Data Replication in Atomic Action Systems

Data replication techniques for atomic action systems to maintain one-copy serialisability (1SR) have been extensively studied (most notably with regard to replicating databases). When designing a replication protocol it is natural to examine those protocols (and systems which use them) that already exist, to determine whether they have any relevance.

• Definition: If the effect of a group of atomic actions executing on a replicated object is equivalent to running those same atomic actions on a single copy of the object then the overall execution is said to be 1SR.
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• Definition: A replica consistency protocol is one which ensures 1SR.

Because most replication protocols have been developed for use in database environments it is important to understand the differences between the way in which operations function in a database system and the way in which similar operations would function in an object-oriented environment. These differences are important as they affect the way in which the replication protocols function.

In a database system, which performs operations on data structures, a read operation is typically implemented as a "read entire data structure", and a write operation is in fact a "read entire data structure, update state locally to the invoker, then write entire new data state back". In this way, a single write operation can also update (or re-initialise) the state of an out-of-date data structure.

In an object-oriented system, the read operation is typically implemented as "read a specific data value". Similarly, the write is "perform some operation which will modify the state of the object". The object simply exports an interface with certain operations through which it is possible to manipulate the object state. Some of these operations may update the state of the object, whilst others will simply leave it unchanged. A write operation in this case may only modify a subset of an object's state, and so cannot be guaranteed to perform an update as in a database system.

In a database system, the fact that a single write operation can update the entire state of a replica is used in replication protocols such as Available Copies. If these protocols are to be used in an object-oriented system then they will require explicit update protocols.

Finally, in a database system the invoker of a given operation knows whether that operation is state modifying or not i.e., it knows which type of lock will be required. However, in an object-oriented system users of a given object only see the exported interface and see nothing of the implementation, and therefore do not know whether a given operation will modify the state of the object. This difference is important as many of the replication protocols to be described implicitly assume that clients have this type of knowledge (it is used to ensure that read operations can be executed faster than write operations).

In the next few entries we shall examine some of the replication protocols which have been proposed for managing replicated data.