corda/docs/source/api-contract-constraints.rst

API: Contract Constraints

Note

Before reading this page, you should be familiar with the key concepts of key-concepts-contracts.

Contract constraints solve two problems faced by any decentralised ledger that supports evolution of data and code:

1. Controlling and agreeing upon upgrades
2. Preventing attacks

Upgrades and security are intimately related because if an attacker can "upgrade" your data to a version of an app that gives them a back door, they would be able to do things like print money or edit states in any way they want. That's why it's important for participants of a state to agree on what kind of upgrades will be allowed.

Every state on the ledger contains the fully qualified class name of a Contract implementation, and also a constraint. This constraint specifies which versions of an application can be used to provide the named class. There are several types:

1. Hash constraint: exactly one version of the app can be used with this state.
2. Zone whitelist constraint: the compatibility zone operator lists the hashes of the versions that can be used with this contract class name.
3. Signature constraint: any version of the app signed by the given CompositeKey can be used.
4. Always accept constraint: any app can be used at all. This is insecure but convenient for testing.

The actual app version used is defined by the attachments on a transaction: the JAR containing the state and contract classes, and optionally its dependencies, are all attached to the transaction. Other nodes will download these JARs from a node if they haven't seen them before, so they can be used for verification. The TransactionBuilder will manage the details of constraints for you, by selecting both constraints and attachments to ensure they line up correctly. Therefore you only need to have a basic understanding of this topic unless you are doing something sophisticated.

The best kind of constraint to use is the signature constraint. If you sign your application it will be used automatically. We recommend signature constraints because they let you release new versions of your application. Hash and zone whitelist constraints still work, but are left over from earlier Corda versions before signature constraints were implemented. They make it harder to upgrade applications than when using signature constraints, so they're best avoided. Signature constraints can specify flexible policies but if you use the automatic support, then a state will require the attached app to be signed by every key that the first attachment was signed by. Thus if the app that was used to issue the states was signed by Alice and Bob, every transaction must use an attachment signed by Alice and Bob.

Constraint propagation. Constraints are picked when a state is created for the first time in an issuance transaction. Once created, the constraint used by the equivalent output state must match the input state, so it can't be changed and you can't combine states with incompatible constraints together in the same transaction.

Implicit vs explicit. There are two ways of handling upgrades to a smart contract in Corda:

1. Implicit: By allowing multiple implementations of the contract ahead of time, using constraints.
2. Explicit: By creating a special contract upgrade transaction and getting all participants of a state to sign it using the contract upgrade flows.

This article focuses on the first approach. To learn about the second please see upgrading-cordapps.

The advantage of pre-authorising upgrades using constraints is that you don't need the heavyweight process of creating upgrade transactions for every state on the ledger. The disadvantage is that you place more faith in third parties, who could potentially change the app in ways you did not expect or agree with. The advantage of using the explicit upgrade approach is that you can upgrade states regardless of their constraint, including in cases where you didn't anticipate a need to do so. But it requires everyone to sign, requires everyone to manually authorise the upgrade, consumes notary and ledger resources, and is just in general more complex.

Contract/State Agreement

Starting with Corda 4, a ContractState must explicitly indicate which Contract it belongs to. When a transaction is verified, the contract bundled with each state in the transaction must be its "owning" contract, otherwise we cannot guarantee that the transition of the ContractState will be verified against the business rules that should apply to it.

There are two mechanisms for indicating ownership. One is to annotate the ContractState with the BelongsToContract annotation, indicating the Contract class to which it is tied:

@BelongToContract(MyContract.class)
public class MyState implements ContractState {
    // implementation goes here
}

@BelongsToContract(MyContract::class)
data class MyState(val value: Int) : ContractState {
    // implementation goes here
}

The other is to define the ContractState class as an inner class of the Contract class

public class MyContract implements Contract {

    public static class MyState implements ContractState {
        // state implementation goes here
    }

    // contract implementation goes here
}

class MyContract : Contract {
    data class MyState(val value: Int) : ContractState
}

If a ContractState's owning Contract cannot be identified by either of these mechanisms, and the targetVersion of the CorDapp is 4 or greater, then transaction verification will fail with a TransactionRequiredContractUnspecifiedException. If the owning Contract can be identified, but the ContractState has been bundled with a different contract, then transaction verification will fail with a TransactionContractConflictException.

App versioning with signature constraints

Signed apps require a version number to be provided, see versioning. You can't import two different JARs that claim to be the same version, export the same contract classes and which are both signed. At runtime the node will throw a DuplicateContractClassException exception if this condition is violated.

