CSE 8343 Adv. OS Group A2 Communication in Distributed File System

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CSE 8343 Adv. OS
Group A2 Communication in
Distributed File System

• by
• Sajida Begum
• Samina Choudhry

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Outline

• Client Server Model
• - Blocking Vs Non-Blocking.
• - Buffered Vs Un-Buffered.
• - Reliable Vs Un-Reliable.
• - Server architecture concurrent Vs.
  sequential.
• - Scalability
• Remote Procedure Call
• Group Communication

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Client Server Computing

• Client Machines Generally single user PCs or
  workstations that provide user friendly interface
  to the end user.
• Server Machines Provide a set of share user
  services to the clients.
• The server enables many clients to share access
  to the same database and enable the use of a
  high-performance computer system to manage the
  database.

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Client/Server Environment

LAN or WAN
Servers
Workstations or Clients
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Client/Server Applications
Client Requests (128.32.)
Server Responds to (128.32)
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Client/Server Model

• Key Concept The client component and the server
  component of the client/server model are software
  applications running on computers.
• The Client requests an operation or service
  from the server.
• -The client may request data or programs
  
• from the server
• -The client may send data for the server
  
• to store
• - The client may send data to be passed
• on to another computer on the network

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Addressing (Locating Servers)

• Hard wired address
• - Not transparent
• - Difficult to migrate
• Broadcast based
• - Server chooses address from address space
• - Client broadcast request.
• - Can Cache response for future.
• Locate Address via Naming Server

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Addressing cont..

• Use of Name Server
• - Name Server located at well-known address
  maintains mapping between servers logical names
  and their physical addresses.
• - Client sends a lookup request to NS
  requesting address of server.
• - NS reply indicates serer address.
• - Client sends request to server
• - Server sends reply to client.

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Blocked Vs Non-Blocked

• Blocking communication (synchronously)
• Sender specifies msg (in buffer) to send and
  destination address.
• Remains blocked until the message has been sent.
• Non Blocked (Asynchronously)
• Returns control to invoker as soon as message to
  be sent has been copied in a (local) kernel
  buffer.
• Transmission process proceeds asynchronously from
  sender.

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How Block Sends Receives

• Block sends
• Message placed in buffer
• Send invokes process blocks
• Message copied into kernel
• Message is transmitted.
• Block receives
• Received invoked by process
• If there is a message then copied into process
  space
• If no message process blocks until a message
  arrives.

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How Un-Block Sends Receives

• Un-Block sends
• Message placed in buffer
• Send invokes process continues
• Message is transmitted.
• Un-Block receives
• Received invoked by process
• If there is a message then copied into process
  space
• If no message process continues.
• Process needs to check for message arrivals.

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Buffered VS. UnBuffered primitives

• Buffered primitives
• - process invoking receive requires kernel to
  create a mailbox for it, specifies its own local
  address(e.g. a port number) and blocks.
• - The kernel maintains the mailbox in kernel
  space.
• - Incoming message containing process address is
  deposited by the kernel in the process mailbox,
  pending receive is terminated unblocking the
  process.
• - mailbox full race conditions in kernel
  space kernel may have discard message.

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Buffered VS. UnBuffered cont..

• UnBuffered Primitives
• receive(addr, m) invoked by process to receive
  a message at address addr into buffer pointed at
  by m, in the invokers memory space.
• Invoker blocks after invocation.
• Kernel unblocks invoker when message is received
  and copied in buffer m.
• If message is received and there are no pending
  receive , the kernel discards message and client
  retries.
• If receive is invoked by server, race conditions
  may occur when this is doing work for other
  client.

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Remote Procedure Call (RPC)

• RPC is a protocol that manages the messages sent
  between the client and the server processes.It is
  
• a set of rules for marshalling and un-marshalling
  parameters and results
• a set of rules for encoding and decoding
  information transmitted between two processes
• a few primitive operations to invoke an
  individual call, to return its results, and to
  cancel it
• provision in the operating system and process
  structure to maintain and reference state that is
  shared by the participating processes.

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RPC Architecture
Server
Client
Local Call
Remote Procedure
Client Stub
Server Stub
RPC
RPC
Send
Receive
Send
Receive
Network
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RPC Remote Invocation mechanism

• 1. Client calls client stub in normal way
• 2. Client stub builds call msg, marshalling
  (Packing) parameters, and calls OS routine for
  msg sending.
• 3. OS sends the message to server node.

Call
Pack params
Unpack params
call
return
return
Unpack results
Pack results
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RPC Remote Invocation cont..

• 4. Server node receives msg, hands it to server
  stub.
• 5. Server stub unmarshalls (unpacks) parameters,
  and calls actual routine.
• 6. Call is executed and results returned to
  server stub.
• 7. Server marshals parameters, builds result msg,
  and hands it to OS for transmission.
• 8. Message is sent to client node.
• 9. Client stub receives message.
• 10.Message is unmarshalled, and the stub returns
  the call to the client.

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RPC Design Issues

• Parameter passing
• Binding (locating servers)
• Treatment of failures
• Parameter languages employ various techniques
• - Call by value The parameter value is copied
  on the stack, in the called procedure, a local
  variable gets initialized to this value. The
  called procedure
• may modify it, but such changes do not affect
  the value of the original value.
• - Call by reference The reference to the
  variable (pointer) is copied on the stack the
  called procedure can therefore access the
  variable and changes are directly reflected in
  the calling program.

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RPC Scheme binds client server

• Binding information may be pre-decided in the
  form of fixed port addresses.
• - at compile time, an RPC call has a fixed port
  number associated with it.
• - once the program is compile the server cannot
  change the port number of the requested service.
• Binding can be done dynamically
• - Operating system provides a daemon on a fixed
  RPC port.
• - Client then sends a message to the daemon
  requesting the port address of the RPC it needs
  to execute.

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RPC Design Issues IDL

• A specification language is necessary for
  specifying service routines
• - For C programs, one can use C class
  definitions, but this becomes rather language
  specific. Special Interface Definition Languages
  (IDLs) have been suggested to allow mixed-
  language programming.
• - you need IDL to your language stub
  compiler.
• - client and servers could be written in
  different languages.

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RPC failures

• Failure Model
• 1. Communication failures functioning
  processes temporarily unable to communicate.
• 2. Node crashes a node ( computer) either
  works or crashes (stops working), a crashed node
  eventually recovers.

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Understanding about failures

• First, we begin by defining the possible failure
  events during a call
• 1. Client is unable to locate server process.
• 2. Clients request to message lost
• 3. Servers reply message lost
• 4. Server crashes during the call
• 5. Client crashes during the call.

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• Any
• Questions ?