corda/docs/source/key-concepts-contract-constraints.rst

Contract Constraints

A basic understanding of contract key concepts, which can be found here </key-concepts-contracts>, is required reading for this page.

Transaction states specify a constraint over the contract that will be used to verify it. For a transaction to be valid, the verify() function associated with each state must run successfully. However, for this to be secure, it is not sufficient to specify the verify() function by name as there may exist multiple different implementations with the same method signature and enclosing class. Contract constraints solve this problem by allowing a contract developer to constrain which verify() functions out of the universe of implementations can be used. (ie the universe is everything that matches the signature and contract constraints restricts this universe to a subset.)

A typical constraint is the hash of the CorDapp JAR that contains the contract and states but will in future releases include constraints that require specific signers of the JAR, or both the signer and the hash. Constraints can be specified when constructing a transaction; if unspecified, an automatic constraint is used.

A TransactionState has a constraint field that represents that state's attachment constraint. When a party constructs a TransactionState without specifying the constraint parameter a default value (AutomaticHashConstraint) is used. This default will be automatically resolved to a specific HashAttachmentConstraint that contains the hash of the attachment which contains the contract of that TransactionState. This automatic resolution occurs when a TransactionBuilder is converted to a WireTransaction. This reduces the boilerplate involved in finding a specific hash constraint when building a transaction.

It is possible to specify the constraint explicitly with any other class that implements the AttachmentConstraint interface. To specify a hash manually the HashAttachmentConstraint can be used and to not provide any constraint the AlwaysAcceptAttachmentConstraint can be used - though this is intended for testing only. An example below shows how to construct a TransactionState with an explicitly specified hash constraint from within a flow;

// Constructing a transaction with a custom hash constraint on a state
TransactionBuilder tx = new TransactionBuilder()

Party notaryParty = ... // a notary party
DummyState contractState = new DummyState()
SecureHash myAttachmentsHash = serviceHub.cordappProvider.getContractAttachmentID(DummyContract.PROGRAM_ID)
TransactionState transactionState = new TransactionState(contractState, DummyContract.Companion.getPROGRAMID(), notaryParty, new AttachmentHashConstraint(myAttachmentsHash))

tx.addOutputState(transactionState)
WireTransaction wtx = tx.toWireTransaction(serviceHub) // This is where an automatic constraint would be resolved
LedgerTransaction ltx = wtx.toLedgerTransaction(serviceHub)
ltx.verify() // Verifies both the attachment constraints and contracts

This mechanism exists both for integrity and security reasons. It is important not to verify against the wrong contract, which could happen if the wrong version of the contract is attached. More importantly when resolving transaction chains there will, in a future release, be attachments loaded from the network into the attachment sandbox that are used to verify the transaction chain. Ensuring the attachment used is the correct one ensures that the verification will not be tamperable by providing a fake contract.

CorDapps as attachments

CorDapp JARs (cordapp-overview) that are installed to the node and contain classes implementing the Contract interface are automatically loaded into the AttachmentStorage of a node at startup.

After CorDapps are loaded into the attachment store the node creates a link between contract classes and the attachment that they were loaded from. This makes it possible to find the attachment for any given contract. This is how the automatic resolution of attachments is done by the TransactionBuilder and how, when verifying the constraints and contracts, attachments are associated with their respective contracts.

Implementations

There are three implementations of AttachmentConstraints with more planned in the future.

AlwaysAcceptAttachmentConstraint: Any attachment (except a missing one) will satisfy this constraint.

AutomaticHashConstraint: This will be resolved to a HashAttachmentConstraint when a TransactionBuilder is converted to a WireTransaction. The HashAttachmentConstraint will include the attachment hash of the CorDapp that contains the ContractState on the TransactionState.contract field.

HashAttachmentConstraint: Will require that the hash of the attachment containing the contract matches the hash stored in the constraint.

We plan to add a future AttachmentConstraint that will only be satisfied by the presence of signatures on the attachment JAR. This allows for trusting of attachments from trusted entities.

Limitations

An AttachmentConstraint is verified by running the AttachmentConstraint.isSatisfiedBy method. When this is called it is provided only the relevant attachment by the transaction that is verifying it.

Testing

Since all tests involving transactions now require attachments it is also required to load the correct attachments for tests. Unit test environments in JVM ecosystems tend to use class directories rather than JARs, and so CorDapp JARs typically aren't built for testing. Requiring this would add significant complexity to the build systems of Corda and CorDapps, so the test suite has a set of convenient functions to generate CorDapps from package names or to specify JAR URLs in the case that the CorDapp(s) involved in testing already exist.

MockNetwork/MockNode

The most simple way to ensure that a vanilla instance of a MockNode generates the correct CorDapps is to use the cordappPackages constructor parameter (Kotlin) or the setCordappPackages method on MockNetworkParameters (Java) when creating the MockNetwork. This will cause the AbstractNode to use the named packages as sources for CorDapps. All files within those packages will be zipped into a JAR and added to the attachment store and loaded as CorDapps by the CordappLoader. An example of this usage would be:

class SomeTestClass {
     MockNetwork network = null

     @Before
     void setup() {
         network = new MockNetwork(new MockNetworkParameters().setCordappPackages(Arrays.asList("com.domain.cordapp")))
     }

     ... // Your tests go here
}

MockServices

If your test uses a MockServices directly you can instantiate it using a constructor that takes a list of packages to use as CorDapps using the cordappPackages parameter.

MockServices mockServices = new MockServices(Arrays.asList("com.domain.cordapp"))

Driver

The driver takes a parameter called extraCordappPackagesToScan which is a list of packages to use as CorDapps.

driver(new DriverParameters().setExtraCordappPackagesToScan(Arrays.asList("com.domain.cordapp"))) ...

Full Nodes

When testing against full nodes simply place your CorDapp into the cordapps directory of the node.