Languages and Compilers (SProg og Overs

1
Languages and Compilers(SProg og
Oversættere)Concurrency and distribution

• Bent Thomsen
• Department of Computer Science
• Aalborg University

With acknowledgement to John Mitchell whose
slides this lecture is based on.
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Concurrency, distributed computing, the Internet

• Traditional view
• Let the OS deal with this
• gt It is not a programming language issue!
• End of Lecture
• Wait-a-minute
• Maybe the traditional view is getting out of
  date?

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Languages with concurrency constructs

• Maybe the traditional view was always out of
  date?
• Simula
• Modula3
• Occam
• Concurrent Pascal
• ADA
• Linda
• CML
• Facile
• Jo-Caml
• Java
• C

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Categories of Concurrency

• Physical concurrency - Multiple independent
  processors ( multiple threads of control)
• Uni-processor with I/O channels
• (multi-programming)
• Multiple CPU
• (parallel programming)
• Network of uni- or multi- CPU machines
• (distributed programming)
• Logical concurrency - The appearance of physical
  concurrency is presented by time-sharing one
  processor (software can be designed as if there
  were multiple threads of control)
• Concurrency as a programming abstraction
• Def A thread of control in a program is the
  sequence of program points reached as control
  flows through the program

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Introduction

• Reasons to Study Concurrency
• It involves a different way of designing software
  that can be very usefulmany real-world
  situations involve concurrency
• Control programs
• Simulations
• Client/Servers
• Mobile computing
• Games
• 2. Computers capable of physical concurrency are
  now widely used
• High-end servers
• Game consoles
• Grid computing

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The promise of concurrency

• Speed
• If a task takes time t on one processor,
  shouldnt it take time t/n on n processors?
• Availability
• If one process is busy, another may be ready to
  help
• Distribution
• Processors in different locations can collaborate
  to solve a problem or work together
• Humans do it so why cant computers?
• Vision, cognition appear to be highly parallel
  activities

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Challenges

• Concurrent programs are harder to get right
• Folklore Need an order of magnitude speedup (or
  more) to be worth the effort
• Some problems are inherently sequential
• Theory circuit evaluation is P-complete
• Practice many problems need coordination and
  communication among sub-problems
• Specific issues
• Communication send or receive information
• Synchronization wait for another process to act
• Atomicity do not stop in the middle and leave a
  mess

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Why is concurrent programming hard?

• Nondeterminism
• Deterministic two executions on the same input
  it always produce the same output
• Nondeterministic two executions on the same
  input may produce different output
• Why does this cause difficulty?
• May be many possible executions of one system
• Hard to think of all the possibilities
• Hard to test program since some may occur
  infrequently

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Traditional C Library for concurrency
System Calls - fork( ) - wait( ) - pipe( ) -
write( ) - read( ) Examples
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Process Creation
Fork( ) NAME fork() create a new
process SYNOPSIS include ltsys/types.hgt
include ltunistd.hgt pid_t fork(void) RETURN
VALUE success parent- child pid child-
0 failure -1
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Fork()- program structure
include ltsys/types.hgt include
ltunistd.hgt include ltstdio.hgt Main() pid_t
pid if((pid fork())gt0) / parent
/ else if ((pid0) /child/ else
/ cannot fork exit(0)
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Wait() system call
Wait()- wait for the process whose pid reference
is passed to finish executing SYNOPSIS includelts
ys/types.hgt includeltsys/wait.hgt pid_t wait(int
stat)loc) The unsigned decimal integer process
ID for which to wait RETURN VALUE success-
child pid failure- -1 and errno is set
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Wait()- program structure
include ltsys/types.hgt include
ltunistd.hgtinclude ltstdlib.hgt include
ltstdio.hgt Main(int argc, char argv) pid_t
childPID if((childPID fork())0) /child/
else / parent wait(0) exit(0)
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Pipe() system call
Pipe()- to create a read-write pipe that may
later be used to communicate with a process
well fork off. SYNOPSIS Int pipe(pfd) int
pfd2 PARAMETER Pfd is an array of 2 integers,
which that will be used to save the two file
descriptors used to access the pipeRETURN
VALUE0 success-1 error.
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Pipe() - structure
/ first, define an array to store the two file
descriptors/Int pipes2/ now, create the
pipe/int rc pipe (pipes) if(rc -1)
/ pipe() failed/ Perror(pipe) exit(1)
If the call to pipe() succeeded, a pipe will be
created, pipes0 will contain the number of its
read file descriptor, and pipes1 will contain
the number of its write file descriptor.
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Write() system call
Write() used to write data to a file or other
object identified by a file descriptor. SYNOPSIS
include ltsys/types.hgt Size_t write(int fildes,
const void buf, size_t nbyte) PARAMETER fildes
is the file descriptor, buf is the base address
of area of memory that data is copied
from, nbyte is the amount of data to
copy RETURN VALUE The return value is the actual
amount of data written, if this differs from
nbyte then something has gone wrong
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Read() system call
Read() read data from a file or other object
identified by a file descriptor SYNOPSIS include
ltsys/types.hgt Size_t read(int fildes, void buf,
size_t nbyte) ARGUMENT fildes is the file
descriptor, buf is the base address of the
memory area into which the data is read, nbyte
is the maximum amount of data to read. RETURN
VALUE The actual amount of data read from the
file. The pointer is incremented by the amount of
data read.
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Solaris 2 Synchronization

