VLDB 2026 Research / reviewers in the wild / expert
Vatche Isahagian
dblp:28/10038 · also Vatche Ishakian
· DBLP profile ↗
34ranked-venue papers
9as first author
12since 2021 · last 2025
0000-0002-9573-9291ORCID · corroborated
Domains — the database's venue-derived domains; a paper can count in several
Artificial intelligence and machine learning · 8 · 6 since 2021Databases, data management, data science and information retrieval · 7 · 1 first-author · 2 since 2021Software engineering, systems software and programming languages · 6 · 1 first-author · 3 since 2021Computer networks · 4 · 2 first-author · 1 since 2021Graphics, computer vision, multimedia, augmented reality and games · 4 · 2 since 2021Systems, architecture and hardware · 2 · 1 first-authorApplied, interdisciplinary, general and emerging computing · 2 · 1 since 2021
| Year | Publication | Venue | Position |
|---|---|---|---|
| 2025 | On Automating Security Policies with Contemporary LLMs (Short Paper)abstractThe complexity of modern computing environments and the growing sophistication of cyber threats necessitate a more robust, adaptive, and automated approach to security enforcement. In this paper, we present a framework leveraging large language models (LLMs) for automating attack mitigation policy compliance through an innovative combination of in-context learning and retrieval-augmented generation (RAG). We begin by describing how our system collects and manages both tool and API specifications, storing them in a vector database to enable efficient retrieval of relevant information. We then detail the architectural pipeline that first decomposes high-level mitigation policies into discrete tasks and subsequently translates each task into a set of actionable API calls. Our empirical evaluation, conducted using publicly available CTI policies in STIXv2 format and Windows API documen-tation, demonstrates significant improvements in precision, recall, and Fl-score when employing RAG compared to a non-RAG baseline. Pablo Fernández Saura, K. R. Jayaram, Vatche Isahagian, Jorge Bernal Bernabé, Antonio F. Skarmeta |
SSE | 3 |
| 2024 | Who Knows the Answer? Finding the Best Model and Prompt for Each Query Using Confidence-Based SearchabstractThere are increasingly many large language models (LLMs) available to the public. While these LLMs have exhibited impressive abilities on a variety of task, any individual LLM in particular may do well on some tasks and worse on others. Additionally, the performance of these models is heavily dependent on the choice of prompt template used. For instance, they exhibit sensitivity to the few shot examples chosen or brittleness to the wording of instructions. Moreover, a prompt template that makes a model perform well for one input may not be the optimal template for another input. This necessitates an approach for adaptively selecting LLM and prompt template pairs for each input. Recent work has shown that the accuracy of LLM's responses is correlated with the LLM's confidence in the response. Thus, a natural choice for selecting which model and prompt template to use is to select the pair that is most confident in its response. However, existing confidence metrics are expensive to calculate - necessitating multiple calls to each LLm and prompt pair. We thus propose an approach to predict the confidence of each pair using an auxiliary regression model that is inexpensive to run. Using this auxiliary model, we select the LLM and prompt template with the highest predicted confidence for a given input. Results on a range of benchmark datasets show that our confidence-based instance-level prompt search method consistently improves the performance of LLMs. Walter Gerych, Yara Rizk, Vatche Isahagian, Vinod Muthusamy, Evelyn Duesterwald, Praveen Venkateswaran |
AAAI | 3 |
| 2023 | FedGen: Generalizable Federated Learning for Sequential DataabstractExisting federated learning models that follow the standard risk minimization paradigm of machine learning often fail to generalize in the presence of spurious correlations in the training data. In many real-world distributed settings, spurious correlations exist due to biases and data sampling issues on distributed devices or clients that can erroneously influence models. Current generalization approaches are designed for centralized training and attempt to identify features that have an invariant causal relationship with the target, thereby reducing the effect of spurious features. However, such invariant risk minimization approaches rely on apriori knowledge of training data distributions which is hard to obtain in many applications. In this work, we present a generalizable federated learning framework called FedGen, which allows clients to identify and distinguish between spurious and invariant features in a collaborative manner without prior knowledge of training distributions. We evaluate our approach on real-world datasets from different domains and show that FedGen results in models that achieve significantly better generalization and can outperform the accuracy of current federated learning approaches by over 24%. Praveen Venkateswaran, Vatche Isahagian, Vinod Muthusamy, Nalini Venkatasubramanian |
