Upgrading contracts

While every care is taken in development of contract code, inevitably upgrades will be required to fix bugs (in either design or implementation). Upgrades can involve a substitution of one version of the contract code for another or changing to a different contract that understands how to migrate the existing state objects. When state objects are added as outputs to transactions, they are linked to the contract code they are intended for via the StateAndContract type. Changing a state’s contract only requires substituting one ContractClassName for another.

Workflow

Here’s the workflow for contract upgrades:

  1. Banks A and B negotiate a trade, off-platform
  2. Banks A and B execute a flow to construct a state object representing the trade, using contract X, and include it in a transaction (which is then signed and sent to the consensus service)
  3. Time passes
  4. The developer of contract X discovers a bug in the contract code, and releases a new version, contract Y. The developer will then notify all existing users (e.g. via a mailing list or CorDapp store) to stop their nodes from issuing further states with contract X
  5. Banks A and B review the new contract via standard change control processes and identify the contract states they agree to upgrade (they may decide not to upgrade some contract states as these might be needed for some other obligation contract)
  6. Banks A and B instruct their Corda nodes (via RPC) to be willing to upgrade state objects with contract X to state objects with contract Y using the agreed upgrade path
  7. One of the parties (the Initiator) initiates a flow to replace state objects referring to contract X with new state objects referring to contract Y
  8. A proposed transaction (the Proposal), with the old states as input and the reissued states as outputs, is created and signed with the node’s private key
  9. The Initiator node sends the proposed transaction, along with details of the new contract upgrade path that it is proposing, to all participants of the state object
  10. Each counterparty (the ``Acceptor``s) verifies the proposal, signs or rejects the state reissuance accordingly, and sends a signature or rejection notification back to the initiating node
  11. If signatures are received from all parties, the Initiator assembles the complete signed transaction and sends it to the notary

Authorising an upgrade

Each of the participants in the state for which the contract is being upgraded will have to instruct their node that they agree to the upgrade before the upgrade can take place. The ContractUpgradeFlow is used to manage the authorisation process. Each node administrator can use RPC to trigger either an Authorise or a Deauthorise flow for the state in question.

@StartableByRPC
class Authorise(
        val stateAndRef: StateAndRef<*>,
        private val upgradedContractClass: Class<out UpgradedContract<*, *>>
) : FlowLogic<Void?>() {
@StartableByRPC
class Deauthorise(val stateRef: StateRef) : FlowLogic<Void?>() {
    @Suspendable
    override fun call(): Void? {

Proposing an upgrade

After all parties have authorised the contract upgrade for the state, one of the contract participants can initiate the upgrade process by triggering the ContractUpgradeFlow.Initiate flow. Initiate creates a transaction including the old state and the updated state, and sends it to each of the participants. Each participant will verify the transaction, create a signature over it, and send the signature back to the initiator. Once all the signatures are collected, the transaction will be notarised and persisted to every participant’s vault.

Example

Suppose Bank A has entered into an agreement with Bank B which is represented by the state object DummyContractState and governed by the contract code DummyContract. A few days after the exchange of contracts, the developer of the contract code discovers a bug in the contract code.

Bank A and Bank B decide to upgrade the contract to DummyContractV2:

  1. The developer creates a new contract DummyContractV2 extending the UpgradedContract class, and a new state object DummyContractV2.State referencing the new contract.
class DummyContractV2 : UpgradedContractWithLegacyConstraint<DummyContract.State, DummyContractV2.State> {
    companion object {
        const val PROGRAM_ID: ContractClassName = "net.corda.testing.contracts.DummyContractV2"
    }

    override val legacyContract: String = DummyContract::class.java.name
    override val legacyContractConstraint: AttachmentConstraint = AlwaysAcceptAttachmentConstraint

    data class State(val magicNumber: Int = 0, val owners: List<AbstractParty>) : ContractState {
        override val participants: List<AbstractParty> = owners
    }

    interface Commands : CommandData {
        class Create : TypeOnlyCommandData(), Commands
        class Move : TypeOnlyCommandData(), Commands
    }

    override fun upgrade(state: DummyContract.State): State {
        return State(state.magicNumber, state.participants)
    }

    override fun verify(tx: LedgerTransaction) {
        // Other verifications.
    }
  1. Bank A instructs its node to accept the contract upgrade to DummyContractV2 for the contract state.
val rpcClient : CordaRPCClient = << Bank A's Corda RPC Client >>
val rpcA = rpcClient.proxy()
rpcA.startFlow(ContractUpgradeFlow.Authorise(<<StateAndRef of the contract state>>, DummyContractV2::class.java))
  1. Bank B initiates the upgrade flow, which will send an upgrade proposal to all contract participants. Each of the participants of the contract state will sign and return the contract state upgrade proposal once they have validated and agreed with the upgrade. The upgraded transaction will be recorded in every participant’s node by the flow.
val rpcClient : CordaRPCClient = << Bank B's Corda RPC Client >>
val rpcB = rpcClient.proxy()
rpcB.startFlow({ stateAndRef, upgrade -> ContractUpgradeFlow(stateAndRef, upgrade) },
    <<StateAndRef of the contract state>>,
    DummyContractV2::class.java)

Note

See ContractUpgradeFlowTest for more detailed code examples.