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Building transactions
Introduction
Understanding and implementing transactions in Corda is key to building and implementing real world smart contracts. It is only through construction of valid Corda transactions containing appropriate data that nodes on the ledger can map real world business objects into a shared digital view of the data in the Corda ledger. More importantly as the developer of new smart contracts it is the code which determines what data is well formed and what data should be rejected as mistakes, or to prevent malicious activity. This document details some of the considerations and APIs used to when constructing transactions as part of a flow.
The Basic Lifecycle Of Transactions
Transactions in Corda are constructed in stages and contain a number of elements. In particular a transaction’s core data structure is the net.corda.core.transactions.WireTransaction, which is usually manipulated via a net.corda.core.transactions.TransactionBuilder and contains:
1. A set of Input state references that will be consumed by the final accepted transaction.
2. A set of Output states to create/replace the consumed states and thus become the new latest versions of data on the ledger.
3. A set of Attachment items which can contain legal documents, contract code, or private encrypted sections as an extension beyond the native contract states.
4. A set of Command items which give a context to the type of ledger transition that is encoded in the transaction. Also each command has an associated set of signer keys, which will be required to sign the transaction.
5. A signers list, which is populated by the TransactionBuilder to be the union of the signers on the individual Command objects.
6. A notary identity to specify the Notary node which is tracking the state consumption. (If the input states are registered with different notary nodes the flow will have to insert additional NotaryChange transactions to migrate the states across to a consistent notary node, before being allowed to mutate any states.)
7. Optionally a timestamp that can used in the Notary to time bound the period in which the proposed transaction stays valid.
Typically, the WireTransaction should be regarded as a proposal and may need to be exchanged back and forth between parties before it can be fully populated. This is an immediate consequence of the Corda privacy model, which means that the input states are likely to be unknown to the other node.
Once the proposed data is fully populated the flow code should freeze the WireTransaction and form a SignedTransaction. This is key to the ledger agreement process, as once a flow has attached a node’s signature it has stated that all details of the transaction are acceptable to it. A flow should take care not to attach signatures to intermediate data, which might be maliciously used to construct a different SignedTransaction. For instance in a foreign exchange scenario we shouldn't send a SignedTransaction with only our sell side populated as that could be used to take the money without the expected return of the other currency. Also, it is best practice for flows to receive back the TransactionSignature of other parties rather than a full SignedTransaction objects, because otherwise we have to separately check that this is still the same SignedTransaction and not a malicious substitute.
The final stage of committing the transaction to the ledger is to notarise the SignedTransaction, distribute this to all appropriate parties and record the data into the ledger. These actions are best delegated to the FinalityFlow, rather than calling the individual steps manually. However, do note that the final broadcast to the other nodes is asynchronous, so care must be used in unit testing to correctly await the Vault updates.
Gathering Inputs
One of the first steps to forming a transaction is gathering the set of input references. This process will clearly vary according to the nature of the business process being captured by the smart contract and the parameterised details of the request. However, it will generally involve searching the Vault via the VaultService interface on the ServiceHub to locate the input states.
To give a few more specific details consider two simplified real world scenarios. First, a basic foreign exchange Cash transaction. This transaction needs to locate a set of funds to exchange. A flow modelling this is implemented in FxTransactionBuildTutorial.kt. Second, a simple business model in which parties manually accept, or reject each other's trade proposals which is implemented in WorkflowTransactionBuildTutorial.kt. To run and explore these examples using the IntelliJ IDE one can run/step the respective unit tests in FxTransactionBuildTutorialTest.kt and WorkflowTransactionBuildTutorialTest.kt, which drive the flows as part of a simulated in-memory network of nodes.
Note
Before creating the IntelliJ run configurations for these unit tests go to Run -> Edit Configurations -> Defaults -> JUnit, add -javaagent:lib/quasar.jar -Dco.paralleluniverse.fibers.verifyInstrumentation to the VM options, and set Working directory to $PROJECT_DIR$ so that the Quasar instrumentation is correctly configured.
For the Cash transaction let’s assume the cash resources are using the standard CashState in the :financial Gradle module. The Cash contract uses FungibleAsset states to model holdings of interchangeable assets and allow the split/merge and summing of states to meet a contractual obligation. We would normally use the Cash.generateSpend method to gather the required amount of cash into a TransactionBuilder, set the outputs and move command. However, to elucidate more clearly example flow code is shown here that will manually carry out the inputs queries by specifying relevant query criteria filters to the tryLockFungibleStatesForSpending method of the VaultService.
example-code/src/main/kotlin/net/corda/docs/FxTransactionBuildTutorial.kt
As a foreign exchange transaction we expect an exchange of two currencies, so we will also require a set of input states from the other counterparty. However, the Corda privacy model means we do not know the other node’s states. Our flow must therefore negotiate with the other node for them to carry out a similar query and populate the inputs (See the ForeignExchangeFlow for more details of the exchange). Having identified a set of Input StateRef items we can then create the output as discussed below.
