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corda/docs/source/quickstart-build.rst
Ed Prosser 0a9ab5a763 spoke too soon 
Signed-off-by: Ed Prosser <edward.prosser@r3.com>
2019-07-12 10:51:12 +02:00

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Building your own CorDapp

After examining a functioning CorDapp, the next challenge is to create one of your own. We're going to build a simple supply chain CorDapp representing a network between a car dealership, a car manufacturer, and a bank.

To model this network, you need to create one state (representing cars), one contract (to control the rules governing cars), and one flow (to create cars). This CorDapp will be very basic, but entirely functional and deployable.

Step One: Download a template CorDapp

The first thing you need to do is clone a CorDapp template to modify.

  1. Open a terminal and navigate to a directory to store the new project.
  2. Run the following command to clone the template CorDapp: git clone https://github.com/corda/cordapp-template-kotlin.git
  3. Open IntelliJ and open the CorDapp template project.

Step Two: Creating states

Since the CorDapp models a car dealership network, a state must be created to represent cars. States are immutable objects representing on-ledger facts. A state might represent a physical asset like a car, or an intangible asset or agreement like an IOU. For more information on states, see the state documentation.

  1. From IntelliJ expand the source files and navigate to the following state template file: contracts > src > main > kotlin > com > template > states > TemplateState.kt.

  2. Right-click on TemplateState.kt in the project navigation on the left. Select Refactor > Copy.

  3. Rename the file to CarState and click OK.

  4. Add the following imports to the top of the state file:

    import com.template.contracts.CarContract
import com.template.contracts.TemplateContract
import net.corda.core.contracts.BelongsToContract
import net.corda.core.contracts.ContractState
import net.corda.core.contracts.UniqueIdentifier
import net.corda.core.identity.AbstractParty
import net.corda.core.identity.Party

It's important to specify what classes are required in each state, contract, and flow. This process must be repeated with each file as it is created.

  1. Update @BelongsToContract(TemplateContract:class) to specify CarContract::class.
6. Double-click the new state file to open it. Add the following fields to the state:
  • owningBank of type Party
  • holdingDealer of type Party
  • manufacturer of type Party
  • vin of type String
  • licensePlateNumber of type String
  • make of type String
  • model of type String
  • dealershipLocation of type String
  • linearId of type UniqueIdentifier

Don't worry if you're not sure exactly how these should appear, you can check your code shortly.

  1. Remove the data and participants parameters.

  2. Add a body to the CarState class that overrides participants to contain a list of owningBank, holdingDealer, and manufacturer.

  3. The CarState file should now appear as follows:

    package com.template.states

import com.template.contracts.CarContract
import com.template.contracts.TemplateContract
import net.corda.core.contracts.BelongsToContract
import net.corda.core.contracts.ContractState
import net.corda.core.contracts.UniqueIdentifier
import net.corda.core.identity.AbstractParty
import net.corda.core.identity.Party

// *********
// * State *
// *********

@BelongsToContract(CarContract::class)
data class CarState(val owningBank: Party,
                    val holdingDealer: Party,
                    val manufacturer: Party,
                    val vin: String,
                    val licensePlateNumber: String,
                    val make: String,
                    val model: String,
                    val dealershipLocation: String,
                    val linearId: UniqueIdentifier) : ContractState {
    override val participants: List<AbstractParty> = listOf(owningBank, holdingDealer, manufacturer)
}

  4. Save the CarState.kt file.

The CarState definition has now been created. It lists the properties and associated types required of all instances of this state.

Step Three: Creating contracts

After creating a state, you must create a contract. Contracts define the rules that transactions are validated against, for example, a contract might define that an issue transaction must have no input states, and one output state of a specified type. To learn more about contracts, see the contracts documentation.

