Dynamic content Web sites consist of a front-end Web server, an application server and a back-end database. In this paper we introduce distributed versioning, a new method for scaling the back-end database through replication.

We present a new lazy replication technique, suitable for scaling the back-end database of a dynamic content site using a cluster of commodity computers. Our technique, called conflict-aware scheduling, provides both throughput scaling and 1-copy serializability. It has generally been believed that this combination is hard to achieve through replication because of the growth of the number of conflicts. We take advantage of the presence in a database cluster of a scheduler through which all incoming requests pass. We require that transactions specify the tables that they access at the beginning of the transaction. Using that information, a conflictaware scheduler relies on a sequence-numbering scheme to implement 1-copy serializability, and directs incoming queries in such a way that the number of conflicts is reduced. We evaluate conflict-aware scheduling using the TPC-W e-commerce benchmark. For small clusters of up to eight database replicas, our evaluation is performed through measurements of a web site implementing the TPC-W specification. We use simulation to extend our measurement results to larger clusters, faster database engines, and lower conflict rates. Our results show that conflict-awareness brings considerable benefits compared to both eager and conflictoblivious lazy replication for a large range of cluster sizes, database speeds, and conflict rates. Conflict-aware scheduling provides near-linear throughput scaling up to a large number of database replicas for the browsing and shopping workloads of TPC-W. For the write-heavy ordering workload, throughput scales, but only to a smaller number of replicas. 1

A Replica Location Service (RLS) allows registration and discovery of data replicas. In earlier work, we proposed an RLS framework and described the performance and scalability of an RLS implementation in Globus Toolkit Version 3.0. In this paper, we present a Peer-to-Peer Replica Location Service (P-RLS) with properties of self-organization, fault-tolerance and improved scalability. P-RLS uses the Chord algorithm to self-organize PRLS servers and exploits the Chord overlay network to replicate P-RLS mappings adaptively. Our performance measurements demonstrate that update and query latencies increase at a logarithmic rate with the size of the P-RLS network, while the overhead of maintaining the P-RLS network is reasonable. Our simulation results for adaptive replication demonstrate that as the number of replicas per mapping increases, the mappings are more evenly distributed among P-RLS nodes. We introduce a predecessor replication scheme and show it reduces query hotspots of popular mappings by distributing queries among nodes.

We study several transparent techniques for scaling dynamic content web sites, and we evaluate their relative impact when used in combination. Full transparency implies strong data consistency as perceived by the user, no modifications to existing dynamic content site tiers and no additional programming effort from the user or site administrator upon deployment.

This paper investigates the use of partial replication in the Database State Machine approach introduced earlier for fully replicated databases. It builds on the order and atomicity properties of group communication primitives to achieve strong consistency and proposes two new abstractions: Resilient Atomic Commit and Fast Atomic Broadcast.

Abstract — The reliable server pooling (RSP) allows a pool of redundant information sources to be viewed as a single transport endpoint, and therefore is able to provide persistent connections and balanced traffic for different applications. The IETF RSerPool Working Group has proposed an architecture to implement the RSP, which defines an overlay network providing an upper layer protocol or an application with a range of reliability services, from simple server selection to a fully automatic session-failover capability. The simulation experiments conducted in both wired and wireless environments show that the current version of the RSerPool works well in fixed and relatively reliable environments, but its performance worsens rapidly as the networks become more unreliable or mobile. The issues we identified in wireless mobile ad-hoc networks include network partition, high signaling overhead, difficulty in synchronization among name servers, and excessive aggressiveness in handling failures. Alternative design options for the RSP in wireless and mobile environments are introduced and evaluated. Index Terms — reliable server pooling; RSerPool; ad-hoc networks; battlefield networks; service overlay networks; failover I.

GlobData is a project that aims to design and implement a middleware tool offering the abstraction of a global object database repository. This tool, called Copla, supports transactional access to geographically distributed persistent objects independent of their location. Additionally, it supports replication of data according to different consistency criteria. For this purpose, Copla implements a number of consistency protocols offering different tradeoffs between performance and fault-tolerance. This paper presents the work on strong consistency protocols for the Glob-Data system. Two protocols are presented: a voting protocol and a nonvoting protocol. Both these protocols rely on the use of atomic broadcast as a building block to serialize conflicting transactions. The paper also introduces the total order protocol being developed to support large-scale replication. 1

Database replication with update-anywhere capability while maintaining global synchronization and isolation has long been thought impractical. Protocols have been proposed for distributed replicated databases that take advantage of atomic broadcast systems to simplify message passing and conflict resolution in hopes of making replication efficient. This paper presents global recovery algorithms to handle site failures when such protocols are used with a broadcast system providing virtual synchrony. 1

GLOBDATA is a project that aims to design and implement a middleware tool offering the abstraction of a global object database repository. This tool, called COPLA, supports transactional access to geographically distributed persistent objects independent of their location. Additionally, it supports replication of data according to different consistency criteria. For this purpose, COPLA implements a number of consistency protocols offering different tradeoffs between performance and fault-tolerance.

A recent trend has seen the extension of objectoriented middleware to component-oriented middleware. A major advantage components offer over objects is that only the business logic of an application needs to be addressed by a programmer with support services required incorporated into the application at deployment time. This is achieved via components (business logic of an application), containers that host components and are responsible for providing the underlying middleware services required by components and application servers that host containers. Well-known examples of component middleware architectures are Enterprise Java Beans (EJBs) and the CORBA Component Model (CCM). Two of the many services available at deployment time in most component architectures are component persistence and atomic transactions. This paper examines, using EJBs, how replication for availability can be supported by containers so that components that are transparently using persistence and transactions can also be made highly available.