SEN974 ClientServer and the Internet

1
SEN974 Client/Server and the Internet

• UDP TCP
• Javas RMI

2
Topics

• Networking and Internetworking
• Inter-process communication
• Distributed objects and remote invocation
• UDP TCP
• Javas RMI

3
Networking Issues for Distributed Systems

• Previous networks had to meet simple application
  requirements
• File transfer, remote login, e-mail and
  newsgroups
• With the growth of the Internet, more stringent
  requirements are necessary
• New modes of use
• Additional and more stringent requirements

4
Current Networking Requirements Terms

• Performance
• Speed with which individual messages can be
  transferred between two interconnected computers
• Latency is the delay that occurs after a send
  operation is executed before data starts to
  arrive at the destination computer (network
  latency)
• Data transfer rate is the speed at which data
  can be transferred between two computers in the
  network once the transmission has begun
• Bits per second
• Message transmission time
• latency length/data transfer rate

5
Current Networking Requirements Terms

• Scalability
• The potential future size of the Internet is
  commensurate with the population of the planet
• Indicates size and load of the Internet
• Networks must scale

6
Current Networking Requirements Terms

• Reliability
• The degree in which an application can recover
  from communication failures
• Security
• Techniques for achieving security in distributed
  systems
• Firewalls creates a protection boundary between
  an organizations intranet and the rest of the
  Internet
• Distributed systems need a secure network
  environment with end-to-end authentication,
  privacy, and security
• Example - encryption

7
Current Networking Requirements Terms

• Mobility
• Wireless networks provide connectivity to devices
  such as laptops, cell phones, PDAs
• Addressing and routing schemes of Internet were
  developed before the advent of wireless devices
  so are not well-adapted to their intermittent
  connection to many different subnets
• Quality of service
• Applications that transmit multimedia data
  require guaranteed bandwidth and latencies

8
Current Networking Requirements Terms

• Multicasting
• One-to-many communication
• Needed in distributed systems

9
Types of Networks in Distributed Systems

• PAN Personal area network
• Various digital devices carried by a user are
  connected to a low-cost, low-energy network
• LAN Local area network
• Carry messages at high speed between computers
  connected by a single communication medium
• Twisted copper wire, coaxial cable, fiber optics
• WAN Wide area network
• Carry messages a lower speeds between nodes that
  are often in different organizations and may be
  separated by large distances
• Uses routers to route the messages
• MAN Metropolitan area network
• High speed network installed in towns/cities for
  transmission of data and video
• DSL and Cable modem connections

10
Types of Networks in Distributed Systems

• WLAN Wireless local area network
• Used in place of wired LANs for wireless mobile
  devices
• IEEE 802.11 (WiFi) standard protocol
• WMAN Wireless metropolitan area network
• Supercedes 802.11
• IEEE 802.16 WiMAX protocol

11
Types of Networks in Distributed Systems

• Internetworks communication subsystem in which
  several networks are linked together to provide
  common data communication facilities
• Overlay the technologies and protocols of the
  individual component networks and methods used
  for their interconnection
• Needed for the development of extensible, open
  distributed systems
• Example Internet

12
Network performance
13
Network Protocols

• Protocol refers to a well-known set of rules
  and formats to be used for communication between
  processes in order to perform a given task
• Two parts
• a specification of the sequence of messages that
  must be exchanged
• a specification of the format of the data in the
  messages

14
Conceptual Layering of Protocol Software
15
Protocol Layers

• Network software is arranged in a hierarchy of
  layers
• Each layer presents an interface to the layers
  above it and below it
• Sending side each layer (except top) accepts
  items of data in a specified format from the
  layer above it and applies transformations to
  encapsulate the data in the format specified for
  that layer
• Receiving side converse transformations are
  applied to data items received from the layer
  below before being passed to layer above

16
Encapsulation as it is applied in Layered
Protocols
The tags in the headers are the protocol type for
the layer above, needed for the receiving
protocol stack to correctly unpack the packets
17
Protocol Suites

• Complete set of protocol layers is referred to as
  a protocol suite or protocol stack
• Reference Model for Open Systems Interconnection
  (OSI)
• Protocol layering brings substantial benefits in
  simplifying and generalizing the software
  interfaces for access to the communication
  services of networks
• Also carries significant performance costs
• Why do you think this is so?

