Putting consistency back into eventual consistency

Abstract

Geo-replicated storage systems are at the core of current Internet services. The designers of the replication protocols used by these systems must choose between either supporting low-latency, eventually-consistent operations, or ensuring strong consistency to ease application correctness. We propose an alternative consistency model, Explicit Consistency, that strengthens eventual consistency with a guarantee to preserve specific invariants defined by the applications. Given these application-specific invariants, a system that supports Explicit Consistency identifies which operations would be unsafe under concurrent execution, and allows programmers to select either violation-avoidance or invariant-repair techniques. We show how to achieve the former, while allowing operations to complete locally in the common case, by relying on a reservation system that moves coordination off the critical path of operation execution. The latter, in turn, allows operations to execute without restriction, and restore invariants by applying a repair operation to the database state. We present the design and evaluation of Indigo, a middleware that provides Explicit Consistency on top of a causally-consistent data store. Indigo guarantees strong application invariants while providing similar latency to an eventually-consistent system in the common case.

CLOTHO: directed test generation for weakly consistent database systems

“…There have been several recent proposals to reason about programs executing under weak consistency [Alvaro et al 2011;Bailis et al 2014a;Balegas et al 2015;Li et al 2014Li et al , 2012. All of them assume a system model that offers a choice between a coordination-free weak consistency level (e.g., eventual consistency [Alvaro et al 2011;Bailis et al 2014a;Balegas et al 2015;Li et al 2014Li et al , 2012) or causal consistency Lesani et al 2016]). In contrast, our focus is agnostic to the particular consistency guarantees provided by the underlying storage system, and is concerned with statically identifying violations of a particular class of invariants, namely those that hold precisely when transactions exhibit serializable behavior.…”

'Cause I'm strong enough: Reasoning about consistency choices in distributed systems

“…The execution X is consistent with T and F; the conditions in the proof rule imply that so are X and X . The condition e ∈ max(X ) (see (9)) reflects the fact that e is the latest event received by r . The condition X = ctxt(e, X ) ensures that X is a part of X = X | X .E−{e} .…”

“…To address this inefficiency, Balegas et al [9] proposed a hybrid model where consistency can be strengthened using multi-level locks, analogous to readers-writer locks from shared memory. In our example, we represent such a lock by a pair of tokens: τc, introduced before, and τp.…”

“…In more detail, our first contribution is a generic hybrid consistency model ( §2) that is flexible enough to encode a variety of consistency models for replicated databases proposed in the literature [9,32,33,42]. It guarantees causal consistency by default and allows the programmer to additionally specify which pairs of operations may not execute without synchronisation by means of a special conflict relation.…”

Lasp

“…We believe we can leverage recent work in contract enforcement and invariant preservation in eventually consistent systems, specifically "invariant-based programming" by Balegas et al [9] and Quelea by Kaki et al [21].…”