Distributed Object-Based Systems

Chapter 9

Distributed Object Based Systems

• Topics discussed in this chapter:– CORBA is an industry-defined standard for

distributed systems.– Distributed COM can be viewed as a

middleware layer implemented on top of various Windows operating systems.

– Globe is a research project being developed as part of a research project on large-scale wide-area distributed systems.

– Comparison of CORBA, DCOM, and Globe

Overview of CORBA• CORBA: Common Object Request Broker

Architecture • Background:

– Developed by the Object Management Group (OMG) in response to industrial demands for object based middleware

– Currently in version #2.4 and #3 – CORBA is a specification: different implementations of

CORBA exist – Very much the work of a committee: there are over 800

members of the OMG and many of them have a say in what CORBA should look like

• CORBA provides a simple distributed object model, with specifications for many supporting services it may be here to stay (for a couple of years)

Overview of CORBA• Architecture of CORBA

– The Object Request Broker (ORB) forms the core of any CORBA distributed system.

– Horizontal facilities consist of general-purpose high-level services that are independent of application domains.

• User interface• Information management• System management• Task management

– Vertical facilities consist of high-level services that are targeted to a specific application domain such as electronic commerce, banking, manufacturing.

Overview of CORBA

The global architecture of CORBA.

Object Model• CORBA has a traditional remote object model

in which an object residing at an object server is remote accessible through proxies

• All CORBA specifications are given by means of interface descriptions, expressed in an Interface Definition Language (IDL).– An interface is a collection of methods, and

objects specify which interfaces they implement.– It provides a precise syntax for expressing methods

and their parameters.

Object Model• CORBA follows an interface based approach to

objects: – Not the objects, but interfaces are the really important

entities – An object may implement one or more interfaces – Interface descriptions can be stored in an interface

repository, and looked up at runtime – Mappings from IDL to specific programming are part of the

CORBA specification (languages include C, C++, Smalltalk, Cobol, Ada, and Java.

• In DCOM and Globe, interfaces can be specified at a lower level in the form of tables, called binary interfaces.

Object Model• Object Request Broker (ORB): CORBA's object

broker that connects clients, objects, and services • Proxy/Skeleton: Precompiled code that takes care of

(un)marshaling invocations and results • Dynamic Invocation/Skeleton Interface (DII/DSI):

To allow clients to construct invocation requests at runtime instead of calling methods at a proxy, and having the server side reconstruct those request into regular method invocations

• Object adapter: Server side code that handles incoming invocation requests.

Object Model• Interface repository:

– Database containing interface definitions and which can be queried at runtime

– Whenever an interface definition is compiled, the IDL compiler assigns a repository identifier to that interface.

• Implementation repository: – Database containing the implementation (code, and

possibly also state) of objects. – Given an object reference, an object adaptor could

contact the implementation repository to find out exactly what needs to be done.

Object Model

The general organization of a CORBA system.

Corba Services

Overview of CORBA services.

Service Description

Collection Facilities for grouping objects into lists, queue, sets, etc.

Query Facilities for querying collections of objects in a declarative manner

Concurrency Facilities to allow concurrent access to shared objects

Transaction Flat and nested transactions on method calls over multiple objects

Event Facilities for asynchronous communication through events

Notification Advanced facilities for event-based asynchronous communication

Externalization Facilities for marshaling and unmarshaling of objects

Life cycle Facilities for creation, deletion, copying, and moving of objects

Licensing Facilities for attaching a license to an object

Naming Facilities for systemwide name of objects

Property Facilities for associating (attribute, value) pairs with objects

Trading Facilities to publish and find the services on object has to offer

Persistence Facilities for persistently storing objects

Relationship Facilities for expressing relationships between objects

Security Mechanisms for secure channels, authorization, and auditing

Time Provides the current time within specified error margins

Communication Models• Object Invocation Models

– Synchronous

– One-way

– Deferred synchronous

• Event and Notification Services – An event is produced by a supplier and received by a

consumer. Events are delivered through an event channel.

– Drawbacks of event services: • The event might be lost.

• Consumers have no way to filter events.

– Filtering capabilities have been added to the notification service.

Object Invocation Models

Invocation models supported in CORBA.