18
Protocol layers in the ISO Open Systems
Interconnection (OSI) model
19
OSI Protocol Summary
20
Internet Protocols

• TCP/IP Transmission Control Protocol/Internet
  Protocol
• Grown now to over 60 million hosts
• Example application services protocols based in
  TCP/IP
• Web (HTTP)
• email (SMTP, POP)
• netnews (NNTP)
• file transfer (FTP)
• Telnet (telnet)

21
TCP/IP Layers
22
TCP/IP Layers

• Two transport protocols
• TCP reliable connection-oriented protocol
• UDP datagram protocol that does not guarantee
  reliable transmission
• Internet Protocol (IP) is the underlying
  network protocol
• IP datagrams provide the basic transmission
  mechanism for the Internet and other TCP/IP
  networks
• Internet protocols are usually layered over other
  network technologies
• i.e, Ethernet

23
Encapsulation in a message transmitted via TCP
over an Ethernet
24
The programmer's conceptual view of a TCP/IP
Internet
25
IP Addressing

• Scheme chosen for naming and addressing hosts and
  routing IP packets to their destination
• Assigns an IP address to each host in the
  Internet a 32-bit numeric identifier
  containing
• network identifier uniquely identifies one of
  the subnetworks in the Internet
• host identifier uniquely identifies the hosts
  connection to that network

26
Internet Address Space
Class A is reserved for very large networks
(i.e.,US NSFNet) Class B is used for
organizations that operate networks likely to
contain more than 255
computers Class C is allocated to all other
network operators Class D is reserved for
Internet multittask communication Class E
contains a range of unallocated addresses
reserved for future use
27
TCP and UDP

• Provide the communication capabilities of the
  Internet in a form that is useful for application
  programs
• TCP and UDP are transport protocols and must
  provide process-to-process communication
• Accomplished by using ports
• Port numbers are used for addressing messages to
  processes within a particular computer and are
  only valid for that computer
• Once an IP packet has been delivered to its
  destination host, the TCP- or UDP-layer software
  dispatches it to a process via a specific port at
  that host

28
UDP Features

• UDP datagram is encapsulated inside an IP packet
• Short header with source and destination port
  numbers, length field and a checksum
• UDP does NOT guarantee delivery
• Adds no additional reliability mechanisms except
  for checksum (which is optional)
• Use is restricted to applications and services
  that do not require reliable delivery of single
  or multiple messages

29
TCP Features

• TCP provides a much more sophisticated transport
  service
• Provides reliable delivery of a long sequences of
  bytes
• Guarantees the delivery to the receiving process
  of all the data presented to the TCP software by
  the sending process in the same order
• Provides retransmission if necessary
• Done when the it does not receive an
  acknowledgement from the receiving process within
  a specified timeout

30
Middleware

• Looking at IPC from the Programmers point of
  view
• Remote Method Invocation (RMI)
• Remote Procedure Call (RPC)

31
Interprocess Communication(IPC)

• Distributed Systems
• rely on exchanging data and achieving
  synchronization amongst autonomous distributed
  processes
• Inter process communication (IPC)
• shared variables
• message passing
• message passing in concurrent programming
  languages
• language extensions
• API calls
• Principles of IPC
• (Concurrent programs collections of two or more
  sequential programs executing concurrently
• Concurrent processes collection of two or more
  sequential programs in operation, executing
  concurrently

32
IPC Characteristics

• Message passing between pairs of processes can be
  supported by two message communication
  operations
• send
• receive
• In order for one process to communicate with
  another, one process sends a message (sequence of
  bytes) to a destination and another process at
  the destination receives the message

33
Types of Communication

• Synchronous
• Sending and receiving process synchronize at
  every message
• Both send and receive are blocking operations
• Whenever a send is issued, the sending process
  (or thread) is blocked until the corresponding
  receive is issued
• Whenever a receive is issued, the process (or
  thread) blocks until a message arrives