  1. From IntelliJ, expand the project source and navigate to: contracts > src > main > kotlin > com > template > contracts > TemplateContract.kt

  2. Right-click on TemplateContract.kt in the project navigation on the left. Select Refactor > Copy.

  3. Rename the file to CarContract and click OK.

  4. Double-click the new contract file to open it.

  5. Add the following imports to the top of the file:

    package com.template.contracts

import com.template.states.CarState
import net.corda.core.contracts.CommandData
import net.corda.core.contracts.Contract
import net.corda.core.contracts.requireSingleCommand
import net.corda.core.contracts.requireThat
import net.corda.core.transactions.LedgerTransaction

  6. Update the class name to: CarContract

  7. Update the ID field to com.template.contracts.CarContract. This ID field is used to identify contracts when building a transaction.

  8. Update the Action command to an Issue command. This represents an issuance of an instance of the CarState state.

  9. Add val command=tx.commands.requireSingleCommand<Commands.Issue>() at the beginning of the verify() method. This line ensures that the command to issue a car state is called.

  10. The final function of the contract is to prevent unwanted behaviour during the flow. After the val command=tx.commands... line, add the following requirement code:

    requireThat {
    "There should be no input state" using (tx.inputs.isEmpty())
}

  11. Inside the requireThat block add additional lines defining the following requirements:

  • There should be one output state.
  • The output state must be of the type CarState.
  • The licensePlateNumber must be seven characters long.

  1. The CarContract.kt file should look as follows:

    class CarContract : Contract {
    companion object {
        const val ID = "com.template.contracts.CarContract"
    }

    override fun verify(tx: LedgerTransaction) {

        val command = tx.commands.requireSingleCommand<Commands.Issue>()
        requireThat {
            "There should be no input state" using (tx.inputs.isEmpty())
            "There should be one input state" using (tx.outputs.size == 1)
            "The output state must be of type CarState" using (tx.outputs.get(0).data is CarState)
            val outputState = tx.outputs.get(0).data as CarState
            "The licensePlateNumber must be seven characters long" using (outputState.licensePlateNumber.length == 7)
        }
    }

    interface Commands : CommandData {
        class Issue : Commands
    }
}

  2. Save the CarContract.kt file. The contract file now defines rules that all transactions creating car states must follow.

Step Four: Creating a flow

  1. From IntelliJ, expand the project source and navigate to: workflows > src > main > kotlin > com > template > flows > Flows.kt

  2. Right-click on Flows.kt in the project navigation on the left. Select Refactor > Copy.

  3. Rename the file to CarFlow and click OK.

  4. Add the following imports to the top of the file:

    import co.paralleluniverse.fibers.Suspendable
import com.template.contracts.CarContract
import com.template.states.CarState
import net.corda.core.contracts.Command
import net.corda.core.contracts.UniqueIdentifier
import net.corda.core.contracts.requireThat
import net.corda.core.flows.*
import net.corda.core.identity.Party
import net.corda.core.node.ServiceHub
import net.corda.core.transactions.SignedTransaction
import net.corda.core.transactions.TransactionBuilder

  5. Double-click the new contract file to open it.

  6. Update the name of the Initiator class to CarIssueInitiator.

  7. Update the name of the Responder class to CarIssueResponder.

  8. Update the @InitiatedBy property of CarIssueResponder to CarIssueInitiator::class.

  9. Add parameters to the CarIssueInitiator class for all the fields of the CarState definition, except for linearId.

  10. Inside the call() function of the initiator, create a variable for the notary node. expand this with some code

  11. Create a variable for an Issue command.

The first parameter of the command must be the command type, in this case Issue.

The second parameter of the command must be a list of keys from the relevant parties, in this case owningBank, holdingDealer, and manufacturer.

  1. Create a CarState object using the parameters of CarIssueInitiator.

The last parameter for CarState must be a new UniqueIdentifier() object.

