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Interprocess
Communication
and
Middleware
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Sockets and Ports
Node 1
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
Node 2
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Crossing Hosts/Platforms
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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Purposes of Middleware
Higher-level abstractions (RPC, RMI, …)
 Location transparency
 Independent of communication protocols
 Independent of hardware/operating
systems
 Use of several programming languages
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Local vs. Remote Modules

Variables
 Variables of

a remote module cannot be directly accessed.
Parameter-passing Mechanisms
 Call
by reference, for input parameters, is not feasible for a
remote procedure/method.

Pointers
 Pointers
of a remote module are not very useful for the
local module.
* For a module in some process, any other module in a
different process, not necessarily a different computer, is a
remote module.
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The Middleware layer
* The operating system includes common network protocols (TCP/IP, …).
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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Marshalling (Serialization) in CORBA CDR
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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Request-Reply Communication
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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Request-Reply Communication Using HTTP
An HTTP request message:
An HTTP reply message:
HTTP is not only a request-reply protocol but also has been used
the transport of other request-reply protocols.
Source of figures: G. Coulouris et al., Distributed Systems: Concepts and Design
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Distributed Objects
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Client/Server: two-tier, three-tier, …
Interfaces and Interface Definition Languages
(IDLs)
Object References (or Identifiers) and Remote
Method Invocation (RMI)
Object Activations and Persistency
Exceptions: timeouts, …
Synchronization, Replication, Migration,
Garbage Collection, …
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Interactions among Distributed Objects
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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A Remote Object and Its Interface
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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A CORBA IDL Example
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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Client Proxy and Server Skeleton
A client proxy is also known as a client stub and server skeleton as server stub.
The remote reference module is mainly for translating between local and remote
object references.
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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The RMI Software
Proxy: the local representative of the
remote object.
 Dispatcher: relays a request to the
appropriate skeleton method.
 Skeleton: unmarshals the request and
invokes the corresponding method in the
remote object.
 These RMI components are generated
automatically by an interface compiler.
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RMI/RPC Semantics and Transparency
Remote invocations may be made syntactically identical to local invocations,
but they have far more implications that both the client and the server designe
have to deal with.
Source of the table: G. Coulouris et al., Distributed Systems: Concepts and Design
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Event-Based Systems

The publish-subscribe paradigm
 publisher
 event
and event type
 subscriber
 notification
Heterogeneity
 Asynchrony

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A Dealing Room System
Dealer’s computer
Dealer
Dealer’s computer
External
source
Notification
Notification
Notification
Information
provider Notification
Notification
Dealer
Notification
Notification
Dealer’s computer
Dealer’s computer
Notification
Information
provider
Notification
Notification
Dealer
Dealer
External
source
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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An Architecture for Event Notification
Event service
subscriber
object of interest
1.
notification
object of interest
2.
object of interest
observer
notification
subscriber
notification
observer
3.
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
subscriber
notification
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The Roles for Observers/Agents
Forwarding
 Filtering
 Patterns of Events
 Notification Mailboxes

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Java Remote Interfaces
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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Parameter Passing in Java

By reference (passed as remote object
references)
 For
parameters whose type is defined as a remote
interface, one that extends Remote
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By value
 For
parameters of serializable types, including
primitive types and classes that implement the
serializable interface
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A Java Server
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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A Java Server (cont.)
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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A Java Client
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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CORBA
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Defined by OMG to facilitate the development of
distributed object-oriented systems.
Language-independency is achieved through
the use of a standard interface definition
language---the CORBA IDL.
An ORB (Object Request Broker) receives
invocations from a client and deliver them to a
target object.
The main communication protocol is GIOP
(General Inter-ORB Protocol), known as IIOP
when implemented over the Internet.
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The CORBA Architecture
*
**
* The implementation repository allows server objects to be activated on dema
** The interface repository gives run-time type information, mainly for dynamic
invocations.
Source of the figure: G. Coulouris et al., Distributed Systems: Concepts and Design
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The Object Adapter
Creates remote object references for
CORBA objects
 Maps the names of CORBA objects to
their servants
 Dispatches each remote invocation via a
skeleton to the appropriate server object
 Activate objects
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CORBA Object Interfaces
Each object has an interface defined in
IDL.
 An interface defines the operations that
can be called by the clients.
 An interface can be implemented in one
language and called from by another.
 The CORBA IDL includes features such as
inheritance of interfaces, exceptions, and
compound data types.
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CORBA Programming with
Java
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Define the interfaces using IDL and compile
them into Java interfaces.
Implement the interfaces with Java classes.
Write a server main function that creates
instances of these classes and then inform
the underlying CORBA implementation.
Register the server.
Write a client main function to connect to the
server and to use server’s objects.
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Shape and Shapelist in CORBA IDL
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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Java Interface Generated from ShapeList
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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Java Implementation of Shapelist
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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Java Implementation of ShapeList (cont.)
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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Java Implementation of a ShapeList Client
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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CORBA Services
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Naming Service
 locate
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objects by their names
Trading Service
 locate
objects by their attributes
Event Service and Notification Service
 Security Service
 Transaction Service
 Persistent Object Service
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The CORBA Naming Service
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Allows (1) a name to be bound to an object and
(2) that object to be found subsequently by
resolving that name.
A name is a sequence of name components and
is resolved within a given naming context.
The IDL interface NamingContext defines the
core of the naming service.
A NamingContext object acts much like a
directory in a filing system.
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CORBA Naming Graph
initial naming context
ShapeList
initial naming context
B
initial naming context
XX
P
V
C
D
E
R
Q
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
S
T
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The NamingContext Interface (partial)
struct NameComponent { string id; string kind; };
typedef sequence <NameComponent> Name;
interface NamingContext {
void bind (in Name n, in Object obj);
binds the given name and remote object reference in my context.
void unbind (in Name n);
removes an existing binding with the given name.
void bind_new_context(in Name n);
creates a new naming context and binds it to a given name in my context.
Object resolve (in Name n);
looks up the name in my context and returns its remote object reference.
void list (in unsigned long how_many, out BindingList bl, out BindingIterator bi);
returns the names in the bindings in my context.
};
Source: G. Coulouris et al., Distributed Systems: Concepts and Design
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COM/DCOM
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COM stands for Component Object Model. Its
distributed version is referred to as DCOM.
It is a programming model for binary
components reuse and a foundation of OLE
(Object Linking and Embedding) and ActiveX
controls.
COM interfaces are defined in the interface
definition language IDL and compiled by
MIDL.EXE.
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COM Objects
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All COM objects implement the IUnknown
interface (defined in unknwn.idl) or one of its
extended interfaces.
Methods of IUnknown:
 QueryInterface: checks
if the named interface is supported
and, if so, returns the corresponding interface reference
 AddRef
 Release
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A COM object may implement multiple interfaces.
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GUIDS
To eliminate name collisions, all COM
interfaces are assigned a unique binary
name at design time that is the physical
name of the interface.
 These physical names are called Globally
Unique Identifiers (GUIDs).
 GUIDs are 128-bit extremely large
numbers that are guaranteed to be unique
in both time and space.
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