Request type Failure semantics Description

Synchronous At-most-once Caller blocks until a response is returned or an exception is raised

One-way Best effort delivery Caller continues immediately without waiting for any response from the server

Deferred synchronous

At-most-once Caller continues immediately and can later block until response is delivered

Event and Notification Services

The logical organization of suppliers and consumers of events, following the push-style model.

Event and Notification Services

The pull-style model for event delivery in CORBA.

Communication Models

• CORBA supports the message-queuing model through the messaging service.– In callback model, A client provides an object with an

interface containing callback methods which can be called by the underlying communication system to pass the result of an asynchronous invocation.

– In polling model, the client is offered a collection of operations to poll its ORB for incoming result.

• General Inter-ORB Protocol (GIOP) is a standard communication protocol between the client and server.

• Internet Inter-ORB Protocol (IIOP) is a GIOP on top of TCP

Messaging

CORBA's callback model for asynchronous method invocation.

Messaging

CORBA'S polling model for asynchronous method invocation.

Interoperability

GIOP message types.

Message type Originator Description

Request Client Contains an invocation request

Reply Server Contains the response to an invocation

LocateRequest Client Contains a request on the exact location of an object

LocateReply Server Contains location information on an object

CancelRequest Client Indicates client no longer expects a reply

CloseConnection Both Indication that connection will be closed

MessageError Both Contains information on an error

Fragment Both Part (fragment) of a larger message

Processes• CORBA distinguishes two types of processes: clients

and servers.• An interceptor is a mechanism by which an

invocation can be intercepted on its way from client to server, and adapted as necessary before letting it continue.– It is designed to allow proxies to adapt the client-side

software.– Request level: Allows you to modify invocation semantics

(e.g., multicasting) – Message level: Allows you to control message passing

between client and server (e.g., handle reliability and fragmentation)

Clients

Logical placement of interceptors in CORBA.

Portable Object Adaptor• In CORBA, the Portable Object Adapter (POA) is a

component that is responsible for making server-side code appear as CORBA objects to clients.

• A servant is that part of an object that implements the methods that clients can invoke.

• activate_object is a function used by a POA to build a CORBA object from a servant. – It takes a reference to a servant as input parameter and

returns a CORBA object identifier as a result.– The object identifier is used as an index into the POA’s

Active Object Map, which points to servants.– A CORBA object identifier is uniquely associated with a

POA.

Portable Object Adaptor

Mapping of CORBA object identifiers to servants.a) The POA supports multiple servants.b) The POA supports a single servant (multiple instances of the same

servant).

Portable Object Adaptor

Changing a C++ object into a CORBA object.

My_servant *my_object; // Declare a reference to a C++ objectCORBA::Objectid_var oid; // Declare a CORBA identifier

my_object = new MyServant; // Create a new C++ objectoid = poa ->activate_object (my_object);

// Register C++ object as CORBA OBJECT

Agents in CORBA• CORBA adopts a model in which agents from different

kinds of systems can cooperate.• An agent system is defined as platform as a platform

that allows the creation, execution, transfer, and termination of agents.

• A place for an agent corresponds to a server where an agent resides.

• Agent systems can be grouped into a region which represents a administrative domain.

• An agent in CORBA is assumed to be constructed from a collection of classes.

• Each region will have an associated finder (directory service), which allows it to find the location of the agents.

Agents

CORBA's overall model of agents, agent systems, and regions.

Naming• In CORBA, it is essential to distinguish specification

level and implementation level object references – Specification level: An object reference is considered to be

the same as a proxy for the referenced object. Having an object reference means you can directly invoke methods. There is no separate client to object binding phase

– Implementation level: When a client gets an object reference, the implementation ensures that, one way or the other, a proxy for the referenced object is placed in the client's address space.

• Conclusion: Object references in CORBA used to be highly implementation dependent. Different implementations of CORBA could normally not exchange their references.

Interoperable Object References• Recognizing that object references are implementation

dependent, we need a separate referencing mechanism to cross ORB boundaries

• Solution: Object references passed from one ORB to another are transformed by the bridge through which they pass (different transformation schemes can be implemented).

• Passing an object reference A from ORB A to ORB B circumventing the A to B bridge may be useless if ORB B doesn't understand A.

• To allow all kinds of different systems to communicate, we standardize the reference which is called Interoperable Object Reference (IOR) and passed between two different ORBs.

