13 research outputs found
Remove-Win: a Design Framework for Conflict-free Replicated Data Collections
Internet-scale distributed systems often replicate data within and across
data centers to provide low latency and high availability despite node and
network failures. Replicas are required to accept updates without coordination
with each other, and the updates are then propagated asynchronously. This
brings the issue of conflict resolution among concurrent updates, which is
often challenging and error-prone. The Conflict-free Replicated Data Type
(CRDT) framework provides a principled approach to address this challenge.
This work focuses on a special type of CRDT, namely the Conflict-free
Replicated Data Collection (CRDC), e.g. list and queue. The CRDC can have
complex and compound data items, which are organized in structures of rich
semantics. Complex CRDCs can greatly ease the development of upper-layer
applications, but also makes the conflict resolution notoriously difficult.
This explains why existing CRDC designs are tricky, and hard to be generalized
to other data types. A design framework is in great need to guide the
systematic design of new CRDCs.
To address the challenges above, we propose the Remove-Win Design Framework.
The remove-win strategy for conflict resolution is simple but powerful. The
remove operation just wipes out the data item, no matter how complex the value
is. The user of the CRDC only needs to specify conflict resolution for
non-remove operations. This resolution is destructed to three basic cases and
are left as open terms in the CRDC design skeleton. Stubs containing
user-specified conflict resolution logics are plugged into the skeleton to
obtain concrete CRDC designs. We demonstrate the effectiveness of our design
framework via a case study of designing a conflict-free replicated priority
queue. Performance measurements also show the efficiency of the design derived
from our design framework.Comment: revised after submissio
Tunable Causal Consistency: Specification and Implementation
To achieve high availability and low latency, distributed data stores often
geographically replicate data at multiple sites called replicas. However, this
introduces the data consistency problem. Due to the fundamental tradeoffs among
consistency, availability, and latency in the presence of network partition, no
a one-size-fits-all consistency model exists. To meet the needs of different
applications, many popular data stores provide tunable consistency, allowing
clients to specify the consistency level per individual operation. In this
paper, we propose tunable causal consistency (TCC). It allows clients to choose
the desired session guarantee for each operation, from the well-known four
session guarantees, i.e., read your writes, monotonic reads, monotonic writes,
and writes follow reads. Specifically, we first propose a formal specification
of TCC in an extended (vis,ar) framework originally proposed by Burckhardt et
al. Then we design a TCC protocol and develop a prototype distributed key-value
store called TCCSTORE. We evaluate TCCSTORE on Aliyun. The latency is less than
38ms for all workloads and the throughput is up to about 2800 operations per
second. We also show that TCC achieves better performance than causal
consistency and requires a negligible overhead when compared with eventual
consistency
Specification and Implementation of Replicated List: The Jupiter Protocol Revisited
The replicated list object is frequently used to model the core functionality of replicated collaborative text editing systems. Since 1989, the convergence property has been a common specification of a replicated list object. Recently, Attiya et al. proposed the strong/weak list specification and conjectured that the well-known Jupiter protocol satisfies the weak list specification. The major obstacle to proving this conjecture is the mismatch between the global property on all replica states prescribed by the specification and the local view each replica maintains in Jupiter using data structures like 1D buffer or 2D state space. To address this issue, we propose CJupiter (Compact Jupiter) based on a novel data structure called n-ary ordered state space for a replicated client/server system with n clients. At a high level, CJupiter maintains only a single n-ary ordered state space which encompasses exactly all states of each replica. We prove that CJupiter and Jupiter are equivalent and that CJupiter satisfies the weak list specification, thus solving the conjecture above
Efficient Black-box Checking of Snapshot Isolation in Databases
Snapshot isolation (SI) is a prevalent weak isolation level that avoids the
performance penalty imposed by serializability and simultaneously prevents
various undesired data anomalies. Nevertheless, SI anomalies have recently been
found in production cloud databases that claim to provide the SI guarantee.
Given the complex and often unavailable internals of such databases, a
black-box SI checker is highly desirable.
In this paper we present PolySI, a novel black-box checker that efficiently
checks SI and provides understandable counterexamples upon detecting
violations. PolySI builds on a novel characterization of SI using generalized
polygraphs (GPs), for which we establish its soundness and completeness. PolySI
employs an SMT solver and also accelerates SMT solving by utilizing the compact
constraint encoding of GPs and domain-specific optimizations for pruning
constraints. As demonstrated by our extensive assessment, PolySI successfully
reproduces all of 2477 known SI anomalies, detects novel SI violations in three
production cloud databases, identifies their causes, outperforms the
state-of-the-art black-box checkers under a wide range of workloads, and can
scale up to large-sized workloads.Comment: 20 pages, 15 figures, accepted by PVLD
The Unsteady-State Response of Tires to Slip Angle and Vertical Load Variations
The tire is the only part that connects the vehicle and the road surface. Many important properties of vehicles are related to the mechanical properties of tires, such as handling stability, braking safety, vertical vibration characteristics, and so on. Although a great deal of research on tire dynamics has been completed, mainly focusing on steady-state tire force and moment characteristics, as well as linear unsteady force characteristics, less research has been conducted on nonlinear unsteady characteristics, especially when the vertical load changes dynamically. Therefore, the main purpose of this paper is to improve the tire unsteady-state model and verify it by experiment. To achieve this goal, we first study the nonlinear unsteady tire cornering theoretical model and obtain clear force and torque frequency response functions. Then, based on the results of the theoretical model, a high-precision and high-efficiency semi-physical model is developed. Finally, model identification and accuracy verification are carried out based on the bench test data. The model developed in this paper has high accuracy, and it significantly improves the expression of the aligning torque, which helps to improve the virtual simulation of transient conditions, such as vehicle handling and dynamic load conditions