By Bettina Kemme, Ricardo Jiménez Peris, Marta Patiño-Martínez, M. Tamer Özsu
Database replication is generic for fault-tolerance, scalability and function. The failure of 1 database copy doesn't cease the approach from operating as to be had replicas can take over the projects of the failed copy. Scalability could be accomplished through allotting the burden throughout all replicas, and including new replicas should still the weight raise. ultimately, database replication gives you quick neighborhood entry, whether consumers are geographically allotted consumers, if information copies can be found as regards to consumers. regardless of its benefits, replication isn't really an easy strategy to practice, and there are various hurdles to beat. on the vanguard is copy keep an eye on: assuring that information copies stay constant whilst updates ensue. There exist many choices in regard to the place updates can take place and while alterations are propagated to facts copies, how adjustments are utilized, the place the replication software is found, and so forth. a specific problem is to mix copy regulate with transaction administration because it calls for a number of operations to be taken care of as a unmarried logical unit, and it presents atomicity, consistency, isolation and sturdiness around the replicated method. The publication presents a categorization of reproduction regulate mechanisms, provides a number of reproduction and concurrency keep watch over mechanisms intimately, and discusses the various matters that come up while such suggestions have to be carried out inside or on best of relational database platforms. desk of Contents: review / 1-Copy-Equivalence and Consistency / uncomplicated Protocols / Replication structure / The Scalability of Replication / keen Replication and 1-Copy-Serializability / 1-Copy-Snapshot Isolation / Lazy Replication / Self-Configuration and Elasticity / different facets of Replication
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Extra resources for Database Replication (Synthesis Lectures on Data Management)
The problem with the protocol described so far is that it is possible that the local replica has sent the commit request to some of the remote replicas but it crashes before all receive it. Or it commits the transaction without anybody having received the commit request. In this case, 1-copy-atomicity would be violated as some would commit, others abort the transaction. The use of an agreement protocol, such as the 2-phase commit protocol described in Appendix A, is necessary. This will guarantee that either all replicas commit the transaction or all abort it.
Thus, if the middleware does its own locking, it is typically on a coarse granularity such as tables. We also see shortly that executing write operations at remote replicas can be better optimized in a kernel based approach. A second advantage of kernel based replication is that clients remain directly connected with the database system. In contrast, middleware systems introduce a level of indirection, leading to more messages in the system. However, middleware and database replica typically remain in the same local area network where this additional message overhead has relatively little impact.
Directory-based replica discovery. An alternative is to rely on a directory service. In this case, a directory node with a well-known IP address keeps a directory of all available replicas and their IP addresses. The node monitors the current set of available replicas and updates the directory content regularly. Thus, the client connectivity component can request the directory information from the directory node and then connect to one of the available replicas. Of course, the directory node now becomes a single point of failure and might require replication by itself for fault-tolerance purposes.