Coordination Avoidance in Database Systems (Extended Version) (1402.2237v4)

Abstract: Minimizing coordination, or blocking communication between concurrently executing operations, is key to maximizing scalability, availability, and high performance in database systems. However, uninhibited coordination-free execution can compromise application correctness, or consistency. When is coordination necessary for correctness? The classic use of serializable transactions is sufficient to maintain correctness but is not necessary for all applications, sacrificing potential scalability. In this paper, we develop a formal framework, invariant confluence, that determines whether an application requires coordination for correct execution. By operating on application-level invariants over database states (e.g., integrity constraints), invariant confluence analysis provides a necessary and sufficient condition for safe, coordination-free execution. When programmers specify their application invariants, this analysis allows databases to coordinate only when anomalies that might violate invariants are possible. We analyze the invariant confluence of common invariants and operations from real-world database systems (i.e., integrity constraints) and applications and show that many are invariant confluent and therefore achievable without coordination. We apply these results to a proof-of-concept coordination-avoiding database prototype and demonstrate sizable performance gains compared to serializable execution, notably a 25-fold improvement over prior TPC-C New-Order performance on a 200 server cluster.

• The paper introduces Invariant Confluence (I), a formal framework providing a necessary and sufficient condition to determine when coordination is required in database applications.
• It analyzes practical database systems, finding many operations are invariant confluent while others, like sequential value generation, inherently need coordination.
• Empirical evaluation shows a prototype adhering to I principles achieves significant performance gains, like a 25-fold improvement over serializable execution.

Coordination Avoidance in Database Systems

The paper addresses a fundamental issue in database systems: balancing coordination with performance, availability, and scalability. Coordination, defined as blocking communication between operations, is a significant factor that can hinder these objectives. Serializability, which provides the illusion of operations executing in a sequential order, is a traditional method for maintaining application correctness. However, it often incurs unnecessary coordination, especially when strict consistency levels exceed the needs of the application. This work introduces a formal framework called invariant confluence (I) to identify when coordination is genuinely necessary.

Main Contributions

1. Invariant Confluence (I): This framework establishes whether an application requires coordination to maintain correctness based on its invariants. It delivers a necessary and sufficient condition for applications to execute without coordination, contingent upon maintaining the application's specific correctness criteria. Such a condition is vital as it determines the degree to which a system can scale and maintain availability without sacrificing correctness.
2. Analysis of Real-World Applications: The authors analyze a spectrum of invariants and operations from practical database systems, finding that many are invariant confluent. Key examples include forms of foreign key constraints and some uniqueness guarantees. However, certain operations, like generating sequential values, inherently necessitate coordination.
3. Performance Implications: The paper empirically evaluates a database prototype that adheres to the principles of I. The prototype achieves significant performance gains, such as a 25-fold improvement over serializable execution with TPC-C New-Order transactions on a 200-server cluster. This empirical evidence underscores the framework's applicability in designing high-throughput, distributed database systems that utilize coordination only when essential.

Theoretical Insights

The paper dives deep into database theory by formalizing notions of correctness through invariants—predicates over database states representing integrity constraints. Invariant confluence is determined by whether two operations that independently preserve these invariants can produce a valid final state after merging. This approach elevates the analysis from low-level I/O conflicts to application-specific correctness, providing a nuanced understanding of when coordination becomes inevitable.

Practical Implications

From a practical standpoint, the framework equips database developers and system architects with the knowledge to avoid unnecessary synchronization overheads while ensuring application consistency. This capability is particularly relevant in distributed databases, where coordination overheads magnify due to network latency and the complexity of maintaining consensus.

Future Directions

The concept of invariant confluence opens multiple avenues for future exploration. There is potential to develop more automated tools or compilers that can analyze application transactions and invariants to determine I confluence automatically. Moreover, expanding this framework to handle more complex data operations, such as those involving temporal or spatial constraints, is an intriguing possibility. Finally, exploring the intersection of I with other computational paradigms, like functional and reactive programming, could yield innovative ways to design distributed systems.

In conclusion, this paper provides a rigorous and actionable strategy for optimizing database systems for scalability, availability, and low-latency performance. It challenges the misconception that strict consistency and high performance are mutually exclusive, presenting invariant confluence as a key to unlocking substantial improvements in distributed database design.