CLOUD | 2 |
| 2023 | Towards Hybrid Automation by Bootstrapping Conversational Interfaces for IT Operation TasksabstractProcess automation has evolved from end-to-end automation of repetitive process branches to hybrid automation where bots perform some activities and humans serve other activities. In the context of knowledge-intensive processes such as IT operations, implementing hybrid automation is a natural choice where robots can perform certain mundane functions, with humans taking over the decision of when and which IT systems need to act. Recently, ChatOps, which refers to conversation-driven collaboration for IT operations, has rapidly accelerated efficiency by providing a cross-organization and cross-domain platform to resolve and manage issues as soon as possible. Hence, providing a natural language interface to bots is a logical progression to enable collaboration between humans and bots. This work presents a no-code approach to provide a conversational interface that enables human workers to collaborate with bots executing automation scripts. The bots identify the intent of users' requests and automatically orchestrate one or more relevant automation tasks to serve the request. We further detail our process of mining the conversations between humans and bots to monitor performance and identify the scope for improvement in service quality. Jayachandu Bandlamudi, Kushal Mukherjee, Prerna Agarwal, Sampath Dechu, Siyu Huo, Vatche Isahagian, Vinod Muthusamy, Naveen Purushothaman, Renuka Sindhgatta |
AAAI | 6 |
| 2023 | TaskDiff: A Similarity Metric for Task-Oriented ConversationsabstractThe popularity of conversational digital assistants has resulted in the availability of large amounts of conversational data which can be utilized for improved user experience and personalized response generation.Building these assistants using popular large language models like ChatGPT also require additional emphasis on prompt engineering and evaluation methods.Textual similarity metrics are a key ingredient for such analysis and evaluations.While many similarity metrics have been proposed in the literature, they have not proven effective for taskoriented conversations as they do not take advantage of unique conversational features.To address this gap, we present TaskDiff, a novel conversational similarity metric that utilizes different dialogue components (utterances, intents, and slots) and their distributions to compute similarity.Extensive experimental evaluation of TaskDiff on a benchmark dataset demonstrates its superior performance and improved robustness over other related approaches. Ankita Bhaumik, Praveen Venkateswaran, Yara Rizk, Vatche Isahagian |
EMNLP | 4 |
| 2023 | DiSTRICT: Dialogue State Tracking with Retriever Driven In-Context TuningabstractDialogue State Tracking (DST), a key component of task-oriented conversation systems, represents user intentions by determining the values of pre-defined slots in an ongoing dialogue.Existing approaches use hand-crafted templates and additional slot information to fine-tune and prompt large pre-trained language models and elicit slot values from the dialogue context.Significant manual effort and domain knowledge is required to design effective prompts, limiting the generalizability of these approaches to new domains and tasks.In this work, we propose DiSTRICT, a generalizable in-context tuning approach for DST that retrieves highly relevant training examples for a given dialogue to fine-tune the model without any hand-crafted templates.Experiments with the MultiWOZ benchmark datasets show that DiSTRICT outperforms existing approaches in various zeroshot and few-shot settings using a much smaller model, thereby providing an important advantage for real-world deployments that often have limited resource availability. Praveen Venkateswaran, Evelyn Duesterwald, Vatche Isahagian |
EMNLP | 3 |
| 2023 | Configuration and Placement of Serverless Applications Using Statistical LearningabstractIn the last decade, serverless computing emerged as a new compelling paradigm for the deployment of cloud applications and services. It represents an evolution of cloud computing with a simplified programming model, that aims to abstract away most operational concerns. Running serverless applications requires users to configure multiple parameters, such as memory, CPU, cloud provider,etc. While relatively simpler, configuring such parameters correctly while minimizing cost and meeting delay constraints is not trivial. In this paper, we present COSE, a framework that uses Bayesian Optimization to find the optimal resource configuration and placement for functions in a serverless application. COSE uses statistical learning techniques to intelligently collect samples and predict the cost and execution time of a serverless function across unseen configuration values. Our framework uses the predicted cost and execution time on available locations to select the “best” configuration parameters and placement for running a serverless application while satisfying customer objectives. We evaluate COSE on AWS Lambda with real-world applications consisting of multiple functions (both linear chains and service graphs), where we successfully found optimal/near-optimal configurations. We also evaluate COSE over a wide range of simulated distributed cloud environments that confirm the efficacy of our approach. Ali Raza 0003, Nabeel Akhtar, Vatche Isahagian, Abraham Matta |
