REBLS 2021: Proceedings of the 8th ACM SIGPLAN International Workshop on Reactive and Event-Based Languages and Systems

Embedded control systems have long been designed using block diagrams and state machines. These models often simply guide the manual implementation of software. But in Model-Based Design they are treated as programs and compiled automatically into low-level code. This is the approach taken in academic languages like Lustre and commercial tools like Simulink and SCADE Suite.

This talk presents results from the ongoing Vélus project that aims to specify a compiler for Model-Based Design (Lustre with features from Scade 6) in an Interactive Theorem Prover (Coq). It will describe the model of synchronous streams that gives a semantics to the input language and features of the proof that links this model to the step-by-step model of the generated assembly code. Our prototype builds on the CompCert C compiler and we will talk about some of the challenges of interfacing with such a low-level model of the underlying machine.

The Internet of Things (IoT) requires applications to deal with a large amount of data - streamed, processed and stored from small devices to analytical systems. Cloud computing offers a hardware solution to this issue, providing on-demand resources to process IoT data. The newer programming paradigms simplify the use of those cloud resources. The Function-as-a-Service (FaaS) and the Serverless paradigm transform the conception of microservices applications to the definition and the composition of several callable functions. Although defined as distributed architectures - mostly publicly available solutions rely on either a gateway or an internal messaging middleware. These architectures create a single point of failure in exchange for more straightforward service to service communication.

In this article, we present Dyninka, a framework to rapidly prototype FaaS-based distributed dataflow applications. Its programming model gathers the definition and the composition of services within a single file using the multitier programming paradigm and compiles them into a multitude of services deployable on cloud computing infrastructure. Dyninka is built without a gateway or a messaging platform, and services communicate directly with each other or with the cloud abstracted infrastructure. As a result, we reduce the network and the computation overheads introduced by commercial FaaS frameworks such as OpenFaaS.

We validate Dyninka on a Fog computing scenario with limited resources and several load profiles. For all scenarios, Dyninka shows better stability, throughput and a reduced overhead compared to OpenFaaS.

This paper presents Poker, a visual instrumentation platform for reactive programs. Similar to other platforms, Poker features a visual dashboard that allows the programmer to inspect the flow of values through the reactive program. The novelty of Poker is that: (a) It features a canvas of so-called probes that can be dynamically wired into a running reactive program in order to instrument the running system. (b) In addition to focusing on the values flowing through the program, a probe can measure a particular property about the way these values flow through the instrumented program. (c) The set of probes is open because a probe is programmed in the same language as the instrumented program.

Poker is implemented for Stella, an experimental reactive programming language. The paper uses an application written in Stella to motivate the concepts provided by Poker. We show 4 different probes that help us understand the behaviour of the application and we measure the overhead of using Poker on the running application with some preliminary benchmarks.

Reactive programming is all about relegating the management of a program’s state changes to the realm of the runtime environment. Nevertheless, sometimes it is still necessary to enrich a reactive program with state variables that are explicitly updated by the programmer. In current reactive languages this is accomplished either by polluting the reactive paradigm with imperative constructs or by relying on built-in operators such as foldp.

This paper introduces trampoline variables, a new general mechanism that allows reactive programs to manipulate state explicitly without resorting to imperative programming. We show that our proposal is at least as powerful as existing built-in reactive operators. We also analyse how reactive programs with trampoline variables can be composed and how they can form the basis to replace stateful constituents of a running reactive program — a.k.a. hotswapping — in a coherent way. The latter is an essential building block towards live IDEs for reactive programming languages.

A program is deterministic if multiple re-executions with the same inputs always lead to the same state. Even concurrent instances of a deterministic program should observe identical behavior---in real time---if assigned the same set of inputs. In this work, we propose real-time reproducibility for distributed programs. Multiple instances of the same interactive application can broadcast asynchronous inputs and yet conform to identical behavior. Collaborative networked applications, such as watch parties, document editing, and video games can benefit from this approach. We name this class of applications as symmetric distributed applications. Using a standard event-driven API to wait and emit events, programmers write code as if the application executes in a single machine. Our middleware intercepts event generation and synchronizes all instances in a consistent timeline so that receipt is identically reproducible. Not only distributed applications benefit from consistency and determinism but also development and testing can be done in a single instance with the same guarantees. In our experiments, the middleware can handle applications with 25 FPS, distributed in up to 25 nodes over the Internet, with an event latency below 350ms.

Modern services running in cloud and edge environments need to be resource-efficient to increase deployment density and reduce operating costs. Asynchronous I/O combined with asynchronous programming provides a solid technical foundation to reach these goals. Reactive programming and reactive streams are gaining traction in the Java ecosystem. However, reactive streams implementations tend to be complex to work with and maintain. This paper discusses the performance of the three major reactive streams compliant libraries used in Java applications: RxJava, Project Reactor, and SmallRye Mutiny. As we will show, advanced optimization techniques such as operator fusion do not yield better performance on realistic I/O-bound workloads, and they significantly increase development and maintenance costs.