Object References

The organization of an IOR with specific information for IIOP.

Interoperable Object References• Each IOR comprise the following fields:

– The repository identifier is assigned to an interface when that interface is stored in the interface repository.

– The tagged profile contains the complete information to invoke an object.

• There are two possible IOR bindings:– An IOR directly referring to an object is referred to direct

binding.

– The indirect binding request is sent to an implementation repository which acts as a registry. Indirect binding is used primarily for persistent objects.

Object References

Indirect binding in CORBA.

Naming Service

• CORBA offers a naming service that allows clients to look up object references using a character-based name. A name in CORBA is a sequence of name components with the form a (id, kind)-pair where id indicates its name and kind indicates its attribute.

• In most CORBA implementations, object references denote servers at specific hosts. Naming makes it easier to relocate objects

Naming Service• In the naming graph all nodes are objects.

There are no restrictions to binding names to objects. CORBA allows arbitrary naming graphs

• A naming context is an object that stores a table mapping name components to object references.

• There is no single root. An initial context node is returned through a special call to the ORB. the naming service can operate across different ORBs interoperable naming service.

Synchronization• The two most important services that facilitate

synchronization in CORBA are its concurrency control service and its transaction service.

• The two services collaborate to implement distributed and nested transactions using two-phase locking.

• There are two types of objects that can be part of transaction:– A recoverable object is an object that is executed by an

object server capable of participating in a two-phase commit protocol.

– The transactional objects are executed by servers that do not participate in a transaction’s two-phase commit protocol.

Caching and Replication

• CORBA offers no support for generic caching and replication.

• CASCADE is built to provide a generic, scalable mechanism that allows any kind of CORBA object to be cached.

• CASCADE offers a caching service implemented as a large collection of object servers referred to as a Domain Caching Server (DCS).

• Each DCS is an object server running on a CORBA ORB. The collection of DCSs may be spread across the Internet.

Caching and Replication

The (simplified) organization of a DCS.

Fault Tolerance • In CORBA version 3, fault tolerance is addressed. The

basic approach for fault tolerance is to replicate objects into object groups.

• Masking failures is achieved through replication by putting objects into object groups. Object groups are transparent to clients. They appear as normal objects.

• This approach requires a separate type of object reference: Interoperable Object Group Reference (IOGR).

• IOGRs have the same structure as IORs. The main difference is that they are used differently. In IORs an additional profile is used as an alternative. In IOGR, it denotes another replica.

Object Groups

A possible organization of an IOGR for an object group having a primary and backups.

Fault Tolerance • To support object groups and to handle

additional failure management in the Eteral system, it is necessary to add components to CORBA:– A replication manager is responsible for creating

and managing a group of replicated objects.– An interceptor intercepts and passes the invocation

to a separate replicas.

• Invocation are subsequently sent to the other group members using reliable, totally-ordered multicasting.

An Example Architecture

An example architecture of a fault-tolerant CORBA system.

Security• The underlying idea is to allow the client and

object to be mostly unaware of all the security policies, except perhaps at binding time. The ORB does the rest.

• Specific policies are passed to the ORB as (local) policy objects and are invoked when necessary. – Examples: Type of message protection, lists of

trusted parties.

• A replaceable security service is a service which can be specified by means of standard interfaces that hide the implementation.

Security

The general organization for secure object invocation in CORBA.

Security• Replaceable security service can be implemented in

combination with two different interceptors:– The access control interceptor is a request-level

interceptor that checks the access right associated with an invocation.

– A secure invocation interceptor is a message-level interceptor takes care of implementing the message protection.

• The secure invocation interceptor sets up a security context (security information) for the client.

• A security association will be established after the client is bound to the target object.

• The value object sets up the security association with a standardized interface.

Security

The role of security interceptors in CORBA.

Distributed COM • DCOM: Distributed Component Object Model

– Microsoft's solution to establishing inter process communication, possibly across machine boundaries.

– Supports a primitive notion of distributed objects – Evolved from early Windows versions to current

NT based systems (including Windows 2000) – Comparable to CORBA's object request broker

DCOM Overview• Somewhat confused? DCOM is related to

many things that have been introduced by Microsoft in the past couple of years:

• DCOM: Adds facilities to communicate across process and machine boundaries.