IEEE Trans. Netw. Serv. Manag. | 3 |
| 2022 | A Process-Aware Decision Support System for Business ProcessesabstractBusiness processes in workflows comprise of an ordered sequence of tasks and decisions to accomplish certain business goals. Each decision point requires the input of a decision-maker to distill complex case information and make an optimal decision given their experience, organizational policy, and external contexts. Overlooking some of the essential factors or lack of knowledge can impact the throughput and business outcomes. Therefore, we propose an end-to-end automated decision support system with explanation for business processes. The system uses the proposed process-aware feature engineering methodology that extracts features from process and business data attributes. The system helps a decision-maker to make quick and quality decisions by predicting the decision and providing an explanation of the factors which led to the prediction. We provide offline and online training methods robust to data drift that can also incorporate user feedback. The system also support predictions with live instance data i.e., allow decision-makers to conduct trials on current data instance by modifying its business data attribute values. We evaluate our system on real-world and synthetic datasets and benchmark the performance, achieving an average of 15% improvement over baselines. Prerna Agarwal, Buyu Gao, Siyu Huo, Prabhat Reddy, Sampath Dechu, Yazan Obeidi, Vinod Muthusamy, Vatche Isahagian, Sebastian Carbajales |
KDD | 8 |
| 2021 | Graph Autoencoders for Business Process Anomaly Detection
Siyu Huo, Hagen Völzer, Prabhat Reddy, Prerna Agarwal, Vatche Isahagian, Vinod Muthusamy |
BPM | 5 |
| 2021 | Robust and Generalizable Predictive Models for Business Processes
Praveen Venkateswaran, Vinod Muthusamy, Vatche Isahagian, Nalini Venkatasubramanian |
BPM | 3 |
| 2021 | LIBRA: An Economical Hybrid Approach for Cloud Applications with Strict SLAsabstractFunction-as-a-Service (FaaS) has recently emerged to reduce the deployment cost of running cloud applications compared to Infrastructure-as-a-Service (IaaS). FaaS follows a serverless “pay-as-you-go” computing model; it comes at a higher cost per unit of execution time but typically application functions experience lower provisioning time (startup delay). IaaS requires the provisioning of Virtual Machines, which typically suffer from longer cold-start delays that cause higher queuing delays and higher request drop rates. We present LIBRA, a balanced (hybrid) approach that leverages both VM-based and serverless resources to efficiently manage cloud resources for the applications. LIBRA closely monitors the application demand and provisions appropriate VM and serverless resources such that the running cost is minimized and Service-Level Agreements are met. Unlike state of the art, LIBRA not only hides VM cold-start delays, and hence reduces response time, by leveraging serverless, but also directs a low-rate bursty portion of the demand to serverless where it would be less costly than spinning up new VMs. We evaluate LIBRA on real traces in a simulated environment as well as on the AWS commercial cloud. Our results show that LIBRA outperforms other resource-provisioning policies, including a recent hybrid approach - LIBRA achieves more than 85% reduction in SLA violations and up to 53% cost savings. Ali Raza 0003, Zongshun Zhang, Nabeel Akhtar, Vatche Isahagian, Abraham Matta |
IC2E | 4 |
| 2021 | Environment Agnostic Invariant Risk Minimization for Classification of Sequential DatasetsabstractThe generalization of predictive models that follow the standard risk minimization paradigm of machine learning can be hindered by the presence of spurious correlations in the data. Identifying invariant predictors while training on data from multiple environments can influence models to focus on features that have an invariant causal relationship with the target, while reducing the effect of spurious features. Such invariant risk minimization approaches heavily rely on clearly defined environments and data being perfectly segmented into these environments for training. However, in real-world settings, perfect segmentation is challenging to achieve and these environment-aware approaches prove to be sensitive to segmentation errors. In this work, we present an environment-agnostic approach to develop generalizable models for classification tasks in sequential datasets without needing prior knowledge of environments. We show that our approach results in models that can generalize to out-of-distribution data and are not influenced by spurious correlations. We evaluate our approach on real-world sequential datasets from various domains. Praveen Venkateswaran, Vinod Muthusamy, Vatche Isahagian, Nalini Venkatasubramanian |
KDD | 3 |
