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Persistent and Transient Objects. Persistent objects continue to exist even if they aren’t in the address space of a server process Transient objects existence depends on having a server. Binding a Client to an Object. Unlike RPC, distributed objects have systemwide object references

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persistent and transient objects
Persistent and Transient Objects
  • Persistent objects continue to exist even if they aren’t in the address space of a server process
  • Transient objects existence depends on having a server
binding a client to an object
Binding a Client to an Object
  • Unlike RPC, distributed objects have systemwide object references
  • The system may support either implicit binding or explicit binding
  • The object reference may contain - IP address, port, object name
  • Or use a location server so we need only address for this server plus the object name
binding a client to an object3
Binding a Client to an Object
  • (a) Example with implicit binding using only global references
  • (b) Example with explicit binding using global and local references

Distr_object* obj_ref; //Declare a systemwide object referenceobj_ref = …; // Initialize the reference to a distributed objectobj_ref-> do_something(); // Implicitly bind and invoke a method


Distr_object objPref; //Declare a systemwide object referenceLocal_object* obj_ptr; //Declare a pointer to local objectsobj_ref = …; //Initialize the reference to a distributed objectobj_ptr = bind(obj_ref); //Explicitly bind and obtain a pointer to the local proxyobj_ptr -> do_something(); //Invoke a method on the local proxy


parameter passing
Parameter Passing
  • Since we have systemwide object refs, we don’t have the same types of problems we had with RPCs and pointers
  • However, for performance motives we may want to treat object ref parameters differently depending on where the object resides
parameter passing5
Parameter Passing
  • The situation when passing an object by reference or by value.


java rmi
Java RMI
  • Java offers remote objects as the only type of distributed object
  • One difference between local and remote objects is that synchronized methods work differently on the two types
    • Blocking applies only to the proxies of the remote objects
  • A parameter passed to an RMI must be serializable
message oriented communication
Message-Oriented Communication
  • Neither RPC nor RMI works when we can’t assure that the receiving side isn’t executing
    • We can use messaging in this case
message oriented communication8
Message-Oriented Communication
  • General organization of a communication system in which hosts are connected through a network


messaging modes
Messaging Modes
  • Messaging systems can be either persistent or transient
    • Are messages retained when the senders and/or receivers stop executing?
  • Can also be either synchronous or asynchronous
    • Blocking vs. non-blocking
persistent communication
Persistent Communication
  • Persistent communication of letters back in the days of the Pony Express.
persistence and synchronicity in communication
Persistence and Synchronicity in Communication
  • Persistent asynchronous communication
  • Persistent synchronous communication


persistence and synchronicity in communication12
Persistence and Synchronicity in Communication
  • Transient asynchronous communication
  • Receipt-based transient synchronous communication


persistence and synchronicity in communication13
Persistence and Synchronicity in Communication
  • Delivery-based transient synchronous communication at message delivery
  • Response-based transient synchronous communication
message oriented transient communication
Message-Oriented Transient Communication
  • Sockets are an example of message-oriented transient communication
  • The Message-Passing Interface (MPI) is a newer set of message-oriented primitives for multicomputers
    • MPI communication takes place within a known group of processes
      • A (groupID, processID) pair uniquely identifies a source or destination of a message
the message passing interface mpi
The Message-Passing Interface (MPI)
  • Some of the most intuitive message-passing primitives of MPI.
message oriented persistent communication
Message-Oriented Persistent Communication
  • Known as message-queuing systems or Message-Oriented Middleware (MOM)
    • Support persistent asynchronous communication
    • Generally have slow communications
    • Similar to e-mail systems
    • Basic model - applications communicate by inserting messages in specific queues
message queuing model
Message-Queuing Model
  • Four combinations for loosely-coupled communications using queues.


message queuing model18
Message-Queuing Model
  • Basic interface to a queue in a message-queuing system.
general architecture of a message queuing system
General Architecture of a Message-Queuing System
  • Messages are inserted into a local source queue
    • The message contains the name of a destination queue
  • The message-queuing system transfers messages to the destination queue
    • Use a db which maps queue names to network locations
general architecture of a message queuing system20
General Architecture of a Message-Queuing System
  • Queues are managed by queue managers
    • Special queue managers act as relays which forward messages to other managers
general architecture of a message queuing system21
General Architecture of a Message-Queuing System
  • The relationship between queue-level addressing and network-level addressing.
general architecture of a message queuing system22
General Architecture of a Message-Queuing System
  • The general organization of a message-queuing system with routers.


