corda/docs/source/using-a-notary.rst

Using a notary service
----------------------

This tutorial describes how to assign a notary to a newly issued state, and how to get a transaction notarised by
obtaining a signature of the required notary. It assumes some familiarity with *flows* and how to write them, as described
in :doc:`flow-state-machines`.

Assigning a notary
==================

The first step is to choose a notary and obtain its identity. Identities of all notaries on the network are kept by
the :ref:`network-map-service`. The network map cache exposes two methods for obtaining a notary:

.. sourcecode:: kotlin

    /**
     * Gets a notary identity by the given name.
     */
    fun getNotary(name: String): Party?

    /**
     * Returns a notary identity advertised by any of the nodes on the network (chosen at random)
     *
     * @param type Limits the result to notaries of the specified type (optional)
     */
    fun getAnyNotary(type: ServiceType? = null): Party?

Currently notaries can only be differentiated by name and type, but in the future the network map service will be
able to provide more metadata, such as location or legal identities of the nodes operating it.

Now, let's say we want to issue an asset and assign it to a notary named "Notary A".
The first step is to obtain the notary identity -- ``Party``:

.. sourcecode:: kotlin

    val ourNotary: Party = serviceHub.networkMapCache.getNotary("Central Bank Notary")

Then we initialise the transaction builder:

.. sourcecode:: kotlin

    val builder: TransactionBuilder = TransactionType.General.Builder(notary = ourNotary)

For any output state we add to this transaction builder, ``ourNotary`` will be assigned as its notary.
Next we create a state object and assign ourselves as the owner. For this example we'll use a
``DummyContract.State``, which is a simple state that just maintains an integer and can change ownership.

.. sourcecode:: kotlin

   val myIdentity = serviceHub.myInfo.legalIdentity
   val state = DummyContract.SingleOwnerState(magicNumber = 42, owner = myIdentity.owningKey)

Then we add the state as the transaction output along with the relevant command. The state will automatically be assigned
to our previously specified "Notary A".

.. sourcecode:: kotlin

   builder.addOutputState(state)
   val createCommand = DummyContract.Commands.Create()
   builder.addCommand(Command(createCommand, myIdentity))

We then sign the transaction, build and record it to our transaction storage:

.. sourcecode:: kotlin

   val mySigningKey: KeyPair = serviceHub.legalIdentityKey
   builder.signWith(mySigningKey)
   val issueTransaction = builder.toSignedTransaction()
   serviceHub.recordTransactions(issueTransaction)

The transaction is recorded and we now have a state (asset) in possession that we can transfer to someone else. Note
that the issuing transaction does not need to be notarised, as it doesn't consume any input states.

Notarising a transaction
========================

Following our example for the previous section, let's say we now want to transfer our issued state to Alice.

First we obtain a reference to the state, which will be the input to our "move" transaction:

.. sourcecode:: kotlin

    val stateRef = StateRef(txhash = issueTransaction.id, index = 0)

Then we create a new state -- a copy of our state but with the owner set to Alice. This is a bit more involved so
we just use a helper that handles it for us. We also assume that we already have the ``Party`` for Alice, ``aliceParty``.

.. sourcecode:: kotlin

    val inputState = StateAndRef(sate, stateRef)
    val moveTransactionBuilder = DummyContract.move(inputState, newOwner = aliceParty.owningKey)

The ``DummyContract.move()`` method will a new transaction builder with our old state as the input, a new state
with Alice as the owner, and a relevant contract command for "move".

Again we sign the transaction, and build it:

.. sourcecode:: kotlin

   moveTransactionBuilder.signWith(mySigningKey)
   // We build it without checking if all signatures are present, because we know that the notary signature is missing
   val moveTransaction = builder.toSignedTransaction(checkSufficientSignatures = false)

Next we need to obtain a signature from the notary for the transaction to be valid. Prior to signing, the notary will
commit our old (input) state so it cannot be used again.

To manually obtain a signature from a notary we can run the ``NotaryFlow.Client`` flow. The flow will work out
which notary needs to be called based on the input states (and the timestamp command, if it's present).

.. sourcecode:: kotlin

    // The subFlow() helper is available within the context of a Flow
    val notarySignature: DigitalSignature = subFlow(NotaryFlow.Client(moveTransaction))

.. note:: If our input state has already been consumed in another transaction, then ``NotaryFlow`` with throw a ``NotaryException``
   containing the conflict details:

   .. sourcecode:: kotlin
        /** Specifies the consuming transaction for the conflicting input state */
        data class Conflict(val stateHistory: Map<StateRef, ConsumingTx>)

        /**
        * Specifies the transaction id, the position of the consumed state in the inputs, and
        * the caller identity requesting the commit
        */
        data class ConsumingTx(val id: SecureHash, val inputIndex: Int, val requestingParty: Party)

   Conflict handling and resolution is currently the responsibility of the flow author.

Note that instead of calling the notary directly, we would normally call ``FinalityFlow`` passing in the ``SignedTransaction``
(including signatures from the participants) and a list of participants to notify. The flow will request a notary signature
if needed, record the notarised transaction, and then send a copy of the transaction to all participants for them to store.
``FinalityFlow`` delegates to ``NotaryFlow.Client`` followed by ``BroadcastTransactionFlow`` to do the
actual work of notarising and broadcasting the transaction. For example:

.. sourcecode:: kotlin

    subFlow(FinalityFlow(moveTransaction, setOf(aliceParty))