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CorDapp constraints migration
=============================

.. note:: Before reading this page, you should be familiar with the key concepts of :doc:`Contract Constraints <api-contract-constraints>`.

Corda 4 introduces and recommends building signed CorDapps that issue states with signature constraints.
When building transactions in Corda 4, existing on ledger states issued before Corda 4 are only automatically transitioned to the new
Signature Constraint if they were originally using the CZ Whitelisted Constraint. This document explains how to modify existing CorDapp flows to
explicitly consume and evolve pre Corda 4 states.

Faced with the exercise of upgrading an existing Corda 3.x CorDapp to Corda 4, you need to consider the following:

* What existing unconsumed states have been issued on ledger by a previous version of this CorDapp and using other constraint types?

   If you have existing **hash** constrained states see :ref:`Migrating hash constraints<hash_constraint_migration>`.

   If you have existing **CZ whitelisted** constrained states see :ref:`Migrating CZ whitelisted constraints<cz_whitelisted_constraint_migration>`.

   If you have existing **always accept** constrained states these are not consumable nor evolvable as they offer no security and should only
   be used in test environments.

* What type of contract states does my CorDapp use?

   **Linear states** typically evolve over an extended period of time (defined by the lifecycle of the associated business use case), and
   thus are prime candidates for constraints migration.

   **Fungible states** are created by an issuer and transferred around a Corda network until explicitly exited (by the same issuer).
   They do not evolve as linear states, but are transferred between participants on a network. Their consumption may produce additional new
   output states to represent adjustments to the original state (e.g. change when spending cash). For the purposes of constraints migration,
   it is desirable that any new output states are produced using the new Corda 4 signature constraint types.

   Where you have long transaction chains of fungible states, it may be advisable to send them back to the issuer for re-issuance (this is
   called "chain snipping" and has performance advantages as well as simplifying constraints type migration).

* Should I use the **implicit** or **explicit** upgrade path?

   The general recommendation for Corda 4 is to use **implicit** upgrades for the reasons described :ref:`here <implicit_vs_explicit_upgrades>`.

   **Implicit** upgrades allow pre-authorising multiple implementations of the contract ahead of time.
   They do not require additional coding and do not incur a complex choreographed operational upgrade process.

.. warning:: The steps outlined in this page assume you are using the same CorDapp Contract (eg. same state definition, commands and verification code) and
   wish to use that CorDapp to leverage the upgradeability benefits of Corda 4 signature constraints. If you are looking to upgrade code within an existing
   Contract CorDapp please read :ref:`Contract and state versioning<contract_upgrading_ref>` and :doc:`cordapp-upgradeability` to understand your options.

Please also remember that *states are always consumable if the version of the CorDapp that issued (created) them is installed*.
In the simplest of scenarios it may be easier to re-issue existing hash or CZ whitelist constrained states (eg. exit them from the ledger using
the original unsigned CorDapp and re-issuing them using the new signed CorDapp).

.. _hash_constraint_migration:

Hash constraints migration
--------------------------

.. note:: These instructions only apply to CorDapp Contract JARs (unless otherwise stated).

