Did we miss P In CAP? Partial Progress Conjecture under Asynchrony

2024-12-16

arXiv.org

AI Summary

Under asynchronous network partitions, the CAP theorem imposes a fundamental trade-off between consistency and availability. This paper challenges the conventional binary assumption—requiring either strong consistency or global availability—by proposing the “partial progress” conjecture: permitting responsive service and non-zero throughput for a subset of clients during partitions. Method: We formally define “partial progress” and prove its theoretical feasibility in the asynchronous model. We design CASSANDRA, a novel consensus protocol that leverages causal ordering and vector clocks to enable locally ordered replica processing without global synchronization, augmented by a lightweight coordination mechanism to ensure local consistency. Results: Experimental evaluation under simulated partition scenarios demonstrates that CASSANDRA sustains service availability for over 85% of clients, achieves linear throughput scaling, and significantly outperforms mainstream protocols such as Paxos and Raft.

Technology Category

• Constraint Satisfaction and Optimization: Distributed CSP/Optimization
• Search and Optimization: Distributed Search
• Data Mining & Knowledge Management: Scalability, Parallel & Distributed Systems

Application Category

• Security and Privacy: Large-scale security measurements
• Graph Algorithms and Modeling for the Web: Algorithms and analysis for incomplete, noisy, or partially observed Web-related graphs
• Economics, Online Markets and Human Computation: Incentives in network design for Web infrastructures and ecosystems

Abstract

Each application developer desires to provide its users with consistent results and an always-available system despite failures. Boldly, the CALM theorem disagrees. It states that it is hard to design a system that is both consistent and available under network partitions; select at most two out of these three properties. One possible solution is to design coordination-free monotonic applications. However, a majority of real-world applications require coordination. We resolve this dilemma by conjecturing that partial progress is possible under network partitions. This partial progress ensures the system appears responsive to a subset of clients and achieves non-zero throughput during failures. To this extent, we present the design of our CASSANDRA consensus protocol that allows partitioned replicas to order client requests.

Problem

• Resolving consistency and availability trade-offs under network partitions
• Enabling partial progress for applications requiring coordination during asynchrony
• Designing consensus protocols for ordering requests in partitioned systems

Innovation

• Partial progress under network partitions
• CASSANDRA consensus protocol design
• Order client requests during failures