• Implements a variety of locks to support
  multitasking, multithreading (including real-time
  threads), and multiprocessing.
• Uses adaptive mutexes for efficiency when
  protecting data from short code segments.
• Uses condition variables and readers-writers
  locks when longer sections of code need access to
  data.
• Uses turnstiles to order the list of threads
  waiting to acquire either an adaptive mutex or
  reader-writer lock.

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Windows 2000 Synchronization

• Uses interrupt masks to protect access to global
  resources on uniprocessor systems.
• Uses spinlocks on multiprocessor systems.
• Also provides dispatcher objects which may act as
  wither mutexes and semaphores.
• Dispatcher objects may also provide events. An
  event acts much like a condition variable.

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Basic question

• Maybe the library approach is not such a good
  idea?
• How can programming languages make concurrent and
  distributed programming easier?

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Language support for concurrency

• Help promote good software engineering
• Allowing the programmer to express solutions more
  closely to the problem domain
• No need to juggle several programming models
  (Hardware, OS, library, )
• Make invariants and intentions more apparent
  (part of the interface and/or type system)
• Allows the compiler much more freedom to choose
  different implementations
• Base the programming language constructs on a
  well-understood formal model gt formal reasoning
  may be less hard and the use tools may be possible

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What could languages provide?

• Abstract model of system
• abstract machine gt abstract system
• Example high-level constructs
• Communication abstractions
• Synchronous communication
• Buffered asynchronous channels that preserve msg
  order
• Mutual exclusion, atomicity primitives
• Most concurrent languages provide some form of
  locking
• Atomicity is more complicated, less commonly
  provided
• Process as the value of an expression
• Pass processes to functions
• Create processes at the result of function call

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Basic issue conflict between processes

• Critical section
• Two processes may access shared resource
• Inconsistent behavior if two actions are
  interleaved
• Allow only one process in critical section
• Deadlock
• Process may hold some locks while awaiting others
• Deadlock occurs when no process can proceed

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Concurrency

• Def A task is disjoint if it does not
  communicate with or affect the execution of any
  other task in the program in any way
• Task communication is necessary for
  synchronization
• Task communication can be through
• 1. Shared nonlocal variables
• 2. Parameters
• 3. Message passing

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Synchronization

• Kinds of synchronization
• 1. Cooperation
• Task A must wait for task B to complete some
  specific activity before task A can continue its
  execution e.g., the producer-consumer problem
• 2. Competition
• When two or more tasks must use some resource
  that cannot be simultaneously used e.g., a shared
  counter
• Competition is usually provided by mutually
  exclusive access (approaches are discussed
  later)

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Design Issues for Concurrency

• How is cooperation synchronization provided?
• How is competition synchronization provided?
• How and when do tasks begin and end execution?
• Are tasks statically or dynamically created?
• Are there any syntactic constructs in the
  language?
• Are concurrency construct reflected in the type
  system?

