corda/docs/source/api-flows.rst

API: Flows

Note

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

An example flow

Let's imagine a flow for agreeing a basic ledger update between Alice and Bob. This flow will have two sides:

• An Initiator side, that will initiate the request to update the ledger
• A Responder side, that will respond to the request to update the ledger

Initiator

In our flow, the Initiator flow class will be doing the majority of the work:

Part 1 - Build the transaction

1. Choose a notary for the transaction
2. Create a transaction builder
3. Extract any input states from the vault and add them to the builder
4. Create any output states and add them to the builder
5. Add any commands, attachments and timestamps to the builder

Part 2 - Sign the transaction

6. Sign the transaction builder
7. Convert the builder to a signed transaction

Part 3 - Verify the transaction

8. Verify the transaction by running its contracts

Part 4 - Gather the counterparty's signature

9. Send the transaction to the counterparty
10. Wait to receive back the counterparty's signature
11. Add the counterparty's signature to the transaction
12. Verify the transaction's signatures

Part 5 - Finalize the transaction

13. Send the transaction to the notary
14. Wait to receive back the notarised transaction
15. Record the transaction locally
16. Store any relevant states in the vault
17. Send the transaction to the counterparty for recording

We can visualize the work performed by initiator as follows:

Responder

To respond to these actions, the responder takes the following steps:

Part 1 - Sign the transaction

1. Receive the transaction from the counterparty
2. Verify the transaction's existing signatures
3. Verify the transaction by running its contracts
4. Generate a signature over the transaction
5. Send the signature back to the counterparty

Part 2 - Record the transaction

6. Receive the notarised transaction from the counterparty
7. Record the transaction locally
8. Store any relevant states in the vault

FlowLogic

In practice, a flow is implemented as one or more communicating FlowLogic subclasses. Each FlowLogic subclass must override FlowLogic.call(), which describes the actions it will take as part of the flow.

So in the example above, we would have an Initiator FlowLogic subclass and a Responder FlowLogic subclass. The actions of the initiator's side of the flow would be defined in Initiator.call, and the actions of the responder's side of the flow would be defined in Responder.call.

FlowLogic annotations

Any flow that you wish to start either directly via RPC or as a subflow must be annotated with the @InitiatingFlow annotation. Additionally, if you wish to start the flow via RPC, you must annotate it with the @StartableByRPC annotation.

Any flow that responds to a message from another flow must be annotated with the @InitiatedBy annotation. @InitiatedBy takes the class of the flow it is responding to as its single parameter.

So in our example, we would have:

@InitiatingFlow
@StartableByRPC
class Initiator(): FlowLogic<Unit>() {

...

@InitiatedBy(Initiator::class)
class Responder(val otherParty: Party) : FlowLogic<Unit>() {

@InitiatingFlow
@StartableByRPC
public static class Initiator extends FlowLogic<Unit> {

...

@InitiatedBy(Initiator.class)
public static class Responder extends FlowLogic<Void> {

Additionally, any flow that is started by a SchedulableState must be annotated with the @SchedulableFlow annotation.

ServiceHub

Within FlowLogic.call, the flow developer has access to the node's ServiceHub, which provides access to the various services the node provides. See api-service-hub for information about the services the ServiceHub offers.

Some common tasks performed using the ServiceHub are:

• Looking up your own identity or the identity of a counterparty using the networkMapCache
• Identifying the providers of a given service (e.g. a notary service) using the networkMapCache
• Retrieving states to use in a transaction using the vaultService
• Retrieving attachments and past transactions to use in a transaction using the storageService
• Creating a timestamp using the clock
• Signing a transaction using the keyManagementService

Common flow tasks

There are a number of common tasks that you will need to perform within FlowLogic.call in order to agree ledger updates. This section details the API for the most common tasks.

Retrieving information about other nodes

We use the network map to retrieve information about other nodes on the network:

val networkMap = serviceHub.networkMapCache

val allNodes = networkMap.partyNodes
val allNotaryNodes = networkMap.notaryNodes
val randomNotaryNode = networkMap.getAnyNotary()

val alice = networkMap.getNodeByLegalName(X500Name("CN=Alice,O=Alice,L=London,C=UK"))
val bob = networkMap.getNodeByLegalIdentityKey(bobsKey)

final NetworkMapCache networkMap = getServiceHub().getNetworkMapCache();

final List<NodeInfo> allNodes = networkMap.getPartyNodes();
final List<NodeInfo> allNotaryNodes = networkMap.getNotaryNodes();
final Party randomNotaryNode = networkMap.getAnyNotary(null);

final NodeInfo alice = networkMap.getNodeByLegalName(new X500Name("CN=Alice,O=Alice,L=London,C=UK"));
final NodeInfo bob = networkMap.getNodeByLegalIdentityKey(bobsKey);

