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Transcript
Understanding Networked Applications:
A First Course
Chapter 16
by
David G. Messerschmitt
Layering a computing
infrastructure
Applications
Application components
Middleware
Operating system
Network
Understanding Networked Applications
2
A First Course
Spanning layer
Application
Distributed object management
Windows NT
TCP UDP
Mac OS
UNIX
TCP UDP
TCP UDP
Internet protocol
Network 1
Network 2
Spanning layers
Understanding Networked Applications
3
A First Course
The new infrastructure:
middleware
• Layer of software between OS and
application
– Hides heterogeneity
– Provides generic common services
– Increases level of abstraction
• By its nature, not single platform, nor
bundled with equipment
– Microsoft attempts to be an exception
Understanding Networked Applications
4
A First Course
The new infrastructure
• Middleware is
– where new capabilities are added by layering
– where much of the experimentation and
innovation happens in the infrastructure
– where the successful approaches have a chance
to become a spanning layer and/or integrated
into a distributed OS
• Boundary to a distributed OS is fuzzy
Understanding Networked Applications
5
A First Course
Middleware objectives
• Hide heterogeneity
• Location independence
• Common functionality needed by many
applications
• Software portability and mobile code
• Help integrate legacy facilities
• Aid application interoperability
• Aid scalability
Understanding Networked Applications
6
A First Course
Some middleware categories
• Transaction processing
– Simplify the coordination of complementary
resource managers
• Message-oriented middleware
– Support message and queuing capabilities
where resource mangers are not available
simultaneously (like workflow)
Understanding Networked Applications
7
A First Course
Some middleware categories
• Distributed object management
– Support applications that are distributed across
heterogeneous platforms and organizations
• Mobile code
– Allow application code to be moved and
executed on heterogeneous platforms
– Without prior software installation
Understanding Networked Applications
8
A First Course
Infrastructure software today
• With networks, new emphasis on:
– Portability: applications run across multiple
platforms (avoid lock-in)
– Interoperability: pieces of application must
work together (benefit from network effects)
What are some examples of each?
Understanding Networked Applications
9
A First Course
Understanding Networked Applications:
A First Course
Transaction processing
by
David G. Messerschmitt
The transaction
Durable
starting
state
Collection of resource actions
Abort
Successful
completion
Durable,
consistent,
ending state
Rollback
Understanding Networked Applications
11
A First Course
Transaction architecture
Join
Application logic
Transaction
manager
Prepare,
commit,
abort
Resource managers
Understanding Networked Applications
12
A First Course
Commit or abort
One or
more “no’s”
Phase 2
Transaction
manager
abort()
Phase 1
Transaction
manager
Rollback
Transaction
manager
prepare() yes_or_no
commit()
All “yes”
Understanding Networked Applications
13
A First Course
Atomic series of resource actions
Client_1
Client_2
Server
Starting state
Inconsistent
state
Group of RMI’s
Final state
Understanding Networked Applications
14
A First Course
Locking to prevent conflicts
Client_1
Client_2
Server
Lock
Starting state
Group of RMI’s
Refused
Final state
Unlock
Understanding Networked Applications
15
A First Course
Abort
Client_1
Client_2
Server
Starting state
Group of RMI’s
Something goes awry
Inconsistent
state
Final state
Understanding Networked Applications
16
A First Course
Rollback
Client_1
Client_2
Server
Starting state
Rollback
Group of RMI’s
Something goes awry
Understanding Networked Applications
17
A First Course
Transaction protocol
Application server
Resource manager
request(tp_ID,….)
Transaction manager
join(tp_ID)
More requests…. Lock
commit?(tp_ID)
commit_or_abort(tp_ID)
Commit or rollback
Understanding Networked Applications
18
A First Course
Understanding Networked Applications:
A First Course
Mobile code and Java
by
David G. Messerschmitt
Reminder: two key requirements
• With networks, new emphasis on:
– Portability: applications run across multiple
platforms (avoid lock-in)
– Interoperability: pieces of application must
work together (benefit from network effects)
Understanding Networked Applications
20
A First Course
Dynamic portability: mobile code
 Send code (as a message) to a host
MC
Mobile code:
Code representing
a software program that
can be moved to
heterogeneous
platforms and executed
there
Understanding Networked Applications
 Execute the program
represented by that code
21
A First Course
Portability can aid
interoperability
MC
Mobile code originating from a common
source can enhance interoperability
Understanding Networked Applications
22
A First Course
Java
• Portability
– “Write once, run anywhere”
• Programming productivity
– Garbage collection (no memory leaks)
– Multi-threaded
• Scalability
– Move execution cycles
• Interoperability
– Software components come from common repository
Understanding Networked Applications
23
A First Course
Mobile code:
Code representing
a software program
MC
Idea of mobile code
 Send as a message to a host
MA
Mobile agent:
Code and data
representing an object
or component
Understanding Networked Applications
 Execute the program
24
A First Course
Some mobile code advantages
Executing program
closer to user can
enhance interactivity
MC
Mobile code originating
from a common source
can enhance
interoperability and
bypass network effects
MC
Shifting location of
computation can
enhance scalability
Understanding Networked Applications
25
A First Course
Idea of mobile agents
 Agent executes in each
host, modifying its state
 Agent launched
MA
 Agent returns
Understanding Networked Applications
26
A First Course
Java virtual machine
Java program
Mobile code
Compilation
JIT
compiler
VM
interpreter
Bytecode:
low level but
machine independent
Native machine
instructions
Understanding Networked Applications
27
A First Course
VM as spanning layer
Applications
JavaBeans
component framework
Java VM spanning layer
Java virtual machine
Operating system
Understanding Networked Applications
28
A First Course
SUN/Java strategy
• License Java freely, even to rival Microsoft
– Why?