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Concurrent Pascal cobegin/coend

• Limited concurrency primitive
• Example
• x 0
• cobegin
• begin x 1 x x1 end
• begin x 2 x x1 end
• coend
• print(x)

execute sequential blocks in parallel
x 1
x x1
x 0
print(x)
x 2
x x1
Atomicity at level of assignment statement
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Mutual exclusion

• Sample action
• procedure sign_up(person)
• begin
• number number 1
• listnumber person
• end
• Problem with parallel execution
• cobegin
• sign_up(fred)
• sign_up(bill)
• end

bob
fred
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Locks and Waiting

• ltinitialze concurrency controlgt
• cobegin
• begin
• ltwaitgt
• sign_up(fred) // critical section
• ltsignalgt
• end
• begin
• ltwaitgt
• sign_up(bill) // critical section
• ltsignalgt
• end
• end

Need atomic operations to implement wait
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Mutual exclusion primitives

• Atomic test-and-set
• Instruction atomically reads and writes some
  location
• Common hardware instruction
• Combine with busy-waiting loop to implement mutex
• Semaphore
• Avoid busy-waiting loop
• Keep queue of waiting processes
• Scheduler has access to semaphore process sleeps
• Disable interrupts during semaphore operations
• OK since operations are short

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Monitor Brinch-Hansen, Dahl, Dijkstra, Hoare

• Synchronized access to private data. Combines
• private data
• set of procedures (methods)
• synchronization policy
• At most one process may execute a monitor
  procedure at a time this process is said to be
  in the monitor.
• If one process is in the monitor, any other
  process that calls a monitor procedure will be
  delayed.
• Modern terminology synchronized object

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Java Concurrency

• Threads
• Create process by creating thread object
• Communication
• shared variables
• method calls
• Mutual exclusion and synchronization
• Every object has a lock (inherited from class
  Object)
• synchronized methods and blocks
• Synchronization operations (inherited from class
  Object)
• wait pause current thread until another thread
  calls notify
• notify wake up waiting threads
• notifyAll

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Java Threads

• Thread
• Set of instructions to be executed one at a time,
  in a specified order
• Java thread objects
• Object of class Thread
• Methods inherited from Thread
• start method called to spawn a new thread of
  control causes VM to call run method
• suspend freeze execution
• interrupt freeze execution and throw exception
  to thread
• stop forcibly cause thread to halt
• Objects can implement the Runnable interface and
  be passed to a thread
• public interface Runnable
• public void run()

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Example subclass of Thread

• class PrintMany extends Thread
• private String msg
• public PrintMany (String m) msg m
• public void run()
• try for () System.out.print(msg
  )
• sleep(10)
• catch (InterruptedException e)
• return
• (inherits start
  from Thread)

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Interaction between threads

• Shared variables
• Two threads may assign/read the same variable
• Programmer responsibility
• Avoid race conditions by explicit
  synchronization!!
• Method calls
• Two threads may call methods on the same object
• Synchronization primitives
• Each object has internal lock, inherited from
  Object
• Synchronization primitives based on object
  locking

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Synchronization example

• Objects may have synchronized methods
• Can be used for mutual exclusion
• Two threads may share an object.
• If one calls a synchronized method, this locks
  object.
• If the other calls a synchronized method on same
  object, this thread blocks until object is
  unlocked.

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Synchronized methods

• Marked by keyword
• public synchronized void commitTransaction()
  
• Provides mutual exclusion
• At most one synchronized method can be active
• Unsynchronized methods can still be called
• Programmer must be careful
• Not part of method signature
• sync method equivalent to unsync method with body
  consisting of a synchronized block
• subclass may replace a synchronized method with
  unsynchronized method

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Join, another form of synchronization

• Wait for thread to terminate
• class Future extends Thread
• private int result
• public void run() result f()
• public int getResult() return result
• Future t new future
• t.start() // start new
  thread
• t.join() x t.getResult() // wait and get
  result

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Aspects of Java Threads

• Portable since part of language
• Easier to use in basic libraries than C system
  calls
• Example garbage collector is separate thread
• General difficulty combining serial/concur code
• Serial to concurrent
• Code for serial execution may not work in
  concurrent sys
• Concurrent to serial
• Code with synchronization may be inefficient in
  serial programs (10-20 unnecessary overhead)
• Abstract memory model
• Shared variables can be problematic on some
  implementations
• Java 1.5 has expanded the definition of the
  memory model

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C Threads

• Basic thread operations
• Any method can run in its own thread, i.e. no
  need to pass a class implementing a run method
• A thread is created by creating a Thread object
• The Thread class is sealed thus no inheritance
  from it
• Creating a thread does not start its concurrent
  execution it must be requested through the
  Start method
• A thread can be made to wait for another thread
  to finish with Join
• A thread can be suspended with Sleep
• A thread can be terminated with Abort

