Paths

Path and Path constraints are probably the most complicated aspect of implementing a CDL Smart contract. To help simplify the implementation of Path constraints, some functions have been abstracted into a ContractUtils.kt file. This leaves a relatively simple implementation of the verifyPathConstraints() function.

The first step is to define Paths and PathConstraints.

A Path represents the transition that a Primary state makes from a given input status within a specific transaction. Path is implemented as follows:

Contractutils.kt:

class Path<T: StatusState>(val command: CommandData,
                val outputStatus: Status?,
                val numberOfInputStates: Int,
                val numberOfOutputStates: Int,
                val additionalStates: Set<AdditionalStates> = setOf())

class AdditionalStates(val type: AdditionalStatesType, val clazz: Class<out ContractState>, val numberOfStates: Int)

enum class AdditionalStatesType {INPUT, OUTPUT, REFERENCE}

Where:

  • command represents the command.value in the transaction which relates to the Primary state’s Contract (there could be other commands in the transaction but they are not dealt with by Paths).
  • outputStatus represents the status of the output Primary state. it will be null if there is no output state.
  • numberOfInputStates represents the number of input states of the Primary state type in the transaction.
  • numberOfOutputStates represents the number of outputs states of the Primary state type in the transaction.
  • additionalStates represents state types other than the Primary States type which are in the transaction, including whether they are inputs, outputs or reference states and how many of each are in the transaction.

In the Agreement CorDapp numberOfInputStates and numberOfOutputStates are going to be set to 0 or 1. This is because for any agreement there should only be a maximum of one AgreementState unconsumed at any point in time representing the latest state of this agreement. However, in other use cases there could be different Multiplicities involved. For example, additionalStates can be used to represent the BillingChips required in the Agree Paths (although this is not yet implemented in the cdl-example CorDapp).