Persistent State Service

1
Persistent State Service

• Concept
• Persistence is the ability of an object state to
  survive termination of the process in which the
  object executes
• State objects have attributes in their
  interfaces, and instance variables in their
  implementation
• Stateless objects
• Server object designer/programmer implement
  persistence
• Persistent state service hide the details of data
  stores from server objects
• PSS is not an interface to a complete,
  object-oriented database management system
• PSS offers an OO data definition language called
  PSDL that is much more appropriate for persisting
  object state than SQL
• Persistence is transparent to client

2
Persistent State Service

• Concept
• Object states, a conflict with encapsulation?
• Need for persistence
• Persistence-related details of data model without
  implementation language specific features
• PSDL (Persistent State Definition Language),
  declarative, independent of a data store
• Super set of IDL
• Second layer of abstraction between abstract
  interface and concrete implementation
• PSDL compiler
• Datastore
• Storage homes
• Storage home incarnation
• Storage objects
• Storage object incarnations
• Catalog

3
Persistent State Service
Storage Home Incarnation

• Concept

Storage Objects
DataStore
Sessions
Catalog
Storage Homes
Storage Object Incarnation
4
Persistent State Service

• Concept
• Datastores are storage technologies that are used
  to achieve persistence of data.
• Storage types define the representation of
  storage objects
• Storage type specifications are interfaces to
  storage objects
• A storage object incarnation is a programming
  language representation of a storage object
• A storage home manages the life cycle of types of
  storage objects. Its interfaces defined in a
  storage home specification
• Storage home incarnations are programming
  language representations of storage homes.
• A key is an attribute of a storage object that
  uniquely identifies the storage object within its
  storage home.
• A session is a logical connection between a
  datastore and a process that executes a server
  object

5
Storage Objects

• Storage objects
• Have types declared statically in PSDL
• Have individual identities
• Resides in storage homes
• Globally accessible
• Not CORBA objects, cannot be accessed using
  CORBA invocations
• represent the persistent state of individual
  CORBA objects, with which they are associated
• Client representative
• Storage object instance
• Storage object instance can be connected to a
  storage object in the datastore ? incarnation
  changes affect the data in the datastore
• Can be disconnected

6
Storage Homes

• Storage homes
• Have types declared statically in PSDL
• For any given type, there can be only a single
  storage home within a datastore
• For any storage home, it contains storage object
  of one type only
• For any given storagetype, there can only be a
  single storage home in a datastore
• Storage homes and storage objects live in
  datastores
• Storage home client representative
• Instance
• Incarnation
• Catalogs
• Scope for storage homes
• Client representative of datastore
• Can be defined in PSDL

7
Persistent State Service

• CORBA Persistent State Service
• PSSDL
• Superset of IDL
• Extends IDL with mechanisms to specify storage
  objects, storage homes and keys
• Transparent to client
• Programming language bindings storage home and
  storage object specifications are mapped to java
  interfaces.
• Server object programmer subclass the generated
  classes to add specific behavior
• Integration with Object Adapter
• Storage home keys can be used in CORBA object
  identifiers.
• Multiple storage objects can be executed in one
  server process.

8
Persistent State Service

• Persistent State Service Architecture

Server Object Implementation
Storage Object Incarnations Storage Home
Incarnations
Connection Session Management API
Datastore AP
Storage Object Storage Home Schema
Datastore Run-Time
9
Persistent State Service

• Architecture Components and Their Interactions
• Datastore APIs are independent of storage types
• Session management of the persistent state
  service has to be integrated with the session
  management of the datastore
• A schema is a type definition of information that
  is defined in a language specific to the
  datastore
• Server object implementations are independent of
  schemas and thus not dependent on particular
  datastore technology
• Storage object and storage home implementations
  are dependent on datastore technology

10
Persistent State Service

• Generating Architecture Components
• Storage object and storage home incarnations can
  be generated automatically

Storage Home and Storage Type Incarnations
(Prog-Lan)
PSSDL Compiler
Storage Home and Storage Type Specification
(PSSDL)
Storage Home and Storage Type schema
11
Persistent State Service

• Datastore Technology for Persistence
• Persistence in file systems
• CORBA externalization service standardize
  interfaces that enable CORBA objects to write
  their state into a sequential byte stream and to
  read the state from the byte stream.
• No concurrency control and transactional access
• Persistence in relational databases
• Mapping objects into tables
• Object-relational mappings are complicated due to
  the mismatch rational and object-oriented
  paradigms
• Persistence in object database

12
The Persistent State Definition Language

• Declarative language
• Stateful, non-CORBA object
• Superset of IDL
• Abstract storagetypes
• // file employee.psdl
• Abstract storagetype Person
• state string full_name
• readonly state long ssn
• readonly state refltPersongt spouse
• void marry(in refltPersongt the_spouse)
• Abstract storagetype Employee Person
• state string phone_number
• state string office

13
The Persistent State Definition Language

• Embedded storagetype state member itself is a
  storagetype
• Reference types state member references to other
  storagetypes
• Operations declared in an abstract storagetype
  follow the same rule as the operations in IDL
  interface
• Arguments must be storagetypes
• Storagetypes can inherit from each other
• The base is CosPersistentState

14
CosPersistentState

• Module CosPersistentState
• typedef CORBAOctetSeq Pid
• typedef CORBAOctetSeq ShortPid
• abstract storagetype StorageObject
• void destroy_object() // associated object is
  destroyed, not the incarnation
• bolean object_exists()
• Pid get_pid()
• ShortPid get_short_pid()
• StorageHomeBase get_storage_home()

15
Abstract Storage Homes

• abstract storagehome PersonHome of Person
• factory create_person(in string name, in long
  ssn)
• key ssn_key social_security_number
• abstract storagehome EmployeeHome of Employee
  PersonHome
• factory create_employee(in string name, in long
  ssn,
• in string phone, in string office)

16
Catalogs

• Catalogs are the units of session management in
  the PSS
• Applications access datastores through sessions
• Catalogs define the scope of a session in terms
  of the storage homes that can be accessed
• Catalogs can define local operations
• catalog HumanResources
• provides PersonHome persons
• provides EmployeeHome employees

17
Key Points

• Stateless and stateful server objects
• Persistence only needs to be implemented for
  stateful server object
• Persistence is transparent to client
• Persistence state service keeps implementation of
  persistence independent of datastore
• PSDL
• File, relational db, oo db for store object state