Multi-phase atomic commitment protocols require long-lived resource locks on the participants and introduce blocking behaviour at the coordinator. They are also pessimistic in nature, preventing reads from executing concurrently with writes. Despite their known shortfalls, multi-phase protocols are the mainstay of transactional integration between autonomous, federated systems. This paper presents a novel atomic commitment protocol, STRIDE (Speculative Transactions in Decentralised Environments), that offers strict serializable certification of distributed transactions across autonomous, replicated sites. The protocol follows the principles of optimistic concurrency control, operating on the premise that conflicting transactions are infrequent. When they do occur, conflicting transactions are identified through antidependency testing on the certifier, which may be replicated for performance and availability. The majority of transactions can be certified entirely in memory. Unlike its multi-phase counterparts, STRIDE is nonblocking, decentralised and does not mandate the use of long-lived resource locks on the participants. It also offers a flexible isolation model for read-only transactions, which can be served directly from the participant sites without undergoing certification. Also, update transactions are Φ-serializable, making the certifier immune to the recently disclosed logical timestamp skew anomaly.

To sidestep reasoning about the complex effects of concurrent execution, many system designers have conveniently embraced strict serializability on the strength of its claims, support from commercial and open-source database communities and ubiquitous levels of industry adoption. Crucially, distributed components are built on this model; multiple schedulers are composed in an event-driven architecture to form larger, ostensibly correct systems. This paper examines the oft-misconstrued position of strict serializability as a composable correctness criterion in the design of such systems. An anomaly is presented wherein a strict serializable scheduler in one system produces a history that cannot be serially applied to even a weak prefix-consistent replica in logical timestamp order. Several solutions are presented under varying isolation properties, including novel isolation properties contributed by this paper. We also distinguish between concurrent schedulers based on their propensity to produce deterministic histories. It is further shown that every nondeterministic scheduler is anomaly-prone, every nonconcurrent scheduler is anomaly-free, and that at least one deterministic concurrent scheduler is anomaly-free.

All existing solutions to distributed consensus are organised around a Paxos-like structure wherein processes contend for exclusive leadership in one phase, and then either use their dominant position to propose a value in the next phase or elect an alternate leader. This approach may be characterised as adversarial and phase-asymmetric, requiring distinct message schemas and process behaviours for each phase. In over three decades of research, no algorithm has diverged from this basic model, alluding to it perhaps being the only viable solution to consensus. This paper presents a new consensus algorithm named Spire, characterised by a phase-symmetric, cooperative structure. Processes do not contend for leadership; instead, they collude to iteratively establish a dominant value and may do so concurrently without conflicting. Each successive iteration is structured identically to the previous, employing the same messages and invoking the same behaviour. By these characteristics, Spire buckles the trend in protocol design, proving that at least two disjoint cardinal solutions to consensus exist. The resulting phase symmetry halves the number of distinct messages and behaviours, offering a clear intuition and an approachable foundation for learning consensus and building practical systems.