1. Deadlock Freedom by Construction
for Distributed Adaptative Applications
Mila Dalla Preda, Ivan Lanese, Jacopo Mauro,
Maurizio Gabbrielli, Saverio Giallorenzo
Milan, Feb 2014

2. Structure of the talk
Choreographies
Adaptation
Our approach:
Motivation
Scopes
Adaptation Rules
Results
Demo → Saverio

3. Choreographies
Allow to describe the behavior of a distributed communicating system from a global perspective
All participants + interactions described together
Composed by operations of the form
Interaction ( op: a → b )
Sequential composition ( ; )
Parallel composition ( | )
Control flow (if then else, while)
Very similar to global types in multiparty
session types

4. Choreography Example

5. Easy to describe distributed systems
Why Choreographies?
Easy to describe distributed systems
Possibility to derive actual code via projection
Input: Choreography code
Output: Code for every participant
Properties: preserves semantics
(when interacting, the participants behave as specified by the choreography)

6. Locations corresponding to participants, containing their code
Where to Project?
Locations corresponding to participants, containing their code
Basic operations: input and output
Composed using
sequential composition ( ; )
parallel composition ( | )
Control flow (if then else, while)

7. Projection Example

8. Users can change their minds
Adaption
Systems should live for long periods of time in ever changing environments
Users can change their minds
The system should adapt to satisfy new requirements
Adaptation happens at runtime
Adaptation details not known when the system has been designed or even started

9. Motivation of Our Work
Lots of works on adaptation exist
Not many formal approaches
Very little guarantees on the properties of the system after adaptation takes place
Using choreographies we can guarantee safe adaptation
Deadlock freedom

10. Our Approach
A system is obtained as projection of a choreography
The system runs on its own
The system interacts with an adaptation middleware composed by distributed adaptation servers and the environment
A single adaptation may involve many participants

11. Adaptation Rules
The adaptation servers contain rules which can be applied to update the running system
Conditions checking whether adaptation is applicable/useful can be specified
New rules can be added at any moment
Application of a rule involves code mobility from the adaptation server to the system

15. Adaptation Scopes
Scopes to enclose code that could be adapted
Contain default code
May tag code with (non-functional) properties
Specify leader of adaptation → who triggers the check of adaptation

16. Adaptation Rules
Contain new code + information on when and where the rule can be applied

17. Projecting a Scope
Semantics of the leader:
Check whether there is a rule whose applicability condition holds
✓ → download the code for each participant and send it to them
x → tell the other participants that no adaptation is needed
Semantics of other participants
Wait for instruction from the leader
✓ adaptation → execute the new code
x adaptation → execute the current code

18. Connectedness
Two syntactic conditions:
For sequence → ensures that projecting a sequence I;I' implies actions in I executed before action in I'
For parallel → ensure that different interactions with the same operation do not interfere
Polynomial check
On line validation while writing code – see tool

19. Results
A choreography and its projection have the same traces
Under all possible adaptations
With environments and sets of applicable rules that may change at any moment during the computation
The adapted system is deadlock free by construction
Developed an Eclipse plug-in + automatic projection in Jolie