  1. The CarFlow.kt file should look like this:

    @InitiatingFlow
@StartableByRPC
class CarIssueInitiator(val owningBank: Party,
                        val holdingDealer: Party,
                        val manufacturer: Party,
                        val vin: String,
                        val licensePlateNumber: String,
                        val make: String,
                        val model: String,
                        val dealershipLocation: String) : FlowLogic<Unit>() {

    @Suspendable
    override fun call() {
        val notary = serviceHub.networkMapCache.notaryIdentities.single()
        val command = Command(CarContract.Commands.Issue(), listOf(owningBank, holdingDealer, manufacturer).map { it.owningKey })
        val carState = CarState(owningBank, holdingDealer, manufacturer, vin, licensePlateNumber, make, model, dealershipLocation, UniqueIdentifier())
    }
}

@InitiatedBy(CarIssueInitiator::class)
class CarIssueResponder(val counterpartySession: FlowSession) : FlowLogic<Unit>() {
    @Suspendable
    override fun call(){

        }
    }
}

  2. Update the FlowLogic<Unit> to FlowLogic<SignedTransaction> in both the initiator and responder class.

  3. Update the return type of both call() transactions to be of type SignedTransaction.

  4. In the call() function, create a TransactionBuilder object similarly. The TransactionBuilder class should take in the notary node. The output state and command must be added to the TransactionBuilder.

  5. Verify the transaction by calling verify(serviceHub) on the TransactionBuilder.

  6. Sign the transaction and store the result in a variable.

  7. Delete the progressTracker as it won't be used in this tutorial.

  8. The CarFlow.kt file should now look like this:

    @InitiatingFlow
@StartableByRPC
class CarIssueInitiator(val owningBank: Party,
                        val holdingDealer: Party,
                        val manufacturer: Party,
                        val vin: String,
                        val licensePlateNumber: String,
                        val make: String,
                        val model: String,
                        val dealershipLocation: String) : FlowLogic<SignedTransaction>() {
    override val progressTracker = ProgressTracker()

    @Suspendable
    override fun call(): SignedTransaction {

        val notary = serviceHub.networkMapCache.notaryIdentities.single()
        val command = Command(CarContract.Commands.Issue(), listOf(owningBank, holdingDealer, manufacturer).map { it.owningKey })
        val carState = CarState(owningBank, holdingDealer, manufacturer, vin, licensePlateNumber, make, model, dealershipLocation, UniqueIdentifier())

        val txBuilder = TransactionBuilder(notary)
                .addOutputState(carState, CarContract.ID)
                .addCommand(command)

        txBuilder.verify(serviceHub)
        val tx = serviceHub.signInitialTransaction(txBuilder)
    }
}

@InitiatedBy(CarIssueInitiator::class)
class CarIssueResponder(val counterpartySession: FlowSession) : FlowLogic<SignedTransaction>() {
    @Suspendable
    override fun call(): SignedTransaction {

        }
    }
}

  9. To finish the initiators call() function, other parties must sign the transaction. Add the following code to send the transaction to the other relevant parties:

    val sessions = (carState.participants - ourIdentity).map { initiateFlow(it as Party) }
val stx = subFlow(CollectSignaturesFlow(tx, sessions))
return subFlow(FinalityFlow(stx, sessions))

The first line creates a List<FlowSession> object by calling initiateFlow() for each party. The second line collects signatures from the relevant parties and returns a signed transaction. The third line calls FinalityFlow(), finalizes the transaction using the notary or notary pool.

  1. Lastly, the body of the responder flow must be completed. The following code checks the transaction contents, signs it, and sends it back to the initiator:

    @Suspendable
override fun call(): SignedTransaction {
    val signedTransactionFlow = object : SignTransactionFlow(counterpartySession) {
        override fun checkTransaction(stx: SignedTransaction) = requireThat {
            val output = stx.tx.outputs.single().data
            "The output must be a CarState" using (output is CarState)
        }
    }
    val txWeJustSignedId = subFlow(signedTransactionFlow)
    return subFlow(ReceiveFinalityFlow(counterpartySession, txWeJustSignedId.id))
}

