Cache Serializability: Reducing Inconsistency in Edge Transactions

TL;DR

T-Cache is a novel caching policy that enhances consistency in read-only edge caches by tolerating certain inconsistencies, thereby reducing latency and load without compromising safety.

Contribution

It introduces cache-serializability and a new caching policy, T-Cache, which improves consistency in asynchronous, unreliable cache environments with limited resources.

Findings

Detects 43-70% of inconsistencies in real workloads

Increases rate of consistent transactions by 33-58%

Operates with nominal overhead in synthetic workloads

Abstract

Read-only caches are widely used in cloud infrastructures to reduce access latency and load on backend databases. Operators view coherent caches as impractical at genuinely large scale and many client-facing caches are updated in an asynchronous manner with best-effort pipelines. Existing solutions that support cache consistency are inapplicable to this scenario since they require a round trip to the database on every cache transaction. Existing incoherent cache technologies are oblivious to transactional data access, even if the backend database supports transactions. We propose T-Cache, a novel caching policy for read-only transactions in which inconsistency is tolerable (won't cause safety violations) but undesirable (has a cost). T-Cache improves cache consistency despite asynchronous and unreliable communication between the cache and the database. We define cache-serializability, a variant of serializability that is suitable for incoherent caches, and prove that with unbounded resources T-Cache implements this new specification. With limited resources, T-Cache allows the system manager to choose a trade-off between performance and consistency. Our evaluation shows that T-Cache detects many inconsistencies with only nominal overhead. We use synthetic workloads to demonstrate the efficacy of T-Cache when data accesses are clustered and its adaptive reaction to workload changes. With workloads based on the real-world topologies, T-Cache detects 43-70% of the inconsistencies and increases the rate of consistent transactions by 33-58%.