Computer Science and Engineering PhD Thesis Defense by Yahya Hassanzadeh Nazarabadi






Title: SEALA: Secure, Efficient, Availability, and Locality Aware Peer-to-Peer Cloud Storage Platform-as-a-Service


Speaker: Yahya Hassanzadeh Nazarabadi


Time: May 9, 2019, 15:00


Place: ENG 208

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Prof. Öznur Özkasap (Advisor, Koç University)

Assist. Prof. Alptekin Küpçü (Advisor, Koç University)

Prof. Emre Alper Yıldırım (Koç University)

Prof. Yücel Yemez (Koç University)

Assoc. Prof. Tolga Ovatman (Istanbul Technical University)

Assist. Prof. Gonca Gürsun (Özyeğin University)




Peer-to-peer (P2P) cloud storage systems are decentralized systems with no centralized or federated administrative domain, where users are in charge of storing each others’ data in exchange for having their data stored on some other peers of the system. In this thesis, a middleware architecture, namely SEALA, which provides P2P cloud storage Platform-as-a-Service (PaaS) functionality is proposed. The components offered by SEALA aim at improving the performance of the P2P cloud storage system with respect to the scalability, availability, security, response time, fault tolerance, and energy efficiency.


In the first part of the thesis, we propose the structural primitives of SEALA, which aim at providing a scalable, low-latency, highly-available, and fault-tolerant Skip Graph-based P2P overlay. We also propose a scalable and efficient DHT-based blockchain as the authentication mechanism of SEALA. In the second part of the thesis, we develop various replication functionalities based on the proposed structural primitives. The replication services of SEALA aim at providing locality-awareness, availability-awareness, and utility-awareness of replicas.


Each module of SEALA presents a novel fully decentralized solution that outperforms the existing state-of-the-art decentralized solutions. For modelling and performance analysis, we develop an offline, scalable, and publicly available Skip Graph simulator as an independent contribution of this thesis. We implement and simulate our proposed modules as well as the existing decentralized solutions that are applicable on the Skip Graph-based P2P overlays, and compare their performances with the associated modules of SEALA. Compared to the best existing solutions in the availability and utility aspects, SEALA provides components that maximize the number of available replicas as well as the available bandwidth of replicas under churn. In the latency aspect, the locality-aware component of SEALA minimizes the average response time of the queries in the P2P overlay. The fault-tolerant component maximizes the overlay connectivity while minimizing the response time of queries under churn. In the energy efficiency aspect, the aggregation component operates as good as the best existing solutions, while minimizing the aggregation latency. The DHT-based blockchain module asymptotically improves the operational complexity of the blockchains.


In addition to its applications on P2P cloud storage systems, we present other configurations of the SEALA’s components for the potential use cases in clusters and grids. Likewise, the individual components of SEALA are capable of being used as alternatives of the associated existing solutions in DHT-based services other than Skip Graphs. Finally, our scalable DHT-based blockchain proposal can be applied in other settings including the cryptocurrencies and smart contracts.