Distributed Computing: Remote Method Invocation

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Distributed ComputingRemote Method Invocation

• Distributed Objects

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Distributed Objects

• What we want
• listener.update( statusEvent )
• What we have
• Send(byte) recieve(byte)

Update(statusEvent)
listener
Update(statusEvent)
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Remote Method Invocation

• RMI is what Tanenbaum and Steen calls a middle
  layer (middleware).
• It is a framework that translates between high
  level application abstractions method calls
  between distributed objects into transport
  protocol layer primitives send and receive.
• There are several challenges in this.

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Challenges

• Complex data types
• Little endian/big endian integers
• Floating point many different representations
• Strings \0 terminated, etc.
• Moving between abstraction levels
• update(statusEvent) -gt bit array -gt
  update(statusEvent)
• Representing method identity on the wire
• Coding that it is update I want to call
• Method calls have blocking semantics
• The caller stops until callee has ended processing

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Challenges

• Object references
• How to a get a reference to an object in the
  first place?
• References only have meaning on the local machine
• Unreliable network
• Is the machine in the other end dead? Or the
  network is down?

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Remote Procedure Call
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A suggestion

• Birrell and Nelson, 1984
• allow calling procedure on remote machines
• A call procedure x on B means
• A suspends, information on x is transmitted to B
• B executes the x procedure
• B sends the result back to A
• A resumes
• Example on machine A
• int x foo(x)
• foo(int) is located on machine B

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Translation Client stub

• Idea
• replace the library containing foo with a foo
  stub
• Actually a proxy pattern but the stub term is
  the traditional one (bad do not mix it with test
  stubs!)

void foo(int x) translate parameter and
method info into message format transmit it to
B block in a receive() call until answer is
returned return answer in return message
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Translation Server stub

• Server side
• block in a receive
• upon message received
• Often these code fragments are called skeletons

unpack the message, find out which procedure is
called and the parameters call the resulting
procedure pack the return value into a
message send it back to the origin
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ORBs

• The middleware layer are often called ORBs in RMI
  systems Object Request Broker

server node
client node
client
serverObj
ORB
ORB
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Parameter passing conventions

• Basically languages have two types of parameter
  passing. Ex. in C
• int x foo( x ) pass-by-value
• int x foo( x ) pass-by-reference
• Exercise What do you have in Java?

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Pass by Value

• Pass-by-value
• easy one simply code value into byte stream
• (you get the right semantics in case the receive
  changes it!)
• But but...
• encoding! What is an integer? 16-bit? 32-bit?
  little endian/big endian.
• Morale You HAVE to decide on the encoding.

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Ex. CORBA IIOP

• CORBA Common Object Request Broker Architecture
• IIOP Internet Inter-ORB Protocol
• defines the on the wire encoding and protocol
• CDR Common Data Representation
• how many bits are an integer little endian?
• Message Format
• how to we pack the message with the information
• Transport layer assumptions
• do we use TCP/IP or UPD or ???

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Terminology

• Marshalling
• Encoding parameters into the message format
• Unmarshalling
• Decoding

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Pass by Reference

• Tricky! My options are
• A) forbid pointers and reference parameters!
• what does memory address 0x00056 mean on another
  machine?
• B) replace pass-by-reference with copy/restore
• copy a memory area into the message
• let Bs foo work on a copy of the memory area on
  B
• ship a copy of the memory area back to A
• copy the received memory area into that on A
• C) pass references to remote objects
• i.e. any method call is a remote call on a remote
  object
• RMI uses option C)

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RMI

• ... getting the last pieces in place

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Object References

• Procedures are global whereas methods are local
  to an object.
• Program to an interface comes to our rescue!
• The caller only needs to know the interface of
  our object on the remote location.

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Proxy

• A proxy can inherit the same interface as the
  real object (i.e. implement the same methods)
  but simply pass the calls on and block just as
  in the RPC case.
• Logical

foo(x)
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What really happens

• The client invokes a method on a proxy. The proxy
  marshals the message to the skeleton on the
  server side.

ltltinterfacegtgt Server foo()
Client
ServerProxy
ServerSkeleton
ServerImpl
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Dynamics (CORBA)
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Interfaces?

• In Java interfaces are well known and found in
  the language. But what if we need a refernce to a
  remote object code in C, Cobol, or C?
• IDL Interface Definition Language
• OMG Standard for declaring interfaces
• Works well with CORBA (also supported by Java)
• Also part of MicroSoft DCOM.