| 2020 | AI Trust in Business Processes: The Need for Process-Aware ExplanationsabstractBusiness processes underpin a large number of enterprise operations including processing loan applications, managing invoices, and insurance claims. The business process management (BPM) industry is expected to grow at approximately 16 Billion dollar by 2023. There is a large opportunity for infusing AI to reduce cost or provide better customer experience with a $15.7 trillion “potential contribution to the global economy by 2030”. To this end, the BPM literature is rich in machine learning solutions including unsupervised learning to gain insights on clusters of process traces, classification models to predict the outcomes, duration, or paths of partial process traces, extracting business process from documents, and models to recommend how to optimize a business process or navigate decision points. More recently, deep learning models including those from the NLP domain have been applied to process predictions.Unfortunately, very little of these innovations have been applied and adopted by enterprise companies. We assert that a large reason for the lack of adoption of AI models in BPM is that business users are risk-averse and do not implicitly trust AI models. There has, unfortunately, been little attention paid to explaining model predictions to business users with process context. We challenge the BPM community to build on the AI interpretability literature, and the AI Trust community to understand what it means to take advantage of business process artifacts in order to provide business level explanations. Steve T. K. Jan, Vatche Isahagian, Vinod Muthusamy |
AAAI | 2 |
| 2020 | COSE: Configuring Serverless Functions using Statistical LearningabstractServerless computing has emerged as a new compelling paradigm for the deployment of applications and services. It represents an evolution of cloud computing with a simplified programming model, that aims to abstract away most operational concerns. Running serverless functions requires users to configure multiple parameters, such as memory, CPU, cloud provider, etc. While relatively simpler, configuring such parameters correctly while minimizing cost and meeting delay constraints is not trivial. In this paper, we present COSE, a framework that uses Bayesian Optimization to find the optimal configuration for serverless functions. COSE uses statistical learning techniques to intelligently collect samples and predict the cost and execution time of a serverless function across unseen configuration values. Our framework uses the predicted cost and execution time, to select the "best" configuration parameters for running a single or a chain of functions, while satisfying customer objectives. In addition, COSE has the ability to adapt to changes in the execution time of a serverless function. We evaluate COSE not only on a commercial cloud provider, where we successfully found optimal/near-optimal configurations in as few as five samples, but also over a wide range of simulated distributed cloud environments that confirm the efficacy of our approach. Nabeel Akhtar, Ali Raza 0003, Vatche Isahagian, Abraham Matta |
INFOCOM | 3 |
| 2019 | The Future of Computing is Boring (and that is exciting!)abstractWe see a trend where computing becomes a metered utility similar to how the electric grid evolved. Initially electricity was generated locally but economies of scale (and standardization) made it more efficient and economical to have utility companies managing the electric grid. Similar developments can be seen in computing where scientific grids paved the way for commercial cloud computing offerings. However, in our opinion, that evolution is far from finished and in this paper we bring forward the remaining challenges and propose a vision for the future of computing. In particular we focus on diverging trends in the costs of computing and developer time, which suggests that future computing architectures will need to optimize for developer time. Aleksander Slominski, Vinod Muthusamy, Vatche Isahagian |
IC2E | 3 |
| 2019 | FfDL: A Flexible Multi-tenant Deep Learning PlatformabstractDeep learning (DL) is becoming increasingly popular in several application domains and has made several new application features involving computer vision, speech recognition and synthesis, self-driving automobiles, drug design, etc. feasible and accurate. As a result, large scale "on-premise" and "cloud-hosted" deep learning platforms have become essential infrastructure in many organizations. These systems accept, schedule, manage and execute DL training jobs at scale. K. R. Jayaram, Vinod Muthusamy, Parijat Dube, Vatche Isahagian, Chen Wang 0039, Benjamin Herta, Scott Boag, Diana Arroyo, Asser N. Tantawi, Archit Verma, Falk Pollok, Rania Khalaf |
Middleware | 4 |
