corda/docs/source/tutorial-contract-clauses.rst

Writing a contract using clauses

This tutorial will take you through restructuring the commercial paper contract to use clauses. You should have already completed "tutorial-contract".

Clauses are essentially micro-contracts which contain independent verification logic, and can be logically composed together to form a contract. Clauses are designed to enable re-use of common logic, for example issuing state objects is generally the same for all fungible contracts, so a common issuance clause can be inherited for each contract's issue clause. This cuts down on scope for error, and improves consistency of behaviour. By splitting verification logic into smaller chunks, they can also be readily tested in isolation.

Clauses can be composed of subclauses, for example the AllClause or AnyClause clauses take list of clauses that they delegate to. Clauses can also change the scope of states and commands being verified, for example grouping together fungible state objects and running a clause against each distinct group.

The commercial paper contract has a Group outermost clause, which contains the Issue, Move and Redeem clauses. The result is a contract that looks something like this:

1. Group input and output states together, and then apply the following clauses on each group:

   1. If an Issue command is present, run appropriate tests and end processing this group.
   2. If a Move command is present, run appropriate tests and end processing this group.
   3. If a Redeem command is present, run appropriate tests and end processing this group.

Commercial paper class

To use the clause verification logic, the contract needs to call the verifyClause function, passing in the transaction, a clause to verify, and a collection of commands the clauses are expected to handle all of. This list of commands is important because verifyClause checks that none of the commands are left unprocessed at the end, and raises an error if they are. The top level clause would normally be a composite clause (such as AnyComposition, AllComposition, etc.) which contains further clauses. The following examples are trimmed to the modified class definition and added elements, for brevity:

class CommercialPaper : Contract {
    override val legalContractReference: SecureHash = SecureHash.sha256("https://en.wikipedia.org/wiki/Commercial_paper")

    override fun verify(tx: TransactionForContract) = verifyClause(tx, Clauses.Group(), tx.commands.select<Commands>())

public class CommercialPaper implements Contract {
    @Override
    public SecureHash getLegalContractReference() {
        return SecureHash.Companion.sha256("https://en.wikipedia.org/wiki/Commercial_paper");
    }

    @Override
    public void verify(@NotNull TransactionForContract tx) throws IllegalArgumentException {
        ClauseVerifier.verifyClause(tx, new Clauses.Group(), extractCommands(tx));
    }

Clauses

We'll tackle the inner clauses that contain the bulk of the verification logic, first, and the clause which handles grouping of input/output states later. The clauses must extend the Clause abstract class, which defines the verify function, and the requiredCommands property used to determine the conditions under which a clause is triggered. Composite clauses should extend the CompositeClause abstract class, which extends Clause to add support for wrapping around multiple clauses.

The verify function defined in the Clause interface is similar to the conventional Contract verification function, although it adds new parameters and returns the set of commands which it has processed. Normally this returned set is identical to the requiredCommands used to trigger the clause, however in some cases the clause may process further optional commands which it needs to report that it has handled.

The Move clause for the commercial paper contract is relatively simple, so we will start there:

class Move: Clause<State, Commands, Issued<Terms>>() {
    override val requiredCommands: Set<Class<out CommandData>>
        get() = setOf(Commands.Move::class.java)

    override fun verify(tx: TransactionForContract,
                        inputs: List<State>,
                        outputs: List<State>,
                        commands: List<AuthenticatedObject<Commands>>,
                        groupingKey: Issued<Terms>?): Set<Commands> {
        val command = commands.requireSingleCommand<Commands.Move>()
        val input = inputs.single()
        requireThat {
            "the transaction is signed by the owner of the CP" by (input.owner in command.signers)
            "the state is propagated" by (outputs.size == 1)
            // Don't need to check anything else, as if outputs.size == 1 then the output is equal to
            // the input ignoring the owner field due to the grouping.
        }
        return setOf(command.value)
    }
}

class Move extends Clause<State, Commands, State> {
    @NotNull
    @Override
    public Set<Class<? extends CommandData>> getRequiredCommands() {
        return Collections.singleton(Commands.Move.class);
    }

    @NotNull
    @Override
    public Set<Commands> verify(@NotNull TransactionForContract tx,
                                   @NotNull List<? extends State> inputs,
                                   @NotNull List<? extends State> outputs,
                                   @NotNull List<? extends AuthenticatedObject<? extends Commands>> commands,
                                   @NotNull State groupingKey) {
        AuthenticatedObject<Commands.Move> cmd = requireSingleCommand(tx.getCommands(), Commands.Move.class);
        // There should be only a single input due to aggregation above
        State input = single(inputs);

        if (!cmd.getSigners().contains(input.getOwner()))
            throw new IllegalStateException("Failed requirement: the transaction is signed by the owner of the CP");

        // Check the output CP state is the same as the input state, ignoring the owner field.
        if (outputs.size() != 1) {
            throw new IllegalStateException("the state is propagated");
        }
        // Don't need to check anything else, as if outputs.size == 1 then the output is equal to
        // the input ignoring the owner field due to the grouping.
        return Collections.singleton(cmd.getValue());
    }
}

Group Clause

We need to wrap the move clause (as well as the issue and redeem clauses - see the relevant contract code for their full specifications) in an outer clause that understands how to group contract states and objects. For this we extend the standard GroupClauseVerifier and specify how to group input/output states, as well as the top-level to run on each group. As with the top level clause on a contract, this is normally a composite clause that delegates to subclauses.

class Group : GroupClauseVerifier<State, Commands, Issued<Terms>>(
    AnyComposition(
        Redeem(),
        Move(),
        Issue())) {
    override fun groupStates(tx: TransactionForContract): List<TransactionForContract.InOutGroup<State, Issued<Terms>>>
            = tx.groupStates<State, Issued<Terms>> { it.token }
}

class Group extends GroupClauseVerifier<State, Commands, State> {
    public Group() {
        super(new AnyComposition<>(
            new Clauses.Redeem(),
            new Clauses.Move(),
            new Clauses.Issue()
        ));
    }

    @NotNull
    @Override
    public List<InOutGroup<State, State>> groupStates(@NotNull TransactionForContract tx) {
        return tx.groupStates(State.class, State::withoutOwner);
    }
}

For the CommercialPaper contract, this is the top level clause for the contract, and is passed directly into verifyClause (see the example code at the top of this tutorial).

Summary

In summary the top level contract CommercialPaper specifies a single grouping clause of type CommercialPaper.Clauses.Group which in turn specifies GroupClause implementations for each type of command (Redeem, Move and Issue). This reflects the flow of verification: In order to verify a CommercialPaper we first group states, check which commands are specified, and run command-specific verification logic accordingly.

Debugging

Debugging clauses which have been composed together can be complicated due to the difficulty in knowing which clauses have been matched, whether specific clauses failed to match or passed verification, etc. There is "trace" level logging code in the clause verifier which evaluates which clauses will be matched and logs them, before actually performing the validation. To enable this, ensure trace level logging is enabled on the Clause interface.