Section: New Results

Distributed systems verification and programming language

Participants : Cezara Drăgoi [correspondant] , Thomas Henzinger [IST Austria, Austria] , Damien Zufferey [MIT, CSAIL, USA] .

Fault-tolerant distributed systems, Programming languages, Verification Fault-tolerant distributed algorithms play an important role in many critical/high-availability applications. These algorithms are notoriously difficult to implement correctly, due to asynchronous communication and the occurrence of faults, such as the network dropping messages or computers crashing. Noteworthy is the lack of automated verification techniques for distributed systems, highly contrasting the mass distribution and development of distributed software. Therefore, our main motivation is to increase the confidence we have in distributed systems using formal verification methods. However, due to the complexity distributed systems have reached, we believe it is no longer realistic nor efficient to assume that high level specifications can be proved when development and verification are two disconnected steps in the software production process. We think that the difficulty does not only come from the algorithms but from the way we think about distributed systems. Therefore, we are interested in finding an appropriate programming model for fault-tolerant distributed algorithms, that increases the confidence we have distributed software. We introduced PSYNC, a domain specific language based on the Heard-Of model, which views asynchronous faulty systems as synchronous ones with an adversarial environment that simulates asynchrony and faults by dropping messages. We defined a runtime system for PSYNC that efficiently executes on asynchronous networks. We formalize the relation between the runtime system and PSYNC in terms of observational refinement. PSYNC introduces a high-level lockstep abstraction (on top of the standard asynchronous semantics), which simplifies the design and implementation of fault-tolerant distributed algorithms and enables automated formal verification. We have implemented an embedding of PSYNC in the SCALA programming language with a runtime system for asynchronous networks. We showed the applicability of PSYNC by implementing several important fault-tolerant distributed algorithms and we compared the implementation of consensus algorithms in PSYNC against implementations in other languages in terms of code size, runtime efficiency, and verification.