  2. The completed CarFlow.kt should look like this:

    @InitiatingFlow
@StartableByRPC
class CarIssueInitiator(val owningBank: Party,
                        val holdingDealer: Party,
                        val manufacturer: Party,
                        val vin: String,
                        val licensePlateNumber: String,
                        val make: String,
                        val model: String,
                        val dealershipLocation: String) : FlowLogic<SignedTransaction>() {
    @Suspendable
    override fun call(): SignedTransaction {

        val notary = serviceHub.networkMapCache.notaryIdentities.single()
        val command = Command(CarContract.Commands.Issue(), listOf(owningBank, holdingDealer, manufacturer).map { it.owningKey })
        val carState = CarState(owningBank, holdingDealer, manufacturer, vin, licensePlateNumber, make, model, dealershipLocation, UniqueIdentifier())

        val txBuilder = TransactionBuilder(notary)
                .addOutputState(carState, CarContract.ID)
                .addCommand(command)

        txBuilder.verify(serviceHub)
        val tx = serviceHub.signInitialTransaction(txBuilder)

        val sessions = (carState.participants - ourIdentity).map { initiateFlow(it as Party) }
        val stx = subFlow(CollectSignaturesFlow(tx, sessions))
        return subFlow(FinalityFlow(stx, sessions))
    }
}

@InitiatedBy(CarIssueInitiator::class)
class CarIssueResponder(val counterpartySession: FlowSession) : FlowLogic<SignedTransaction>() {

    @Suspendable
    override fun call(): SignedTransaction {
        val signedTransactionFlow = object : SignTransactionFlow(counterpartySession) {
            override fun checkTransaction(stx: SignedTransaction) = requireThat {
                val output = stx.tx.outputs.single().data
                "The output must be a CarState" using (output is CarState)
            }
        }
        val txWeJustSignedId = subFlow(signedTransactionFlow)
        return subFlow(ReceiveFinalityFlow(counterpartySession, txWeJustSignedId.id))
    }
}

Step Five: Update the Gradle build

The Gradle build files must be updated to change how the nodes are deployed. (how)

  1. Navigate to the build.gradle file in the root cordapp-template-kotlin directory.

  2. In the deployNodes task, update the nodes to read as follows:

    node {
    name "O=Notary,L=London,C=GB"
    notary = [validating : false]
    p2pPort 10002
    rpcSettings {
        address("localhost:10003")
        adminAddress("localhost:10043")
    }
}
node {
    name "O=Dealership,L=London,C=GB"
    p2pPort 10005
    rpcSettings {
        address("localhost:10006")
        adminAddress("localhost:10046")
    }
    rpcUsers = [[ user: "user1", "password": "test", "permissions": ["ALL"]]]
}
node {
    name "O=Manufacturer,L=New York,C=US"
    p2pPort 10008
    rpcSettings {
        address("localhost:10009")
        adminAddress("localhost:10049")
    }
    rpcUsers = [[ user: "user1", "password": "test", "permissions": ["ALL"]]]
}
node {
    name "O=BankofAmerica,L=New York,C=US"
    p2pPort 10010
    rpcSettings {
        address("localhost:10007")
        adminAddress("localhost:10047")
    }
    rpcUsers = [[ user: "user1", "password": "test", "permissions": ["ALL"]]]
}

  3. Save the updated build.gradle file and click Import Changes when the pop-up message appears in the lower-right corner.

Step Six: Deploying your CorDapp locally

Now that the CorDapp code has been completed and the build file updated, the CorDapp can be deployed.

  1. Open a terminal and navigate to the root directory of the project.
  2. To deploy the nodes on Windows run the following command: gradlew clean deployNodes

To deploy the nodes on Mac or Linux run the following command: ./gradlew clean deployNodes

  1. To start the nodes on Windows run the following command: build\nodes\runnodes

To start the nodes on Mac/Linux run the following command: build/nodes/runnodes

  1. To run flows in your CorDapp, enter the following flow command from any node terminal window: flow start CarIssueInitiator owningBank: Bank of America, holdingDealer: Dealership, manufacturer: Manufacturer, vin:"abc", licensePlateNumber: "abc1234", make: "Honda", model: "Civic", dealershipLocation: "NYC"
  2. To check that the state was correctly issued, query the node using the following command:

run vaultQuery contractStateType: com.template.states.CarState

The vault is the node's repository of all information from the ledger that involves that node, stored in a relational model. After running the query, the terminal should display the state created by the flow command. This command can be run from the terminal window of any node, as all parties are participants in this transaction.

Next steps

The getting started experience is designed to be lightweight and get to code as quickly as possible, for more detail, see the following documentation:

For operational users, see the following documentation: