• you want to write Corda contracts with minimum code
• you want to write Corda contracts which are easy to read and understand
• you want to visualize your Corda contracts as described here: Automatic visualization of Corda contracts

The Contract SDK itself is a CorDapp. In order to use its power, you need to add it as a dependency in your build.gradle file. Typically, you would be referring to the Contract SDK from within the contracts module of your CorDapp. So adding the dependency to the build.gradle of the contracts module makes most sense.

You will have a choice to make, whether you want to:

1. Include the Contract SDK CorDapp code into your contracts jar (aka "fat jar" it) or
2. Keep the Contract SDK CorDapp out of your contracts jar, and manually add the Contract SDK jar to the Corda transactions as attachment when needed. Using, for example, this helper function.

If you decide for the second option, you need to provide checks stopping a potential attacker from using different (and possibly malicious) version of the Contract SDK jar to the one intended.

In order to include the Contract SDK code into your contracts jar, you would declare your dependency as:

compile "com.r3.corda.lib.contracts:contract-sdk:$corda_contracts_sdk_version"

In order to keep the Contract SDK jar separate, you would declare your dependency as:

cordaCompile "com.r3.corda.lib.contracts:contract-sdk:$corda_contracts_sdk_version"

The only version available at the moment is 0.9-SNAPSHOT. Make sure you have this repository declared in your build.gradle:

maven { url 'https://software.r3.com/artifactory/corda-lib-dev' }

Normally, you would go writing your contract like this:

class MyContract : Contract {

  override fun verify(tx: LedgerTransaction) {
      /*
        Verification logic here. Typically following the lines of:
        
        get the command from the transaction
        when (command) {
         is Action -> do some checks
         is SomeOtherAction -> do some other checks
        }
      */
  }

  interface Commands : CommandData {
      class Action : Commands
      class SomeOtherAction : Commands
  }

}

With the Contract SDK, you don't need to write the verify method of your contract. Instead, you extend from the StandardContract and you annotate your contract and your commands with annotations defining what you want the contract and commands to accept and enforce. In the below snippet, placeholder annotations are used to illustrate the concept. The full list of annotations available to you is in the next section of this document.

@Annotation_Defining_What_The_Contract_Requires_Or_Enforces_Always_1
@Annotation_Defining_What_The_Contract_Requires_Or_Enforces_Always_2
class MyContract : Contract, StandardContract() {

  interface Commands : CommandData {
      @Annotation_Defining_What_The_Command_Requires_Or_Enforces_When_Present_On_The_Transaction_1
      @Annotation_Defining_What_The_Command_Requires_Or_Enforces_When_Present_On_The_Transaction_2
      class Action : Commands

      @Annotation_Defining_What_The_Command_Requires_Or_Enforces_When_Present_On_The_Transaction_3
      class SomeOtherAction : Commands
  }

}

Note that MyContract here still must extend the Contract class, else it won't be recognized by the Corda's mechanism loading CorDapps.

As of now, there are over 30 annotations that you can use on the contract or command level. You can view them further down in this readme or in Annotations.kt. Their names follow a convention that should make it easier to find the annotation you are after. Each annotation follows this pattern:

@[Require|Permit|Forbid][Subject][Scope][AnyOtherQualifier](parameter1, parameter2...)

such as, for example:

• @RequireNumberOfStatesOnInputAtLeast(1)
• @PermitStatusOnInput('Pending')
• @RequireSignersFromEachOutputState('buyer','seller')
• @RequireStatusChangeInCoupledLinearStates('Pending','Active')
• @ForbidChangeInCoupledLinearStatesExcept('status','authorizedBy')

The only states in the transaction under verification which the annotations apply to are the states which belong to the contract (@BelongsToContract). So for example, if you defined contract MyContract and contract state MyState which belongs to MyContract and MyContract is annotated with @RequireNumberOfStatesOnInputBetween(0,5), then whilst the contract will only allow 0 to 5 of MyStates on the input, it won't care if you have 100 of input states of type SomeOtherState which doesn't belong to MyContract. Even this can be further narrowed down by using the targetClasses parameter of the annotations. More about it later in this readme.

The way the Contract SDK recognizes statuses of the Corda states is by utilizing the StateWithStatus interface. A state that wants to expose its status to the Contract SDK must implement that interface.

Similarly, the Contract SDK translates roles (e.g. 'buyer') to parties by utilizing the StateWithRoles interface. A state which wants to expose the translation of the roles it recognizes to parties must implement that interface.

This means that if you want to use annotations referring to statuses or roles (e.g. @PermitStatusOnInput or @RequireDistinctPartiesWithinEachInputState), you will also need to use those two interfaces when defining the Contract states. You can see an example of that in the examples/test-app, where the Membership states implement both of the interfaces (for convenience collapsed into StandardState interface).

Below is the whole list of annotations currently available for use. Click on the black triangle for a short description.

• Require
  • RequireNumberOfStatesOnInput

    The number of input states must be the one provided as parameter.
  • RequireNumberOfStatesOnInputAtLeast

    The number of input states must be at least the one provided as parameter.
  • RequireNumberOfStatesOnInputAtMost

    The number of input states must be at most the one provided as parameter.
  • RequireNumberOfStatesOnInputBetween

    The number of input states must be in the range provided in parameters.
  • RequireNumberOfStatesOnOutput

    The number of output states must be the one provided as parameter.
  • RequireNumberOfStatesOnOutputAtLeast

    The number of output states must be at least the one provided as parameter.
  • RequireNumberOfStatesOnOutputAtMost

    The number of output states must be at most the one provided as parameter.
  • RequireNumberOfStatesOnOutputBetween

    The number of output states must be in the range provided in parameters.
  • RequireSignersFromEachInputState

    Takes list of roles as parameter. Each input state is expected to implement interface StateWithRoles. During verification of the transaction each role is translated into a party (or parties) using the method from the StateWithRoles interface. And each such party must be a signer on the transaction.
  • RequireSignersFromEachOutputState

    Takes list of roles as parameter. Each output state is expected to implement interface StateWithRoles. During verification of the transaction each role is translated into a party (or parties) using the method from the StateWithRoles interface. And each such party must be a signer on the transaction.
  • RequireStatusChangeInCoupledLinearStates

    States on input and output must be linear and they must form pairs by linear id. They must also implement interface StateWithStatus. The status on the input side must be as defined in the first parameter and the status on the output must be as defined in the second parameter.
  • RequireDistinctPartiesWithinEachInputState

    Each state on input must implement interface StateWithRoles. Each state on input is asked to provide parties for each of the roles listed among the parameters. All these parties must be distinct.
  • RequireDistinctPartiesWithinEachOutputState

    Each state on output must implement interface StateWithRoles. Each state on output is asked to provide parties for each of the roles listed among the parameters. All these parties must be distinct.
  • RequirePartyToAssumeAtLeastOneOtherRoleWithinEachInputState

    Each state on input must implement interface StateWithRoles. Each state on input is asked to provide party for the role set in the first parameter and parties for the roles provided in the second parameter. The party resolved from the first parameter must be found among the parties resolved from the second parameter.
  • RequirePartyToAssumeAtLeastOneOtherRoleWithinEachOutputState

    Each state on output must implement interface StateWithRoles. Each state on output is asked to provide party for the role set in the first parameter and parties for the roles provided in the second parameter. The party resolved from the first parameter must be found among the parties resolved from the second parameter.
  • RequirePropertySetOnInput

    Each state on the input must have the property specified as the parameter set.
  • RequirePropertiesSetOnInput

    Each state on the input must have all the properties specified in the parameters set.
  • RequirePropertySetOnOutput

    Each state on the output must have the property specified as the parameter set.
  • RequirePropertiesSetOnOutput

