Current Implementations

PerDiS

PerDiS [] is a persistent distributed store, supporting persistence by reachability (PBR) and garbage collection. PerDiS adopts a two-level address scheme, which organizes objects into clusters. Invisible to programmers, a cluster is implemented as a collection of segments, consisting of contiguous pages. PerDiS uses a special variant of malloc to allocate persistent objects. To access a persistent object, the programmer first uses a pathname to open a cluster, stored as a file, and then uses a character name to obtain a pointer the object. On a page fault, a process named PerDiS Daemon (PD) fetches the requested object and takes a pointer swizzling. For keeping consistency, transaction control is supported.

PJama

PJama [] is formerly called PJava or Persistent Java, supporting persistence by reachability. A PJama program has a flat persistent address space, where PJavaStore is the root of persistence from which all persistent objects are reachable. To access a persistent object, the programmer uses a character name to obtain a reference of the root object that the requested object is organized in, and references the requested object through the root object. On an object fault, the requested object is fetched and an early swizzling or a lazy swizzling [7] will occur.

ObjectStore

ObjectStore [] is a client-server object database, supporting persistence by declaration: it uses a special variant of operator new and delete to create and destroy persistent objects. ObjectStore adopts a four-level address scheme, which organizes objects into clusters, clusters into segments, segments into databases. To access a persistent object, programmers first use a pathname to open a database; then use a character name and a type descriptor named typespec to retrieve a pointer of the root object that the requested object is organized in; finally retrieve a pointer to the requested object from the root object. Besides, the programmer can pass a 32-bit integer to os_database::get_segment() to obtain a pointer of a segment, while calling os_segment::get_all_object_clusters() to enumerate a pointer to a cluster [69]. On a page fault, the requested object is fetched and a pointer swizzling will occur.

Texas

Texas [] is a single machine object store, supporting persistence by declaration: it uses a special variant of operator new and delete to create and destroy persistent objects. Similar to PJama, Texas supports a flat persistent space, where all persistent objects are stored in a persistent heap associated with a file. Invisible to programmers, a store is structured as a collection of pages. The programmer uses a normal pointer to access a persistent object, fetched on a page fault followed by a pointer swizzling.

Mneme

Mneme [] is a persistent distributed store, supporting persistence by declaration: it uses functions MnObjectCreate and MnObjectCreatePtr to create persistent objects. Mneme adopts a two-level address scheme, organizing objects into file. To enable multiple managing strategies for objects, Mneme introduces pool, where persistent objects are created. To access a persistent object, programmers first use a pathname to open a file, and then use an object identifier, a 32-bit integer, to obtain a pointer to the requested object through a handle. The latter process is called an object fault. Mneme supports transactions and recovery.

Other Persistent Systems

The following table is a brief survey of some existing persistent object systems. For lack of documents, there may be something missed. However, the purpose of this survey is to compare some major features in various persistent object system designs. Some abbreviations are listed in the note below the table.

Comparisons of some persistent object systems

Name	Type	Supported languages	Distribution Support	Persistence Model	Approaches
PerDiS		C, C++	Transaction support	PBR	Pointer swizzling based on page fault
PJama		Java	Yes	PBR	Swizzling on object fault
ObjectStore		C++, Java	Client/server	PBD	Pointer swizzling based on page fault
Khazana		C++			Flat shared address space
Shore		C++	Peer-to-Peer	PBR	Registered objects
Thor		Theta, C++	Client/server	PBR	Lazy pointer swizzling + surrogate
Mneme		Smalltalk, Modula-3	Transaction support	PBD	Object fault
Texas		C++	No	PBD	Pointer swizzling at page-fault time
Objectivity		C++, Java, Smalltalk	Client/server	PBR for Smalltalk; PBI for DB/C++/Java	No swizzling
O2	Database	C, C++	Client/server	PBD and PBR	No swizzling Hash tables are used for mapping
Cricket	Database
QuickStore		C++	Client/server	-	Pointer swizzling based on page fault
LOOM		Smalltalk	-	-	Swizzling based on object fault

Note:

An Address scheme refers to a persistent address scheme but not a memory address scheme

An ID refers to an identifier

PBR refers to persistence by reachability

PBD refers to persistence by declaration

PBI refers to persistence by inheritance