Communication between parties

FlowLogic instances communicate using three functions:

• send(otherParty: Party, payload: Any)

  • Sends the payload object to the otherParty

• receive(receiveType: Class<R>, otherParty: Party)

  • Receives an object of type receiveType from the otherParty

• sendAndReceive(receiveType: Class<R>, otherParty: Party, payload: Any)

  • Sends the payload object to the otherParty, and receives an object of type receiveType back

Each FlowLogic subclass can be annotated to respond to messages from a given counterparty flow using the @InitiatedBy annotation. When a node first receives a message from a given FlowLogic.call() invocation, it responds as follows:

• The node checks whether they have a FlowLogic subclass that is registered to respond to the FlowLogic that is sending the message:

  1. If yes, the node starts an instance of this FlowLogic by invoking FlowLogic.call()
  2. Otherwise, the node ignores the message

• The counterparty steps through their FlowLogic.call() method until they encounter a call to receive(), at which point they process the message from the initiator

Upon calling receive()/sendAndReceive(), the FlowLogic is suspended until it receives a response.

UntrustworthyData

send() and sendAndReceive() return a payload wrapped in an UntrustworthyData instance. This is a reminder that any data received off the wire is untrustworthy and must be verified.

We verify the UntrustworthyData and retrieve its payload by calling unwrap:

val partSignedTx = receive<SignedTransaction>(otherParty).unwrap { partSignedTx ->
        val wireTx = partSignedTx.verifySignatures(keyPair.public, notaryPubKey)
        wireTx.toLedgerTransaction(serviceHub).verify()
        partSignedTx
    }

final SignedTransaction partSignedTx = receive(SignedTransaction.class, otherParty)
    .unwrap(tx -> {
        try {
            final WireTransaction wireTx = tx.verifySignatures(keyPair.getPublic(), notaryPubKey);
            wireTx.toLedgerTransaction(getServiceHub()).verify();
        } catch (SignatureException ex) {
            throw new FlowException(tx.getId() + " failed signature checks", ex);
        }
        return tx;
    });

Subflows

Corda provides a number of built-in flows that should be used for handling common tasks. The most important are:

• CollectSignaturesFlow, which should be used to collect a transaction's required signatures
• FinalityFlow, which should be used to notarise and record a transaction
• ResolveTransactionsFlow, which should be used to verify the chain of inputs to a transaction
• ContractUpgradeFlow, which should be used to change a state's contract
• NotaryChangeFlow, which should be used to change a state's notary

These flows are designed to be used as building blocks in your own flows. You invoke them by calling FlowLogic.subFlow from within your flow's call method. Here is an example from TwoPartyDealFlow.kt:

../../core/src/main/kotlin/net/corda/flows/TwoPartyDealFlow.kt

In this example, we are starting a CollectSignaturesFlow, passing in a partially signed transaction, and receiving back a fully-signed version of the same transaction.

Subflows in our example flow

In practice, many of the actions in our example flow would be automated using subflows:

• Parts 2-4 of Initiator.call should be automated by invoking CollectSignaturesFlow
• Part 5 of Initiator.call should be automated by invoking FinalityFlow
• Part 1 of Responder.call should be automated by invoking SignTransactionFlow
• Part 2 of Responder.call will be handled automatically when the counterparty invokes FinalityFlow

FlowException

Suppose a node throws an exception while running a flow. Any counterparty flows waiting for a message from the node (i.e. as part of a call to receive or sendAndReceive) will be notified that the flow has unexpectedly ended and will themselves end. However, the exception thrown will not be propagated back to the counterparties.

If you wish to notify any waiting counterparties of the cause of the exception, you can do so by throwing a FlowException:

../../core/src/main/kotlin/net/corda/core/flows/FlowException.kt

The flow framework will automatically propagate the FlowException back to the waiting counterparties.

There are many scenarios in which throwing a FlowException would be appropriate:

• A transaction doesn't verify()
• A transaction's signatures are invalid
• The transaction does not match the parameters of the deal as discussed
• You are reneging on a deal

Suspending flows

In order for nodes to be able to run multiple flows concurrently, and to allow flows to survive node upgrades and restarts, flows need to be checkpointable and serializable to disk.

This is achieved by marking any function invoked from within FlowLogic.call() with an @Suspendable annotation.

We can see an example in CollectSignaturesFlow:

../../core/src/main/kotlin/net/corda/flows/CollectSignaturesFlow.kt