In this this paper we focus on the expressiveness of smart contracts (SC) and its role in blockchain technologies (BCT), by presenting Tenderfone, a prototypical blockchain platform providing SC as pro-active, time-aware, and asynchronous entities.

1. Smart Contracts are more than Objects:
Pro-activeness on the Blockchain
Giovanni Ciatto1 Alfredo Maffi1 Stefano Mariani2
Andrea Omicini1
giovanni.ciatto@unibo.it alfredo.maffi@studio.unibo.it
stefano.mariani@unimore.it andrea.omicini@unibo.it
1Dipartimento di Informatica – Scienza e Ingegneria—Universit`a di Bologna
2Dipartimento di Scienze e Metodi dell’Ingegneria, Universit`a di Modena e Reggio Emilia
1st International Congress on Blockchain and Applications
´Avila, Spain – June 27, 2019
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 1 / 20

2. Outline
1 The Problem with Blockchain Smart Contracts
2 Towards Pro-activity & Asynchrony
3 The Tenderfone Language & Interpreter
4 Tenderfone Internals: Overview
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 2 / 20

3. The Problem with Blockchain Smart Contracts
Next in Line. . .
1 The Problem with Blockchain Smart Contracts
2 Towards Pro-activity & Asynchrony
3 The Tenderfone Language & Interpreter
4 Tenderfone Internals: Overview
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 3 / 20

4. The Problem with Blockchain Smart Contracts
Context
Most recent and interesting BCT come with Smart Contracts (SC)
i.e., trusted, autonomous intermediaries for users transacting on BCT
inspired to Szabo’s idea of self-enforcing contracts [Szabo, 1997]
an idea which is interesting even beyond financial applications
e.g. for enforcing interaction protocols in multi agent systems
computationally interpreted through object orientated programming
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 4 / 20

5. The Problem with Blockchain Smart Contracts
Motivation
Our claim
The mainstream interpretation of SC as objects is inherently flawed
the method call semantics for SC comm. is subtle and bug-prone
e.g. Ethereum’s unexpected recursion [Luu et al., 2016, Atzei et al., 2017]
as objects, SC are constrained in the sorts of things they can do
e.g. they are purely user-reative =⇒ no user invocation means no action
e.g. lack of time-reactiveness =⇒ no time-dependent behaviour
e.g. lack of pro-activeness =⇒ no initiative in rule enforcement
Main research questions
is OOP the best programming paradigm for SC?
is it possible to implement different paradigms? How?
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 5 / 20

6. The Problem with Blockchain Smart Contracts
Motivation
Our claim
The mainstream interpretation of SC as objects is inherently flawed
the method call semantics for SC comm. is subtle and bug-prone
e.g. Ethereum’s unexpected recursion [Luu et al., 2016, Atzei et al., 2017]
as objects, SC are constrained in the sorts of things they can do
e.g. they are purely user-reative =⇒ no user invocation means no action
e.g. lack of time-reactiveness =⇒ no time-dependent behaviour
e.g. lack of pro-activeness =⇒ no initiative in rule enforcement
Main research questions
is OOP the best programming paradigm for SC?
is it possible to implement different paradigms? How?
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 5 / 20

7. Towards Pro-activity & Asynchrony
Next in Line. . .
1 The Problem with Blockchain Smart Contracts
2 Towards Pro-activity & Asynchrony
3 The Tenderfone Language & Interpreter
4 Tenderfone Internals: Overview
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 6 / 20

8. Towards Pro-activity & Asynchrony
Our Proposal
Endowing SC with pro-activity and asynchronous communication support
In this work we
model a minimal language for SC, supporting such features
develop a proof-of-concept SC interpreter (or VM) for it
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 7 / 20

9. Towards Pro-activity & Asynchrony
Our Proposal
Endowing SC with pro-activity and asynchronous communication support
In this work we
model a minimal language for SC, supporting such features
develop a proof-of-concept SC interpreter (or VM) for it
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 7 / 20

10. The Tenderfone Language & Interpreter
Next in Line. . .
1 The Problem with Blockchain Smart Contracts
2 Towards Pro-activity & Asynchrony
3 The Tenderfone Language & Interpreter
4 Tenderfone Internals: Overview
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 8 / 20

11. The Tenderfone Language & Interpreter
Tenderfone in a Nutshell
Layered perspective
Pro-active, logic SC
Tenderfone
Tendermint + tuProlog
Network
Our prototype for a SC interpreter
consists of a language for pro-active,
asynchronous, and logic-based smart
contracts
plus its operational semantics & an
interpreter implementing it
based on the Tendermint [Kwon, 2014]
consensus engine and the tuProlog
framework [Denti et al., 2001]
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 9 / 20

12. The Tenderfone Language & Interpreter
Execution Model – Toy Program Example
§
init([Delay | OtherArgs]) :-
now(Time),
Next is Time + Delay,
when(Next, foo).
receive(foo, Me) :-
self(Me),
periodically(10 min, bar).
receive(bar, Me) :-
/* does something
once every 10 min */.
receive(Message, Sender) :-
Answer = /* compute answer */,
send(Answer, Sender).
¦ ¥
U A B
deploy(Delay)
init([Delay])
when(Now + Delay, foo)
after Delay seconds
receive(foo, A)
periodically(10 min, bar)
once every 10 minutes
receive(bar, A)
do something periodic
in any moment
Message
receive(Message, B)
compute Answer
send(Answer, B)
Answer
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 10 / 20