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C Threads

• Synchronizing threads
• The Interlock class
• The lock statement
• The Monitor class
• Evaluation
• An advance over Java threads, e.g., any method
  can run its own thread
• Thread termination cleaner than in Java
• Synchronization is more sophisticated

42
Polyphonic C

• An extension of the C language with new
  concurrency constructs
• Based on the join calculus
• A foundational process calculus like the
  p-calculus but better suited to asynchronous,
  distributed systems
• A single model which works both for
• local concurrency (multiple threads on a single
  machine)
• distributed concurrency (asynchronous messaging
  over LAN or WAN)
• It is different
• But its also simple if Mort can do any kind of
  concurrency, he can do this

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In one slide

• Objects have both synchronous and asynchronous
  methods.
• Values are passed by ordinary method calls
• If the method is synchronous, the caller blocks
  until the method returns some result (as usual).
• If the method is async, the call completes at
  once and returns void.
• A class defines a collection of chords
  (synchronization patterns), which define what
  happens once a particular set of methods have
  been invoked. One method may appear in several
  chords.
• When pending method calls match a pattern, its
  body runs.
• If there is no match, the invocations are queued
  up.
• If there are several matches, an unspecified
  pattern is selected.
• If a pattern containing only async methods fires,
  the body runs in a new thread.

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Extending C with chords

• Classes can declare methods using generalized
  chord-declarations instead of method-declarations
  .

chord-declaration method-header
method-header body method-header
attributes modifiers return-type async name
(parms)

• Interesting well-formedness conditions
• At most one header can have a return type (i.e.
  be synchronous).
• Inheritance restriction.
• ref and out parameters cannot appear in async
  headers.

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A Simple Buffer

• class Buffer
• String get() async put(String s)
• return s

• Calls to put() return immediately (but are
  internally queued if theres no waiting get()).
• Calls to get() block until/unless theres a
  matching put()
• When theres a match the body runs, returning the
  argument of the put() to the caller of get().
• Exactly which pairs of calls are matched up is
  unspecified.

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OCCAM

• Program consists of processes and channels
• Process is code containing channel operations
• Channel is a data object
• All synchronization is via channels
• Formal foundation based on CSP

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Channel Operations in OCCAM

• Read data item D from channel C
• D ? C
• Write data item Q to channel C
• Q ! C
• If reader accesses channel first, wait for
  writer, and then both proceed after transfer.
• If writer accesses channel first, wait for
  reader, and both proceed after transfer.

48
Concurrent ML

• Threads
• New type of entity
• Communication
• Synchronous channels
• Synchronization
• Channels
• Events
• Atomicity
• No specific language support

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Threads

• Thread creation
• spawn (unit ? unit) ? thread_id
• Example code
• CIO.print "begin parent\n"
• spawn (fn () gt (CIO.print "child
  1\n"))
• spawn (fn () gt (CIO.print "child
  2\n"))
• CIO.print "end parent\n
• Result

child 1
child 2
begin parent
end parent
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Channels

• Channel creation
• channel unit ? a chan
• Communication
• recv a chan ? a
• send ( a chan a ) ? unit
• Example
• ch channel()
• spawn (fn()gt ltAgt send(ch,0) ltBgt )
• spawn (fn()gt ltCgt recv ch ltDgt )
• Result

ltAgt
ltBgt
send/recv
ltCgt
ltDgt
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CML programming

• Functions
• Can write functions channels ? threads
• Build concurrent system by declaring channels and
  wiring together sets of threads
• Events
• Delayed action that can be used for
  synchronization
• Powerful concept for concurrent programming
• Sample Application
• eXene concurrent uniprocessor window system

52
A CML implementation (simplified)

• Use queues with side-effecting functions
• datatype 'a queue Q of front 'a list ref,
  rear 'a list ref
• fun queueIns (Q()) ( insert into queue )
• fun queueRem (Q()) ( remove from queue )
• And continuations
• val enqueue queueIns rdyQ
• fun dispatch () throw (queueRem rdyQ) ()
• fun spawn f callcc (fn parent_k gt
• ( enqueue parent_k f ()
  dispatch()))

Source Appel, Reppy
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Language issues in client/server programming

• Communication mechanisms
• RPC, Remote Objects, SOAP
• Data representation languages
• XDR, ASN.1, XML
• Parsing and deparsing between internal and
  external representation
• Stub generation

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Client/server example

• A major task of most clients is to interact
  with a human user and a remote server.

