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• Distributed applications programming
- distributed objects model
- RMI, invocation semantics
- RPC
Products
- Java RMI,CORBA,DCOM
- Sun RPC
- JINI
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• Location transparency
- client/server need not know their location
• Sits on top of OS, independent of:
communication protocols : use abstract request-reply protocols over UDP,TCP
computer hardware: use external data representation e.g. CORBA CDR
operating system: use e.g. socket abstraction available in most systems
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programming language: e.g. CORBA supports Java, C++

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Applications
RMI , RPC and events
Request-reply protocol
External data representation
Operating System
Middleware layer

object
Data
Implementation of methods m1 m2 m3 interface m4 m5 object
Data
Implementation of methods
•
Objects = data + methods
• Interact via interfaces :
- define types of arguments and exceptions of methods
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•
Programs logically partitioned into objects
- distributing objects natural and easy
• Interfaces
- the only means to access data, make them remote?
•
Actions
- via method invocation
-interaction, chains of invocations
-may lead to exceptions, part of interface
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-exceptions system as well as programmer-defined reactions on unexpected or error situations
Garbage collection
-disposal of resources (memory) that are occupied by objects that are no longer referenced and will not be needed any more in the future
- reduced effort, error-free (Java, not C++)

A
REMOTE
INVOCATION
LOCAL
INVOCATION
C
B
LOCAL
INVOCATION
D
LOCAL
INVOCATION
REMOTE
INVOCATION
E
F
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Objects distributed (client-server models)
•
Extend with
- Remote object reference
- Remote interfaces
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- Remote method invocation (RMI)

• Data encapsulation gives better protection
- concurrent processes, interference
• Method invocations
- can be remote or local
• Objects
- can act as clients, servers, etc
- can be replicated for fault-tolerance and performance
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• Object References
- used to access objects which live in processes
- can be passed as arguments, stored in variables,…
• Remote Object References
- object identifiers in a distributed system
- must be unique in space and time
- error returned if accessing a deleted object
- can allow relocation
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• Constructing unique remote object reference
- IP address, port, interface name
- time of creation, local object number (new for each object)
• Use the same as for local object references
• If used as addresses
- cannot support relocation
32 bit 32 bit 32 bit 32 bit
Internet address Port number time Object number
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Interface of remote object

Remote interface m1 m2 m3
Remote object
Data
Implementation
Of method m4 m5 m6
Local interface
• CORBA: Interface Definition Language (IDL)
• Java RMI: as other interfaces, keyword remote
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 if implemented over UDP, then the following failures may occur:
– message omission
– messages not delivered in send order
– message duplication
 additionally, process failures are possible (crash)
 consequence: cannot be guaranteed, that remote operations are executed exactly once (as it can be guaranteed for local operations)

• Local invocations
-executed exactly once
• Remote invocations
- via Request-Reply
- may suffer from communication failures!
• retransmission of request/reply
• message duplication, duplication filtering
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Fault tolerance measures Invocation semantics
Retransmit request Duplicate Re-execute procedure message filtering or retransmit reply
No Not applicable Not applicable Maybe
Yes No Re-execute procedure At-least-once
Yes Yes Retransmit reply At-most-once
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Re-executing a method sometimes dangerous...

Invocation Semantics choices in the implementation
– retransmit request message until reply or server presumed failed
– filtering of duplicates at server if retransmit is used
– when retransmitted request arrives:
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result retransmission if retransmitted request arrives (requires keeping of history at server) or
2. repeated execution of procedure
 leads to different invocation semantics

•
Remote method
- may execute or not at all, invoker cannot tell
- useful only if occasional failures
• Invocation message lost…
- method not executed
• Result not received…
- was method executed or not?
• Server crash…
- before or after method executed?
- if timeout, result could be received after timeout...
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•
Remote method
- invoker receives result (executed exactly) or exception (no result, executed once or not at all)
- retransmission of request message
• Invocation message retransmitted…
- method may be executed more than once
- arbitrary failure (wrong result possible)
- method must be non-indempotent(repeated execution has the same effect as a single execution)
•
Server crash…

• Remote method
- invoker receives result (executed once) or exception (no result)
- retransmission of reply & request messages
- duplicate filtering
• Best fault-tolerance …
- arbitrary failures prevented if method called at most once
• Used by CORBA and Java RMI
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Server
Client object A proxy for B
Request
Skeleton
&dispatcher for B’s class
Remote object B
Remote reference module
Reply
Communication module
Communication module
Remote reference module
Object A invokes a method in a remote object B : communication module reference module, RMI software.
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• Translates remote to local references (object table):
– correspondence between remote and local object references (proxies)
• Directs requests to proxy (if exists)
•when marshalling / un marshalling
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• Proxy
– behaves like local object to client
– forwards requests to remote object
• Dispatcher
– receives request
– selects method and passes on request to skeleton
• skeleton
– implements methods in remote interface
– unmarshals data, invokes remote object
– Waits for result, marshals it and returns reply
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• RPC
– historically first,now little used
– over Request-Reply protocol
– usually at-least-once or at-most-once semantics
– can be seen as a restricted form of RMI
–sun RPC
• RPC software architecture
– similar to RMI (communication,dispatcher and stub in place of proxy / skeleton )
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Client process
Server process
Request
Client stub procedure
Client program
Reply
Communication module
Server stub
Communication module procedure dispatcher
Service procedure
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(cont.)
• Client procedure calls the client stub in a normal way
• Client stub builds a message and traps to the kernel
• Kernel sends the message to remote kernel
• Remote kernel gives the message to server stub
• Server stub unpacks parameters and calls the server
• Server computes results and returns it to server stub
• Server stub packs results in a message and traps to kernel
• Remote kernel sends message to client kernel
• Client kernel gives message to client stub
• Client stub unpacks results and returns to client

• Distributed object model
– capabilities for handling remote objects (remote references,etc)
– RMI:maybe,at-least-once,at-most-once semantics
– RMI implementation,software architecture
• Other distributed programming paradigms
– RPC,restricted form of RMI, less often used
– event notification (for heterogeneous,asynchronous systems)
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