| 2018 | Serving Deep Learning Models in a Serverless PlatformabstractServerless computing has emerged as a compelling paradigm for the development and deployment of a wide range of event based cloud applications. At the same time, cloud providers and enterprise companies are heavily adopting machine learning and Artificial Intelligence to either differentiate themselves, or provide their customers with value added services. In this work we evaluate the suitability of a serverless computing environment for the inferencing of large neural network models. Our experimental evaluations are executed on the AWS Lambda environment using the MxNet deep learning framework. Our experimental results show that while the inferencing latency can be within an acceptable range, longer delays due to cold starts can skew the latency distribution and hence risk violating more stringent SLAs. Vatche Isahagian, Vinod Muthusamy, Aleksander Slominski |
IC2E | 1 |
| 2017 | Serverless Programming (Function as a Service)abstractIn this tutorial, we will present serverless computing, survey existing serverless platforms from industry, academia, and open source projects, identify key characteristics and use cases, and describe technical challenges and open problems. Our tutorial will involve a hands-on experience of using the serverless technologies available from different cloud providers (e.g. IBM, Amazon, Google and Microsoft). We expect our users to have basic knowledge of programming and basic knowledge of cloud computing. Paul C. Castro, Vatche Isahagian, Vinod Muthusamy, Aleksander Slominski |
ICDCS | 2 |
| 2017 | AngelCast: Cloud-based peer-assisted live streaming using optimized multi-tree construction
Vatche Isahagian, Raymond Sweha, Azer Bestavros |
Comput. Commun. | 1 |
| 2016 | Process Trace Clustering: A Heterogeneous Information Network ApproachabstractProcess mining is the task of extracting information from event logs, such as ones generated from workflow management or enterprise resource planning systems, in order to discover models of the underlying processes, organizations, and products. As the event logs often contain a variety of process executions, the discovered models can be complex and difficult to comprehend. Trace clustering helps solve this problem by splitting the event logs into smaller subsets and applying process discovery algorithms on each subset, resulting in per-subset discovered processes that are less complex and more accurate. However, the state-of-the-art clustering techniques are limited: the similarity measures are not process-aware and they do not scale well to high-dimensional event logs. In this paper, we propose a conceptualization of process's event logs as a heterogeneous information network, in order to capture the rich semantic meaning, and thereby derive better process-specific features. In addition, we propose SeqPathSim, a meta path-based similarity measure that considers node sequences in the heterogeneous graph and results in better clustering. We also introduce a new dimension reduction method that combines event similarity with regularization by process model structure to deal with event logs of high dimensionality. The experimental results show that our proposed approach outperforms state-of-the-art trace clustering approaches in both accuracy and structural complexity metrics. Phuong Nguyen 0002, Aleksander Slominski, Vinod Muthusamy, Vatche Isahagian, Klara Nahrstedt |
SDM | 4 |
| 2015 | Case Analytics Workbench: Platform for Hybrid Process Model Creation and Evolution
Yiqin Yu, Xiang Li 0013, Haifeng Liu 0005, Jing Mei, Nirmal Mukhi, Vatche Isahagian, Guo Tong Xie, Geetika T. Lakshmanan, Mike Marin |
BPM | 6 |
| 2015 | A Divide-and-Conquer Algorithm for Betweenness CentralityabstractGiven a set of target nodes S in a graph G we define the betweenness centrality of a node v with respect to S as the fraction of shortest paths among nodes in S that contain v. For this setting we describe Brandes++, a divide-and-conquer algorithm that can efficiently compute the exact values of betweenness scores. Brandes++ uses Brandes− the most widely-used algorithm for betweenness computation – as its subroutine. It achieves the notable faster running times by applying Brandes on significantly smaller networks than the input graph, and many of its computations can be done in parallel. The degree of speedup achieved by Brandes++ depends on the community structure of the input network as well as the size of S. Our experiments with real-life networks reveal Brandes++ achieves an average of 10-fold speedup over Brandes, while there are networks where this speedup is 75-fold. We have made our code public to benefit the research community. Dóra Erdös, Vatche Isahagian, Azer Bestavros, Evimaria Terzi |
SDM | 2 |
| 2014 | Secure and QoS-Managed Information Exchange Between Enterprise and Constrained EnvironmentsabstractMobile devices performing mission-critical functions at the tactical edge, such as those employed by first responders, military personnel, and law enforcement, operate in environments that are vastly different from enterprise computing environments. In spite of the differences in resource availability, threat models, vulnerabilities, information formats, and communication protocols, there is a great advantage to (and great demand for) enabling information exchange between the tactical edge and enterprise environments. Creating a specialized mobile version of each desired service that incorporates an appropriate level of security protection and quality of service (QoS) for the tactical users is one possibility. Such an approach is not cost effective, however, as