34
Types of Communication

• Asynchronous
• Send is non-blocking sending process is allowed
  to proceed as soon as the message has been copied
  to a local buffer
• Receive operation can have blocking and
  non-blocking variants
• Non-blocking receiving process proceeds with
  its program after issuing a receive operation
  which provides a buffer to be filled in the
  background
• Must separately receive notification that is
  buffer has been filled by polling or interrupt
• Todays systems do not generally provide the
  non-blocking form of receive because it adds
  complexity to the application

35
Sockets

• Both UDP and TCP use socket abstraction
• Provides an endpoint for communication between
  processes
• Originated from BSD UNIX but in all Unix flavors
  (i.e., Linux)
• IPC consists of transmitting a message between a
  socket in one process and a socket in another
  process

36
Sockets and Ports
37
Sockets and Ports

• For a process to receive messages, its socket
  must be bound to a local port and one of the
  Internet addresses of the computer on which it
  runs
• Messages sent to a particular Internet address
  and port number can be received only by a process
  whose socket is associated with that Internet
  address and port number
• Processes may use the same socket for sending and
  receiving messages

38
Moving from Interprocess to Distributed
Communication

• A distributed system breaks up a previously
  monolithic application into different components
  and then proceeds to place them on different
  systems (i.e. tiers).
• We are decoupling the consumer and the supplier
  we are spreading the activities of an application
  onto team players (systems)

39
Moving from Interprocess to Distributed
Communication

• Analogy
• Let us imagine for a moment a small business run
  by a single entrepreneur. This one person,
  Suzette Sellers, does everything, much like a
  monolithic application. She sells to customers,
  orders new product, manages the books, pays the
  bills, and does anything else. Internally, she
  has compartmentalized her job function (much like
  we program in a modular fashion) selling,
  ordering, and administration. There is no
  overhead associated with external communication
  she does everything.
• When her store becomes successful, she finds that
  she simply cannot do it all and requires
  assistance. Suzette hires Henry Helpers, a
  capable and likeable chap. This is now similar
  to our two-tier architecture. Because Suzette
  and Henry work in the same physical location,
  this is a logical two-tier system. Communication
  is quick and easy.
• Think about two programs on a computer they can
  communicate via files, shared memory, pipes, or
  other constructs.
• If you are familiar with processing on a UNIX- or
  Linix-based system, multiple commands are often
  strung together with pipes.

40
Moving from Interprocess to Distributed
Communication

• Suzettes store does so well that she moves her
  office into another building to increase the
  amount of retail floor space in her original
  store. Now Suzette and Henry are in different
  physical locations but still need to complete the
  same tasks of selling, ordering, and
  administration. We have moved into a physical
  two tier distributed system and things just
  become more complicated. Suzette and Henry still
  need to communicate but the previous methods will
  no longer work.
• The obvious solution is the telephone, but this
  changes the dynamic of the entire conversation
• In our computer analogy, we now have a client and
  a server tier where the previous solutions of
  files, shared memory, or pipes are no longer
  solutions as they only work within a single
  system.

41
Moving from Interprocess to Distributed
Communication

• Suzette and Henry must now specifically locate
  each other (i.e. phone numbers) before
  communicating.
• Conversations will be slower (network latency).
• Data will need to be shared in different ways
  (difficult to draw a picture on the phonethe
  crayon keeps breaking on the handset).
• And finally, the phone lines can go down or have
  interference (network errors or corruption).

42
Moving from Interprocess to Distributed
Communication

• Yes, we have gained some benefits by spreading
  out Suzette and Henry (more retail floor space)
  but we have introduced complexity and latency
  into the business.
• Similarly, when a distributed application is
  placed on different systems (whether physically
  or logically separated), we also introduce
  complexity and latency into the application.
• However, the ability to take advantage of
  multiple computers is such a powerful incentive
  that we have devised many ways to accommodate our
  new problems
• The stage is now set to discover how to implement
  this distributed communication.

