As you write CorDapps, you'll also want to learn more about the Corda API <corda-api>.

If you get stuck at any point, please reach out on Slack or Stack Overflow.