Pronto: High Availability for Standard Off-the-shelf Databases

Enterprise applications typically store their state in databases. If a database fails, the application is unavailable while the database recovers. Database recovery is time consuming because it involves replaying the persistent transaction log. To isolate end-users from database failures we introduce Pronto, a protocol to orchestrate the transaction processing by multiple, standard databases so that they collectively implement the illusion of a single, highly-available database. Pronto is a novel replication protocol that handles non-determinism without relying on perfect failure detection, does not require any modifications in existing applications and databases, and allows databases from different providers to be part of the replicated compound

Exactly-Once Transactions

A three-tier application is organized as three layers: human users interact with front-end clients (e.g browsers); middle-tier application servers (e.g. Web servers) contain the application's business logic, and perform transactions against back-end databases. Although three-tier applications are becoming mainstream, they usually fail to provide sufficient reliability guaranteesto the users. Usually, replication and transaction-processing techniques are applied to specific parts of the application, but their combination does not provide end-to-end reliability. The aim of this paper is to precisely define a desirable, yet realistic, specification of end-to-end reliability in three-tier applications. We present the specification in the form of a problem, called Exactly-Once Transactions, which encompasses both safety and liveness properties in such environments. We also describe a practical protocol that solves the problem and we discuss its implementation and performance in a practical setting

Democratrizing the Parliament

This paper presents a replication algorithm that implements a highly-available, non-deterministic state machine. Our algorithm generalizes the Paxos parliament algorithm of Lamport to cope with non-deterministic computations, while preserving its nice resilience and efficiency properties. The algorithm is surprisingly simple, thanks to the use of two powerful underlying abstractions: weak consensus and weak leader election, together with a generic data structure: consensus bag. As a side-effect of our work, we discuss some similarities and differences between replicating deterministic and non-deterministic state machines. Indirectly, we revisit the traditional classification between state-machine replication and primary-backup

X-ability: A Theory of Replication

This paper presents x-ability (Exactly-once-ability): a correctness criteria for replicated services. X-ability provides the illusion that the actions executed by a replicated service are executed exactly-once, even if these actions have been actually executed several times and by various replicas. A client can treat a x-able replicated service as if it was not replicated, even if this service executes actions that are non-deterministic and have side-effects on the environment, e.g., invoke other services. X-ability is a local property: replicated services can be specified and implemented independently, and later composed in the implementation of more complex replicated services. We illustrate our theory through an asynchronous replication protocol that handles non-determinism and external side-effects. The replication protocol is asynchronous in the sense that it may vary, at run-time and according to the asynchrony of the system, between some form of primary-backup and some form of active replication

Implementing E-transactions with asynchronous replication

An e-Transaction is one that executes exactly-once despite failures. This paper describes a distributed protocol that implements the abstraction of e-Transactionsin three-tier architectures. Three-tier architectures are typically Internet-oriented architectures, where the end-user interacts with front-end clients (e.g., browsers) that invoke middle-tier application servers (e.g., web servers) to access back-end databases. We implement the e-Transaction abstraction using an asynchronous replication scheme that preserves the three-tier nature of the architecture and introduces a very acceptable overhead with respect to unreliable solutions

A Scatterer Localization Method Using Large-Scale Antenna Array Systems

As ultra-massive multiple-input multiple-output (UM-MIMO) has emerged as a key technology for millimeter-wave and terahertz communications, the spherical wave propagation should be considered for channel modeling. Therefore, it is critical to identify the locations and evolving behaviors of scatterers, i.e., the sources of the spherical wavefronts. In this contribution, a novel space-alternating generalized expectation-maximization (SAGE) based scatterer localization algorithm is proposed, where a large-scale antenna array is divided into multiple sub-arrays. Due to the decreased aperture of each sub-array, plane wave assumption can be applied to estimate the angles of departure/arrival, delays and amplitudes of multipath components (MPCs). Based on the angle variations of MPCs observed at different sub-arrays, the corresponding scatterers can be located. The proposed algorithm is verified in a simulation using a large-scale uniform circular array (UCA) system. Moreover, we apply this algorithm to an indoor measurement campaign conducted at 27-29\,GHz in a hall scenario. Dominant scatterers are identified, which can be used for the development of further geometry-based stochastic channel models

Geometry-Based Clustering Characteristics for Outdoor Measurements at 28-30 GHz

This paper presents an analysis of channel characteristics for an outdoor scenario at the frequency band of 28-30 GHz. A geometry-based clustering algorithm is proposed to group the measured multipath components (MPCs) based on the interaction points on the surrounding walls or objects obtained from Ray Tracing 3-D simulations, which is more physically interpretable than the traditional clustering algorithm. The cluster-level characteristics at a total of 15 positions along a pre-defined route covering both line-of-sight (LoS) and non-LoS (NLoS) scenarios are investigated. Moreover, power contributions of MPCs with multiple bounces (up to 7 bounces in NLoS condition) from the interaction objects (IOs) are also investigated

Making Consensus Practical

This paper presents the abstraction of lazy consensus and argues for its use as an effective component for building distributed agreement protocols in practical asynchronous systems where processes and links can crash and recover. Lazy consensus looks like consensus, is equivalent to consensus, but is not consensus. The specification of lazy consensus has an on-demand and a re-entrant flavors that makes its use very efficient, especially in terms of forced logs, which are known to be major sources of overhead in distributed systems. We illustrate the use of lazy consensus as a building block to develop efficient atomic broadcast and atomic commitment protocols: two central abstractions in our DACE middleware environment