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About

I am a senior researcher at INESC TEC (HASLab research unit) and invited assistant professor at the Department of Informatics of University of Minho, Braga, Portugal. I hold a PhD from the University of Toulouse, advised by Jean-Paul Bahsoun and mentored by Rachid Guerraoui. I've spent some time at IRIT (France), INSA de Lyon (France), and EPFL (Switzerland) twice in the past.

I am interested in understanding and building correct, scalable, available, resilient, secure, and efficient distributed systems. 
Currently, I am working Edge Computing (based on Conflict-free Replicated DataTypes), and Blockchain and Cryptocurrencies (PoW,PoX, and BFT).

Interest
Topics
Details

Details

001
Publications

2018

Delta State replicated data types

Authors
Almeida, PS; Shoker, A; Baquero, C;

Publication
JOURNAL OF PARALLEL AND DISTRIBUTED COMPUTING

Abstract
Conflict-free Replicated Data Types (CRDTs) are distributed data types that make eventual consistency of a distributed object possible and non ad-hoc. Specifically, state-based CRDTs ensure convergence through disseminating the entire state, that may be large, and merging it to other replicas. We introduce Delta State Conflict-Free Replicated Data Types (delta-CRDT) that can achieve the best of both operation-based and state-based CRDTs: small messages with an incremental nature, as in operation-based CRDTs, disseminated over unreliable communication channels, as in traditional state-based CRDTs. This is achieved by defining delta-mutators to return a delta-state, typically with a much smaller size than the full state, that to be joined with both local and remote states. We introduce the delta-CRDT framework, and we explain it through establishing a correspondence to current state-based CRDTs. In addition, we present an anti-entropy algorithm for eventual convergence, and another one that ensures causal consistency. Finally, we introduce several delta-CRDT specifications of both well-known replicated datatypes and novel datatypes, including a generic map composition.

2018

Delta state replicated data types

Authors
Almeida, PS; Shoker, A; Baquero, C;

Publication
J. Parallel Distrib. Comput.

Abstract

2017

As Secure as Possible Eventual Consistency: Work in Progress

Authors
Shoker, A; Yactine, H; Baquero, C;

Publication
Proceedings of the 3rd International Workshop on Principles and Practice of Consistency for Distributed Data, PaPoC@EuroSys 2017, Belgrade, Serbia, April 23 - 26, 2017

Abstract
Eventual consistency (EC) is a relaxed data consistency model that, driven by the CAP theorem, trades prompt consistency for high availability. Although, this model has shown to be promising and greatly adopted by industry, the state of the art only assumes that replicas can crash and recover. However, a Byzantine replica (i.e., arbitrary or malicious) can hamper the eventual convergence of replicas to a global consistent state, thus compromising the entire service. Classical BFT state machine replication protocols cannot solve this problem due to the blocking nature of consensus, something at odd with the availability via replica divergence in the EC model. In this work in progress paper, we introduce a new secure highly available protocol for the EC model that assumes a fraction of replicas and any client can be Byzantine. To respect the essence of EC, the protocol gives priority to high availability, and thus Byzantine detection is performed off the critical path on a consistent data offset. The paper concisely explains the protocol and discusses its feasibility. We aim at presenting a more comprehensive and empirical study in the future. © 2017 ACM.

2017

Aggregation protocols in light of reliable communication

Authors
Kassam, Z; Shoker, A; Almeida, PS; Baquero, C;

Publication
16th IEEE International Symposium on Network Computing and Applications, NCA 2017, Cambridge, MA, USA, October 30 - November 1, 2017

Abstract

2017

Sustainable blockchain through proof of exercise

Authors
Shoker, A;

Publication
16th IEEE International Symposium on Network Computing and Applications, NCA 2017, Cambridge, MA, USA, October 30 - November 1, 2017

Abstract

Supervised
thesis

2016

Optimizing Operation-based Conflict-free Replicated Data Types

Author
Georges Younes

Institution
UM

2016

Extending Conflict free Replicated DataTypes fault models

Author
Houssam Yactine

Institution
UM

2015

Author
André Filipe Faria dos Santos

Institution
UM