Parameterized Concurrent Multi-Party Session Types

Session types have been proposed as a means of statically verifying implementations of communication protocols. Although prior work has been successful in verifying some classes of protocols, it does not cope well with parameterized, multi-actor scenarios with inherent asynchrony. For example, the sliding window protocol is inexpressible in previously proposed session type systems. This paper describes System-A, a new typing language which overcomes many of the expressiveness limitations of prior work. System-A explicitly supports asynchrony and parallelism, as well as multiple forms of parameterization. We define System-A and show how it can be used for the static verification of a large class of asynchronous communication protocols.Comment: In Proceedings FOCLASA 2012, arXiv:1208.432

Scalable Termination Detection for Distributed Actor Systems

Automatic garbage collection (GC) prevents certain kinds of bugs and reduces programming overhead. GC techniques for sequential programs are based on reachability analysis. However, testing reachability from a root set is inadequate for determining whether an actor is garbage because an unreachable actor may send a message to a reachable actor. Instead, it is sufficient to check termination (sometimes also called quiescence): an actor is terminated if it is not currently processing a message and cannot receive a message in the future. Moreover, many actor frameworks provide all actors with access to file I/O or external storage; without inspecting an actor's internal code, it is necessary to check that the actor has terminated to ensure that it may be garbage collected in these frameworks. Previous algorithms to detect actor garbage require coordination mechanisms such as causal message delivery or nonlocal monitoring of actors for mutation. Such coordination mechanisms adversely affect concurrency and are therefore expensive in distributed systems. We present a low-overhead reference listing technique (called DRL) for termination detection in actor systems. DRL is based on asynchronous local snapshots and message-passing between actors. This enables a decentralized implementation and transient network partition tolerance. The paper provides a formal description of DRL, shows that all actors identified as garbage have indeed terminated (safety), and that all terminated actors--under certain reasonable assumptions--will eventually be identified (liveness).Comment: 23 pages, 7 figures. To appear in the proceedings of CONCUR 2020. Version 2: Fixed TeX error that omitted predicates in the third line of the Send rule: Actor $A$ must have active refobs $x$ and $y_1 \dots y_n

Targeted Test Generation for Actor Systems

This paper addresses the problem of targeted test generation for actor systems. Specifically, we propose a method to support generation of system-level tests to cover a given code location in an actor system. The test generation method consists of two phases. First, static analysis is used to construct an abstraction of an entire actor system in terms of a message flow graph (MFG). An MFG captures potential actor interactions that are defined in a program. Second, a backwards symbolic execution (BSE) from a target location to an "entry point" of the actor system is performed. BSE uses the MFG constructed in the first phase of our targeted test generation method to guide execution across actors. Because concurrency leads to a huge search space which can potentially be explored through BSE, we prune the search space by using two heuristics combined with a feedback-directed technique. We implement our method in Tap, a tool for Java Akka programs, and evaluate Tap on the Savina benchmarks as well as four open source projects. Our evaluation shows that the Tap achieves a relatively high target coverage (78% on 1,000 targets) and detects six previously unreported bugs in the subjects

Dynamic Voltage Scaling Techniques for Energy Efficient Synchronized Sensor Network Design

Building energy-efficient systems is one of the principal challenges in wireless sensor networks. Dynamic voltage scaling (DVS), a technique to reduce energy consumption by varying the CPU frequency on the fly, has been widely used in other settings to accomplish this goal. In this paper, we show that changing the CPU frequency can affect timekeeping functionality of some sensor platforms. This phenomenon can cause an unacceptable loss of time synchronization in networks that require tight synchrony over extended periods, thus preventing all existing DVS techniques from being applied. We present a method for reducing energy consumption in sensor networks via DVS, while minimizing the impact of CPU frequency switching on time synchronization. The system is implemented and evaluated on a network of 11 Imote2 sensors mounted on a truss bridge and running a high-fidelity continuous structural health monitoring application. Experimental measurements confirm that the algorithm significantly reduces network energy consumption over the same network that does not use DVS, while requiring significantly fewer re-synchronization actions than a classic DVS algorithm.unpublishedis peer reviewe

