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Upgrading apps to Corda 4
=========================

These notes provide instructions for upgrading your CorDapps from previous versions. Corda provides backwards compatibility for public,
non-experimental APIs that have been committed to. A list can be found in the :doc:`corda-api` page.

This means that you can upgrade your node across versions *without recompiling or adjusting your CorDapps*. You just have to upgrade
your node and restart.

However, there are usually new features and other opt-in changes that may improve the security, performance or usability of your
application that are worth considering for any actively maintained software. This guide shows you how to upgrade your app to benefit
from the new features in the latest release.

.. contents::
   :depth: 3

Step 1. Adjust the version numbers in your Gradle build files
-------------------------------------------------------------

Alter the versions you depend on in your Gradle file like so:

.. sourcecode:: groovy

    ext.corda_release_version = '4.0'
    ext.corda_gradle_plugins_version = '4.0.38'
    ext.kotlin_version = '1.2.71'
    ext.quasar_version = '0.7.10'

.. note:: You may wish to update your kotlinOptions to use language level 1.2, to benefit from the new features. Apps targeting Corda 4
   may not at this time use Kotlin 1.3, as it was released too late in the development cycle
   for us to risk an upgrade. Sorry! Future work on app isolation will make it easier for apps to use newer Kotlin versions than
   the node itself uses.

You should also ensure you're using Gradle 4.10 (but not 5). If you use the Gradle wrapper, run::

    ./gradlew wrapper --gradle-version 4.10.3

Otherwise just upgrade your installed copy in the usual manner for your operating system.

Step 2. Update your Gradle build file
-------------------------------------

There are several adjustments that are beneficial to make to your Gradle build file, beyond simply incrementing the versions
as described in step 1.

**Provide app metadata.** This is used by the Corda Gradle build plugin to populate your app JAR with useful information.
It should look like this::

    cordapp {
        targetPlatformVersion 4
        minimumPlatformVersion 4
        contract {
            name "MegaApp Contracts"
            vendor "MegaCorp"
            licence "A liberal, open source licence"
            versionId 1
        }
        workflow {
            name "MegaApp flows"
            vendor "MegaCorp"
            licence "A really expensive proprietary licence"
            versionId 1
        }
    }

.. important:: Watch out for the UK spelling of the word licence (with a c).

Name, vendor and licence can be set to any string you like, they don't have to be Corda identities.

Target versioning is a new concept introduced in Corda 4. Learn more by reading :doc:`versioning`.
Setting a target version of 4 opts in to changes that might not be 100% backwards compatible, such as
API semantics changes or disabling workarounds for bugs that may be in your apps, so by doing this you
are promising that you have thoroughly tested your app on the new version. Using a high target version is
a good idea because some features and improvements are only available to apps that opt in.

The minimum platform version is the platform version of the node that you require, so if you
start using new APIs and features in Corda 4, you should set this to 4. Unfortunately Corda 3 and below
do not know about this metadata and don't check it, so your app will still be loaded in such nodes and
may exhibit undefined behaviour at runtime. However it's good to get in the habit of setting this
properly for future releases.

.. note:: Whilst it's currently a convention that Corda releases have the platform version number as their
   major version i.e. Corda 3.3 implements platform version 3, this is not actually required and may in
   future not hold true. You should know the platform version of the node releases you want to target.

The new ``versionId`` number is a version code for **your** app, and is unrelated to Corda's own versions.
It is used to block state downgrades: when a state constraint can be satisfied
by multiple attachments, the version is tracked in the ledger and cannot decrement. This ensures security
fixes in CorDapps stick and can't be reversed by downgrading states to an earlier version. See
":ref:`contract_non-downgrade_rule_ref`" for more information.

**Split your app into contract and workflow JARs.** The duplication between ``contract`` and ``workflow`` blocks exists because you should split your app into
two separate JARs/modules, one that contains on-ledger validation code like states and contracts, and one
for the rest (called by convention the "workflows" module although it can contain a lot more than just flows:
services would also go here, for instance). For simplicity, here we use one JAR for both, but this is in
general an anti-pattern and can result in your flow logic code being sent over the network to arbitrary
third party peers, even though they don't need it.

