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145 lines
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ReStructuredText
145 lines
6.6 KiB
ReStructuredText
.. highlight:: kotlin
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.. raw:: html
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<script type="text/javascript" src="_static/jquery.js"></script>
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<script type="text/javascript" src="_static/codesets.js"></script>
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Updating the flow
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=================
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We now need to update our flow to achieve three things:
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* Verifying that the transaction proposal we build fulfills the ``IOUContract`` constraints
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* Updating the lender's side of the flow to request the borrower's signature
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* Creating a response flow for the borrower that responds to the signature request from the lender
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We'll do this by modifying the flow we wrote in the previous tutorial.
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Verifying the transaction
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-------------------------
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In ``IOUFlow.java``/``Flows.kt``, change the imports block to the following:
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.. container:: codeset
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.. literalinclude:: example-code/src/main/kotlin/net/corda/docs/kotlin/tutorial/twoparty/IOUFlow.kt
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:language: kotlin
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:start-after: DOCSTART 01
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:end-before: DOCEND 01
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.. literalinclude:: example-code/src/main/java/net/corda/docs/java/tutorial/twoparty/IOUFlow.java
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:language: java
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:start-after: DOCSTART 01
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:end-before: DOCEND 01
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And update ``IOUFlow.call`` to the following:
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.. container:: codeset
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.. literalinclude:: example-code/src/main/kotlin/net/corda/docs/kotlin/tutorial/twoparty/IOUFlow.kt
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:language: kotlin
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:start-after: DOCSTART 02
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:end-before: DOCEND 02
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:dedent: 8
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.. literalinclude:: example-code/src/main/java/net/corda/docs/java/tutorial/twoparty/IOUFlow.java
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:language: java
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:start-after: DOCSTART 02
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:end-before: DOCEND 02
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:dedent: 8
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In the original CorDapp, we automated the process of notarising a transaction and recording it in every party's vault
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by invoking a built-in flow called ``FinalityFlow`` as a subflow. We're going to use another pre-defined flow,
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``CollectSignaturesFlow``, to gather the borrower's signature.
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First, we need to update the command. We are now using ``IOUContract.Create``, rather than
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``TemplateContract.Commands.Action``. We also want to make the borrower a required signer, as per the contract
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constraints. This is as simple as adding the borrower's public key to the transaction's command.
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We also need to add the output state to the transaction using a reference to the ``IOUContract``, instead of to the old
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``TemplateContract``.
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Now that our state is governed by a real contract, we'll want to check that our transaction proposal satisfies these
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requirements before kicking off the signing process. We do this by calling ``TransactionBuilder.verify`` on our
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transaction proposal before finalising it by adding our signature.
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Requesting the borrower's signature
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-----------------------------------
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Previously we wrote a responder flow for the borrower in order to receive the finalised transaction from the lender.
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We use this same flow to first request their signature over the transaction.
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We gather the borrower's signature using ``CollectSignaturesFlow``, which takes:
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* A transaction signed by the flow initiator
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* A list of flow-sessions between the flow initiator and the required signers
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And returns a transaction signed by all the required signers.
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We can then pass this fully-signed transaction into ``FinalityFlow``.
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Updating the borrower's flow
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----------------------------
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On the lender's side, we used ``CollectSignaturesFlow`` to automate the collection of signatures. To allow the borrower
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to respond, we need to update its responder flow to first receive the partially signed transaction for signing. Update
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``IOUFlowResponder.call`` to be the following:
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.. container:: codeset
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.. literalinclude:: example-code/src/main/kotlin/net/corda/docs/kotlin/tutorial/twoparty/IOUFlowResponder.kt
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:language: kotlin
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:start-after: DOCSTART 1
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:end-before: DOCEND 1
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:dedent: 4
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.. literalinclude:: example-code/src/main/java/net/corda/docs/java/tutorial/twoparty/IOUFlowResponder.java
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:language: java
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:start-after: DOCSTART 1
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:end-before: DOCEND 1
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:dedent: 4
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We could write our own flow to handle this process. However, there is also a pre-defined flow called
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``SignTransactionFlow`` that can handle the process automatically. The only catch is that ``SignTransactionFlow`` is an
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abstract class - we must subclass it and override ``SignTransactionFlow.checkTransaction``.
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CheckTransactions
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^^^^^^^^^^^^^^^^^
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``SignTransactionFlow`` will automatically verify the transaction and its signatures before signing it. However, just
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because a transaction is contractually valid doesn't mean we necessarily want to sign. What if we don't want to deal
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with the counterparty in question, or the value is too high, or we're not happy with the transaction's structure?
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Overriding ``SignTransactionFlow.checkTransaction`` allows us to define these additional checks. In our case, we are
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checking that:
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* The transaction involves an ``IOUState`` - this ensures that ``IOUContract`` will be run to verify the transaction
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* The IOU's value is less than some amount (100 in this case)
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If either of these conditions are not met, we will not sign the transaction - even if the transaction and its
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signatures are contractually valid.
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Once we've defined the ``SignTransactionFlow`` subclass, we invoke it using ``FlowLogic.subFlow``, and the
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communication with the borrower's and the lender's flow is conducted automatically.
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``SignedTransactionFlow`` returns the newly signed transaction. We pass in the transaction's ID to ``ReceiveFinalityFlow``
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to ensure we are recording the correct notarised transaction from the lender.
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Conclusion
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----------
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We have now updated our flow to verify the transaction and gather the lender's signature, in line with the constraints
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defined in ``IOUContract``. We can now re-run our updated CorDapp, using the
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:doc:`same instructions as before <hello-world-running>`.
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Our CorDapp now imposes restrictions on the issuance of IOUs. Most importantly, IOU issuance now requires agreement
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from both the lender and the borrower before an IOU can be created on the blockchain. This prevents either the lender or
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the borrower from unilaterally updating the ledger in a way that only benefits themselves.
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After completing this tutorial, your CorDapp should look like this:
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* Java: https://github.com/corda/corda-tut2-solution-java
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* Kotlin: https://github.com/corda/corda-tut2-solution-kotlin
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You should now be ready to develop your own CorDapps. You can also find a list of sample CorDapps
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`here <https://www.corda.net/samples/>`_. You are now ready to learn more about the Corda API.
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If you get stuck at any point, please reach out on `Slack <https://slack.corda.net/>`_ or
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`Stack Overflow <https://stackoverflow.com/questions/tagged/corda>`_.
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