43
Middleware Layer

• By choosing Java, we are explicitly ignoring the
  platform because Java (in theory) runs exactly
  the same on any platform that has a
  Java-compatible runtime environment.
• We are not yet concerned with what capabilities
  an application will exhibit other than we will
  create a server component that holds the
  desired properties and a client component that
  requests these capabilities.
• Assume that our distributed Java-based
  application will communicate via the TCP/IP
  protocol

44
UDP Communication

• Basis of TCP/IP protocol stack is the Internet
  Protocol (IP) data packet
• IP is a connectionless and stateless protocol
  (fire and forget)
• Once a IP packet is sent, the sender simply does
  not care about it anymore
• IP packet can become lost, destroyed, misplaced,
  or corrupted on its way to its destination
• If multiple IP packets are sent they can arrive
  out of order
• IP is quick to use but not reliable
• Think of IP as simple postal envelopes they can
  be easily damaged, lost, and delivered out of
  order.
• However, IP envelopes are cheap and convenient.
  (The analogy to a postal service is very
  appropriate as the cost is low per message and
  the service is usually reliable but there is no
  capability to trace a wayward envelope.)

45
UDP Communication

• The User Datagram Protocol (UDP) is implemented
  on top of IP layer.
• UDP is still a fairly lightweight protocol which
  means that most of its effort is spent on
  delivering data.
• UDP adds two important features larger
  payloads and data integrity.
• If a message is larger than the size of a single
  IP envelope, UDP will split the message into
  multiple IP envelopes, taking care to remember
  how it split up the original message.
• UDP guarantees that the data in a single UDP
  packet in the will arrive exactly as it was sent.
  
• What happens if one of the IP packets that made
  up the UDP packet is lost? Then UDP discards the
  rest of the IP packets and thus the entire UDP
  packet rather than deliver a damaged UDP packet.
  
• This is analogous to sending a four page letter
  in separate envelopes if envelope 3 does not
  arrive, then UDP will throw array the remaining
  envelopes that did arrive as it has no way to
  reconstruct the original four page letter.

46
UDP Communication

• UDP is also a stateless and connectionless
  protocol
• Stateless means that once a message is sent, the
  sender does not care about it anymore.
• If the message arrives, then that is fine. If it
  does not arrive, we do not feel any grief
• Connectionless means that there is no continued
  conversation.
• The message is complete in and of itself.
• Think about sending postcards. If we send one
  postcard from Europe and another from South
  America, neither of the postcards depend upon
  each other. They are separate communications

47
UDP Design Considerations

• For client-server applications that transfer
  relatively small amounts of data, UDP is
  acceptable
• For the simplest application, a client will send
  a single message to the server and expect a
  response
• If the client does not receive a response within
  a pre-determined amount of time, the client may
  send the request again.
• This is similar to asking a person to repeat an
  answer. Of course, after asking for the answer
  so many times, it is likely that we (the client)
  will just give up.
• If a DNS server does not respond after a couple
  of seconds, a client will simply accept that no
  answer is forthcoming and either ask another DNS
  server or return an error.

48
UDP Example

• UDP communications can be implemented in Java
  quite easily
• However, it should be pointed out that UDP is
  simply a service to ferry data from point A to
  point B.
• UDP does not understand data types or other Java
  constructs, such as objects.
• To accommodate this understanding, we will use
  the capability of the String class to convert
  itself into an array of bytes and back.
• We are implicitly relying on the String class to
  correctly convert an array of bytes using the
  correct code set.
• Both the server and client will perform many of
  the same operations but the server has the
  additional task of existing in a state where it
  can receive requests at any time and must respond
  to them. A client, on the other hand, will only
  be executed on demand and exit as soon as it
  receives a response.

49
UDP Server

• The UDP server may look complicated but there is
  actually very little to it.
• The server first creates a socket, which is a
  logical construct that we may attach to an actual
  port.
• We have hard-coded the port upon which our server
  will listen, 7777.
• You may choose any port between 1024 and 65535
  (assuming that it is not already in use).
• The server attaches to the port and then performs
  an endless loop of listening for any incoming
  message and sending the same message (known as a
  datagram) back to its sender.
• The effect is similar to a mirror
• See UDPServerSimple.java

50
UDP Client

• The UDP client seems to be much longer in length
  but that is because we have added in the ability
  to specify the message to send, the server name,
  and the port on which the UDP server is listening
  on the command line, rather than hard-coding it.
  