• The basic organization of the X Window System

55
Client-Side Software for Distribution Transparency

• A possible approach to transparent replication of
  a remote object using a client-side solution.

56
The Stub Generation Process

Compiler / Linker
Server Program
Server Stub
Server Source
Interface Specification
Common Header
RPC LIBRARY
RPC LIBRARY
Stub Generator
Client Stub
Client Source
Client Program
Compiler / Linker
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RPC and the OSI Reference Model
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Representation

• Data must be represented in a meaningful format.
• Methods
• Sender or Receiver makes right (NDR).
• Network Data Representation (NDR).
• Transmit architecture tag with data.
• Represent data in a canonical (or standard) form
• XDR
• ASN.1
• Note these are languages, but traditional DS
  programmers dont like programming languages,
  except C

59
XDR - eXternal Data Representation

• XDR is a universally used standard from Sun
  Microsystems used to represent data in a network
  canonical (standard) form.
• A set of conversion functions are used to encode
  and decode data for example, xdr_int( ) is used
  to encode and decode integers.
• Conversion functions exist for all standard data
  types
• Integers, chars, arrays,
• For complex structures, RPCGEN can be used to
  generate conversion routines.

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RPC Example
61
XDR Example

• include ltrpc/xdr.hgt
• ..
• XDR sptr // XDR stream pointer
• XDR xdrs // Pointer to XDR stream pointer
• char bufBUFSIZE // Buffer to hold XDR data
• xdrs (sptr)
• xdrmem_create(xdrs, buf, BUFSIZE, XDR_ENCODE)
• ..
• int i 256
• xdr_int(xdrs, i)
• printf(position d. \n, xdr_getpos(xdrs))

62
Abstract Syntax Notation 1 (ASN.1)

• ASN.1 is a formal language that has two features
  
• a notation used in documents that humans read
• a compact encoded representation of the same
  information used in communication protocols.
• ASN.1 uses a tagged message format
• lt tag (data type), data length, data value gt
• Simple Network Management Protocol (SNMP)
  messages are encoded using ASN.1.

63
Distributed Objects

• CORBA
• Java RMI
• SOAP and XML

64
Distributed ObjectsProxy and Skeleton in Remote
Method Invocation
65
CORBA

• Common Object Request Broker Architecture
• An industry standard developed by OMG to help in
  distributed programming
• A specification for creating and using
  distributed objects
• A tool for enabling multi-language,
  multi-platform communication
• A CORBA based-system is a collection of objects
  that isolates the requestors of services
  (clients) from the providers of services
  (servers) by an encapsulating interface

66
CORBA objects

• They are different from typical programming
  objects in three ways
• CORBA objects can run on any platform
• CORBA objects can be located anywhere on the
  network
• CORBA objects can be written in any language that
  has IDL mapping.

67
Client
Client
Object Implementation
Object Implementation
IDL
IDL
IDL
IDL
ORB
ORB
NETWORK
A request from a client to an Object
implementation within a network
68
IDL (Interface Definition Language)

• CORBA objects have to be specified with
  interfaces (as with RMI) defined in a special
  definition language IDL.
• The IDL defines the types of objects by defining
  their interfaces and describes interfaces only,
  not implementations.
• From IDL definitions an object implementation
  tells its clients what operations are available
  and how they should be invoked.
• Some programming languages have IDL mapping (C,
  C, SmallTalk, Java,Lisp)

69
IDL File
IDL Compiler
Client Stub File
Server Skeleton File
Object Implementation
Client Implementation
ORB
70
The IDL compiler

• It will accept as input an IDL file written using
  any text editor (fileName.idl)
• It generates the stub and the skeleton code in
  the target programming language (ex Java stub
  and C skeleton)
• The stub is given to the client as a tool to
  describe the server functionality, the skeleton
  file is implemented at the server.

