This document introduces the CorDapp Design Language (CDL), a standard way to represent complex CorDapp designs using different views. It discusses how CDL can be used to represent smart contract state machines, depict smart contract transactions and state evolutions, and reason about privacy. The document provides examples of using CDL views like state machine view, transaction instance view, and state evolution view to model elements of a smart contract for selling goods. It argues that CDL helps build more robust CorDapps by enabling clear documentation and analysis of key design aspects.

1. 11
CorDapp Design Language
(CDL)
How to Draw a CorDapp and why you should?
Matt Bradbury – Oct 19

2. A declaration ...
I love designing CorDapps 
If you’re here at CordaCon dev day…
(Most of) You love designing CorDapps too 

3. However… its not 2017
We’re not doing PoCs anymore… production grade Cordapps are more complex
• Need more functionality
• Must included non-happy path
• Privacy must be maintained
• Design must be provably robust
We need a way to represent and reason about
non-trivial CorDapp designs

4. How to represent CorDapp design?

5. (Semi) Real Client example
WYSINWISI see.. So the
data is all in
the same
Corda state
I see.. So the
data is split
across multiple
Corda states
I have no Idea
what that box
is meant to be
What’s for
lunch

6. The problem
Working out the idea Working out how to represent the idea
VS

7. Conclusion…
CorDapp Design Language
(Previously Corda Modelling Notation)
We need a standardised way to represent and reason
about non-trivial CorDapp designs, that encourages
good CorDapp design

8. CorDapp Design Language - Objectives
• Allow non-trivial CorDapp designs to be represented in an accurate, clear and concise
way
• Enable reasoning about key aspects of CorDapp design:
• Shared data and state
• Shared logic
• Permissioning (via digital signatures)
• Privacy (Visibility of data to different participants)
• Encourage good design – distinguishes Ledger vs Orchestration Layer
• Facilitate reusable Design Patterns
• Degrade gracefully
• Accurate design documentation – Not just Marketechture

9. Let do some examples
• Represent a Smart Contract using State Machine View
• Show a Smart Contract in action using Transaction Instance view and State Evolution View
• Analyse Smart Contract privacy using Privacy Overlays
• Introduce a few design patterns along the way

10. How to represent a Smart Contract
• Smart Contracts in Corda are made up of
• ContractState (Data), and
• Contract (Rule book)
• You can consider the Smart Contract behaviour as a Finite State Machine
• Think washing machine
• Example: An agreement for a Seller to sell goods to a buyer for an agreed price

11. Lets use the…
State Machine View
Or ‘drawing a Smart Contract’

14. Constraints -> Verify()

23. However….. Complication 1

24. This is not a true
agreement of two
parties

25. Node A
However…..
A
Node B
B

26. Node A
However…..
A Buy B’s House for £1
A
Node B
B
Initiate

27. Node A
However…..
Sign: A
A Buy B’s house for £1
A
• Verify()
• Sign
Node B
B
Initiate
???

28. Node A
However…..
Sign: A
A Buy B’s house for £1
A
• Verify()
• Sign
Node B
Sign: A
A Buy B’s house for £1
B
Initiate

29. Node A
However…..
Sign: A
A Buy B’s house for £1
A
• Verify()
• Sign
Node B
Sign: A
A Buy B’s house for £1
Sign: B
• Verify()
• checkTransaction()
• Sign
B
Initiate

30. Node A
However…..
Sign: A
A Buy B’s house for £1
A
• Verify()
• Sign
Node B
Sign: A
A Buy B’s house for £1
Sign: B
• Verify()
• checkTransaction()
• Sign
B
Initiate
Sign: B

31. Node A
However…..
Sign: A
A Buy B’s house for £1
A
Node B
Sign: A
A Buy B’s house for £1
Sign: B
B
Initiate
Sign: B
???
Signing is automatic for the responder to a transaction

33. Solution – Propose-Consent Pattern
Transaction Instance View
Or ‘drawing transactions’

44. Complication 2
• The Business Network Operator wants paying
• Specifically they want to charge each time a deal is initially agreed

46. Problem solved… or is it?
State Evolution View
Or… ‘State Machines in Action’

54. Party C ends up with Party B’s
private Deal in their vault

55. Party C ends up with Party B’s
private Deal in their vault
This is NOT okay!!

56. So… How do we fix this?

67. Takeaways
• CorDapp Design Language (CDL) is here to help you write better, more robust
CorDapps
• It allows you to
• Describe complex production grade Smart Contracts
• Show Smart Contracts in action
• Reason about Privacy
• Also views for the Orchestration layer (not covered here)

68. For more information
• See the Corda Solutions DocSite: https://solutions.corda.net/
• Publishing a CDL template soon

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