• SCM: Service Control Manager, responsible for activating objects (cf., to CORBA's implementation repository).

• Proxy marshaler: handles the way that object references are passed between different machines.

Overview of DCOM

The general organization of ActiveX, OLE, and COM.

COM Object Model• An interface is a collection of semantically related

operations • Each interface is typed, and therefore has a globally

unique interface identifier • A client always requests an implementation of an

interface: – Locate a class that implements the interface – Instantiate that class, i.e., create an object – Throw the object away when the client is done

• Note: COM+ is effectively COM plus services that were previously available in an ad hoc fashion

Object Model

The difference between language-defined and binary interfaces.

Tape Library and Registry

The overall architecture of DCOM.

DCOM Services

Overview of DCOM services in comparison to CORBA services.

CORBA Service DCOM/COM+ Service Windows 2000 Service

Collection ActiveX Data Objects -

Query None -

Concurrency Thread concurrency -

Transaction COM+ Automatic Transactions Distributed Transaction Coordinator

Event COM+ Events -

Notification COM+ Events -

Externalization Marshaling utilities -

Life cycle Class factories, JIT activation -

Licensing Special class factories -

Naming Monikers Active Directory

Property None Active Directory

Trading None Active Directory

Persistence Structured storage Database access

Relationship None Database access

Security Authorization SSL, Kerberos

Time None None

Communication Models• Object invocations: Synchronous remote method calls with at

most once semantics. Asynchronous invocations are supported through a polling model, as in CORBA.

• Event communication: Similar to CORBA's push style model: • Messaging: Completely analogous to CORBA messaging. • Observation: Objects are referenced by means of a local

interface pointer. The question is how such pointers can be passed between different machines:

• Question: Where does the proxy marshaler come from? Do we always need it?

Events

Event processing in DCOM.

Clients

Passing an object reference in DCOM with custom marshaling.

Naming: Monikers• Observation: DCOM can handle only objects as tem porary

instances of a class. To accommodate objects that can outlive their client, something else is needed.

• Moniker: A hack to support real objects – A moniker associates data (e.g., a file), with an application

or program – Monikers can be stored – A moniker can contain a binding protocol, specifying how

the associated program should be ``launched'' with respect to the data.

Monikers

Binding to a DCOM object by means of file moniker.

Step Performer Description

1 Client Calls BindMoniker at moniker

2 MonikerLooks up associated CLSID and instructs SCM to create object

3 SCM Loads class object

4 Class objectCreates object and returns interface pointer to moniker

5 Moniker Instructs object to load previously stored state

6 Object Loads its state from file

7 Moniker Returns interface pointer of object to client

Monikers

DCOM-defined moniker types.

Moniker type Description

File moniker Reference to an object constructed from a file

URL moniker Reference to an object constructed from a URL

Class moniker Reference to a class object

Composite moniker Reference to a composition of monikers

Item moniker Reference to a moniker in a composition

Pointer moniker Reference to an object in a remote process

Active Directory• Essence: a worldwide distributed directory

service, but one that does not provide location transparency.

• Basics: Associate a directory service (called domain controller) with each domain; look up the controller using a normal DNS query:

• Note: Controller is implemented as an LDAP server

Active Directory

The general organization of Active Directory.

Fault Tolerance• Automatic transactions: Each class object

(from which objects are created), has a transaction attribute that determines how its objects behave as part of a transaction:

• Note: Transactions are essentially executed at the level of a method invocation.

Fault Tolerance

Transaction attribute values for DCOM objects.

Attribute value Description

REQUIRES_NEW A new transaction is always started at each invocation

REQUIRED A new transaction is started if not already done so

SUPPORTED Join a transaction only if caller is already part of one

NOT_SUPPORTED Never join a transaction

DISABLED Never join a transaction, even if told to do so

Security • Declarative security: Register per object what

the system should enforce with respect to authentication. Authentication is associated with users and user groups. There are different authentication levels:

• Delegation: A server can impersonate a client depending on a level:

• Note: There is also support for programmatic security by which security levels can be set by an application, as well as the required security services (see book).

Declarative Security

Authentication levels in DCOM.

