Dos unit 2

 A process is a program in execution. When we say
that two computers of a distributed system are
communicating with each other, we mean that two
processes, one running on each computer, are in
communication with each other.
 The two basic methods for information sharing are
as follows:
1. Original sharing or shared-data approach
2. Copy sharing or message-passing approach

 Simplicity
 Uniform semantics
 Efficiency
 Reliability
 Correctness
 Flexibility
 Security
 Portability

 A message is a block of information formatted by a
sending process in such a manner that it is
meaningful to the receiving process.
1. Address
2. Sequence number
3. Structural information

 Address: It contains characters that uniquely
identify the sending and receiving process in the
network.
 Sequence number: This is the message
identifier(ID), which is very useful for identifying
lost messages and duplicate messages in case of
system failures.
 Structural information: This element also has two
parts.

 A central issue in the communication structure is the
synchronization imposed on the communicating
processes by the communication primitives.
 One of the following two methods is commonly
used for this purpose:
1. Polling
2. Interrupt

 In interprocess communication, the message-
buffering strategy is strongly related to
synchronization strategy.
 The synchronous and asynchronous modes of
communication correspond respectively to the two
extremes of bufffering: a null buffer and a buffer
with unbounded capacity.

 In case of no buffering, there is no place to
temporarily store the message.
1. The message remains in the sender process
address space and the execution of the send is
delayed until the receiver executes the
corresponding receive.
2. The message is simply discarded and the
timeout mechanism is used to resend the message
after a timeout period.

 The null buffer strategy is generally not suitable for
synchronous communication between two process
in a distributed system because if the receiver is not
ready, a message has to be transferred two or more
times, and the receiver of the message has to wait
for the entire time taken to transfer the message
across the network.

 In the asynchronous mode of communication, since
a sender does not wait for the receiver to be ready,
there may be served pending messages that have not
yet been accepted by the receiver.
 Therefore, an bounded-capacity message buffer that
can store all unreceived messages is needed to
support asynchronous communication with the
assurance that all the messages sent to the receiver
will be delivered.

 Unbounded capacity of a buffer is practically
impossible.
 Therefore, in practice, systems using asynchronous
mode of communication use finite-bound buffers,
also known as multiple-message buffers.
1. Unsuccessful communication
2. Flow-controlled communication

 Almost all networks have an upper bound on the
size of data that can be transmitted at a time.
 This size is known as the maximum transfer
unit(MTU) of a network.
 On the other hand, messages larger than the MTU of
the network have to be fragmented and sent in
multiple packets.
 Such messages are known as multidatagram
messages.

 Due to the problems in transferring program objects in
their original form, they are first converted to stream
form that is suitable for transmission and placed into a
message buffer.
 This conversion process takes place on the sender side
and is known as encoding of a message data.
 The encoded message, when received by the receiver,
must be converted back from the stream form to the
original program objects before it can be used.
 The process of reconstruction of program objects from
message data on the receiver side is known as decoding
of the message data.

 Another important issue in message-based
communication is addressing of the parties involved
in an interation.
 A message-passing system usually supports two
types of process addressing:
1) Explicit addressing
2) Implicit addressing

 While a distributed system may offer potential for
parallelism, it is also prone to partial failures such as
a node crash or a communication link failure.
1) Loss of request message
2) Loss of response message
3) Unsuccessful execution of the request

1) Loss of request message: This may happen either
due to the failure communication link between the
sender and receiver or because the receiver’s node
is down at the time the request message reaches
there.
2) Loss of response message: This may happen either
due to the failure of communication link between
the sender and receiver or because the sender’s
node is down at the time the response message
reaches there.
3) Unsuccessful execution of the request: This
happens due to the receiver’s node crashing while
the request is being processed.

 The most elementary form of message-based
interaction is one-to-one communication in which a
single-sender process sends a message to a single-
receiver process.
1) One to many(single sender and multiple
receivers)
2) Many to one(multiple senders and single
receiver)
3)Many to many(multiple senders and multiple
receivers)

 In this scheme, there are multiple receivers for a
message sent by a single sender.
 One-to-many scheme is also known as multicast
communication.
 A special case of multicast communication is
broadcast communication, in which the message is
sent to all processors connected to a network.

 In this scheme, multiple senders send messages to a
single receiver.
 The single receiver may be selective or nonselective.
 A selective receiver specifies a unique sender, a
message exchange takes place only if that sender
sends a message.
 On the other hand, a non selective receiver specifies
a set of senders, and if any one sender in the set
sends a message to this receiver, a message
exchange takes place.

 In this scheme, multiple senders send messages to
multiple receivers.
 The one-to-many and many-to-one schemes are
implicit in this scheme.
 Hence the issues related to one-to-many and many-
to-one schemes, which already been described
above, also apply to the many-to-many
communication scheme.
 In addition, an important issue related to many-to-
many communication scheme is that of ordered
message delivery.

 The commonly used semantics for ordered delivery
of multicast messages are :
1) Absolute ordering
2) Consistent ordering
3) Casual ordering

Dos unit 2

More Related Content

What's hot (20)

Similar to Dos unit 2 (20)

Recently uploaded (20)

Dos unit 2