In future, the version ID attached to the workflow JAR will also be used to help implement smoother upgrade
and migration features. You may directly reference the gradle version number of your app when setting the
CorDapp specific versionId identifiers if this follows the convention of always being a whole number
starting from 1.

If you use the finance demo app, you should adjust your dependencies so you depend on the finance-contracts
and finance-workflows artifacts from your own contract and workflow JAR respectively. Although a single
finance jar still exists in Corda 4 for backwards compatibility, it should not be installed or used for
updated apps. This way, only the code that needs to be on the ledger actually will be.

Step 3. Security: Upgrade your use of FinalityFlow
--------------------------------------------------

The previous ``FinalityFlow`` API is insecure. It doesn't have a receive flow, so requires counterparty nodes to accept any and
all signed transactions that are sent to it, without checks. It is **highly** recommended that existing CorDapps migrate
away to the new API, as otherwise things like business network membership checks won't be reliably enforced.

This is a three step process:

1. Change the flow that calls ``FinalityFlow``.
2. Change or create the flow that will receive the finalised transaction.
3. Make sure your application's minimum and target version numbers are both set to 4 (see step 2).

Upgrading a non-initiating flow
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

As an example, let's take a very simple flow that finalises a transaction without the involvement of a counterpart flow:

.. container:: codeset

    .. literalinclude:: example-code/src/main/kotlin/net/corda/docs/kotlin/FinalityFlowMigration.kt
        :language: kotlin
        :start-after: DOCSTART SimpleFlowUsingOldApi
        :end-before: DOCEND SimpleFlowUsingOldApi

    .. literalinclude:: example-code/src/main/java/net/corda/docs/java/FinalityFlowMigration.java
        :language: java
        :start-after: DOCSTART SimpleFlowUsingOldApi
        :end-before: DOCEND SimpleFlowUsingOldApi
        :dedent: 4

To use the new API, this flow needs to be annotated with ``InitiatingFlow`` and a ``FlowSession`` to the participant(s) of the transaction must be
passed to ``FinalityFlow`` :

.. container:: codeset

    .. literalinclude:: example-code/src/main/kotlin/net/corda/docs/kotlin/FinalityFlowMigration.kt
        :language: kotlin
        :start-after: DOCSTART SimpleFlowUsingNewApi
        :end-before: DOCEND SimpleFlowUsingNewApi

    .. literalinclude:: example-code/src/main/java/net/corda/docs/java/FinalityFlowMigration.java
        :language: java
        :start-after: DOCSTART SimpleFlowUsingNewApi
        :end-before: DOCEND SimpleFlowUsingNewApi
        :dedent: 4

If there are more than one transaction participants then a session to each one must be initiated, excluding the local party
and the notary.

A responder flow has to be introduced, which will automatically run on the other participants' nodes, which will call ``ReceiveFinalityFlow``
to record the finalised transaction:

.. container:: codeset

    .. literalinclude:: example-code/src/main/kotlin/net/corda/docs/kotlin/FinalityFlowMigration.kt
        :language: kotlin
        :start-after: DOCSTART SimpleNewResponderFlow
        :end-before: DOCEND SimpleNewResponderFlow

    .. literalinclude:: example-code/src/main/java/net/corda/docs/java/FinalityFlowMigration.java
        :language: java
        :start-after: DOCSTART SimpleNewResponderFlow
        :end-before: DOCEND SimpleNewResponderFlow
        :dedent: 4

.. note:: All the nodes in your business network will need the new CorDapp, otherwise they won't know how to receive the transaction. **This
   includes nodes which previously didn't have the old CorDapp.** If a node is sent a transaction and it doesn't have the new CorDapp loaded
   then simply restart it with the CorDapp and the transaction will be recorded.

Upgrading an initiating flow
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For flows which are already initiating counterpart flows then it's a matter of using the existing flow session.
Note however, the new ``FinalityFlow`` is inlined and so the sequence of sends and receives between the two flows will
change and will be incompatible with your current flows. You can use the flow version API to write your flows in a
backwards compatible manner.