• This approach should allow you to run the program
  as-is, even though you will run it on your system
  (which has a different network name and IP
  address than everybody elses system).
• The client, similar to the server, first creates
  a socket that can be bound to a network port.
• However, we will not bind it to a particular port
  because we do not carejust as when you send a
  letter, the mailbox you use is irrelevant to the
  receiver.

51
UDP Client

• The client program then resolves the name of the
  server (first argument) to an IP address, which
  is what the UDP protocol requires.
• The second argument is the port that the UDP
  server is listening on, and the third argument is
  the actual message (remember to enclose it in
  double quotes if it contains a space).
• A datagram message is then created and sent to
  the server.
• Immediately after sending the message, the client
  then acts like the server, waiting for a reply.
• Although the client did not request a specific
  port, the port that was automatically assigned is
  included in the outgoing message so that the
  server can successfully respond back to us.
• See UDPClientSimple.java

52
UDP Example In Action

• We first start the UDP server which opens the
  port and waits for the first incoming message.
• It will wait until the program is terminated
• Although not shown, the UDP server is running on
  a system with the name Alpha (as in the first
  Greek letter).

53
UDP Example In Action

• The server gets up and running quickly and then
  immediately starts waiting for eternity
• Then run the UDP client which sends a message and
  then itself waits for a reply.

54
UDP Example In Action

• Notice that the client runs and then exits.
• This is typical of most client-server
  communications where the client makes a request,
  receives a reply from the server and is done.

55
UDP Example In Action

• Meanwhile, the server has recorded the incoming
  message, responded to it, and begun waiting for
  another incoming request.

56
TCP Communication

• TCP network communication is similar to UDP but
  it fixes the flaws inherent in UDP by providing
  message delivery guarantee and concurrency.
• TCP goes a step beyond UDP and is a stateful and
  connection-oriented protocol.
• Stateful means that the TCP remembers what it has
  sent and ensures that it arrives in the same
  order
• Connection-oriented means that each packet sent
  is actually part of a larger conversation, or
  stream.

57
TCP Communication

• A TCP-based server differs from a UDP-based
  server in that a single thread does not perform
  all of the communication.
• The usual solution to handle TCP communications
  is to create a new thread that is dedicated to
  incoming requests from a particular client.
• A server can distinguish multiple requests from
  the multiple clients on the same client system
  because each client will be forced to use a
  different port

58
TCP Example

• The programs for the TCP client and server are
  similar in nature although the details vary a
  bit.
• Because TCP is stateful, we actually open up two
  streams one inbound and one outbound
• These act like conveyor belts for data to flow
  between the client and the server.
• From a programming standpoint, we no longer have
  to use datagrams but can simply read and write a
  simple data type or a string object.

59
TCP Server

• For the server, the largest change is that
  additional of threads in order to maintain each
  connection as a separate conversation.
• However, once inside of a thread, the commands
  are simply reading and writing data.
• See TCPServer.java

60
TCP Client

• The TCP client is very similar to the UDP client
  except that instead of sending datagrams, we hook
  up to the streams-based plumbing.
• However, the rest of the logic is pretty much the
  same.
• See TCPClient.java

61
TCP Example in Action

• Set CLASSPATH first!
• Start with the server first

62
TCP Example in Action

• Next we invoke the TCP client to send a simple
  message.
• We can see that the client was able to acquire a
  port to communicate out and received a reply that
  was different than what was originally sent,
  proving that something happened.

63
TCP Example in Action

• The server first received a connection from the
  remote port of 4304 (the client).
• It then created new clientConnection object (the
  two lines with the word setup in the brackets)
  and then went back to listening for more
  connection requests.
• Meanwhile, the new thread was run and it picked
  up the message sent by the client and then sent
  it back with the prefix Received.