13. The Tenderfone Language Interpreter
How to Support the Desired Features
asynchronous communication
achieved by changing the semantics of the send/2 primitive
asynchronous computation
achieved by sending messages to self
time-awareness
provided by the now/1 predicate
time-reactiveness
provided by the delay/2 and periodically/2 primitives
pro-activeness
achieved through (delay/2 | periodically/2 | send/2) + init/1
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 11 / 20

14. Tenderfone Internals: Overview
Next in Line. . .
1 The Problem with Blockchain Smart Contracts
2 Towards Pro-activity Asynchrony
3 The Tenderfone Language Interpreter
4 Tenderfone Internals: Overview
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 12 / 20

15. Tenderfone Internals: Overview
Tenderfone System Architecture (coarse-grained) I
a number of clients, issuing transactions, served by a number of cores
which engage consensus, update the blockchain, and execute SC
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 13 / 20

16. Tenderfone Internals: Overview
Tenderfone System Architecture (coarse-grained) II
each core is actually composed by a validator and an interpreter
validators handle transactions, blocks, cryptography, etc
interpreters execute smart contracts
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 14 / 20

17. Tenderfone Internals: Overview
Asynchronous Pro-Active SC through Spontaneous TX
beginBlock(Index, Time)
Scheduled := get_scheduled(Time)
to_tx(Scheduled) spontaneous!
for each Tx in current block
deliverTx(Tx)
State := execute(Tx, State)
Outbox += get_outgoing(State)
commitBlock()
to_tx(Outbox) spontaneous!
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 15 / 20

18. Tenderfone Internals: Overview
About Spontaneous TX
conceptually, they carry the messages / scheduled tasks of SC
in practice, they are produced by interpreters
technically, interpreters act as clients
for each message / scheduled task, n TX are actually produced
where n is the total amount of interpreters
Problem
Corrupt interpreters may forge spurious TX, thus faking SC triggers
→ spontaneous TX are valid only if n copies exist
signed by as many interpreters
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 16 / 20

19. Tenderfone Internals: Overview
About Spontaneous TX
conceptually, they carry the messages / scheduled tasks of SC
in practice, they are produced by interpreters
technically, interpreters act as clients
for each message / scheduled task, n TX are actually produced
where n is the total amount of interpreters
Problem
Corrupt interpreters may forge spurious TX, thus faking SC triggers
→ spontaneous TX are valid only if n copies exist
signed by as many interpreters
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 16 / 20

20. Conclusions and Future Work
Conclusion
Summarising, we
discuss how SC need pro-activity, time-reactiveness and asynchrony
define a language for SC with such features
describe how an interpreter for such SC should work
provide a prototype implementation
In the future, we plan to
provide a formal operational semantics for Tenderfone
endow SC with higher-level coordination/synchronisation mechanisms
extend Tenderfone to support HyperLedger Fabric’s architecture
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 17 / 20

21. Conclusions and Future Work
Conclusion
Summarising, we
discuss how SC need pro-activity, time-reactiveness and asynchrony
define a language for SC with such features
describe how an interpreter for such SC should work
provide a prototype implementation
In the future, we plan to
provide a formal operational semantics for Tenderfone
endow SC with higher-level coordination/synchronisation mechanisms
extend Tenderfone to support HyperLedger Fabric’s architecture
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 17 / 20

22. Smart Contracts are more than Objects:
Pro-activeness on the Blockchain
Giovanni Ciatto1 Alfredo Maffi1 Stefano Mariani2
Andrea Omicini1
giovanni.ciatto@unibo.it alfredo.maffi@studio.unibo.it
stefano.mariani@unimore.it andrea.omicini@unibo.it
1Dipartimento di Informatica – Scienza e Ingegneria—Universit`a di Bologna
2Dipartimento di Scienze e Metodi dell’Ingegneria, Universit`a di Modena e Reggio Emilia
1st International Congress on Blockchain and Applications
´Avila, Spain – June 27, 2019
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 18 / 20

23. References
References I
Atzei, N., Bartoletti, M., and Cimoli, T. (2017).
A survey of attacks on Ethereum smart contracts (SoK).
In Principles of Security and Trust, volume 10204 of LNCS, pages 164–186.
Springer.
Denti, E., Omicini, A., and Ricci, A. (2001).
tuProlog: A light-weight Prolog for Internet applications and infrastructures.
In Practical Aspects of Declarative Languages, volume 1990 of LNCS, pages
184–198. Springer.
Kwon, J. (2014).
Tendermint: Consensus without mining.
Luu, L., Chu, D.-H., Olickel, H., Saxena, P., and Hobor, A. (2016).
Making smart contracts smarter.
In 2016 ACM SIGSAC Conference on Computer and Communications Security
(CCS’16), pages 254–269, New York, NY, USA. ACM Press.
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 19 / 20

24. References
References II
Szabo, N. (1997).
Formalizing and securing relationships on public networks.
First Monday, 2(9).
Ciatto et al. (UniBo, UniMoRe) Pro-activeness on the Blockchain BCC 2019, Jun 27 20 / 20