71
IDL Example
module katytrail module weather struct
WeatherData float temp string
wind_direction_and_speed float
rain_expected float humidity
typedef sequenceltWeatherDatagt WeatherDataSeq
interface WeatherInfo WeatherData
get_weather( in string site
) WeatherDataSeq find_by_temp(
in float temperature )
72
IDL Example Cont.
interface WeatherCenter
register_weather_for_site ( in string
site, in WeatherData site_data )

Both interfaces will have Object
Implementations. A different type of Client will
talk to each of the interfaces. The Object
Implementations can be done in one of two ways.
Through Inheritance or through a Tie.
73
Stubs and Skeletons

• In terms of CORBA development, the stubs and
  skeleton files are standard in terms of their
  target language.
• Each file exposes the same operations specified
  in the IDL file.
• Invoking an operation on the stub file will cause
  the method to be executed in the skeleton file
• The stub file allows the client to manipulate the
  remote object with the same ease with each a
  local file is manipulated

74
Java RMI

• Overview
• Supports remote invocation of Java objects
• Key Java Object SerializationStream objects
  over the wire
• Language specific
• History
• Goal RPC for Java
• First release in JDK 1.0.2, used in Netscape 3.01
• Full support in JDK 1.1, intended for applets
• JDK 1.2 added persistent reference, custom
  protocols, more support for user control.

75
Java RMI

• Advantages
• True object-orientation Objects as arguments
  and values
• Mobile behavior Returned objects can execute on
  caller
• Integrated security
• Built-in concurrency (through Java threads)
• Disadvantages
• Java only
• Advertises support for non-Java
• But this is external to RMI requires Java on
  both sides

76
Java RMI Components

• Base RMI classes
• Extend these to get RMI functionality
• Java compiler javac
• Recognizes RMI as integral part of language
• Interface compiler rmic
• Generates stubs from class files
• RMI Registry rmiregistry
• Directory service
• RMI Run-time activation system rmid
• Supports activatable objects that run only on
  demand

77
RMI Implementation
Client Host
Server Host
Stub
Skeleton
78
Java RMI Object Serialization

• Java can send object to be invoked at remote site
• Allows objects as arguments/results
• Mechanism Object Serialization
• Object passed must inherit from serializable
• Provides methods to translate object to/from byte
  stream
• Security issues
• Ensure object not tampered with during
  transmission
• Solution Class-specific serializationThrow it
  on the programmer

79
Building a Java RMI Application

• Define remote interface
• Extend java.rmi.Remote
• Create server code
• Implements interface
• Creates security manager, registers with registry
• Create client code
• Define object as instance of interface
• Lookup object in registry
• Call object
• Compile and run
• Run rmic on compiled classes to create stubs
• Start registry
• Run server then client

80
Parameter Passing

• Primitive types
• call-by-value
• Remote objects
• call-by-reference
• Non-remote objects
• call-by-value
• use Java Object Serialization

81
Java Serialization

• Writes object as a sequence of bytes
• Writes it to a Stream
• Recreates it on the other end
• Creates a brand new object with the old data
• Objects can be transmitted using any byte stream
  (including sockets and TCP).

82
Codebase Property

• Stub classpaths can be confusing
• 3 VMs, each with its own classpath
• Server vs. Registry vs. Client
• The RMI class loader always loads stubs from the
  CLASSPATH first
• Next, it tries downloading classes from a web
  server
• (but only if a security manager is in force)
• java.rmi.server.codebase specifies which web
  server

83
CORBA vs. RMI

• CORBA was designed for language independence
  whereas RMI was designed for a single language
  where objects run in a homogeneous environment
• CORBA interfaces are defined in IDL, while RMI
  interfaces are defined in Java
• CORBA objects are not garbage collected because
  they are language independent and they have to be
  consistent with languages that do not support
  garbage collection, on the other hand RMI objects
  are garbage collected automatically

84
SOAP Introduction

• SOAP is simple, light weight and text based
  protocol
• SOAP is XML based protocol (XML encoding)
• SOAP is remote procedure call protocol, not
  object oriented completely
• SOAP can be wired with any protocol
• SOAP is a simple lightweight protocol with
  minimum set of rules for invoking remote services
  using XML data representation and HTTP wire.
• Main goal of SOAP protocol Interoperability
• SOAP does not specify any advanced distributed
  services.