Corda 4.0
~~~~~~~~~

Corda 4.0 requires some additional steps to consume and evolve pre-existing on-ledger **hash** constrained states:

1. All Corda Nodes in the same CZ or business network that may encounter a transaction chain with a hash constrained state must be started using
   relaxed hash constraint checking mode as described in :ref:`relax_hash_constraints_checking_ref`.

2. CorDapp flows that build transactions using pre-existing *hash-constrained* states must explicitly set output states to use *signature constraints*
   and specify the related public key(s) used in signing the associated CorDapp Contract JAR:

.. container:: codeset

    .. sourcecode:: kotlin

        // This will read the signers for the deployed CorDapp.
        val attachment = this.serviceHub.cordappProvider.getContractAttachmentID(contractClass)
        val signers = this.serviceHub.attachments.openAttachment(attachment!!)!!.signerKeys

        // Create the key that will have to pass for all future versions.
        val ownersKey = signers.first()

        val txBuilder = TransactionBuilder(notary)
                // Set the Signature constraint on the new state to migrate away from the hash constraint.
                .addOutputState(outputState, constraint = SignatureAttachmentConstraint(ownersKey))

    .. sourcecode:: java

        // This will read the signers for the deployed CorDapp.
        SecureHash attachment = this.getServiceHub().getCordappProvider().getContractAttachmentID(contractClass);
        List<PublicKey> signers = this.getServiceHub().getAttachments().openAttachment(attachment).getSignerKeys();

        // Create the key that will have to pass for all future versions.
        PublicKey ownersKey = signers.get(0);

        TransactionBuilder txBuilder = new TransactionBuilder(notary)
                // Set the Signature constraint on the new state to migrate away from the hash constraint.
                .addOutputState(outputState, myContract, new SignatureAttachmentConstraint(ownersKey))

3. As a node operator you need to add the new signed version of the contracts CorDapp to the ``/cordapps`` folder together with the latest version of the flows jar.
   Please also ensure that the original unsigned contracts CorDapp is removed from the ``/cordapps`` folder (this will already be present in the
   nodes attachments store) to ensure the lookup code in step 2 retrieves the correct signed contract CorDapp JAR.

Later releases
~~~~~~~~~~~~~~

The next version of Corda will provide automatic transition of *hash constrained* states. This means that signed CorDapps running on a Corda 4.x node will
automatically propagate any pre-existing on-ledger *hash-constrained* states (and generate *signature-constrained* outputs) when the system property
to break constraints is set.

.. _cz_whitelisted_constraint_migration:

CZ whitelisted constraints migration
-------------------------------------

.. note:: These instructions only apply to CorDapp Contract JARs (unless otherwise stated).

Corda 4.0
~~~~~~~~~

Corda 4.0 requires some additional steps to consume and evolve pre-existing on-ledger **CZ whitelisted** constrained states:

1. As the original developer of the CorDapp, the first step is to sign the latest version of the JAR that was released (see :doc:`cordapp-build-systems`).
   The key used for signing will be used to sign all subsequent releases, so it should be stored appropriately. The JAR can be signed by multiple keys owned
   by different parties and it will be expressed as a ``CompositeKey`` in the ``SignatureAttachmentConstraint`` (See :doc:`api-core-types`).

2. Any flow that builds transactions using this CorDapp will automatically transition states to use the ``SignatureAttachmentConstraint`` if
   no other constraint is specified. Therefore, there are two ways to alter the existing code.

   * Do not specify a constraint
   * Explicitly add a Signature Constraint

The code below details how to explicitly add a Signature Constraint:

.. container:: codeset

    .. sourcecode:: kotlin

        // This will read the signers for the deployed CorDapp.
        val attachment = this.serviceHub.cordappProvider.getContractAttachmentID(contractClass)
        val signers = this.serviceHub.attachments.openAttachment(attachment!!)!!.signerKeys

        // Create the key that will have to pass for all future versions.
        val ownersKey = signers.first()

        val txBuilder = TransactionBuilder(notary)
                // Set the Signature constraint on the new state to migrate away from the WhitelistConstraint.
                .addOutputState(outputState, constraint = SignatureAttachmentConstraint(ownersKey))

    .. sourcecode:: java

        // This will read the signers for the deployed CorDapp.
        SecureHash attachment = this.getServiceHub().getCordappProvider().getContractAttachmentID(contractClass);
        List<PublicKey> signers = this.getServiceHub().getAttachments().openAttachment(attachment).getSignerKeys();

        // Create the key that will have to pass for all future versions.
        PublicKey ownersKey = signers.get(0);

        TransactionBuilder txBuilder = new TransactionBuilder(notary)
                // Set the Signature constraint on the new state to migrate away from the WhitelistConstraint.
                .addOutputState(outputState, myContract, new SignatureAttachmentConstraint(ownersKey))

3. As a node operator you need to add the new signed version of the contracts CorDapp to the ``/cordapps`` folder together with the latest version of the flows jar.
   Please also ensure that the original unsigned contracts CorDapp is removed from the ``/cordapps`` folder (this will already be present in the
   nodes attachments store) to ensure the lookup code in step 3 retrieves the correct signed contract CorDapp JAR.