Transcript Binding a Client to an Object

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
• 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 Object
Distr_object* obj_ref;
obj_ref = …;
obj_ref-> do_something();
//Declare a systemwide object reference
// Initialize the reference to a distributed object
// Implicitly bind and invoke a method
(a)
Distr_object objPref;
Local_object* obj_ptr;
obj_ref = …;
obj_ptr = bind(obj_ref);
obj_ptr -> do_something();
//Declare a systemwide object reference
//Declare a pointer to local objects
//Initialize the reference to a distributed object
//Explicitly bind and obtain a pointer to the local proxy
//Invoke a method on the local proxy
(b)
a)
b)
(a) Example with implicit binding using only global references
(b) Example with explicit binding using global and local references
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 Passing
• The situation when passing an object by reference or by
value.
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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
• 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 Communication
• General organization of a communication system in which
hosts are connected through a network
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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 of letters back in the days of the
Pony Express.
Persistence and Synchronicity in
Communication
a)
b)
Persistent asynchronous communication
Persistent synchronous communication
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Persistence and Synchronicity in
Communication
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c)
d)
Transient asynchronous communication
Receipt-based transient synchronous communication
Persistence and Synchronicity in
Communication
e)
f)
Delivery-based transient synchronous communication at message delivery
Response-based transient synchronous 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)
• Some of the most intuitive message-passing primitives of
MPI.
Primitive
Meaning
MPI_bsend
Append outgoing message to a local send buffer
MPI_send
Send a message and wait until copied to local or remote buffer
MPI_ssend
Send a message and wait until receipt starts
MPI_sendrecv
Send a message and wait for reply
MPI_isend
Pass reference to outgoing message, and continue
MPI_issend
Pass reference to outgoing message, and wait until receipt starts
MPI_recv
Receive a message; block if there are none
MPI_irecv
Check if there is an incoming message, but do not block
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
• Four combinations for loosely-coupled communications
using queues.
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Message-Queuing Model
• Basic interface to a queue in a message-queuing system.
Primitive
Meaning
Put
Append a message to a specified queue
Get
Block until the specified queue is nonempty, and remove the first message
Poll
Check a specified queue for messages, and remove the first. Never block.
Notify
Install a handler to be called when a message is put into the specified
queue.
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
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
System
•
The relationship between queue-level addressing and
network-level addressing.
General Architecture of a Message-Queuing
System
• The general organization of a message-queuing system
with routers.
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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 Brokers
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• The general organization of a message broker in a
message-queuing
•
system.
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 MQSeries
• General organization of IBM's MQSeries message-queuing
system.
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Channels
• Some attributes associated with message channel agents.
Attribute
Description
Transport type
Determines the transport protocol to be used
FIFO delivery
Indicates that messages are to be delivered in the order they are sent
Message length
Maximum length of a single message
Setup retry
count
Specifies maximum number of retries to start up the remote MCA
Delivery retries
Maximum times MCA will try to put received message into queue
Aliases
• 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
• The general organization of an MQSeries queuing network
using routing tables and aliases.
Message Transfer
Primitive
Description
MQopen
Open a (possibly remote) queue
MQclose
Close a queue
MQput
Put a message into an opened queue
MQget
Get a message from a (local) queue
• Primitives available in an IBM MQSeries MQI
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
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
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
• Setting up a stream between two processes across a
network.
Data Stream
• Setting up a stream directly between two devices.
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Data Stream
• An example of multicasting a stream to several receivers.
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
Characteristics of the Input
Service Required
•maximum data unit size (bytes)
•Token bucket rate (bytes/sec)
•Token bucket size (bytes)
•Maximum transmission rate
(bytes/sec)
•Loss sensitivity (bytes)
•Loss interval (sec)
•Burst loss sensitivity (data units)
•Minimum delay noticed (sec)
•Maximum delay variation (sec)
•Quality of guarantee
• A flow specification.
Specifying QoS
• The principle of a token bucket algorithm.
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 Stream
• The basic organization of RSVP for resource reservation in
a distributed
• system.
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 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 Mechanisms
• The principle of explicit synchronization on the level of
data units.
Synchronization Mechanisms
• The principle of synchronization as supported by highlevel interfaces.
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