Issues when using the HashAttachmentConstraint

When setting up a new network, it is possible to encounter errors when states are issued with the HashAttachmentConstraint, but not all nodes have that same version of the CorDapp installed locally.

In this case, flows will fail with a ContractConstraintRejection, and the failed flow will be sent to the flow hospital. From there it's suspended waiting to be retried on node restart. This gives the node operator the opportunity to recover from those errors, which in the case of constraint violations means adding the right cordapp jar to the cordapps folder.

CorDapps as attachments

CorDapp JARs (see cordapp-overview) that contain classes implementing the Contract interface are automatically loaded into the AttachmentStorage of a node, and made available as ContractAttachments.

They are retrievable by hash using AttachmentStorage.openAttachment. These JARs can either be installed on the node or will be automatically fetched over the network when receiving a transaction.

Warning

The obvious way to write a CorDapp is to put all you states, contracts, flows and support code into a single Java module. This will work but it will effectively publish your entire app onto the ledger. That has two problems: (1) it is inefficient, and (2) it means changes to your flows or other parts of the app will be seen by the ledger as a "new app", which may end up requiring essentially unnecessary upgrade procedures. It's better to split your app into multiple modules: one which contains just states, contracts and core data types. And another which contains the rest of the app. See cordapp-structure.

Constraints propagation

As was mentioned above, the TransactionBuilder API gives the CorDapp developer or even malicious node owner the possibility to construct output states with a constraint of his choosing.

For the ledger to remain in a consistent state, the expected behavior is for output state to inherit the constraints of input states. This guarantees that for example, a transaction can't output a state with the AlwaysAcceptAttachmentConstraint when the corresponding input state was the SignatureAttachmentConstraint. Translated, this means that if this rule is enforced, it ensures that the output state will be spent under similar conditions as it was created.

Before version 4, the constraint propagation logic was expected to be enforced in the contract verify code, as it has access to the entire Transaction.

Starting with version 4 of Corda, the constraint propagation logic has been implemented and enforced directly by the platform, unless disabled using @NoConstraintPropagation - which reverts to the previous behavior.

For contracts that are not annotated with @NoConstraintPropagation, the platform implements a fairly simple constraint transition policy to ensure security and also allow the possibility to transition to the new SignatureAttachmentConstraint.

During transaction building the AutomaticPlaceholderConstraint for output states will be resolved and the best contract attachment versions will be selected based on a variety of factors so that the above holds true. If it can't find attachments in storage or there are no possible constraints, the TransactionBuilder will throw an exception.

Debugging

If an attachment constraint cannot be resolved, a MissingContractAttachments exception is thrown. There are three common sources of MissingContractAttachments exceptions:

Not setting CorDapp packages in tests

You are running a test and have not specified the CorDapp packages to scan. When using MockNetwork ensure you have provided a package containing the contract class in MockNetworkParameters. See api-testing.

Similarly package names need to be provided when testing using DriverDSl. DriverParameters has a property cordappsForAllNodes (Kotlin) or method withCordappsForAllNodes in Java. Pass the collection of TestCordapp created by utility method TestCordapp.findCordapp(String).

Example of creation of two Cordapps with Finance App Flows and Finance App Contracts in Kotlin:

Driver.driver(DriverParameters(cordappsForAllNodes = listOf(TestCordapp.findCordapp("net.corda.finance.schemas"),
        TestCordapp.findCordapp("net.corda.finance.flows"))) {
    // Your test code goes here
})

The same example in Java:

Driver.driver(new DriverParameters()
        .withCordappsForAllNodes(Arrays.asList(TestCordapp.findCordapp("net.corda.finance.schemas"),
        TestCordapp.findCordapp("net.corda.finance.flows"))), dsl -> {
    // Your test code goes here
});

Staring a node missing CorDapp(s)

When running the Corda node ensure all CordDapp JARs are placed in cordapps directory of each node. By default Gradle Cordform task deployNodes copies all JARs if CorDapps to deploy are specified. See generating-a-node for detailed instructions.

Wrong fully-qualified contract name

You are specifying the fully-qualified name of the contract incorrectly. For example, you've defined MyContract in the package com.mycompany.myapp.contracts, but the fully-qualified contract name you pass to the TransactionBuilder is com.mycompany.myapp.MyContract (instead of com.mycompany.myapp.contracts.MyContract).