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IDL eksempel
module SalesChart interface Observer
void update() string uniqueIdentity()
interface SalesChart void addSale(
in string person, in long amount ) long
sumSales( ) long mySales( in string person
) void attach( in Observer obs ) void detach(
in Observer obs )
module er et heterogent navnerum!
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Last bits of the puzzle

• Summary
• we can marshal and unmarshal
• can translate in and out of bit streams
• we have proxies and skeletons
• can translate from OO methods to messages
• but how do I get the reference to the server
  object?

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Registry

• We need a separate process that holds a mapping
  between abstract names (often strings) and remote
  references
• Basically a dictionary/associate array/map
• (key,value) (name, remote reference)
• CORBA Naming Service
• Java RMI Registry (part of JDK)
• Tanenbaum Location server

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Network Failures

• All method calls may fail when distributed
• Java exception handling mechanism comes in handy.
• RemoteException!

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Parameter types

• Pass by reference ( object references in java)
• proxy/skeleton pairs
• Pass by value
• do you have any idea?
• Hint you need to translate object state into a
  streamable format. Java already supports this!

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Example

• Echo Server

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Declaring the interface

• Must import java.rmi.
• Must extend Remote
• Methods must throw RemoteException

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Binding to the server

• You need to
• install a security manager
• ask the registry for the remote reference using a
  name in URL form //localhost/EchoServer

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The Server

• Server object that exists first
• extend UnicastRemoteObject
• tell my name to registry
• Alternative export it
• UnicastRemoteObject.exportObject(obj,port)
• and tell registry
• Exercise
• What is the advantage ofthe second form?

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Object references from here on

• Once two objects have established contact by
  rebind/bind over the registry they may exchange
  object references as they like
• Either remote references or by-value references.
• But note that the semantics is different from
  that of normal Java when pass by value! Tricky
  errors occur!

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Running

• Remember to start the registry first!
• start rmiregistry
• start ant runSesrver
• start ant runClient

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How to make it

• We need to generate the proxy
• rmic rmi compiler
• We need to set the security settings...

ltltinterfacegtgt Server foo()
Client
ServerProxy
ServerSkeleton
Java 5 onwards Not necessary!
ServerImpl
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Rmic compiler

• rmic - The Java RMI Compiler
• rmic generates stubs, skeletons, ties for remote
  objects using either the JRMP or IIOP protocols.
  Also generates OMG IDL.

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RMIC

• RMIC require a class file for generating the stub
  files.
• Thus you must compile all files first
• Next call the RMIC on the .class file that
  represents the implementation of the remote
  interface
• Here EchoServer
• Bit backwards ? - The clients proxy (stub) is
  generated by the final server implementation
• Therefore EchoServer_stub

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Peeking behind the curtain

• -keep

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Security

• Tell which security policy to use
• And define that policy a lenient one here ?

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Codebases
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Why hostname and codebase?

• When the client is passed a reference of a remote
  object it must get hold of an implementation of
  the remote interface the stub class (i.e. the
  proxy ?)
• Java use dynamic class loading and simply request
  the stub implementation from the server
• give me the stub that allows me to talk to the
  remote object reference that you gave me
• The hostnamecodebase tell the server the URL to
  feed to the client the client then dynamically
  loads the classes from that URL.
• Quite cool but a bit tricky ?

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Concurrency
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?

• Distributed programs are inherently concurrent
  systems.
• Consider two echo clients that invoke echo on
  the server at the very same time.
• Exercise what happens?

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Critical Regions

• Therefore we have to declare critical regions
  using the synchronized keyword.
• The question is where?
• On the proxy?
• On the server?
• What is the right answer here?

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RMI Spec

• From the RMI spec
• A method dispatched by the RMI runtime to a
  remote object implementation may or may not
  execute in a separate thread. The RMI runtime
  makes no guarantees with respect to mapping
  remote object invocations to threads. Since
  remote method invocation on the same remote
  object may execute concurrently, a remote object
  implementation needs to make sure its
  implementation is thread-safe.

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Summary
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Problem Statement

• Complex data types
• Little endian/big endian integers
• Floating point many different representations
• Strings \0 terminated, etc.
• Moving between abstraction levels
• Update(statusEvent) -gt bit array -gt
  update(statusEvent)
• Representing method identity on the wire
• Coding that it is update I want to call
• Method calls have blocking semantics
• The caller stops until callee has ended processing

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Problem Statement

• Object references
• How to a get a reference to an object in the
  first place
• References only have meaning on the local machine
• Unreliable network
• Is the machine in the other end dead? Or the
  network is down?

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