• License terms give Sun a modicum of
control over the “standard”
– Why?
• How does Sun expect to make money?
Understanding Networked Applications
29
A First Course
Understanding Networked Applications:
A First Course
Distributed object management
by
David G. Messerschmitt
Distributed object management
• Emphasis is on interoperability
– Allows objects on one host to invoke methods of
objects on another host
– Platform, language independent
• CORBA vs DCOM
• Portability is not the emphasis
Understanding Networked Applications
31
A First Course
Interoperability
Client
Server
Client object can invoke methods of server object,
even if they are running on different platforms,
written in different languages
What else is needed?
Understanding Networked Applications
32
A First Course
Interoperability
Client
Server
Interoperability also requires:
• Common structure of data
• Common interpretation of data
• Agreement on protocols
Understanding Networked Applications
33
A First Course
Before and after
Distributed application
OS 1
OS 2
OS 3
IP
Distributed application
Distributed object management middleware
OS 1
OS 2
OS 3
Potential
spanning
layer
Spanning
layer
IP
Understanding Networked Applications
34
A First Course
What is the acronym?
• Common Object Request Broker
Architecture
or
• Concerned Off-Road Bicyclist Association?
Understanding Networked Applications
35
A First Course
CORBA architecture
Client
objects
Server objects
Object
adapter
Object request broker core
Interface
repository
Understanding Networked Applications
CORBA services
36
A First Course
Protocol layer
Location-independent
application
Application
Object Management
Group CORBA
standard
Object request broker
Internet Inter-ORB Protocol (IIOP)
User datagram
protocol (UDP)
Transmission control
protocol (TCP)
Internet protocol (IP)
Subnetworks
Understanding Networked Applications
37
A First Course
Portability not promised
Location-dependent application
Application 1
Interoperability
ORB 1
Application 2
ORB 2
Internet Inter-ORB Protocol (IIOP)
CORBA standard does not insure
ORB-to-ORB portability
Understanding Networked Applications
38
A First Course
OMG process
• Identify need
• Request for proposals
• Process to
– choose best
– or ask proposal advocates to work together
Understanding Networked Applications
39
A First Course
Which is most effective?
• Industry de facto standard effort (CORBA)
or
• Single vendor integrated solution (DCOM)?
Understanding Networked Applications
40
A First Course
CORBA vs DCOM
CORBA
• Integrate best ideas
• Multi-vendor support
• Cross-platform and
language
Understanding Networked Applications
DCOM
• Fast, no consensus
required
• No vendor
interoperability issues
41
A First Course
Two methods for application
interaction
• CORBA and DCOM
• Exchange documents (XML)
What are their relative merits?
Understanding Networked Applications
42
A First Course
CORBA vs XML
CORBA
XML
• Natural OOP extension
• Flexible data-sharing
• No document
interpretation
• Good for document-like
objects
• Good for back-andforth protocols
• Natural cross-platform
capability
Both have need for
standardization of data or
document interpretation
• No protocol standardization
Understanding Networked Applications
43
A First Course
What CORBA provides
• Java
– Language bindings
– Transportable objects
• “Inter-galactic” software bus
– Cross-platform and language
– Interoperability (but not portability)
• High levels of abstraction
– Remote method invocation on objects
Understanding Networked Applications
44
A First Course
What CORBA provides (con’t)
• Run-time flexibility
– Everything self-describing
– Interface discovery
– Dynamic data structures and binding
• Useful services
– Naming
– Security
– Many others
Understanding Networked Applications
45
A First Course
Importance of CORBA
• Inter-enterprise computing
– Platform and language independence
– Electronic commerce, network management,
etc
• Reduction of network effects
– Another spanning layer
– Significance of platform reduced
Understanding Networked Applications
46
A First Course
Are Java and CORBA
competitive or complementary?
• Both offer interoperability across different
platforms
• Java offers portability and transportability
• CORBA offers heterogeneous language bindings
• CORBA offers many services, metadata, etc.
• Bottom line: they are complementary!
– (but some Java proponents may not agree)
Understanding Networked Applications
47
A First Course
Who favors what?
Would these vendors be
in favor of:
Microsoft
Intel
• Interoperability?
SUN
• Portability?
Novell
Iona
Understanding Networked Applications
48
A First Course