message brokers
Message Brokers
  • Message-queuing systems can be used to integrate existing and new applications
    • These diverse applications have different message formats
    • Since we have old apps, can’t use a standard message format
    • So use message brokers which convert messages from one format to another
message brokers24
Message Brokers
  • The general organization of a message broker in a message-queuing
  • system.


example ibm mqseries
Example: IBM MQSeries
  • IBM MQSeries is used to integrate old apps (generally running on IBM mainframes)
    • Queues are managed by queue managers
    • Queue managers are connected through message channels
    • Each of the two ends of the message channel is managed by a message channel agents (MCA)
    • Queue managers can be linked into the same process as the application using the queue
    • Queue managers implemented using RPC
example ibm mqseries26
Example: IBM MQSeries
  • General organization of IBM's MQSeries message-queuing system.


  • Some attributes associated with message channel agents.
  • In order to be able to change the name of a queue manager or to replace it with another without having to recompile all of the applications which send messages to it, local aliases are used for queue manager names.
message transfer
Message Transfer
  • The general organization of an MQSeries queuing network using routing tables and aliases.
message transfer30
Message Transfer
  • Primitives available in an IBM MQSeries MQI
stream oriented communication
Stream-Oriented Communication
  • Multimedia systems use stream-oriented communications
    • The timing of the data delivery is critical in such systems
    • Such communication is used for continuous media such as audio where the temporal relationships between different data items are meaningful as opposed to discrete media such as text
stream oriented communication32
Stream-Oriented Communication
  • Data streams have several modes
    • Asynchronous transmission mode places no timing constraints on the data items in a stream
    • Synchronous transmission mode gives a maximum end-to-end delay for each item in a data stream
    • Isochronous transmission mode gives both maximum and minimum delays
      • Bounded jitter
stream oriented communication33
Stream-Oriented Communication
  • Streams can be either simple or complex (with several related simple substreams)
    • Related substreams will need to be synchronized
  • Streams can be be seen as a channel between a source and a sink
    • Source could be a file or multimedia capture device
    • Sink could be a file or multimedia rendering device
data stream
Data Stream
  • Setting up a stream between two processes across a network.
data stream35
Data Stream
  • Setting up a stream directly between two devices.


data stream36
Data Stream
  • An example of multicasting a stream to several receivers.
streams and qos
Streams and QoS
  • Time-dependent requirements are generally expressed as Quality of Service (QoS) requirements
    • The underlying distributed system and network must ensure that these are met
    • We can express such requirements using a flow specification
    • Partridge’s model uses a token bucket algorithm
specifying qos
Specifying QoS
  • A flow specification.
specifying qos39
Specifying QoS
  • The principle of a token bucket algorithm.
setting up a stream
Setting up a Stream
  • Before a stream is opened between source and sink resources through the network must be reserved in order to meet the QoS requirements
    • Bandwidth
    • Buffers
    • Processing capability
    • Figuring out how much of each is required is difficult since they aren’t specified directly in the QoS
    • RSVP is a protocol for enabling resource reservations in network routers
setting up a stream41
Setting Up a Stream
  • The basic organization of RSVP for resource reservation in a distributed
  • system.
stream synchronization
Stream Synchronization
  • An important issue is that different streams (possibly substreams of a complex stream) must be synchronized
    • Continuous with discrete
    • Continuous with continuous (more difficult)
    • Different levels of granularity for syncing required depending on situation
synchronization mechanisms
Synchronization Mechanisms
  • Synchronization can be carried out by the application
  • Can also be supplied by a middleware layer
  • Complex streams are multiplexed according to a given synchronization specification (e.g. MPEG)
  • Syncing can occur either at the sending or receiving end.
synchronization mechanisms44
Synchronization Mechanisms
  • The principle of explicit synchronization on the level of data units.
synchronization mechanisms45
Synchronization Mechanisms
  • The principle of synchronization as supported by high-level interfaces.