3 Eloadas Corba Poa

8/10/2019 3 Eloadas Corba Poa

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Portable Object Adaptor

Martin Senger

[email protected]


2/31

Significant news in CORBA

Objects By Value

CORBA Components

CORBA scripting

Metadata and modeling

Quality of Service Portable Object Adaptor

BOA deprecated now


3/31

CORBA Object Adaptors

a piece of CORBA architecture dispatching

client requests to theservants(implementations)

object adaptors are only on the server side

key functions are

they create CORBA object references

they ensure that target objects are incarnated by

servants

they direct requests to the servants (they connect orb

with the servants - locating and marshaling)

remember this term,use this term

remember: object references and

servants are two different things


4/31

BOA and POA

BOA was the original CORBA object adaptor

BOA was under-specified and therefore less

portable between ORBs

BOA had no servant management

BOA has been removed from CORBA!

POA is everything else

portable

providing many more features


5/31

POA abstract model

Client Server

servantPOA

POA

ORB

Object Id

logical connection

to the targetCORBA object

ObjectReference

servant

servant


6/31

Three key Entities[CORBA Object is a programming entity with an identity, an

interface and an implementation]

Servant

a programming language object implementing requests on

one or more CORBA objects (modifier native in IDL)

Object Id

a value identifying a particular CORBA object within the

scope of its POA

Ids are hidden from clients, encapsulated by objectreferences

Object Reference

a usual reference to a CORBA object

encapsulates an Object Id and a POA identity


7/31

A simplified picture

ORBPOA

Manager

POAPOAPOA

ServantServant

Servant

ServantServant

Servant

ServantServant

Servant

Server Application

Incoming

requests


8/31

How to code Server

import org.omg.CORBA.*;

// Initialize ORB and BOA

ORB orb = ORB.init (args, props);BOA boa = orb.BOA_init(args, props);

// Create a servant and activate itHelloWorldImpl hw = new HelloWorldImpl();

// Wait for requests

boa.impl_is_ready(null);with BOA

import org.omg.CORBA.*;

import org.omg.PortableServer.*;

// Initialize ORB and POA

ORB orb = ORB.init(args, props);POA rootPOA = POAHelper.narrow(

orb.resolve_initial_references("RootPOA"));

// Get a reference to the POA manager

POAManager manager = rootPOA.the_POAManager();

// Create a servant and activate itHelloWorldImpl hwImpl = new HelloWorldImpl();

HelloWorld hw = hwImpl._this(orb);

// Wait for incoming requests ("run the implementation")

manager.activate();

orb.run(); with POA


9/31

Server

overview

CORBA.Object

HelloWorld

CORBA.

_HelloWorldImplBase

HelloWorldImplget_text()

extends

extends

implements

extends

The old way - with BOA

HelloWorld

Operations

PortableServer.Servant

HelloWorldPOA

HelloWorldImpl

get_text()

extends

extends

implements

extends

HelloWorld

CORBA.Object

portable.IDLEntity

extends

The new way - with POA

this is a servant

this is an interfaceof the same type as

the object reference

How to connect

your implementation

classes withthe generated code

_this() or

activate_object()


10/31

ORB

Yet another picture

extract POA name

from Object Key

POA

find

POA

Object Reference

(with Object Key)request

from a client

or

Adaptor Activator

call Adaptor

Activator if

POA not found

create POA

extract Object Id

from Object Key Active Object Map

Object ID

Object ID

Object ID

Servant

Servant

incarnate

servant

Default Servant

or

Servant Manager

or

Servant

Activator

Servant

Locator

create

servant

update

map


11/31

POA Policies

there can be more POAs and each of them

can have different features =policies

the policies are given to the POA in itscreation time, they are not changeablelater

the policies are not inheritedfrom parent

POA to its children POA the policies are locally-constrainedobjects

they cannot be passed as arguments to normal

CORBA operations, or be converted to strings


12/31

policy:CORBA Object Life Span

TRANSIENT

the objects created in the POA cannot outlive

this POA

they are more modest regarding resources

PERSISTENT

the objects can outlive the process in whichthey were first created

their POA name is stored in the object

references

enum LifespanPolicyValue { TRANSIENT, PERSISTENT };