64
Remote Method Invocation (RMI)

• Provides for communication between distributed
  objects by means of remote method invocation
• RMI objects that can receive remote method
  invocations are called remote objects and they
  implement a remote interface
• Is an extension of local method invocation that
  allows an object living in one process to invoke
  methods of an object living in another process

65
Middleware Layer for RMI
66
Interfaces in Distributed Systems

• An interface specifies the procedures and
  variables that can be accessed from other modules
• Modules are implemented so as to hide all the
  information about them except that which is
  available through its interface.
• As long as the interface does NOT change,
  implementations may change without affecting the
  users of the module
• In a distributed program, the modules can run in
  separate processes.
• Cannot access the variables in a module in
  another process not direct variable access

67
Interfaces in Distributed Systems

• Other differences
• Parameter-passing mechanisms (i.e., call by
  value, call by reference) used in local procedure
  calls do not work in a distributed system
• Uses input and output parameters that are passed
  between processes
• Pointers in one process are not valid in another
  process on a remote computer
• No pointer passing allowed

68
Remote and Local Method Invocations
69
Java RMI

• Java RMI provides an adapter that effectively
  hides the network from us, instead allowing us to
  simply lookup functionality that we want
  (regardless of where it actually exists) and use
  it as if it was locally available.
• The level of complexity definitely increases once
  we decide to distribute the objects in a Java
  system.
• Assuming that we started with two Java programs,
  with one calling the other, we now introduce the
  following elements
• an interface that explicitly states what
  functionality is remotely available
• a server that provides access to the remote
  object
• the RMI registry that links up requests from a
  client to a remote object

70
Overview of RMI Interfaces and Classes
71
Java RMI Example Foundation

• We will start with two Java classes that are not
  RMI-enabled and then adapt them to operate with
  RMI so we can see exactly what changes are
  needed.
• Let us imagine that we have devised an automated
  registration process class which, upon knowing
  which class for which we want to register, will
  tell us if we are registered or if the class is
  full.

72
AskTheRegistrar.java
If the class includes the word CS483 (case
sensitive) then the class must have some spots
open. Otherwise, the class is full.
73
Java Application to Invoke the Functionality
LocalStudent.java
In order to work, both classes just need to be
in a directory within the current CLASSPATH
environment variable
74
Result when run...

• By passing the argument CS483, the
  AskTheRegistrar service concludes that the
  student is registered

75
Java RMI Example (with a server)

• We now seek to move the AskTheRegistrar service
  to a remote server so that it can be accessed
  from multiple locations within the university.
• The first step is to decide what functionality
  should be made available.
• Currently, the AskTheRegistrar class has two
  member functions the constructor and
  getRegistrationStatus.
• The constructor will not be available remotely
  and so we need to create an alternative method to
  set the location.
• These two functions, setClass and
  getRegistrationStatus, will be available to
  remote invocation and we can now create an
  interface detailing this decision.
• Need to enhance the object AskTheRegistrar so
  that it can be remotely used

76
RMI Server Example Sequence Diagram
77
AskTheRegistrarInterface.java
Remember an interface is a contract that a class
can choose to implement so there is no actual
code in the interface itself
78
New Version of AskTheRegistrar

• See AskTheRegistrarRemote.java
• Imported java.rmi and java.rmi.server packages to
  enable remote invocation capability
• Class inherits from the UnicaseRemoteObject
  parent class
• This object means that a single instance (as
  opposed to one instance per request) of the
  AskRemoteRegistrarRemote object will be enabled
  to be called remotely
• Provides methods outlined in the interface
• Added an additional constructor that takes no
  arguments and sets the private theClass variable

79
RMI Registry

• A client wishing to use a remote object will need
  to know exactly where it is located
• RMI registry
• Coordinated phone book that records every
  objects name and address
• Publishes objects info in a well-known location
• rmiregistry.exe
• In the same directory as other java executables
• Can be run by itself as a background process
• We will run in its own command window

80
RMI Registry

• RMI Registry starts off with an empty directory
• Has no way of actually invoking any objects
• Its only job is to provide locations to any
  program that asks (like directory assistance)

81
RMI Server Example

• We now need a Java program similar to our UDP
  server that accepts requests for a remote object
• This server program must also communicate to the
  RMI registry that it exists so clients can find it