the market for a given tactical application is small compared to the commercial user base for mobile applications and services. Furthermore, the need for information or services from the enterprise by tactical users can be too ad hoc and time critical, e.g., during disaster response, to support developing a specialized version. Finally, service specialization for mobile web access covers only one of multiple information dissemination and access patterns that arise in tactical operations. This paper presents the design and a prototype implementation of a gateway solution that provides secure tactical-enterprise information exchange and handles the differences in resource availability, QoS requirements, communication formats, and protocols. Partha P. Pal, Michael Atighetchi, Nathaniel Soule, Vatche Isahagian, Joseph P. Loyall, Robert Grant, Asher Sinclair |
ISORC | 4 |
| 2013 | Repetition-aware content placement in navigational networksabstractArguably, the most effective technique to ensure wide adoption of a concept (or product) is by repeatedly exposing individuals to messages that reinforce the concept (or promote the product). Recognizing the role of repeated exposure to a message, in this paper we propose a novel framework for the effective placement of content: Given the navigational patterns of users in a network, e.g., web graph, hyperlinked corpus, or road network, and given a model of the relationship between content-adoption and frequency of exposition, we define the repetition-aware content-placement (RACP) problem as that of identifying the set of B nodes on which content should be placed so that the expected number of users adopting that content is maximized. The key contribution of our work is the introduction of memory into the navigation process, by making user conversion dependent on the number of her exposures to that content. This dependency is captured using a conversion model that is general enough to capture arbitrary dependencies. Our solution to this general problem builds upon the notion of absorbing random walks, which we extend appropriately in order to address the technicalities of our definitions. Although we show the RACP problem to be NP-hard, we propose a general and efficient algorithmic solution. Our experimental results demonstrate the efficacy and the efficiency of our methods in multiple real-world datasets obtained from different application domains. Dóra Erdös, Vatche Isahagian, Azer Bestavros, Evimaria Terzi |
KDD | 2 |
| 2012 | MORPHOSYS: Efficient Colocation of QoS-Constrained Workloads in the CloudabstractAbstract—In hosting environments such as IaaS clouds, desirable application performance is usually guaranteed through the use of Service Level Agreements (SLAs), which specify minimal fractions of resource capacities that must be allocated for unencumbered use for proper operation. Arbitrary colocation of applications with different SLAs on a single host may result in inefficient utilization of the host’s resources. In this paper, we propose that periodic resource allocation and consumption models – often used to characterize real-time workloads – be used for a more granular expression of SLAs. Our proposed SLA model has the salient feature that it exposes flexibilities that enable the infrastructure provider to safely transform SLAs from one form to another for the purpose of achieving more efficient colocation. Towards that goal, we present MORPHOSYS: a framework for a service that allows the manipulation of SLAs to enable efficient colocation of arbitrary workloads in a dynamic setting. We present results from extensive trace-driven simulations of colocated Video-on-Demand servers in a cloud setting. These results show that potentially-significant reduction in wasted resources (by as much as 60%) are possible using MORPHOSYS. Vatche Isahagian, Azer Bestavros |
CCGRID | 1 |
| 2012 | CloudPack - Exploiting Workload Flexibility through Rational Pricing
Vatche Isahagian, Raymond Sweha, Azer Bestavros, Jonathan Appavoo |
Middleware | 1 |
| 2012 | AngelCast: cloud-based peer-assisted live streaming using optimized multi-tree constructionabstractIncreasingly, commercial content providers (CPs) offer streaming and IPTV solutions that leverage an underlying peer-to-peer (P2P) stream distribution architecture. The use of P2P protocols promises significant scalability and cost savings by leveraging the local resources of clients -- specifically, uplink capacity. A major limitation of P2P live streaming is that playout rates are constrained by the uplink capacities of clients, which are typically much lower than downlink capacities, thus limiting the quality of the delivered stream. Thus, to leverage P2P architectures without sacrificing the quality of the delivered stream, CPs must commit additional resources to complement those available through clients. In this paper, we propose a cloud-based service -- AngelCast -- that enables CPs to elastically complement P2P streaming "as needed". By subscribing to AngelCast, a CP is able to deploy extra resources ("angels"), on-demand from the cloud, to maintain a desirable stream (bit-rate) quality. Angels need not download the whole