Object Stores

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The persistence abstraction may abstract over all the physical properties of data. Therefore, the implementation of a persistent store must be transparent to its users. That is, the size, speed and components of a persistent store are all hidden from a user by the persistence abstraction. As a consequence of this, a persistent store has certain perceived attributes such as unbounded size, infinite speed and stability. The main technical problems encountered when constructing a persistent store involve the simulation of these perceived attributes.

The ideal persistent store is of unbounded size. If it were not, its size could be determined by a program that created objects until the store was full. This would violate the persistence abstraction. Unbounded size can be simulated by using mechanisms such as garbage collectors and stacks to reclaim unused space. However, the particular choice of mechanisms is dependent on the type of computation being performed and the scale of the persistent store. For example, a persistent store supporting first class procedures may not be able to use a stack allocation scheme for recording all procedure activations. Similarly, an extremely large persistent store that is in continual use, may be constrained to use a garbage collector that operates concurrently with the store's users.

The conceptual requirement on a store is that it is infinitely fast but, technologically, this is not possible. The only solution to this problem is to divide the persistent store into components that can operate concurrently. In this way, the total performance of the store may be increased although it can never approach the ideal performance.

The components of the persistent store are hidden so any failures must also be hidden. There are a wide variety of techniques that can be employed to simulate stable storage, but they all use the basic technique of maintaining more than one copy of the persistent store on non-volatile media. This may be performed by employing a strategy for regularly dumping the persistent store or always maintaining multiple copies of the persistent store on different disks. However, no stability mechanism can guarantee total stability, since all the copies of a persistent store may be subject to simultaneous damage.

This research is concerned with designing, implementing and measuring such persistent object stores.

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