Balakrishnan Dasarathy

Achieving and assuring high availability

We discuss availability aspects of large software- based systems. We classify faults into Bohrbugs, Mandelbugs and aging-related bugs, then examine mitigation methods for the last two bug types. We also consider quantitative approaches to availability assurance.

Performance analysis of the CORBA event service using stochastic reward nets

The event service is the earliest CORBA solution to the message queue model of communication in distributed systems. Typical implementations, however, suffer from the lack of event delivery guarantees. The loss of messages is aggravated by the presence of burstiness in the input to the event service, and occurrences of isolated bursts of traffic could also have serious effects. In this paper, we develop stochastic reward net (SRN) models that can aid in the study and configuration of the event service to conform to design specifications. To capture burstiness in the input, a Markov-modulated Poisson process (MMPP) is used as the input source. Erlang distributed event consumption times are used in the models to accommodate more general distributions and a wider range of variances. The models also take into consideration the FIFO discard policy adopted in many event service implementations. The SRN models are solved using the tool SPNP (Stochastic Petri Net Package). The applicability of the models to the CORBA notification service is also briefly discussed.

The past, present, and future of MOOCs and their relevance to software engineering

Massive open online courses (MOOCs) are a recent development in online education aimed at unlimited participation and open access via the Web. They are a potentially disruptive technology, changing how education is delivered and funded around the world. MOOCs are relevant to software researchers and practitioners, not only because they will increasingly receive lifelong education through MOOCs and related technologies, but also because content creation, delivery, and enhancement of MOOCs is evolving into a new form of socially- and cognitively-embedded software development. This paper discusses how education is being enhanced by MOOCs and other digital learning technology. In particular, we distinguish the free educational content provided by MOOCs from the emerging collaborative processes through which MOOCs are created, which is arguably more transformative on education than the content itself. We discuss blended models of higher education to suit different learner communities, as well as nascent moves toward the creation of instructional communities of educators that transcend institutional boundaries. We also explore MOOCs and their evolution as a subject for research in the learning sciences and implications for R&D in software and systems engineering.

Network QoS assurance in a multi-layer adaptive resource management scheme for mission-critical applications using the CORBA middleware framework

We present adaptive network QoS (quality of service) technology that provides ongoing, end-to-end assurance that critical traffic belonging to admitted flows has bounded queuing loss, delay, and jitter. Our technology uses a bandwidth broker to provide admission control, and leverages differentiated services and class of service functionality of high-end routers and switches for enforcement. The technology employs an integrated QoS treatment across a hybrid layer-2/layer-3 network and adapts to changes in mission requirements, work load and configurations; it uses discovery algorithms in these layers to maintain a current view of resource availability. Under the DARPA ARMS (adaptive and reflective middleware systems) program, our technology is being developed, integrated and validated in a CORBA-based multilayer resource management framework.

NetQoPE: A Model-Driven Network QoS Provisioning Engine for Distributed Real-time and Embedded Systems

This paper provides two contributions to the study of quality of service (QoS)-enabled middleware that supports the network QoS requirements of distributed real-time and embedded (DRE) systems. First, we describe the design and implementation of NetQoPE, which is a model-driven component middleware framework that shields applications from the details of network QoS mechanisms by (1) specifying per-flow network QoS requirements, (2) performing resource allocation and validation decisions (such as admission control), and (3) enforcing per-flow network QoS at runtime. Second, we evaluate the effort required and flexibility of using NetQoPE to provide network QoS assurance to end-to-end application flows. Our results demonstrate that NetQoPE can provide network-level differentiated performance to each application flow without modifying its programming model or source code, thereby providing greater flexibility in leveraging network-layer mechanisms.

Reliable messaging using the CORBA Notification Service

With the growing popularity of the CORBA architecture as a distributed computing infrastructure standard, the need for a reliable CORBA messaging solution is being increasingly felt. The Event Service, which is the first such solution, provides inadequate event delivery guarantees and lacks many important features such as configurable quality of service and filtering. A Notification Service has since been specified to address these shortcomings and to extend the Event Service functionality. This paper briefly looks at the features of the Notification Service and explains in detail the Notification Service support for reliable messaging. The highest event delivery guarantee possible with the CORBA Notification Service is deduced and the delivery semantics provided is examined. A framework for architecting reliable messaging systems using the CORBA Notification Service is proposed. The performance of the Notification Service is analyzed to determine the performance penalty, for increased reliability, and the effect of event size, number of clients and filtering on throughput.

Adaptive network QoS in layer-3/layer-2 networks as a middleware service for mission-critical applications

We present adaptive network Quality of Service (QoS) technology that provides delay bounds and capacity guarantees for traffic belonging to mission-critical tasks. Our technology uses a Bandwidth Broker to provide admission control and leverages the differentiated aggregated traffic treatment provided by todays high-end COTS layer-3/2 switches. The technology adapts to changes in network resources, work load and mission requirements, using two components that are a particular focus of this paper: Fault Monitor and Performance Monitor. Our technology is being developed and applied in a CORBA-based multi-layer resource management framework.

Performance analysis of the CORBA notification service

As CORBA (Common Object Request Broker Architecture) gains popularity as a standard for portable, distributed, object-oriented computing, the need for a CORBA messaging solution is being increasingly felt. This led the Object Management Group (OMQ) to specify a Notification Service that aims to provide a more flexible and robust messaging solution than the earlier Event Service. The Notification Service provides several configurable quality of service (QoS) and administrative settings that deal with issues such as reliability, event (message) delivery order and discard policies. Unlike in conventional queuing systems, some Notification Service QoS configurations can lead to discards from within the internal queues, requiring careful analysis and configuration if such discards are to be avoided or minimized. This paper presents stochastic models (based on continuous time Markov chains and queuing theory) to analyze the Notification Service delivery and discard policies in detail.

Building a Reliable Message Delivery System Using the CORBA Event Service

In this paper we study the suitability of the CORBA Event Service as a reliable message delivery mechanism. We first show that products built to the CORBA Event Service specification will not guarantee against loss of messages or guarantee order. This is not surprising, as the CORBA Event Service specification does not deal with Quality of Service (QoS) and monitoring issues. The CORBA Notification Service, although it provides much of the QoS features, may not be an option. Therefore, we examine application-level reliability schemes to build a reliable communication means over the existing CORBA Event Service. Our end-to-end reliability schemes are applicable to management applications where state resynchronization is possible and sufficient. The reliability schemes proposed provide resilience in the face of failures of the supplier, consumer, and the Event Service processes.

Fast Recovery and QoS Assurance in the Presence of Network Faults for Mission-Critical Applications in Hostile Environments

In a hostile military environment, systems must be able to detect and react to catastrophes in a timely manner in order to provide assurance that critical tasks will continue to meet their timeliness requirements. Our research focuses on achieving network quality of service (QoS) assurance using a Bandwidth Broker in the presence of network faults in layer-3 networks. Passive discovery techniques using the link-state information from routers provide for rapid path discovery which, in turn, leads to fast failure impact analysis and QoS restoration. In addition to network fault tolerance, the Bandwidth Broker must be fault tolerant and must be able to recover quickly. This is accomplished using a modified commercially available and open-source in- memory database cluster technology.