stream (they are not "leachers"), nor are they in possession of it (they are not "seeders"). Rather, angels only relay (download once and upload as many times as needed) the minimal possible fraction of the stream that is necessary to achieve the desirable stream quality, while maximally utilizing available client resources. We provide a lower bound on the minimum amount of angel capacity needed to maintain a certain bit-rate to all clients, and develop a fluid model construction that achieves this lower bound. Realizing the limitations of the fluid model construction -- namely, susceptibility to potentially arbitrary start-up delays and significant degradation due to churn -- we present a practical multi-tree construction that captures the spirit of the optimal construction, while avoiding its limitations. In particular, our AngelCast protocol achieves near optimal performance (compared to the fluid-model construction) while ensuring a low startup delay by maintaining a logarithmic-length path between any client and the provider, and while gracefully dealing with churn by adopting a flexible membership management approach. We present the blueprints of a prototype implementation of AngelCast, along with experimental results confirming the feasibility and performance potential of our AngelCast service when deployed on Emulab and PlanetLab. Raymond Sweha, Vatche Isahagian, Azer Bestavros |
MMSys | 2 |
| 2012 | A Framework for the Evaluation and Management of Network CentralityabstractNetwork-analysis literature is rich in node-centrality measures that quantify the centrality of a node as a function of the (shortest) paths of the network that go through it.Existing work focuses on defining instances of such measures and designing algorithms for the specific combinatorial problems that arise for each instance.In this work, we propose a unifying definition of centrality that subsumes all path-counting based centrality definitions: e.g., stress, betweenness or paths centrality.We also define a generic algorithm for computing this generalized centrality measure for every node and every group of nodes in the network.Next, we define two optimization problems: k-Group Centrality Maximization and k-Edge Centrality Boosting.In the former, the task is to identify the subset of k nodes that have the largest group centrality.In the latter, the goal is to identify up to k edges to add to the network so that the centrality of a node is maximized.We show that both of these problems can be solved efficiently for arbitrary centrality definitions using our general framework.In a thorough experimental evaluation we show the practical utility of our framework and the efficacy of our algorithms. Vatche Isahagian, Dóra Erdös, Evimaria Terzi, Azer Bestavros |
SDM | 1 |
| 2012 | On supporting mobility and multihoming in recursive internet architectures
Vatche Isahagian, Joseph Akinwumi, Flavio Esposito, Abraham Matta |
Comput. Commun. | 1 |
| 2012 | The Filter-Placement Problem and its Application to Minimizing Information MultiplicityabstractIn many information networks, data items -- such as updates in social networks, news flowing through interconnected RSS feeds and blogs, measurements in sensor networks, route updates in ad-hoc networks -- propagate in an uncoordinated manner: nodes often relay information they receive to neighbors, independent of whether or not these neighbors received the same information from other sources. This uncoordinated data dissemination may result in significant, yet unnecessary communication and processing overheads, ultimately reducing the utility of information networks. To alleviate the negative impacts of thisinformation multiplicityphenomenon, we propose that a subset of nodes (selected at key positions in the network) carry out additional information filtering functionality. Thus, nodes are responsible for the removal (or significant reduction) of the redundant data items relayed through them. We refer to such nodes asfilters. We formally define the Filter Placement problem as a combinatorial optimization problem, and study its computational complexity for different types of graphs. We also present polynomial-time approximation algorithms and scalable heuristics for the problem. Our experimental results, which we obtained through extensive simulations on synthetic and real-world information flow networks, suggest that in many settings a relatively small number of filters are fairly effective in removing a large fraction of redundant information. Dóra Erdös, Vatche Isahagian, Andrei Lapets, Evimaria Terzi, Azer Bestavros |
Proc. VLDB Endow. | 2 |
