Session 1: Keynote

[9:20] Opening Remarks

[9:30] 20+ Years of data replication and consistency. Have we learned anything? (Keynote by Gustavo Alonso, ETH Zurich)

Data consistency is a fundamental topic in computer science, cutting across many areas and applications. Surprisingly, a significant part of ongoing research revolves around ideas pursued decades ago. The fact that published papers systematically ignore all this previous work does not mean it does not exist. Maybe the context has changed (cloud, geo-replication, massive scale, etc.) but the principles behind consistency for replicated data remain the same and have been known for a long time. In this talk, I will argue that there is a corpus of basic principles governing data consistency and that the mapping of such principles to mechanisms and system implications is also well established after decades of research. By providing a historical perspective of how data replication, and the subsequent problem of consistency, has evolved over the years, I will enumerate these principles, discuss the performance aspects associated to them, and relate these ideas to existing systems and trends. The goal of the talk is to open up new perspectives in an area where there is still much to be done and that has become highly relevant in the era of large scale computing infrastructures.

[10:30] A Modular Design for Geo-Distributed Querying (Dimitrios Vasilas, Marc Shapiro and Bradley King)

Most distributed storage systems provide limited abilities for querying data by attributes other than their primary keys. Supporting efficient search on secondary attributes is challenging as applications pose varying requirements to query processing systems, and no single system design can be suitable for all needs. In this paper, we show how to overcome these challenges in order to extend distributed data stores to support queries on secondary attributes. We propose a modular architecture that is flexible and allows query processing systems to make trade-offs according to different use case requirements. We describe adaptive mechanisms that make use of this flexibility to enable query processing systems to dynamically adjust to query and write operation workloads.

[11:00] End of Session, Coffee Break

Session 2 : Causality

[11:30] The Pitfalls in Achieving Tagged Causal Delivery (Georges Younes, Paulo Sérgio Almeida and Carlos Baquero)

Causal delivery middleware respect causality when delivering messages, but do not provide information about it to the client process. When two messages, m1 and m2 are delivered in sequence to a given process, either they are concurrent or m1 happens-before m2, but the client application is not told which is the case. In some domains, like operation-based CRDTs, this information is useful, and previously we have proposed exposing it in the API of a Tagged Causal Delivery middleware. One could think this would be just a matter of taking some current middleware and exposing the vector-clocks which are internally kept. In this paper we identify some obstacles in doing so, and describe how to overcome them. The essence lies in the role of current middleware, allowing it to tag as ordered some messages that are concurrent, and we describe how it can happen in several interaction models between middleware and client code, either callback-based or with independent threads/processes. This means that vector-clocks in current middleware are not precise to describe happens-before, and cannot be simply exposed in the API.

[12:00] Ensuring referential integrity under causal consistency (Marc Shapiro, Annette Bieniusa, Peter Zeller and Gustavo Petri)

Referential integrity (RI) is an important correctness property of a shared, distributed object storage system. It is sometimes thought that enforcing RI requires a strong form of consistency. In this paper, we argue that causal consistency suffices to maintain RI. We support this argument with pseudocode for a reference CRDT data type that maintains RI under causal consistency. QuickCheck has not found any errors in the model.

[12:30] Towards Causal Datacenter Networks (Ellis Michael and Dan R. K. Ports)

Traditionally, distributed systems conservatively assume an asynchronous network. However, recent work on the codesign of networks and distributed systems has shown that stronger ordering properties are achievable in datacenter networks and yield performance improvements for the distributed systems they support. We build on that trend and ask whether it is possible for the datacenter network to order all messages in a protocol-agnostic way. This approach, which we call omnisequencing, would ensure causal delivery of all messages, making consistency a network-level guarantee.

[13:00] End of Session, Lunch Break

Session 3: Consistency Models

[14:00] Towards Affordable Externally Consistent Guarantees for Geo-Replicated Systems (Manuel Bravo and Luis Rodrigues)

We propose a novel consistency model called external causality, which aims at making external consistency affordable for georeplicated systems. In this short paper, we (i) outline this idea— informally defining external causality, and (ii) motivate it with two simple examples that illustrate how developers of cloud services may benefit from it.