For the trade approval flow we need to implement a simple workflow pattern. We start by recording the unconfirmed trade details in a state object implementing the LinearState interface. One field of this record is used to map the business workflow to an enumerated state. Initially the initiator creates a new state object which receives a new UniqueIdentifier in its linearId property and a starting workflow state of NEW. The Contract.verify method is written to allow the initiator to sign this initial transaction and send it to the other party. This pattern ensures that a permanent copy is recorded on both ledgers for audit purposes, but the state is prevented from being maliciously put in an approved state. The subsequent workflow steps then follow with transactions that consume the state as inputs on one side and output a new version with whatever state updates, or amendments match to the business process, the linearId being preserved across the changes. Attached Command objects help the verify method restrict changes to appropriate fields and signers at each step in the workflow. In this it is typical to have both parties sign the change transactions, but it can be valid to allow unilateral signing, if for instance one side could block a rejection. Commonly the manual initiator of these workflows will query the Vault for states of the right contract type and in the right workflow state over the RPC interface. The RPC will then initiate the relevant flow using StateRef, or linearId values as parameters to the flow to identify the states being operated upon. Thus code to gather the latest input state for a given StateRef would use the VaultService as follows:
example-code/src/main/kotlin/net/corda/docs/WorkflowTransactionBuildTutorial.kt
Generating Commands
For the commands that will be added to the transaction, these will need to correctly reflect the task at hand. These must match because inside the Contract.verify method the command will be used to select the validation code path. The Contract.verify method will then restrict the allowed contents of the transaction to reflect this context. Typical restrictions might include that the input cash amount must equal the output cash amount, or that a workflow step is only allowed to change the status field. Sometimes, the command may capture some data too e.g. the foreign exchange rate, or the identity of one party, or the StateRef of the specific input that originates the command in a bulk operation. This data will be used to further aid the Contract.verify, because to ensure consistent, secure and reproducible behaviour in a distributed environment the Contract.verify, transaction is the only allowed to use the content of the transaction to decide validity.
Another essential requirement for commands is that the correct set of CompositeKeys are added to the Command on the builder, which will be used to form the set of required signers on the final validated transaction. These must correctly align with the expectations of the Contract.verify method, which should be written to defensively check this. In particular, it is expected that at minimum the owner of an asset would have to be signing to permission transfer of that asset. In addition, other signatories will often be required e.g. an Oracle identity for an Oracle command, or both parties when there is an exchange of assets.
Generating Outputs
Having located a set of StateAndRefs as the transaction inputs, the flow has to generate the output states. Typically, this is a simple call to the Kotlin copy method to modify the few fields that will transitioned in the transaction. The contract code may provide a generateXXX method to help with this process if the task is more complicated. With a workflow state a slightly modified copy state is usually sufficient, especially as it is expected that we wish to preserve the linearId between state revisions, so that Vault queries can find the latest revision.
For fungible contract states such as Cash it is common to distribute and split the total amount e.g. to produce a remaining balance output state for the original owner when breaking up a large amount input state. Remember that the result of a successful transaction is always to fully consume/spend the input states, so this is required to conserve the total cash. For example from the demo code:
example-code/src/main/kotlin/net/corda/docs/FxTransactionBuildTutorial.kt
Building the WireTransaction
Having gathered all the ingredients for the transaction we now need to use a TransactionBuilder to construct the full WireTransaction. The initial TransactionBuilder should be created by calling the TransactionBuilder method. At this point the Notary to associate with the states should be recorded. Then we keep adding inputs, outputs, commands and attachments to fill the transaction. Examples of this process are:
example-code/src/main/kotlin/net/corda/docs/WorkflowTransactionBuildTutorial.kt
example-code/src/main/kotlin/net/corda/docs/FxTransactionBuildTutorial.kt
Completing the SignedTransaction
Having created an initial WireTransaction and converted this to an initial SignedTransaction the process of verifying and forming a full SignedTransaction begins and then completes with the notarisation. In practice this is a relatively stereotypical process, because assuming the WireTransaction is correctly constructed the verification should be immediate. However, it is also important to recheck the business details of any data received back from an external node, because a malicious party could always modify the contents before returning the transaction. Each remote flow should therefore check as much as possible of the initial SignedTransaction inside the unwrap of the receive before agreeing to sign. Any issues should immediately throw an exception to abort the flow. Similarly the originator, should always apply any new signatures to its original proposal to ensure the contents of the transaction has not been altered by the remote parties.
The typical code therefore checks the received SignedTransaction using the verifySignaturesExcept method, excluding itself, the notary and any other parties yet to apply their signature. The contents of the WireTransaction inside the SignedTransaction should be fully verified further by expanding with toLedgerTransaction and calling verify. Further context specific and business checks should then be made, because the Contract.verify is not allowed to access external context. For example the flow may need to check that the parties are the right ones, or that the Command present on the transaction is as expected for this specific flow. An example of this from the demo code is:
example-code/src/main/kotlin/net/corda/docs/WorkflowTransactionBuildTutorial.kt
After verification the remote flow will return its signature to the originator. The originator should apply that signature to the starting SignedTransaction and recheck the signatures match.
Committing the Transaction
Once all the party signatures are applied to the SignedTransaction the final step is notarisation. This involves calling NotaryFlow.Client to confirm the transaction, consume the inputs and return its confirming signature. Then the flow should ensure that all nodes end with all signatures and that they call ServiceHub.recordTransactions. The code for this is standardised in the FinalityFlow, or more explicitly an example is:
example-code/src/main/kotlin/net/corda/docs/WorkflowTransactionBuildTutorial.kt
Partially Visible Transactions
The discussion so far has assumed that the parties need full visibility of the transaction to sign. However, there may be situations where each party needs to store private data for audit purposes, or for evidence to a regulator, but does not wish to share that with the other trading partner. The tear-off/Merkle tree support in Corda allows flows to send portions of the full transaction to restrict visibility to remote parties. To do this one can use the WireTransaction.buildFilteredTransaction extension method to produce a FilteredTransaction. The elements of the SignedTransaction which we wish to be hide will be replaced with their secure hash. The overall transaction txid is still provable from the FilteredTransaction preventing change of the private data, but we do not expose that data to the other node directly. A full example of this can be found in the NodeInterestRates Oracle code from the irs-demo project which interacts with the RatesFixFlow flow. Also, refer to the merkle-trees documentation.