A Critique of Snapshot Isolation

TL;DR

This paper introduces write-snapshot isolation, a new lock-free transactional isolation level that achieves serializability with performance comparable to snapshot isolation, addressing limitations of existing weaker guarantees.

Contribution

The paper proposes write-snapshot isolation, a novel lock-free approach that ensures serializability without the overhead of traditional serializable implementations.

Findings

Write-snapshot isolation prevents read-write conflicts.

Performance of write-snapshot isolation is comparable to snapshot isolation.

It provides serializability without the high overhead of traditional methods.

Abstract

The support for transactions is an essential part of a database management system (DBMS). Without this support, the developers are burdened with ensuring atomic execution of a transaction despite failures as well as concurrent accesses to the database by other transactions. Ideally, a transactional system provides serializability, which means that the outcome of concurrent transactions is equivalent to a serial execution of them. Based on experiences on lock-based implementations, nevertheless, serializability is known as an expensive feature that comes with high overhead and low concurrency. Commercial systems, hence, compromise serializability by implementing weaker guarantees such as snapshot isolation. The developers, therefore, are still burdened with the anomalies that could arise due to the lack of serializability. There have been recent attempts to enrich large-scale data stores, such as HBase and BigTable, with transactional support. Not surprisingly, inspired by traditional database management systems, serializability is usually compromised for the benefit of efficiency. For example, Google Percolator, implements lock-based snapshot isolation on top of BigTable. We show in this paper that this compromise is not necessary in lock-free implementations of transactional support. We introduce write-snapshot isolation, a novel isolation level that has a performance comparable with that of snapshot isolation, and yet provides serializability. The main insight in write-snapshot isolation is to prevent read-write conflicts in contrast to write-write conflicts that are prevented by snapshot isolation.