Authentication level Description

NONE No authentication is required

CONNECT Authenticate client when first connected to server

CALL Authenticate client at each invocation

PACKET Authenticate all data packets

PACKET_INTEGRITY Authenticate data packets and do integrity check

PACKET_PRIVACY Authenticate, integrity-check, and encrypt data packets

Declarative Security

Impersonation levels in DCOM.

Impersonation level Description

ANONYMOUS The client is completely anonymous to the server

IDENTIFYThe server knows the client and can do access control checks

IMPERSONATE The server can invoke local objects on behalf of the client

DELEGATE The server can invoke remote objects on behalf of the client

Programmatic Security

a) Default authentication services supported in DCOM.b) Default authorization services supported in DCOM.

Service Description

NONE No authentication

DCE_PRIVATE DCE authentication based on shared keys

DCE_PUBLIC DEC authentication based on public keys

WINNT Windows NT security

GSS_KERBEROS Kerberos authentication

(a)

Service Description

NONE No authorization

NAME Authorization based on the client's identity

DCE Authorization using DEC Privilege Attribute Certificates (PACs)

(b)

Globe• Experimental wide area system currently being

developed at Vrije Universiteit

• Unique for its focus on scalability by means of truly distributed objects

• Prototype version up and running across multiple machines distributed in NL and across Europe and the US.

Object Model• Essence: A Globe object is a physically distributed

shared object: the object's state may be physically distributed across several machines

• Local object: A nondistributed object residing a single address space, often representing a distributed shared object

• Contact point: A point where clients can contact the distributed object; each contact point is described through a contact address

• Observation: Globe attempts to separate functionality from distribution by distinguishing different local subobjects:

Globe Object Model

The organization of a Globe distributed shared object.

Globe - Object Model• Semantics subobject: Contains the methods that

implement the functionality of the distributed shared object

• Communication subobject: Provides a (relatively simple), network independent interface for communication between local objects

• Replication subobject: Contains the implementation of an object specific consistency protocol that controls exactly when a method on the semantics subobject may be invoked

• Control subobject: Connects the user defined interfaces of the semantics subobject to the generic, predefined interfaces of the replication subobject

Globe Object Model

The general organization of a local object for distributed shared objects in Globe.

Globe Object Model

Interfaces implemented by the semantics subobject of a GlobeDoc object.

Document Interface

Method Description

AddElement Add an element to the current set of elements

DeleteElement Remove an element from the Web document

AllElements Return a list of the elements currently in the document

SetRoot Set the root element

GetRoot Return a reference to the root element

Content Interface

Method Description

GetCotent Return the content of an element as an array of bytes

PutContent Replace the content of an element with a given array of bytes

PutAllContent Replace the content of an entire document

Globe Object Model

Interfaces implemented by the semantics subobject of a GlobeDoc Object.

Property Interface

Method Description

GetProperties Return the list of (attribute, value)-pairs of an element

SetProperties Provide a list of (attribute, value)-pairs for an element

Lock Interface

Method Description

CheckOutElements Check out a series of elements that require modification

CheckInElements Check in a series of modified elements

GetCheckedElements Get a list of elements that are currently checked out

Process-to-Object Binding

Binding a process to an object in Globe.

Globe Services

Overview of possible Globe implementations of typical distributes-systems services.

Service Possible Implementation in Globe Available

Collection Separate object that holds references to other objects No

Concurrency Each object implements its own concurrency control strategy No

Transaction Separate object representing a transaction manager No

Event/Notification Separate object per group of events (as in DCOM) No

Externalization Each object implements its own marshaling routines Yes

Life cycle Separate class objects combined with per-object implementations Yes

Licensing Implemented by each object separately No

Naming Separate service, implemented by a collection of naming objects Yes

Property/Trading Separate service, implemented by a collection of directory objects No

Persistence Implemented on a per-object basis Yes

Security Implemented per object, combined with (local) security services Yes

Replication Implemented on a per-object basis Yes

Fault tolerance Implemented per object combined with fault-tolerant services Yes

Communication

Invoking an object in Globe that uses active replication.

Globe Server

Operations on a Globe object server.