| 2011 | Angels in the Cloud: A Peer-Assisted Bulk-Synchronous Content Distribution ServiceabstractLeveraging client upload capacity through peer assisted content distribution was shown to decrease the load on content providers, while also improving average distribution times. These benefits, however, are limited by the disparity between client upload and download speeds, especially in scenarios requiring a minimum distribution time (MDT) of a fresh piece of content to a set of clients. Achieving MDT is crucial for bulk-synchronous applications, when every client in a set must wait for all other clients in the set to finish their downloads before being able to make use of the downloaded content. In this paper, we propose the use of dedicated servers, which we call angels to accelerate peer-assisted content distribution in general, and to minimize MDT in particular. An angel is not itself the content origin, nor is it interested in fully downloading the content, its only purpose is to enable a peer assisted content distribution scheme to approach the theoretical lower-bound for MDT. To overcome scalability issues inherent in an optimal MDT construction, we propose and evaluate a content exchange strategy involving angels, which we call Group Tree. In addition to simulation results that demonstrate the near optimal performance of our proposed approach, we present the architecture and implementation of CLOUDANGELS -- a service that allows the elastic, on-the-fly deployment of angels (in the cloud) to assist a content provider (off the cloud) in realizing its MDT objective. Raymond Sweha, Vatche Isahagian, Azer Bestavros |
IEEE CLOUD | 2 |
| 2011 | Formal Verification of SLA TransformationsabstractDesirable application performance is typically guaranteed through the use of Service Level Agreements (SLAs) that specify fixed fractions of resource capacities that must be allocated for unencumbered use by the application. The mapping between what constitutes desirable performance and SLAs is not unique: multiple SLA expressions might be functionally equivalent. Having the flexibility to transform SLAs from one form to another in a manner that is provably safe would enable hosting solutions to achieve significant efficiencies. This paper demonstrates the promise of such an approach by proposing a type-theoretic framework for the representation and safe transformation of SLAs. Based on that framework, the paper describes a methodical approach for the inference of efficient and safe mappings of periodic, real-time tasks to the physical and virtual hosts that constitute a hierarchical scheduler. Extensive experimental results support the conclusion that the flexibility afforded by safe SLA transformations has the potential to yield significant savings. Vatche Isahagian, Andrei Lapets, Azer Bestavros, Assaf J. Kfoury |
SERVICES | 1 |
| 2010 | Colocation as a Service: Strategic and Operational Services for Cloud ColocationabstractBy colocating with other tenants of an Infrastructure as a Service (IaaS) offering, IaaS users could reap significant cost savings by judiciously sharing their use of the fixed-size instances offered by IaaS providers. This paper presents the blueprints of a Colocation as a Service (CaaS) framework. CaaS strategic services identify coalitions of self-interested users that would benefit from colocation on shared instances. CaaS operational services provide the information necessary for, and carry out the reconfigurations mandated by strategic services. CaaS could be incorporated into an IaaS offering by providers; it could be implemented as a value-added proposition by IaaS resellers; or it could be directly leveraged in a peer-to-peer fashion by IaaS users. To establish the practicality of such offerings, this paper presents XCS - a prototype implementation of CaaS on top of the Xen hypervisor. XCS makes specific choices with respect to the various elements of the CaaS framework: it implements strategic services based on a game-theoretic formulation of colocation; it features novel concurrent migration heuristics which are shown to be efficient; and it offers monitoring and accounting services at both the hypervisor and VM layers. Extensive experimental results obtained by running PlanetLab trace-driven workloads on the XCS prototype confirm the premise of CaaS - by demonstrating the efficiency and scalability of XCS, and by quantifying the potential cost savings accrued through the use of XCS. Vatche Isahagian, Raymond Sweha, Jorge Londoño, Azer Bestavros |
NCA | 1 |
| 2010 | A Type-Theoretic Framework for Efficient and Safe Colocation of Periodic Real-Time SystemsabstractDesirable application performance is typically guaranteed through the use of Service Level Agreements (SLAs) that specify fixed fractions of resource capacities that must be allocated for unencumbered use by the application. The mapping between what constitutes desirable performance and SLAs is not unique: multiple SLA expressions might be functionally equivalent. Having the flexibility to transform SLAs from one form to another in a manner that is provably safe would enable hosting solutions to achieve significant efficiencies. This paper demonstrates the promise of such an approach by proposing a type-theoretic framework for the representation and safe transformation of SLAs. Based on that framework, the paper describes a methodical approach for the inference of efficient and safe mappings of periodic, real-time tasks to the physical and virtual hosts that constitute a hierarchical scheduler. Extensive experimental results support the conclusion that the flexibility afforded by safe SLA transformations has the potential to yield significant savings. Vatche Isahagian, Azer Bestavros, Assaf J. Kfoury |
RTCSA | 1 |