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Why SOAP Whats wrong with existing distributed
technologies

• Platform and vendor dependent solutions
  (DCOM Windows) (CORBA
  ORB vendors) (RMI Java)
• Different data representation schemes
  (CDR NDR)
• Complex client side deployment
  
• Difficulties with firewall
  
  Firewalls allows only specific ports ( port 80 ),
  but DCOM and CORBA assigns port numbers
  dynamically.
• In short, these distributed technologies do not
  communicate easily with each other because of
  lack of standards between them.

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Base Technologies HTTP and XML

• SOAP uses the existing technologies, invents no
  new technology.
• XML and HTTP are accepted and deployed in all
  platforms.
• Hypertext Transfer Protocol (HTTP)
• HTTP is very simple and text-based protocol.
• HTTP layers request/response communication over
  TCP/IP. HTTP supports fixed set of methods like
  GET, POST.
• Client / Server interaction
• Client requests to open connection to server on
  default port number
• Server accepts connection
• Client sends a request message to the Server
• Server process the request
• Server sends a reply message to the client
• Connection is closed
• HTTP servers are scalable, reliable and easy to
  administer.
• SOAP can be bind any protocol HTTP , SMTP, FTP

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Extensible Markup Language (XML)

• XML is platform neutral data representation
  protocol.
• HTML combines data and representation, but XML
  contains just structured data.
• XML contains no fixed set of tags and users can
  build their own customized tags.
• ltstudentgt
• ltfull_namegtBhavin Parikhlt/full_namegt
• ltemailgtbgp4_at_psu.edult/emailgt
• lt/studentgt
• XML is platform and language independent.
• XML is text-based and easy to handle and it can
  be easily extended.

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Architecture diagram
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Parsing XML Documents

• Remember XML is just text
• Simple API for XML (SAX) Parsing
• SAX is typically most efficient
• No Memory Implementation!
• Left to the Developer
• Document Object Model (DOM) Parsing
• Parsing is not fundamental emphasis.
• A DOM Object is a representation of the XML
  document in a binary tree format.

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Parsing Examples

• SaxParseExample
• Callback functions to process Nodes
• DomParseExample
• Use of JAXP (Java API for XML Parsing)
• Implementations can be swapped, such as
  replacing Apache Xerces with Sun Crimson.
• JAXP does not include some advanced features
  that may be useful.
• SAX used behind the scenes to create object model

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Web-based applications today
Presentation HTML, CSS, Javascript, Flash, Java
applets, ActiveX controls
Application server Web server Content management
system
Business logic C, Java, VB, PHP, Perl,
Python,Ruby
Beans, servlets, CGI, ASP.NET,
Operating System
Database SQL File system
Sockets, HTTP, email, SMS, XML, SOAP, REST,
Rails, reliable messaging, AJAX,
Replication, distribution, load-balancing,
security, concurrency
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Languages for distributed computing

• Motivation
• Why all the fuss about language and platform
  independence?
• It is extremely inefficient to parse/deparse
  to/from external/internal representation
• 95 of all computers run Windows anyway
• There is a JVM for almost any processor you can
  think of
• Few programmers master more than one programming
  language anyway
• Develop a coherent programming models for all
  aspects of an application

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Facile Programming Language

• Integration of Multiple Paradigms
• Functions
• Types/complex data types
• Concurrency
• Distribution/soft real-time
• Dynamic connectivity
• Implemented as extension to SML
• Syntax for concurrency similar to CML

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Facile implementation

• Pre-emptive scheduler implemented at the lowest
  level
• Exploiting CPS translation gt state characterised
  by the set of registers
• Garbage collector used for linearizing data
  structures
• Lambda level code used as intermediate language
  when shipping data (including code) in
  heterogeneous networks
• Native representation is shipped when possible
• i.e. same architecture and within same trust
  domain
• Possibility to mix between interpretation or JIT
  depending on usage

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Conclusion

• Concurrency may be an order of magnitude more
  difficult to handle
• Programming language support for concurrency may
  help make the task easier
• Which concurrency constructs to add to the
  language is still a very active research area
• If you add concurrency construct, be sure you
  base them on a formal model!

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The guiding principle
Put important features in the language itself,
rather than in libraries

• Provide better level of abstraction
• Make invariants and intentions more apparent
• Part of the language syntax
• Part of the type system
• Part of the interface
• Give stronger compile-time guarantees (types)
• Enable different implementations and
  optimizations
• Expose structure for other tools to exploit (e.g.
  static analysis)