Method Description

Bind Lets the server bind to a given object, unless it is already bound

AddBinding Lets the server bind to an object, even if it is already bound

CreateLR Lets the server create a local object for a new distributed object

RemoveLR Lets the server remove a local object of a given object

UnbindDSO Lets the server remove all local objects of a given object

ListAll Returns a list of all local objects

ListDSO Returns a list of all local objects for a given objects

StatLR Get the status of a specific local object

Object References • Observation: Globe's contact addresses correspond to

CORBA's object references • Essence: Globe uses location independent object

handles which are to be resolved to contact addresses (which describes where and how an object can be contacted).

• Associated with a contact point of the distributed object.

• Specifies (for example) a transport level network address to which the object will listen

• Contains an implementation handle, specifying exactly what the client should implement if it wants to communicate through the contact point.

Object References and Contact Addresses (1)

The representation of a protocol layer in a stacked contact address.

Field Description

Protocol identifier A constant representing a (known) protocol

Protocol address A protocol-specific address

Implementation handle Reference to a file in a class repository

Object References and Contact Addresses (2)

The representation of an instance contact address.

Field Description

Implementation handle Reference to a file in a class repository

Initialization string String that is used to initialize an implementation

Naming Objects• Observation: Objects in Globe have their own object

specific implementations; there is no ``standard'' proxy that is implemented for all clients

• Observation: Globe separates naming from locating objects (as described in Chapter 04). The current naming service is based on DNS, using TXT records for storing object handles

• Observation: The location service is implemented as a generic, hierarchical tree, similar to the approach explained in Chapter 04.

Globe Naming Service

Iterative DNS-based name resolution in Globe.

Caching and Replication• Observation: Here's where Globe differs from

many other systems: – The organization of a local object is such that

replication is inherently part of each distributed shared object

– All replication subobjects have the same interface:– This approach allows to implement any object

specific caching/replication strategy

Replication (1)

The interface of the replication subobject as made available to the control subobject.

Method Description

Start Indicate that a new method invocation has been locally requested

Send Pass the marshaled invocation request to the replication subobject

Invoked Indicate that the invocation on the semantics object has completed

Replication (2)

The behavior of the control subobject as a finite state machine.

Examples of Replication in Globe (1)

State transitions and actions for active replication.

Read method

State Action to take Method call Next state

START None Start INVOKE

INVOKE Invoke local method Invoked RETURN

RETURN Return results to caller None START

Modify method

State Action to take Method call Next state

START None Start SEND

SEND Pass marshaled invocations Send INVOKE

INVOKE invoke local method Invoked RETURN

RETURN Return results to caller None START

Examples of Replication in Globe (2)

State transitions and actions with primary-backup replication.

Read method

State Action to take Method call Next state

START None Start INVOKE

INVOKE Invoke local method Invoked RETURN

RETURN Return results to caller None START

Modify method at backup replica

State Action to take Method call Next state

START None Start SEND

SEND Pass marshaled invocation Send RETURN

RETURN Return results to caller None START

Modify method at primary replica

State Action to take Method call Next state

START none Start INVOKE

INVOKE invoke local method Invoked RETURN

RETURN Return results to caller None START

Security

• Essence: Additional security subobject checks for authorized communication, invocation, and parameter values. Globe can be integrated with existing security services:

Security (1)

The position of a security subobject in a Globe local object.

Security (2)

Using Kerberos to establish secure distributed shared objects.

Comparison

Comparison of CORBA, DCOM, and Globe.

Issue CORBA DCOM Globe

Design goals Interoperability Functionality Scalability

Object model Remote objects Remote objects Distributed objects

Services Many of its own From environment Few

Interfaces IDL based Binary Binary

Sync. communication Yes Yes Yes

Async. communication Yes Yes No

Callbacks Yes Yes No

Events Yes Yes No

Messaging Yes Yes No

Object server Flexible (POA) Hard-coded Object dependent

Directory service Yes Yes No

Trading service yes No No

Continued …

Comparison

Comparison of CORBA, DCOM, and Globe.

Issue CORBA DCOM Globe

Naming service Yes Yes Yes

Location service No No Yes

Object reference Object's location Interface pointer True identifier

Synchronization Transactions Transactions Only intra-object

Replication support Separate server None Separate subobject

Transactions Yes Yes No

Fault tolerance By replication By transactions By replication

Recovery support Yes By transactions No

Security Various mechanisms Various mechanisms More work needed