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1. ParBlockchain: Leveraging Transaction Parallelism in Permissioned Blockchain Systems

   Mohammad Javad Amiri, Divyakant Agrawal ( April 2019 , IEEE International Conference on Distributed Computing Systems)

   Many existing blockchains do not adequately address all the characteristics of distributed system applications and suffer from serious architectural limitations resulting in performance and confidentiality issues. While recent permissioned blockchain systems, have tried to overcome these limitations, their focus has mainly been on workloads with no-contention, i.e., no conflicting transactions. In this paper, we introduce OXII, a new paradigm for permissioned blockchains to support distributed applications that execute concurrently. OXII is designed for workloads with (different degrees of) contention. We then present ParBlockchain, a permissioned blockchain designed specifically in the OXII paradigm. The evaluation of ParBlockchain using a series of benchmarks reveals that its performance in workloads with any degree of contention is better than the state of the art permissioned blockchain systems. 

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2. PUFchain: Hardware-Assisted Scalable Blockchain

   https://doi.org/10.1109/iSES47678.2019.00081  

   Mohanty, Saraju ; Yanambaka, Venkata ; Kougianos, Elias ; Puthal, Deepak ( December 2019 , Proceedings of the 5th IEEE International Symposium on Smart Electronic Systems (iSES))

   his is an extended abstract for the research demo of a novel hardware-assisted scalable blockchain called PUFChain. This work presents a scalable energy-efficient private/permissioned blockchain (integrated with Physical Unclonable Functions or PUFs) which can bedeployed in the IoT. PUFs have a multiple of roles in the blockchain: higher security, lower latency, and reduced energy consumption. Experimental validations of PUFChain show a transaction time of 198ms.To the best of authors knowledge this is the first ever work that presents a comprehensive framework integrating PUFs in a blockchain. 

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3. FlexChain: an elastic disaggregated blockchain

   https://doi.org/10.14778/3561261.3561264  

   Wu, Chenyuan ; Amiri, Mohammad Javad ; Asch, Jared ; Nagda, Heena ; Zhang, Qizhen ; Loo, Boon Thau ( September 2022 , Proceedings of the VLDB Endowment)

   While permissioned blockchains enable a family of data center applications, existing systems suffer from imbalanced loads across compute and memory, exacerbating the underutilization of cloud resources. This paper presents FlexChain , a novel permissioned blockchain system that addresses this challenge by physically disaggregating CPUs, DRAM, and storage devices to process different blockchain workloads efficiently. Disaggregation allows blockchain service providers to upgrade and expand hardware resources independently to support a wide range of smart contracts with diverse CPU and memory demands. Moreover, it ensures efficient resource utilization and hence prevents resource fragmentation in a data center. We have explored the design of XOV blockchain systems in a disaggregated fashion and developed a tiered key-value store that can elastically scale its memory and storage. Our design significantly speeds up the execution stage. We have also leveraged several techniques to parallelize the validation stage in FlexChain to further improve the overall blockchain performance. Our evaluation results show that FlexChain can provide independent compute and memory scalability, while incurring at most 12.8% disaggregation overhead. FlexChain achieves almost identical throughput as the state-of-the-art distributed approaches with significantly lower memory and CPU consumption for compute-intensive and memory-intensive workloads respectively. 

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4. On Sharding Permissioned Blockchains.

   Mohammad Javad Amiri, Divyakant Agrawal ( July 2019 , IEEE International Conference on Blockchain)

   Permissioned Blockchain systems rely mainly on Byzantine fault-tolerant protocols to establish consensus on the order of transactions. While Byzantine fault-tolerant protocols mostly guarantee consistency (safety) in an asynchronous network using 3f+1 machines to overcome the simultaneous malicious failure of any f nodes, in many systems, e.g., blockchain systems, the number of available nodes (resources) is much more than 3f + 1. To utilize such extra resources, in this paper we introduce a model that leverages transaction parallelism by partitioning the nodes into clusters (partitions) and processing independent transactions on different partitions simultaneously. The model also shards the blockchain ledger, assigns different shards of the blockchain ledger to different clusters, and includes both intra-shard and cross-shard transactions. Since more than one cluster is involved in each cross-shard transaction, the ledger is formed as a directed acyclic graph. 

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5. Blockchains and Databases: Opportunities and Challenges for the Permissioned and the Permissionless

   https://doi.org/10.1007/978-3-030-54832-2_1  

   Agrawal, D. ; El Abbadi, A. ; Amiri, M.J. ; Maiyya, S. ; Zakhary, V. ( August 2020 , Advances in Databases and Information Systems)

   Darmont, J ; Novikov, B. ; Wrembel, R. (Ed.)

   Bitcoin [12] is a successful and interesting example of a global scale peer-to-peer cryptocurrency that integrates many techniques and protocols from cryptography, distributed systems, and databases. The main underlying data structure is blockchain, a scalable fully replicated structure that is shared among all participants and guarantees a consistent view of all user transactions by all participants in the system. In a blockchain, nodes agree on their shared states across a large network of untrusted participants. Although originally devised for cryptocurrencies, recent systems exploit its many unique features such as transparency, provenance, fault tolerance, and authenticity to support a wide range of distributed applications. Bitcoin and other cryptocurrencies use permissionless blockchains. In a permissionless blockchain, the network is public, and anyone can participate without a specific identity. Many other distributed applications, such as supply chain management and healthcare, are deployed on permissioned blockchains consisting of a set of known, identified nodes that still might not fully trust each other. This paper illustrates some of the main challenges and opportunities from a database perspective in the many novel and interesting application domains of blockchains. These opportunities are illustrated using various examples from recent research in both permissionless and permissioned blockchains. Two main themes unite the various examples: (1) the important role of distribution and consensus in managing large scale systems and (2) the need to tolerate malicious failures. The advent of cloud computing and large data centers shifted large scale data management infrastructures from centralized databases to distributed systems. One of the main challenges in designing distributed systems is the need for fault-tolerance. Cloud-based systems typically assume trusted infrastructures, since data centers are owned by the enterprises managing the data, and hence the design typically only assumes and tolerates crash failures. The advent of blockchain and the underlying premise that copies of the blockchain are distributed among untrusted entities has shifted the focus of fault-tolerance from tolerating crash failures to tolerating malicious failures. These interesting and challenging settings pose great opportunities for database researchers. 

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