Types for Progress in Actor Programs

Properties in the actor model can be described in terms of the message-passing behavior of actors. In this paper, we address the problem of using a type system to capture liveness properties of actor programs. Specifically, we define a simple actor language in which demands for certain types of messages may be generated during execution, in a manner specified by the programmer. For example, we may want to require that each request to an actor eventually results in a reply. The difficulty lies in that such requests can be generated dynamically, alongside the associated requirements for replies. Such replies might be sent in response to intermediate messages that never arrive, but the property may also not hold for more trivial reasons; for instance, when the code of potential senders of the reply omit the required sending command in some branches of a conditional statement. We show that, for a restricted class of actor programs, a system that tracks typestates can statically guarantee that such dynamically generated requirements will eventually be satisfied.Ope

Complexity analysis for DROPLET: Distributed Operator Placement for IoT Applications Spanning Edge and Cloud Resources

Internet of Things (IoT) applications generate massive amounts of real-time data. Owners of such data strive to make predictions/inference from large streams of complex input such as video feeds, often by deploying applications that involve machine learning and image processing operations. A typical deployment of IoT applications includes edge devices to acquire the input data and provide processing/storage capacity closer to the location where the data is captured. An important challenge for IoT applications is deciding which operations to be executed on an edge device and which operations should be carried out on the cloud, in order to minimize the completion time of all operations. We call this the distributed operator placement problem. In this report, we show that the distributed operator placement problem is NP-complete. We then provide computational complexity analysis of a dynamic programming algorithm called Droplet, which is a heuristic to scalably partition operations in IoT applications across shared edge and cloud resources, while minimizing the completion time of all operations. Our analysis shows that Droplet scales log-linearly in the total number of operations.Ope

Garbage Collection of Actors

This paper considers the garbage collection of concurrent objects for which it is necessary to know not only "reachability", the usual criterion for reclaiming data, but also the "state" (active or blocked) of the object. For the actor model, a more comprehensive definition than previously available is given for reclaimable actors. Two garbage collection algorithms, implementing a set of "coloring" rules, are presented and their computational complexity is analyzed. Extensions are briefly described to allow incremental, concurrent, distributed and real-time collection. It is argued that the techniques used for the actor model applies to other object-oriented concurrent models

Measuring service quality and assessing its relationship to contraceptive discontinuation: A prospective cohort study in Pakistan and Uganda

Background: The quality of contraceptive counseling that women receive from their provider can influence their future contraceptive continuation. We examined (1) whether the quality of contraceptive service provision could be measured in a consistent way by using existing tools from 2 large-scale social franchises, and (2) whether facility quality measures based on these tools were consistently associated with contraceptive discontinuation.Methods: We linked existing, routinely collected facility audit data from social franchise clinics in Pakistan and Uganda with client data. Clients were women aged 15-49 who initiated a modern, reversible contraceptive method from a sampled clinic. Consented participants completed an exit interview and were contacted 3, 6, and 12 months later. We collapsed indicators into quality domains using theory-based categorization, created summative quality domain scores, and used Cox proportional hazards models to estimate the relationship between these quality domains and discontinuation while in need of contraception.Results: The 12-month all-modern method discontinuation rate was 12.5% among the 813 enrolled women in Pakistan and 5.1% among the 1,185 women in Uganda. We did not observe similar associations between facility-level quality measures and discontinuation across these 2 settings. In Pakistan, an increase in the structural privacy domain was associated with a 60% lower risk of discontinuation, adjusting for age and baseline method (PP=.005).Conclusions: We were not able to leverage existing, widely used quality measurement tools to create quality domains that were consistently associated with discontinuation in 2 study settings. Given the importance of contraceptive service quality and recent advances in indicator standardization in other areas, we recommend further effort to harmonize and simplify measurement tools to measure and improve contraceptive quality of care for all