82
AskTheRegistrarServer.java
83
RMI Server Example

• This seems like a very small amount of code
  compared to our previous UDP server.
• The magic happens on a single line

84
RMI Server Example

• The rebind command is what actually informs the
  RMI registry that a remote object is now
  available for remote invocation.
• The first argument is the remote objects name in
  the directory as well as which RMI registry will
  be informed.
• In our example, we are contacting the registry on
  the local server (i.e. localhost).
• If the registry was on a port other than its
  default 1098, this could be specified by placing
  a colon and the new port number directly after
  the server name.
• The identifier after the server is the name that
  the object will be listed as in the registry.
• This directory name does not have to be the same
  as the actual remote object.

85
RMI Server Example

• The second argument creates the actual remote
  object that will accept requests from clients.
• Because we are unsure of what location a client
  will specify, we invoke the no-argument version
  of the constructor which will set the internal
  location to an empty string (and thus require the
  client to specify an actual location later).
• Most important point to heed is that in our
  non-RMI setup (previous example), each instance
  of the client application (LocationStudent)
  created its own instance of the AskTheRegistrar
  service class.
• In the RMI version, we have a single instance of
  the AskTheRegistrar class that will be used by
  any and all clients.

86
RMI Server Example

• Before we can run the server, we need to generate
  a stub class for the remote object that will be
  used by the server.
• We use the rmic utility to do this.
• Server does not exit RMI registry still has an
  open reference to it continuously available to
  any incoming requests
• Now that we have a server, we run it from the
  command line like so

87
Client Example

• Create a client that accesses the AskTheRegistrar
  remote object
• Two critical lines

88
Client Example

• The first uses the same class, Naming, that the
  server did to contact the RMI registry.
• In this case, we are asking the registry for the
  location of a remote object that should be known
  as AskTheRegistrar.
• Note that what is returned is a reference to the
  interface we initially created.
• This interface hides the fact that there is no
  code locally but simply provides some stubs that
  will make a call to the remote object to perform
  the actual work.
• The second line uses the resulting object
  reference as if it were localwhich was our
  original intent

89
Client Example

• We can now see the result of running the client

90
Server Output

• This is the activity on the server
• Server displays the callers (client) IP address

91
Java RMI Example (with a daemon)

• The previous example requires us to run a
  separate Java application in a server capacity to
  register and make a remote object available.
• If we wish to make multiple objects available as
  remote objects, we will either need to run a
  separate server for each or create super-server
  that registers one of each object.
• To solve this problem use the RMI daemon
  (background process), or rmid.
• This one process will replace all of the other
  server processes and allow any remote object to
  be dynamically invoked
• No adjustments to the interface or the client
  code are required.

92
Java RMI with the Daemon Sequence Diagram
93
Java RMI Example (with a daemon)

• The remote object that we created in
  AskTheRegistrarRemote last time needs to be
  modified to inherit from a different, but related
  objectthe Activatable object (which implies that
  the remote object may be activated on demand).
• For clarity, we will rename the class
  AskTheRegistrarRemote to AskTheRegistrarActivation
  .

94
AskTheRegistrarActivation.java
95
Java RMI Example (with a daemon)

• Instead of inheriting from UnicastRemoteObject,
  the remote object now inherits from Activatable.
• Both of the constructors that previously existed
  have been removed and replaced with one that has
  two arguments Activation ID MarshalledObject
  
• Any initialization code we need is placed after
  the parent constructor is called. Other than
  those changes, the code for the remote object is
  the same.
• What has changed the most is that instead of a
  server process, we only need to register the
  object as available for remote invocation and
  then create a stub for use with the new daemon
  process.

96
Java RMI Example (with a daemon)Server
Application

• See AskTheRegistrarRMIDSetup.java
• The server application has undergone some
  dramatic change.
• The first statement inside the main class
  provides the first hint of the alterations.
• We are now operating with enhanced security
  restrictions that we will need to alter in order
  for our remote object to accept any connections
  (a remote object that communicates with nobody is
  not very useful).

