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About

About

I am a researcher at HASLab and professor at the U. Minho. My research focuses on dependable distributed systems. I am interested mainly in data management, including database replication and SQL processing over NoSQL systems, and in group communication, including consensus and gossip-based protocols for large-scale systems. I am also interested in tools for testing, evaluating, and monitoring dependable distributed systems. More information is available at my personal home page.

Interest
Topics
Details

Details

  • Name

    José Orlando Pereira
  • Role

    Research Coordinator
  • Since

    01st November 2011
010
Publications

2025

CRDV: Conflict-free Replicated Data Views

Authors
Faria, N; Pereira, J;

Publication
Proc. ACM Manag. Data

Abstract
There are now multiple proposals for Conflict-free Replicated Data Types (CRDTs) in SQL databases aimed at distributed systems. Some, such as ElectricSQL, provide only relational tables as convergent replicated maps, but this omits semantics that would be useful for merging updates. Others, such as Pg\_crdt, provide access to a rich library of encapsulated column types. However, this puts merge and query processing outside the scope of the query optimizer and restricts the ability of an administrator to influence access paths with materialization and indexes. Our proposal, CRDV, overcomes this challenge by using two layers implemented as SQL views: The first provides a replicated relational table from an update history, while the second implements varied and rich types on top of the replicated table. This allows the definition of merge semantics, or even entire new data types, in SQL itself, and enables global optimization of user queries together with merge operations. Therefore, it naturally extends the scope of query optimization and local transactions to operations on replicated data, can be used to reproduce the functionality of common CRDTs with simple SQL idioms, and results in better performance than alternatives.

2024

Databases in Edge and Fog Environments: A Survey

Authors
Ferreira, LMM; Coelho, F; Pereira, J;

Publication
ACM COMPUTING SURVEYS

Abstract
While a significant number of databases are deployed in cloud environments, pushing part or all data storage and querying planes closer to their sources (i.e., to the edge) can provide advantages in latency, connectivity, privacy, energy, and scalability. This article dissects the advantages provided by databases in edge and fog environments by surveying application domains and discussing the key drivers for pushing database systems to the edge. At the same time, it also identifies the main challenges faced by developers in this new environment and analyzes the mechanisms employed to deal with them. By providing an overview of the current state of edge and fog databases, this survey provides valuable insights into future research directions.

2024

When Amnesia Strikes: Understanding and Reproducing Data Loss Bugs with Fault Injection

Authors
Ramos, M; Azevedo, J; Kingsbury, K; Pereira, J; Esteves, T; Macedo, R; Paulo, J;

Publication
PROCEEDINGS OF THE VLDB ENDOWMENT

Abstract
We present LAZYFS, a new fault injection tool that simplifies the debugging and reproduction of complex data durability bugs experienced by databases, key-value stores, and other data-centric systems in crashes. Our tool simulates persistence properties of POSIX file systems (e.g., operations ordering and atomicity) and enables users to inject lost and torn write faults with a precise and controlled approach. Further, it provides profiling information about the system's operations flow and persisted data, enabling users to better understand the root cause of errors. We use LAZYFS to study seven important systems: PostgreSQL, etcd, Zookeeper, Redis, LevelDB, PebblesDB, and Lightning Network. Our fault injection campaign shows that LAZYFS automates and facilitates the reproduction of five known bug reports containing manual and complex reproducibility steps. Further, it aids in understanding and reproducing seven ambiguous bugs reported by users. Finally, LAZYFS is used to find eight new bugs, which lead to data loss, corruption, and unavailability.

2024

TADA: A Toolkit for Approximate Distributed Agreement

Authors
da Conceiçao, EL; Alonso, AN; Oliveira, RC; Pereira, J;

Publication
SCIENCE OF COMPUTER PROGRAMMING

Abstract
TADA is a unique toolkit designed to foster the use and implementation of approximate distributed agreement primitives. Developed in Java, TADA provides ready-to-use implementations of several approximate agreement algorithms, as well as the tools to enable programmers/researchers to easily implement further protocols: A template that enables new protocol implementations to be created by simply changing specific functions; and high-level abstractions for communication and concurrency control. As an example, the toolkit includes a ready-to-use implementation for clock synchronisation between distributed processes. Further use cases can include sensor input stabilisation and distributed machine learning, or other instances of distributed agreement where network synchrony cannot be assumed, byzantine fault tolerance may be required and a bounded divergence in decision values can be tolerated.

2023

MRVs: Enforcing Numeric Invariants in Parallel Updates to Hotspots with Randomized Splitting

Authors
Faria, N; Pereira, J;

Publication
Proc. ACM Manag. Data

Abstract
Performance of transactional systems is degraded by update hotspots as conflicts lead to waiting and wasted work. This is particularly challenging in emerging large-scale database systems, as latency increases the probability of conflicts, state-of-the-art lock-based mitigations are not available, and most alternatives provide only weak consistency and cannot enforce lower bound invariants. We address this challenge with Multi-Record Values (MRVs), a technique that can be layered on existing database systems and that uses randomization to split and access numeric values in multiple records such that the probability of conflict can be made arbitrarily small. The only coordination needed is the underlying transactional system, meaning it retains existing isolation guarantees. The proposal is tested on five different systems ranging from DBx1000 (scale-up) to MySQL GR and a cloud-native NewSQL system (scale-out). The experiments explore design and configuration trade-offs and, with the TPC-C and STAMP Vacation benchmarks, demonstrate improved throughput and reduced abort rates when compared to alternatives.

Supervised
thesis

2023

User-level software-defined storage data planes

Author
Ricardo Gonçalves Macedo

Institution
UM

2023

Distributed and Dependable SDS Control Plane for HPC

Author
Mariana Martins de Sá Miranda

Institution
UM

2022

Epidemic broadcast algorithms In a Byzantine environment

Author
Tomás Francisco Cruz Costa

Institution
UM

2022

Visualização de centro de dados

Author
Pedro Miguel da Costa Capa

Institution
UM

2022

Autonomous Optimization for a Transactional Middleware

Author
Susana Vitória Sá Silva Marques

Institution
UM