Global Serializability - Relaxing Global Serializability

Relaxing Global Serializability

Some techniques have been developed for relaxed global serializability (i.e., they do not guarantee global serializability; see also Relaxing serializability). Among them (with several publications each):

• Quasi serializability
• Two-level serializability

While local (to a database system) relaxed serializability methods compromise serializability for performance gain (and are utilized only when the application can tolerate possible resulting inaccuracies, or its integrity is unharmed), it is unclear that various proposed relaxed global serializability methods which compromise global serializability, provide any performance gain over commitment ordering which guarantees global serializability. Typically, the declared intention of such methods has not been performance gain over effective global serializability methods (which apparently have been unknown to the inventors), but rather correctness criteria alternatives due to lack of a known effective global serializability method. Oddly, some of them were introduced years after CO had been introduced, and some even quote CO without realizing that it provides an effective global serializability solution, and thus without providing any performance comparison with CO to justify them as alternatives to global serializability for some applications (e.g., Two-level serializability). Two-level serializability is even presented as a major global concurrency control method in a 2010 edition of a text-book on databases (authored by two of the original authors of Two-level serializability, where one of them, Avi Silberschatz, is also an author of the original Strong recoverability articles). This book neither mentions CO nor references it, and strangely, apparently does not consider CO a valid Global serializability solution.

Another common reason nowadays for Global serializability relaxation is the requirement of availability of internet products and services. This requirement is typically answered by large scale data replication. The straightforward solution for synchronizing replicas' updates of a same database object is including all these updates in a single atomic distributed transaction. However, with many replicas such a transaction is very large, and may span several computers and networks that some of them are likely to be unavailable. Thus such a transaction is likely to end with abort and miss its purpose. Consequently Optimistic replication (Lazy replication) is often utilized (e.g., in many products and services by Google, Amazon, Yahoo, and alike), while Global serializability is relaxed and compromised for Eventual consistency. In this case relaxation is done only for applications that are not expected to be harmed by it.

Classes of schedules defined by relaxed global serializability properties either contain the global serializability class, or are incomparable with it. What differentiates techniques for relaxed global conflict serializability (RGCSR) properties from those of relaxed conflict serializability (RCSR) properties that are not RGCSR is typically the different way global cycles (span two or more databases) in the global conflict graph are handled. No distinction between global and local cycles exists for RCSR properties that are not RGCSR. RCSR contains RGCSR. Typically RGCSR techniques eliminate local cycles, i.e., provide local serializability (which can be achieved effectively by regular, known concurrency control methods), however, obviously they do not eliminate all global cycles (which would achieve global serializability).