[Bernstein09] Chapter 9. Replication

Chapter 9. Replication

Introduction

Replicated Servers

Synchronizing Updates to Replicated Data

Single-Master Primary-Copy Replication

Multimaster Replication

Other Replication Techniques

Data Sharing Systems

Summary

9.1. Introduction

Replication is the technique of using multiple copies of a server or a resource for better availability and performance. Each copy is called a replica.

The main goal of replication is to improve availability, since a service is available even if some of its replicas are not. This helps mission critical services, such as many financial systems or reservation systems, where even a short outage can be very disruptive and expensive. It helps when communications is not always available, such as a laptop computer that contains a database replica and is connected to the network only intermittently. It is also useful for making a cluster of unreliable servers into a highly-available system, by replicating data on multiple servers.

Replication can also be used to improve performance by creating copies of databases, such as data warehouses, which are snapshots of TP databases that are used for decision support. Queries on the replicas can be processed without interfering with updates to the primary database server. If applied to the primary server, such queries would degrade performance, as discussed in Section 6.6, Query-Update Problems in two-phase locking.

In each of these cases, replication can also improve response time. The overall capacity of a set of replicated servers can be greater than the capacity of a single server. Moreover, replicas can be distributed over a wide area network, ensuring that some replica is near each user, thereby reducing communications delay.