Communication

1
Communication
Part II Message-Oriented Communication
2
Giving credit where credit is due
CSCE455/855 Distributed Operating Systems

• Most of the lecture notes are based on slides by
  Prof. Jalal Y. Kawash at Univ. of Calgary
• I have modified them and added new slides

3
RPC and Message Passing

• RPC enhances access transparency by hiding
  communication
• RPC is inherently synchronous and is not always
  appropriate
• What if the receiving side is not executing when
  the request is sent?
• Sometimes, we need to resort to message-passing

4
Persistence and Synchronicity in Communication (1)
2-20

• General organization of a communication system in
  which hosts are connected through a network

5
Persistence and Synchronicity in Communication (2)

• Persistent communication of letters back in the
  days of the Pony Express.

6
Persistence in Communication

• Persistence A (sent) message is stored by the
  communication system until it is delivered.
• Transient A (sent) message is stored by the
  communication system as long as both the sender
  and receiver applications are executing

7
Synchrony in Communication

• Asynchronous The sender does not block until the
  message is delivered, blocks until stored in a
  buffer at the sending host, or the first
  communication server
• Synchronous The sender blocks until the message
  is stored in a buffer at the receiving host, or
  delivered to the receiver

8
Persistent Asynchronous Communication
2-22.1
9
Persistent Synchronous Communication
2-22.1
10
Transient Asynchronous Communication
2-22.2
11
Transient Synchronous Communication (1)
2-22.2
12
Transient Synchronous Communication (2)

• Delivery-based transient synchronous
  communication at message delivery
• Response-based transient synchronous communication

13
Message-Oriented Transient Communication

• Berkley (UNIX) Sockets
• An abstraction of the actual communication
  endpoints
• Message Passing Interface (MPI)
• a standard for message passing
• Makes applications platform independent
• Short-term storage for messages
• Milliseconds to seconds transfer time

14
Berkeley Sockets (1)
Java

• Socket primitives for
  TCP/IP.

15
Berkeley Sockets (2)
ServerSocket()
Socket()

• Connection-oriented communication pattern using
  sockets.

16
MPI (1)

• Processes are grouped (non-disjoint groups)
• A process is identified by (groupID, processID)
• Used instead of a transport-level address
• Offers communication primitives for all kinds
  forms of communication

17
MPI (2)

• Some of the most intuitive message-passing
  primitives of MPI.

18
Message-Oriented Persistent Communication

• Message Queuing systems
• Offers intermediate-term storage capacity for
  messages
• Without requiring any of the sender or receiver
  to be active during transmission
• Intermediate-term minutes of transfer time

19
Message-Queuing Model (1)

• Applications communicate by inserting messages
  into queues
• Each application has a private queue
• Other applications send message to that queue
• The private queue can be read only by the
  associated application
• Applications can share a queue
• A queue has a system-wide name (address)

20
Message-Queuing Model (2)

• A messages travels through communication servers
  to get to its destination
• It is popular to have direct connections between
  each pair of communication servers
• Persistence a message will eventually be
  inserted into the recipients queue

21
Loosely-Coupled Communication

• Receiver not necessarily executing when the
  message is added to its queue
• Sender not necessarily executing when message
  is picked up by the receiver

Sender
Receiver
4 execution modes while transmitting messages
22
Execution Modes
2-26

• Four combinations for loosely-coupled
  communications using queues.

23
Queuing System Interface

• Basic interface to a queue in a message-queuing
  system.

24
General Architecture of a Message-Queuing System
(1)
A
B

• Nearby the sender
• Messages can be put to this queue only

25
General Architecture of a Message-Queuing System
(2)

• The collection of queues is distributed across
  multiple machines
• The system must maintain a (possibly distributed)
  database of queues (queuing tables)

Queuing table entry
26
General Architecture of a Message-Queuing System
(3)

• The relationship between queue-level addressing
  and network-level addressing.

27
General Architecture of a Message-Queuing System
(4)

• Queues are managed by queue managers
• A queue manager interacts directly with the
  sending/receiving application
• Some (special) queue managers act like routers or
  relays

28
General Architecture of a Message-Queuing System
(5)
2-29

• Routing is static
• Medium-level scalability (manually configure the
  routing tables)

29
Integrating Differing Message-Based Systems
A-specific message format
B-specific message format
30
Message Brokers
2-30

• The general organization of a message broker in a
  message-queuing system.

31
In-Class Exercises
1. C has a construction called a union, in which
a field of a record (called a struct in C) can
hold any one of several alternatives. At run
time, there is no sure-fire way to tell which one
is in there. Does this feature of C have any
implications for remote procedure call? Explain
your answer. 2. One way to handle parameter
conversion in RPC systems is to have each machine
send parameters in its native representation,
with the other one doing the translation, if need
be. The native system could be indicated by a
code in the first byte. However, since locating
the first byte in the first word is precisely the
problem, can this actually work?
32
In-Class Exercises
3. With persistent communication, a receiver
generally has its own local buffer where messages
can be stored when the receiver is not executing.
To create such a buffer, we may need to specify
its size. Give an argument why this is
preferable, as well as one against specification
of the size. 4. Explain why transient
synchronous communication has inherent
scalability problems.