Comp9243 — Week 3a (08s1) Replication and Consistency

Replication involves creating and maintaining copies of services and data provided by a distributed system. Unlike communication, without which it is impossible to build a distributed system, replication is not a fundamental principle. This means that it is possible to build a distributed system that does not make use of replication. Replication does, however, become important when reliability, performance, and scalability of a distributed system are key concerns. In the case of reliability, creating many redundant copies of a service improves that service’s availability. With multiple servers available to clients, it is less likely that a malfunction of one of them will render the whole service unavailable. Likewise, if the data on a server becomes corrupt, data stored at replicas can be used to restore the correct state. With regards to performance, replicating services helps to reduce the load on individual servers. Likewise, by placing replicas close to clients the impact of communication can be greatly reduced. Finally, replication is a key technique for improving a system’s scalability. As a service grows, creating more replicas allows the service to scale along with the growth. When considering the replication of services, there are two types of replication possible: data replication and control replication. In the first case, only a service’s data is replicated. Processing and manipulation of the data is performed either by a non-replicated server, or by clients accessing the data. A typical example of data replication is a replicated (also known as mirrored) FTP site. Web browsers with caches are another example of data replication. In the second case, only the control part of the service is replicated while the data remains at a single centralised server. This form of replication is generally used to to improve or maintain performance by spreading the computational load over multiple servers. It is also possible to combine data and control replication, in which case both the data and control are replicated. They may be replicated together (i.e., both control and data are placed on the same replica servers), or separately (i.e., data is replicated on different servers than control). During the design and implementation of replication in a distributed system, there are a number of issues that must be addressed. The most important of these is keeping the copies of replicated data consistent. Furthermore, it is important to decide how replicas propagate updates amongst each other, where to place the replicas, how many replicas to create, when to add and remove replicas, etc.