Here's what an upgraded initiating flow may look like:

.. container:: codeset

    .. literalinclude:: example-code/src/main/kotlin/net/corda/docs/kotlin/FinalityFlowMigration.kt
        :language: kotlin
        :start-after: DOCSTART ExistingInitiatingFlow
        :end-before: DOCEND ExistingInitiatingFlow

    .. literalinclude:: example-code/src/main/java/net/corda/docs/java/FinalityFlowMigration.java
        :language: java
        :start-after: DOCSTART ExistingInitiatingFlow
        :end-before: DOCEND ExistingInitiatingFlow
        :dedent: 4

For the responder flow, insert a call to ``ReceiveFinalityFlow`` at the location where it's expecting to receive the
finalised transaction. If the initiator is written in a backwards compatible way then so must the responder.

.. container:: codeset

    .. literalinclude:: example-code/src/main/kotlin/net/corda/docs/kotlin/FinalityFlowMigration.kt
        :language: kotlin
        :start-after: DOCSTART ExistingResponderFlow
        :end-before: DOCEND ExistingResponderFlow
        :dedent: 8

    .. literalinclude:: example-code/src/main/java/net/corda/docs/java/FinalityFlowMigration.java
        :language: java
        :start-after: DOCSTART ExistingResponderFlow
        :end-before: DOCEND ExistingResponderFlow
        :dedent: 12

You may already be using ``waitForLedgerCommit`` in your responder flow for the finalised transaction to appear in the local node's vault.
Now that it's calling ``ReceiveFinalityFlow``, which effectively does the same thing, this is no longer necessary. The call to
``waitForLedgerCommit`` should be removed.

Step 4. Security: Upgrade your use of SwapIdentitiesFlow
--------------------------------------------------------

The :ref:`confidential_identities_ref` API is experimental in Corda 3 and remains so in Corda 4. In this release, the ``SwapIdentitiesFlow``
has been adjusted in the same way as ``FinalityFlow`` above, to close problems with confidential identities being injectable into a node
outside of other flow context. Old code will still work, but it is recommended to adjust your call sites so a session is passed into
the ``SwapIdentitiesFlow``.

Step 5. Possibly, adjust test code
----------------------------------

``MockNodeParameters`` and functions creating it no longer use a lambda expecting a ``NodeConfiguration`` object.
Use a ``MockNetworkConfigOverrides`` object instead. This is an API change we regret, but unfortunately in Corda 3 we accidentally exposed
large amounts of the node internal code through this one API entry point. We have now insulated the test API from node internals and
reduced the exposure.

If you are constructing a MockServices for testing contracts, and your contract uses the Cash contract from the finance app, you
now need to explicitly add ``net.corda.finance.contracts`` to the list of ``cordappPackages``. This is a part of the work to disentangle
the finance app (which is really a demo app) from the Corda internals. Example::

    val ledgerServices = MockServices(
        listOf("net.corda.examples.obligation", "net.corda.testing.contracts"),
        identityService = makeTestIdentityService(),
        initialIdentity = TestIdentity(CordaX500Name("TestIdentity", "", "GB"))
    )

becomes::

    val ledgerServices = MockServices(
        listOf("net.corda.examples.obligation", "net.corda.testing.contracts", "net.corda.finance.contracts"),
        identityService = makeTestIdentityService(),
        initialIdentity = TestIdentity(CordaX500Name("TestIdentity", "", "GB"))
    )

You may need to use the new ``TestCordapp`` API when testing with the node driver or mock network, especially if you decide to stick with the
pre-Corda 4 ``FinalityFlow`` API. The previous way of pulling in CorDapps into your tests does not honour CorDapp versioning.

Step 6. Security: Add BelongsToContract annotations
---------------------------------------------------

In versions of the platform prior to v4, it was the responsibility of contract and flow logic to ensure that ``TransactionState`` objects
contained the correct class name of the expected contract class. If these checks were omitted, it would be possible for a malicious counterparty
to construct a transaction containing e.g. a cash state governed by a commercial paper contract. The contract would see that there were no
commercial paper states in a transaction and do nothing, i.e. accept.