13/31

policy:Object Id Uniqueness

UNIQUE_ID

each servant support exactly one Object Id

MULTIPLE_ID

a servant may support one or more Object Ids

good, for example, for many database objects

which may all be served by a single servant(assuming that the state of the objects is kept in

the database)

enum IdUniquenessPolicyValue { UNIQUE_ID, MULTIPLE_ID };


14/31

policy:Id Assignment

USER_ID

Object IDs are assigned by the application, not

by the POAtypical for persistent objects (the same ID is

used for keeping track where the object state is

stored)

SYSTEM_ID

Object IDs are assigned by the POA

enum IdAssignmentPolicyValue { USER_ID, SYSTEM_ID };


15/31

policy:Request Processing

USE_ACTIVE_OBJECT_MAP_ONLY

an Object Id must be found in the active object map

USE_DEFAULT_SERVANT

if an Object Id is not found in the active object map

the request is dispatched to the default servant

USE_SERVANT_MANAGERif an Object Id is not found in the active object map

the servant manager gets control

enum RequestProcessingPolicyValue {

USE_ACTIVE_OBJECT_MAP_ONLY, USE_DEFAULT_SERVANT, USE_SERVANT_MANAGER };


16/31

policy:ObjectId to Servant Associations

RETAIN

the POA will keep active servants in its active

object mapvarious combinations of this policy and Request

Processing policy

NON_RETAINservants are not retained by the POA

it is up to the application to keep track about active

(existing) servants

enum ServantRetentionPolicyValue { RETAIN, NON_RETAIN };


17/31

policy:Implicit Activation

IMPLICIT_ACTIVATION

the POA will support implicit activation of servants

also requires SYSTEM_ID and RETAIN policies

implicit activation is done (in Java) using a shortcut

method _this

NO_IMPLICIT_ACTIVATIONthe POA will not support implicit activation of

servant

typically used to avoid accidental object creation

enum ImplicitActivationPolicyValue { IMPLICIT_ACTIVATION, NON_IMPLICIT_ACTIVATION };


18/31

policy:Thread

ORB_CTRL_MODEL

servants can be called in parallel from separate

threads (created and controlled by the ORB)

servants must be written as thread-aware

SINGLE_THREAD_MODEL

a model which guarantees that all requests for

all objects in this POA will be dispatchedsequentially

but the application still can have more single-threaded

POAs running in parallel (in separate threads)

enum ThreadPolicyValue { ORB_CTRL_MODEL, SINGLE_THREAD_MODEL };


19/31

How to create policies

each policy has its own factory:

an example how to use the policy factory:

typically, all necessary policy objects are passedin a PolicyListto the POA creation function

and then invoke destroyon the policy objects

interface POA {

LifespanPolicy create_lifespan_policy (

in LifespanPolicyValue value);

};

Policy policy =

parentPOA.create_lifespan_policy (LifespanPolicyValue.ORB_CTRL_MODEL);


20/31

ORB

Yet another picture(now lets show where

different policies apply)

extract POA name

from Object Key

POA

find

POA

Object Reference

(with Object Key)request

from a client

or

Adaptor Activator

call Adaptor

Activator if

POA not found

create POA

extract Object Id

from Object Key Active Object Map

Object ID

Object ID

Object IDServant

Servant

incarnate

servant

Default Servant

or

Servant Manager

or

Servant

Activator

Servant

Locator

create

servant

update

map


21/31

POA Creation

each POA is created as a child of another

POA

all server applications have a RootPOA

POAs are created in a tree, in a hierarchy

each POA has its own namespace for its

Object Ids

remember: if you need a different policy you

need a new POA


22/31

code:POA Creation

interface POA {

POA create_POA (

in string adapter_name,

in POAManager manager,

in CORBA::PolicyList policies)

raises (AdapterAlreadyExists, InvalidPolicy);};

// get a reference to the Root POA

POA rootPOA = POAHelper.narrow (orb.resolve_initial_references ("RootPOA"));

// get a reference to the POA manager

POAManager manager = rootPOA.the_POAManager();

// create an empty PolicyList for child POA

Policy[] policies = new Policy [0];

// create the child POA

POA child = rootPOA.create_POA (child, manager, policies);


23/31

Activation: dont be confuse...

