Chapter 2. Communication

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Chapter 2. Communication
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Layered Protocol
Not always 7-layered Protocol But some other
protocols if necessary
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Client-Server Model
Client-Server Model
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Basic Operations of Client-Server Model(Send and
Receive Protocol)
Server While(1) waitMessage(FromAny,rxmsg)
sourcerxmsg.source opcoderxmsg.opcode
paramrxmsg.params resultfunctionsopcode(para
m) txmsg.resultresult txmsg.sourceme
sendMessage(source,txmsg)
Client txmsg.opcodeopcode
txmsg.sourceme txmsg.paramparam
sendMessage(serverID,txmsg) waitMessage(serverI
D,rxmsg) checkAndProcess(rxmsg)
Client
Client
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Steps of RPC

• 1. Client procedure calls client stub in normal
  way
• 2. Client stub builds message, calls local OS
• 3. Client's OS sends message to remote OS
• 4. Remote OS gives message to server stub
• 5. Server stub unpacks parameters, calls server
• 6. Server does work, returns result to the stub
• 7. Server stub packs it in message, calls local
  OS
• 8. Server's OS sends message to client's OS
• 9. Client's OS gives message to client stub
• 10. Stub unpacks result, returns to client

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RPC Remote Procedure Call
Server
Client txmsg.opcodeopcode
txmsg.sourceme txmsg.paramparam
sendMessage(serverID,txmsg) waitMessage(serverI
D,rxmsg) checkAndProcess(rxmsg)
resultfuncA(param)
STUB
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Binding a Client to a Server

• Binding in DCE (Distributed Computing Environment)

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Middleware
Client
Server
Stub for Client
Stub for Server
Binding (Dynamic)
Middleware
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Middleware Protocol
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Some Issues Reliable vs. Non-Reliable
Client
Server
Process
Process
OS
OS
If reply message is lost?
1. Blocking
2. Acknowledge
3. Idempotent
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Some Issues Blocking vs. Non-Blocking

• Blocking (No Buffer, Synchronous)

Send
Client
Server
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Some Issues Blocking vs. Non-Blocking

• Non-Blocking (No Buffer, Asynchronous)

Send
Client
Server
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Asynchronous RPC

• If client does not receive return value,
• No reason to be blocked.
• Asynchronous RPC

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Asynchronous RPC

• Even though client does receive return value,
• Two asynchronous RPC

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Some Issues Parameter Passing
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Parameter Passing for Objects

• Two types of Object Parameter Passing
• Object Copy vs. Reference Copy
• Should it be transparent ?

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Remote Object Invocation

• Generalization of RPC to Object Invocation

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Marshalling
System A
System B
Object in Main Memory
Object in Main Memory
Object
Marshalling
Demarshalling
When objects in memory are to be passed across a
network to another host or persisted to storage,
their in-memory representation must be converted
to a suitable out-of-memory format. This process
is called marshalling, and converting back to an
in memory representation is called demarshalling
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Extended RPC Door for Single Machine

• Client and Server are located in a single machine
• RPC for processes on the same machine

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RMI Object Binding

• Implicit Binding
• Distr_object obj_ref //Declare a system-wide
  object referenceobj_ref // Initialize the
  reference to a distributed objectobj_ref-gt
  do_something() // Implicitly bind and invoke a
  method
• Explicit Binding
• Distr_object objPref //Declare a system-wide
  object referenceLocal_object obj_ptr
  //Declare a pointer to local objectsobj_ref
  //Initialize the reference to a distributed
  objectobj_ptr bind(obj_ref) //Explicitly bind
  and obtain a pointer to the local proxyobj_ptr
  -gt do_something() //Invoke a method on the local
  proxy

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Stream-Oriented Communication

• In distributed environment
• Continuous Media e.g. video
• Discrete Media e.g. Simple File Transfer
• Temporal Aspects of Data Stream
• Asynchronous Transmission
• Synchronous Transmission
• Isochronous Transmission
• Composition of Stream
• Simple Stream
• Complex Stream composed of simple or complex
  streams
• Synchronization is very important

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Data Streaming
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QoS Quality of Service

• Timeline, Volume and Reliability

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Group Communication

• Send to multiple receivers

Send to a machine
Send to a group
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Group Communication Design Issues

• Closed or Open
• Peer or Hierarchical
• Addressing
• Multi-Casting or Broadcasting
• Uni-casting
• Predicate Addressing

Open to outside
Closed to outside
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Group Communication Design Issues

• Group Membership
• Send/Receive Protocol
• How to distinguish it from single receive
• Atomicity
• Message ordering
• Scalability