In Corda 4 the platform takes over this responsibility from the app, if the app has a target version of 4 or higher. A state is expected
to be governed by a contract that is either:

1. The outer class of the state class, if the state is an inner class of a contract. This is a common design pattern.
2. Annotated with ``@BelongsToContract`` which specifies the contract class explicitly.

Learn more by reading ":ref:`implicit_constraint_types`". If an app targets Corda 3 or lower (i.e. does not specify a target version),
states that point to contracts outside their package will trigger a log warning but validation will proceed.

Step 7. Learn about signature constraints and JAR signing
---------------------------------------------------------

:doc:`design/data-model-upgrades/signature-constraints` are a new data model feature introduced in Corda 4. They make it much easier to
deploy application upgrades smoothly and in a decentralised manner. Signature constraints are the new default mode for CorDapps, and
the act of upgrading your app to use the version 4 Gradle plugins will result in your app being automatically signed, and new states
automatically using new signature constraints selected automatically based on these signing keys.

You can read more about signature constraints and what they do in :doc:`api-contract-constraints`. The ``TransactionBuilder`` class will
automatically use them if your application JAR is signed. **We recommend all JARs are signed**. To learn how to sign your JAR files, read
:ref:`cordapp_build_system_signing_cordapp_jar_ref`. In dev mode, all JARs are signed by developer certificates. If a JAR that was signed
with developer certificates is deployed to a production node, the node will refuse to start. Therefore to deploy apps built for Corda 4
to production you will need to generate signing keys and integrate them with the build process.

Step 8. Security: Package namespace handling
--------------------------------------------

Almost no apps will be affected by these changes, but they're important to know about.

There are two improvements to how Java package protection is handled in Corda 4:

1. Package sealing
2. Package namespace ownership

**Sealing.** App isolation has been improved. Version 4 of the finance CorDapp (*corda-finance.jar*) is now built as a set of sealed and
signed JAR files. This means classes in your own CorDapps cannot be placed under the following package namespace:  ``net.corda.finance``

In the unlikely event that you were injecting code into ``net.corda.finance.*`` package namespaces from your own apps, you will need to move them
into a new package, e.g. ``net/corda/finance/flows/MyClass.java`` can be moved to ``com/company/corda/finance/flows/MyClass.java``.
As a consequence your classes are no longer able to access non-public members of finance CorDapp classes.

When recompiling your JARs for Corda 4, your own apps will also become sealed, meaning other JARs cannot place classes into your own packages.
This is a security upgrade that ensures package-private visibility in Java code works correctly. If other apps could define classes in your own
packages, they could call package-private methods, which may not be expected by the developers.

**Namespace ownership.** This part is only relevant if you are joining a production compatibility zone. You may wish to contact your zone operator
and request ownership of your root package namespaces (e.g. ``com.megacorp.*``), with the signing keys you will be using to sign your app JARs.
The zone operator can then add your signing key to the network parameters, and prevent attackers defining types in your own package namespaces.
Whilst this feature is optional and not strictly required, it may be helpful to block attacks at the boundaries of a Corda based application
where type names may be taken "as read". You can learn more about this feature and the motivation for it by reading
":doc:`design/data-model-upgrades/package-namespace-ownership`".


Step 9. Consider adding extension points to your flows
------------------------------------------------------

In Corda 4 it is possible for flows in one app to subclass and take over flows from another. This allows you to create generic, shared
flow logic that individual users can customise at pre-agreed points (protected methods). For example, a site-specific app could be developed
that causes transaction details to be converted to a PDF and sent to a particular printer. This would be an inappropriate feature to put
into shared business logic, but it makes perfect sense to put into a user-specific app they developed themselves.

If your flows could benefit from being extended in this way, read ":doc:`flow-overriding`" to learn more.

Step 10. Explore other new features that may be useful
------------------------------------------------------

Corda 4 adds several new APIs that help you build applications. Why not explore:

* The `new withEntityManager API <api/javadoc/net/corda/core/node/ServiceHub.html#withEntityManager-block->`_ for using JPA inside your flows and services.
* :ref:`reference_states`, that let you use an input state without consuming it.
* :ref:`state_pointers`, that make it easier to 'point' to one state from another and follow the latest version of a linear state.