Object activation is not the same as

Server activation

Server activation is about how to start aseparate process on the server side

also called: Implementation repository issue (see

another talk)

Object activation is about how to make objects

visible within already running process


24/31

Object creation and activation

POA provides several options for creating

objects and activating them

an application can create objects without

creating any servants

a servant can be explicitly or implicitly

registered to incarnate an object - and POA can

remember it in its Active Object Map

an application can supply servant managers

an application can provide a default servant


25/31

Creation and activation methods

Object create_reference(in CORBA::RepositoryId intf)

raises (WrongPolicy);

Object create_reference_with_id(in ObjectId oid,

in CORBA::RepositoryId intf)raises (WrongPOlicy);

Creating CORBA Objectswithout creating servants

ServantManager get_servant_manager()

raises (WrongPolicy);

void set_servant_manager(in ServantManager imgr)

raises (WrongPOlicy);

Registration ofServant managers

Servant get_servant()

raises (NoServant, WrongPolicy);

void set_servant(in Servant serv)

raises (WrongPOlicy);

Registration ofDefault Servant

ObjectId activate_object(in Servant serv)

raises (ServantAlreadyActive,

WrongPolicy);

void activate_object_with_id(in ObjectId oid,in Servant serv)

raises (ServantAlreadyActive,

ObjectAlreadyActive, WrongPolicy);

Object activation


26/31

Conversions among key entitiesObject Reference

Object Id

Servant

ObjectId reference_to_id(in Object ref)

raises (WrongAdapter, WrongPolicy);

Object id_to_reference(in ObjectId oid)

raises (ObjectNotActive, WrongPolicy);

ObjectId servant_to_id(in Servant serv)raises (ServantNotActive, WrongPolicy);

Servant id_to_servant(in ObjectId oid)


Servant reference_to_servant(in Object ref)

raises (ObjectNotActive, WrongAdapter, WrongPolicy);

Object servant_to_reference(in Servant serv)

raises (ServantNotActive, WrongPolicy);


27/31

Servant managers

Used when an explicit servant registration is

difficult or impossible e.g. dealing with too many objects

or finding objects dynamically

Servant Activator Servant incarnate (in ObjectId oid, in POA adapter)

void etherealize (in ObjectId oid, in POA adapter, )

the resulting servant retains in the Active Object Map

Servant Locator Servant preinvoke (in ObjectId oid, in POA adapter, in CORBA::Identifier operation, out Cookie the_cookie)

void postinvoke (in ObjectId oid, in POA adapter, in CORBA::Identifier operation, out Cookie the_cookie,)

the resulting servant does not appear in the AOM


28/31

Common for both Servant managers

The incarnate()and preinvoke()may raise any system

exception

e.g. OBJECT_NOT_EXIST if the object corresponding

to the Object Id cannot be incarnated

SeqFactory.get_sequence()normally would raise at once exceptionWrongAccessionbut if the real access to the database is postponed

some other exception must be used

Both operations may also raise a ForwardRequest

then the ORB is responsible for delivering the current

and subsequent requests to the object denoted in

forward_referencemember


29/31

Default Servant

a single Servant for many CORBA objects

typically of the same type

ideally suited to objects in large databases

the invoking POA and ObjectId isaccessible using Currentinterface

org.omg.CORBA obj =

orb.resolve_initial_references (POACurrent"));

Current current= CurrentHelper.narrow (obj);

How to get Current

interface Current {

POA get_POA();

ObjectID get_object_id();

};

What to get from Current


30/31

Object deactivation

deactivation is breaking object-to-servantassociation

after deactivation the object is not visible

any more but the corresponding servant can still be available and

may be activated later again

an administrative tool for controlling thecontents of the Active Object Map

void deactivate_object(in ObjectId oid)



31/31

Notes on hands-on

POA/stage1plain differences between BOA and POA

POA/stage2-persistent

create persistent object reference POA/stage3-servantActivator

create a factory for sequence references,

the servants are created later only when amethod on a sequence reference is called

3 Eloadas Corba Poa

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