    Each state on the output must have all the properties specified in the parameters set.
  • RequirePropertyNotSetOnInput

    Each state on the input must have the property specified as the parameter set to null.
  • RequirePropertiesNotSetOnInput

    Each state on the input must have all the properties specified in the parameters set to null.
  • RequirePropertyNotSetOnOutput

    Each state on the output must have the property specified as the parameter set to null.
  • RequirePropertiesNotSetOnOutput

    Each state on the output must have all the properties specified in the parameters set to null.
• Permit
  • PermitStatusOnInput

    All input states must be in the status set as parameter. To understand the status, the state must implement the StateWithStatus interface.
  • PermitStatusesOnInput

    All input states must be in one of the statuses listed in the parameter. To understand the status, the state must implement the StateWithStatus interface.
  • PermitStatusOnOutput

    All output states must be in the status set as parameter. To understand the status, the state must implement the StateWithStatus interface.
  • PermitStatusesOnOutput

    All output states must be in one of the statuses listed in the parameter. To understand the status, the state must implement the StateWithStatus interface.
• Forbid
  • ForbidChangeInCoupledLinearStatesExcept

    States on input and output must be linear and they must form pairs by linear id. No change in values of properties between input and output is allowed except for the exempt properties listed in the parameter.

Occasionally, you may want to tag the same command (or contract) with the same annotation more than once. For example the @RequireDistinctPartiesWithinEachInputState could be a candidate for such use. Kotlin (at least in the version supported by Corda) won't let you do this. But you can still achieve the same effect by using a @*List annotation, such as @RequireDistinctPartiesWithinEachInputStateList, where you provide the multiple annotations of the same type as a parameter. Here is an example of such use.

If you need to provide further verification logic, which is not offered by the annotations, then you have two options:

1. Implement the verifyFurther method of the StandardContract class, which you are extending your contract from. This method will be called on every transaction the contract verifies.
2. Have any of your commands implement the interface StandardCommand and its method verifyFurther. This method will be called if the transaction being verified contains that command.

It may be useful to know in which order the verification happens, so that you know what you can count on as already checked.

1. First the verification as per the annotations happens
2. Then the StandardContract.verifyFurther is called
3. Followed by StandardCommand.verifyFurther

Imagine you have a contract looking like this:

class MyContract : Contract, StandardContract() {

  interface Commands : CommandData {
      class Issue : Commands
  }

}

And two state types belonging to this contract.

@BelongsToContract(MyContract::class)
class StateOne(override val participants: List<AbstractParty>) : ContractState

@BelongsToContract(MyContract::class)
class StateTwo(override val participants: List<AbstractParty>) : ContractState

You can target the annotations driving the contract behaviour specifically to a list of state types using the targetClasses parameter. Below we allow the Issue command issue exactly one state of type StateOne but at least one state of type StateTwo.

class MyContract : Contract, StandardContract() {

  interface Commands : CommandData {
      @RequireNumberOfStatesOnOutput(1, targetClasses = [StateOne::class])
      @RequireNumberOfStatesOnOutputAtLeast(1, targetClasses = [StateTwo::class])
      class Issue : Commands
  }

}

Remember, if you don't provide the targetClasses parameter then the default behaviour is such that the annotation targets all the states which belong to the contract.

The Contract SDK is an open-source project and contributions are welcome as seen here: Contributing

Any suggestions / issues are welcome in the issues section: https://github.com/corda/contract-sdk/issues/new

• latest code is in main branch
• tags on main branch mark versions (e.g. v0.9)

Please note that this project is in proof-of-concept stage and the only testing it has gone through are the unit tests present in the project and some ad-hoc testing on the deployed test-app. Bear that in mind should you want to use this project in, say, production. Always test your contract logic and your entire CorDapp independently and thoroughly. Also, please note that R3 doesn't provide any support for this project. Responding to any issues raised will be on best effort basis.