97
Java RMI Example (with a daemon)Server
Application

• To accomplish these alterations, we need a
  security manager to be available like so

98
Java RMI Example (with a daemon)Server
Application

• Next, we need to associate a security policy file
  with the remote object when it is activated.
• The policy file simply states that the remote
  object is allowed to accept connections via the
  network from any address as long as the
  originating port is unprivileged (i.e. above
  1023).
• It should be noted that although the policy file
  kind of looks like Java code, it is a
  configuration file and not compiled it remains a
  text file called AskTheRegistrarActivation.policy

99
Java RMI Example (with a daemon)

• The RMI registry is still operating as it was
  previously therefore it must be run first of all
  the server-based processes.
• The RMI daemon must be run next.
• However, it will default to being very
  restrictive with any of the remote objects that
  will be registered with it as available to be
  activated.
• So, we must define a security policy for the RMI
  daemon itself that allows future remote objects
  to be allowed to register their own security
  policy.
• This could potentially open the door for
  arbitrary permissions which is why this step is
  explicitly required.

100
Java RMI Example (with a daemon)

• Our security policy will look like the following
  RMID.policy
• This policy has only one purpose future remote
  objects that will be registered may use the
  security policy located in a file
  Z\CS483\AskTheRegistrarActivation.policy. Any
  other request for elevated permissions will be
  expressly denied.

101
Run the daemon

• By default, the RMI daemon uses a security policy
  of none (which is like selling a padlock with
  no key) so we must change this when running the
  daemon process.
• Start the daemon

102
Java RMI Example (with a daemon)

• Next, we must register the AskTheRegistrar remote
  object with the daemon and registry.
• The registration program itself needs permission
  to contact both the daemon and the registry.
• Here is its policy file

103
Java RMI Example (with a daemon)

• Assuming that the remote AskTheRegistrar object
  will reside on the server known as Alpha, the
  policy file allows the setup program to contact
  the registry on the default port 1098 and the
  daemon on the default port of 1099
• If the registration succeeds like we sincerely
  hope it will, we should get two status messages

104
Java RMI Example (with a daemon)

• It is worthwhile to note here that the remote
  object, AskTheRegistrarActivation, has not yet
  been instantiated and will remain so until the
  first request.
• This is one of the main benefits of the daemon
  implementation in that it conserves resources
  until they are required.

105
Java RMI Example (with a daemon)

• We need to make two more adjustments.
• First, we need to provide the client with a
  stub copy of the remote object (in addition to
  the compiled interface definition).
• After compiling the remote object (which is
  AskTheRegistrarActivation in our example), we
  employ the remote invocation compiler to whittle
  it down to just a stub.

106
Java RMI Example (with a daemon)

• This will produce a file named AskTheRegistrarActi
  vation_stub.class that needs to be distributed
  with the client application (in addition to the
  AskTheRegistrarInterface.class and
  LocalStudentClient.class files).
• Secondly, while the client is unchanged, it now
  requires a security policy file in order to
  access the RMI daemon and registry

107
Java RMI Example (with a daemon)

• The policy file for the client says that
  connections may be made to any non-privileged
  port on the server that contains the remote
  object.
• When executing the client application, we now
  need to include a reference to the security
  policy file.

108
Executed Client Applicationwith Reference to
Policy File
109
LocalStudent Client

• Our invocation of a remote object is now
  functional again!
• That was certainly a lot of work just to learn
  that we are registered for CS483.
• If we look back at the RMI daemon, we note that
  the status messages are displayed, with some
  extra logging information about execution groups
  (which we did not cover).

110
RMI Daemon

• Again, the benefit to this setup is that we can
  run with just the daemon and dynamically register
  objects with the daemon and the object will be
  immediately available for use by clients.

111
Java RMI Resources

• http//java.sun.com/docs/books/tutorial/rmi/index.
  html
• http//java.sun.com/j2se/1.5.0/docs/guide/rmi/inde
  x.html
• http//java.sun.com/j2se/1.5.0/docs/guide/rmi/spec
  /rmiTOC.html
• http//java.sun.com/j2se/1.5.0/docs/tooldocs/windo
  ws/rmic.html