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

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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``/``IOUFlow.kt``, change the imports block to the following:

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    .. literalinclude:: example-code/src/main/kotlin/net/corda/docs/tutorial/twoparty/flow.kt
        :language: kotlin
        :start-after: DOCSTART 01
        :end-before: DOCEND 01

    .. literalinclude:: example-code/src/main/java/net/corda/docs/java/tutorial/twoparty/IOUFlow.java
        :language: java
        :start-after: DOCSTART 01
        :end-before: DOCEND 01

And update ``IOUFlow.call`` by changing the code following the creation of the ``TransactionBuilder`` as follows:

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    .. literalinclude:: example-code/src/main/kotlin/net/corda/docs/tutorial/twoparty/flow.kt
        :language: kotlin
        :start-after: DOCSTART 02
        :end-before: DOCEND 02
        :dedent: 8

    .. literalinclude:: example-code/src/main/java/net/corda/docs/java/tutorial/twoparty/IOUFlow.java
        :language: java
        :start-after: DOCSTART 02
        :end-before: DOCEND 02
        :dedent: 8

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.Commands.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:

.. container:: codeset

    .. literalinclude:: example-code/src/main/kotlin/net/corda/docs/tutorial/twoparty/flowResponder.kt
        :language: kotlin
        :start-after: DOCSTART 01
        :end-before: DOCEND 01

    .. literalinclude:: example-code/src/main/java/net/corda/docs/java/tutorial/twoparty/IOUFlowResponder.java
        :language: java
        :start-after: DOCSTART 01
        :end-before: DOCEND 01

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
:doc:`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.

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

If you get stuck at any point, please reach out on `Slack <https://slack.corda.net/>`_ or
`Stack Overflow <https://stackoverflow.com/questions/tagged/corda>`_.