API: Contract Constraints

Contract constraints

Corda separates verification of states from their definition. Whilst you might have expected the ContractState interface to define a verify method, or perhaps to do verification logic in the constructor, instead it is primarily done by a method on a Contract class. This is because what we’re actually checking is the validity of a transaction, which is more than just whether the individual states are internally consistent. The transition between two valid states may be invalid, if the rules of the application are not being respected. For instance, two cash states of $100 and $200 may both be internally valid, but replacing the first with the second isn’t allowed unless you’re a cash issuer - otherwise you could print money for free.

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. This normally will happen as applications evolve, but could also happen maliciously.

Contract constraints solve this problem by allowing a contract developer to constrain which verify functions out of the universe of implementations can be used (i.e. the universe is everything that matches the signature and contract constraints restrict this universe to a subset). Constraints are satisfied by attachments (JARs). You are not allowed to attach two JARs that both define the same application due to the no overlap rule. This rule specifies that two attachment JARs may not provide the same file path. If they do, the transaction is considered invalid. Because each state specifies both a constraint over attachments and a Contract class name to use, the specified class must appear in only one attachment.

So who picks the attachment to use? It is chosen by the creator of the transaction that has to satisfy input constraints. The transaction creator also gets to pick the constraints used by any output states, but the contract logic itself may have opinions about what those constraints are - a typical contract would require that the constraints are propagated, that is, the contract will not just enforce the validity of the next transaction that uses a state, but all successive transactions as well. The constraints mechanism creates a balance of power between the creator of data on the ledger and the user who is trying to edit it, which can be very useful when managing upgrades to Corda applications.

There are two ways of handling upgrades to a smart contract in Corda:

  • Implicit: By allowing multiple implementations of the contract ahead of time, using constraints.
  • 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 a CorDapp (outside of platform version upgrades).

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.

How constraints work

Starting from Corda 3 there are two types of constraint that can be used: hash and zone whitelist. In future releases a third type will be added, the signature constraint.

Hash constraints. The behaviour provided by public blockchain systems like Bitcoin and Ethereum is that once data is placed on the ledger, the program that controls it is fixed and cannot be changed. There is no support for upgrades at all. This implements a form of “code is law”, assuming you trust the community of that blockchain to not release a new version of the platform that invalidates or changes the meaning of your program.

This is supported by Corda using a hash constraint. This specifies exactly one hash of a CorDapp JAR that contains the contract and states any consuming transaction is allowed to use. Once such a state is created, other nodes will only accept a transaction if it uses that exact JAR file as an attachment. By implication, any bugs in the contract code or state definitions cannot be fixed except by using an explicit upgrade process via ContractUpgradeFlow.

Zone constraints. Often a hash constraint will be too restrictive. You do want the ability to upgrade an app, and you don’t mind the upgrade taking effect “just in time” when a transaction happens to be required for other business reasons. In this case you can use a zone constraint. This specifies that the network parameters of a compatibility zone (see Network Map) is expected to contain a map of class name to hashes of JARs that are allowed to provide that class. The process for upgrading an app then involves asking the zone operator to add the hash of your new JAR to the parameters file, and trigger the network parameters upgrade process. This involves each node operator running a shell command to accept the new parameters file and then restarting the node. Node owners who do not restart their node in time effectively stop being a part of the network.

Signature constraints. These are not yet supported, but once implemented they will allow a state to require a JAR signed by a specified identity, via the regular Java jarsigner tool. This will be the most flexible type and the smoothest to deploy: no restarts or contract upgrade transactions are needed.

Defaults. The default constraint type is either a zone constraint, if the network parameters in effect when the transaction is built contain an entry for that contract class, or a hash constraint if not.

A TransactionState has a constraint field that represents that state’s attachment constraint. When a party constructs a TransactionState, or adds a state using TransactionBuilder.addOutput(ContractState) without specifying the constraint parameter, a default value (AutomaticHashConstraint) is used. This default will be automatically resolved to a specific HashAttachmentConstraint or a WhitelistedByZoneAttachmentConstraint. This automatic resolution occurs when a TransactionBuilder is converted to a WireTransaction. This reduces the boilerplate that would otherwise be involved.

Finally, an AlwaysAcceptAttachmentConstraint can be used which accepts anything, though this is intended for testing only.

Please note that the AttachmentConstraint interface is marked as @DoNotImplement. You are not allowed to write new constraint types. Only the platform may implement this interface. If you tried, other nodes would not understand your constraint type and your transaction would not verify.

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 myAttachmentHash = SecureHash.parse("2b4042aed7e0e39d312c4c477dca1d96ec5a878ddcfd5583251a8367edbd4a5f");
TransactionState transactionState = new TransactionState(contractState, DummyContract.Companion.getPROGRAMID(), notaryParty, new AttachmentHashConstraint(myAttachmentHash));

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

Hard-coding the hash of your app in the code itself can be pretty awkward, so the API also offers the AutomaticHashConstraint. This isn’t a real constraint that will appear in a transaction: it acts as a marker to the TransactionBuilder that you require the hash of the node’s installed app which supplies the specified contract to be used. In practice, when using hash constraints, you almost always want “whatever the current code is” and not a hard-coded hash. So this automatic constraint placeholder is useful.

CorDapps as attachments

CorDapp JARs (see What is a CorDapp?) 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.


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. You can also just use AlwaysAcceptAttachmentConstraint in your tests to disable the constraints mechanism.


The simplest 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;

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

     ... // Your tests go here


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"))

However - there is an easier way! If your unit tests are in the same package as the contract code itself, then you can use the no-args constructor of MockServices. The package to be scanned for CorDapps will be the same as the the package of the class that constructed the object. This is a convenient default.


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.


If an attachment constraint cannot be resolved, a MissingContractAttachments exception is thrown. There are two 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. See the instructions above.

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).