[14:30] Constraining the Eventual in Eventual Consistency (Jim Bauwens, Florian Myter and Elisa Gonzalez Boix)

CRDTs are highly available replicated data structures which offer strong eventual consistency in the face of concurrent operations. By their definition, CRDTs eventually converge to a consistent state given enough time. However, this is not strict enough for some distributed applications. Current state-of-the-art CRDT implementations fail to provide programmers with the means to specify these constraints. As a result, programmers need to write application-level code which ignores stale or timed-out operations. In this paper, we introduce a leasing model which allows programmers to declaratively specify timing constraints for CRDTs. In short, programmers are able to attach leases to operations on a CRDT instance. When such a lease expires the underlying implementation ensures that the operation is eventually canceled for all replicas.

[15:00] Making Consistency More Consistent: A Unified Model for Coherence, Consistency and Isolation (Adriana Szekeres and Irene Zhang)

Ordering guarantees are often defined using abstract execution models. Unfortunately, these models are complex and make different assumptions about system semantics. As a result, researchers find it impossible to compare the ordering guarantees of coherence, consistency and isolation. This paper presents a simple, unified model for defining ordering guarantees that is sufficiently general to model a wide range of systems, including processor memory, distributed storage, and databases. We define a new single constraint relationship, result visibility, which formalizes the “appears to execute before” relationship between operations. Using only result visibility, we define more than 20 ordering guarantees from different research areas, including PRAM, snapshot isolation and eventual consistency session guarantees. To our knowledge, these definitions form the broadest survey of ordering guarantees using a single constraint in the current literature.

[15:30] Fine-grained Distributed Consistency Guarantees with Effect Orchestration (Kiarash Rahmani, Gowtham Kaki and Suresh Jagannathan)

Highly-available distributed applications typically require data to be replicated over geo-distributed stores that offer weak consistency guarantees by default. Unfortunately, undesirable behaviors may arise under weak consistency that can violate application correctness, forcing designers to either implement complex ad-hoc mechanisms to avoid these anomalies or by sacrificing performance, choose to run applications using stronger levels of consistency. In this paper, we describe a lightweight runtime system that relieves developers from having to make such tradeoffs. Instead, our approach leverages declarative axiomatic specifications that reflect the necessary constraints any correct implementation must satisfy to guide a runtime consistency enforcement and monitoring mechanism. Experimental results show that the performance of our (provably optimal and safe) automatically derived fine-grained consistency enforcement mechanisms is better than common storeoffered consistency guarantees.

[16:00] End of Session, Coffee Break

Session 4: Open Session

[16:30] A Set CRDT with Multiple Conflict Resolution Policies (André Rijo, Carla Ferreira and Nuno Preguiça)

Conflict-Free Replicated Data Types (CRDTs) allow objects to be replicated and concurrently modified without coordination. CRDTs solve conflicts automatically and provide eventual consistency. Typically each CRDT uses a specific policy for solving conflicts. For example, in a add-wins set CRDT, when a concurrent add and remove of the same element happen concurrently, priority is given to the add, i.e., the element stays in the set. Unfortunately, this may be inadequate for some applications – it may be desired to overrule the default policy for some operation executions. For example, an application using an add-wins set may want some removes to win over concurrent adds. This paper present the design of a set CRDT that implements such semantics.

[16:50] On the Consistency of Heterogeneous Clinical Data Sources (Bilal Arshad)

We propose a novel approach to bridge the gap between Strong and Weak Consistency models using a powerful graph model. It tries to combine the benefits of both weak and strong consistency, much like a hybrid consistency model. In this work in progress paper, we introduce a new model ‘GraphCon’ to ensure structural consistency in a distributed clinical environment. This paper concisely explains the framework and discusses its feasibility. We aim to present a more comprehensive and empirical study in the future.

[17:10] End of Workshop