What is a CorDapp?

CorDapps (Corda Distributed Applications) are distributed applications that run on the Corda platform. The goal of a CorDapp is to allow nodes to reach agreement on updates to the ledger. They achieve this goal by defining flows that Corda node owners can invoke over RPC:

node diagram

CorDapp components

CorDapps take the form of a set of JAR files containing class definitions written in Java and/or Kotlin.

These class definitions will commonly include the following elements:

  • Flows: Define a routine for the node to run, usually to update the ledger (see Key Concepts - Flows). They subclass FlowLogic
  • States: Define the facts over which agreement is reached (see Key Concepts - States). They implement the ContractState interface
  • Contracts, defining what constitutes a valid ledger update (see Key Concepts - Contracts). They implement the Contract interface
  • Services, providing long-lived utilities within the node. They subclass SingletonSerializationToken
  • Serialisation whitelists, restricting what types your node will receive off the wire. They implement the SerializationWhitelist interface

But the CorDapp JAR can also include other class definitions. These may include:

  • APIs and static web content: These are served by Corda‚Äôs built-in webserver. This webserver is not production-ready, and should be used for testing purposes only
  • Utility classes

An example

Suppose a node owner wants their node to be able to trade bonds. They may choose to install a Bond Trading CorDapp with the following components:

  • A BondState, used to represent bonds as shared facts on the ledger

  • A BondContract, used to govern which ledger updates involving BondState states are valid

  • Three flows:>

    • An IssueBondFlow, allowing new BondState states to be issued onto the ledger
    • A TradeBondFlow, allowing existing BondState states to be bought and sold on the ledger
    • An ExitBondFlow, allowing existing BondState states to be exited from the ledger

After installing this CorDapp, the node owner will be able to use the flows defined by the CorDapp to agree ledger updates related to issuance, sale, purchase and exit of bonds.