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Section: New Results

Parallel and Distributed Verification

Distributed Code Generation for LNT

Participants : Hugues Evrard, Frédéric Lang.

Rigorous development and prototyping of a distributed verification algorithm in LNT involves the automatic generation of a distributed implementation. For the latter, a protocol realizing process synchronization is required. As far as possible, this protocol must itself be distributed, so as to avoid the bottleneck that would inevitably arise if a unique process would have to manage all synchronizations in the system. A particularity of such a protocol is its ability to support branching synchronizations, corresponding to situations where a process may offer a choice of synchronizing actions (which themselves may nondeterministically involve several sets of synchronizing processes) instead of a single one. Therefore, a classical barrier protocol is not sufficient and a more elaborate synchronization protocol is needed.

Using a synchronization protocol that we verified formally in 2013, we developed a prototype distributed code generator, named DLC (Distributed LNT Compiler), which takes as input the model of a distributed system described as a parallel composition of LNT processes.

In 2014, we continued the development of DLC. We improved the performances of DLC generated code by reducing the number of protocol messages when one or several processes are ready on a single gate. We experimented this optimization on a set of processes running on different computers and synchronizing all together on a single barrier interaction (i.e., all processes are ready on a single gate). In this situation, DLC now generates code that is faster than Java or Erlang.

The distributed program generated by DLC would be of little interest if it could not interact with its environment (e.g., users through human-computer interfaces, or other systems, such as databases, Web services, etc.). Therefore, we designed a mechanism to embed user-defined C functions, called hook functions, into the code generated by DLC. Hook functions are triggered on events related to actions in the system. This allows system actions to be, e.g., monitored by the user or controlled by external conditions. Using hook functions, the code generated by DLC can thus both take an account of and have an effect on its environment.

In order to demonstrate DLC on a real-world example, we applied it to the recent Raft  ( ) consensus algorithm  [60] . We wrote an LNT specification of a simple key-value store made fault tolerant by replication of commands using the Raft consensus algorithm. During the modeling phase, we found a missing transition in the TLA+ specification of the protocol. We signaled it to the authors  (!topic/raft-dev/yu-wOUx-gnA ), who corrected the TLA+ specification. We used hook functions to implement interaction with the replicated store from external clients. The distributed implementation generated by DLC was successfully tested on clusters of the Grid5000 platform. We presented an overview of DLC, the hook functions and the Raft experiment in an article that has been accepted for publication in an international conference [12] .