1. Block 2: Interprocess Communications
Jin Sa
22-Apr-19
Client-Server Programming

2. Client-Server Programming
Outline of block 2
Characteristics of IPC
Unicast and multicast
Operations in IPC
Event synchronisation
Synchronous and asynchronous
Event diagram
Deadlocks and timeout
Data representation and data marshalling
External representation
Data marshalling and unmarshalling
Text-based protocols
22-Apr-19
Client-Server Programming

3. Interprocess Communications
The backbone of distributed computing is interprocess communications (IPC).
Distributed computing requires information to be exchanged among independent processes.
In distributed computing, two or more processes engage in IPC in a protocol agreed upon by the processes. A process may be a sender at some points during a protocol, a receiver at other points.
Interprocess Communications in Unix, the Nook and Crannies by John Shapley Gray. Prentice Hall.
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Client-Server Programming

4. Interprocess Communications in Distributed Computing
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Client-Server Programming

5. Characteristics of IPC
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Client-Server Programming

6. IPC – unicast and multicast
When communication is from one process to a single other process, the IPC is said to be a unicast. When communication is from one process to a group of processes, the IPC is said to be a multicast. In this semseter, We will be mainly concerned with unicast.
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Client-Server Programming

7. Client-Server Programming
Unicast vs. Multicast
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Client-Server Programming

8. Operations provided in Interprocess Communications API
Send transmitting data to a receiving process
Receive accept data from a sending process
Connect establish a logical connection between two processes. The sending process issues a request for connection, the other process issues an accept connection operation
Disconnect de-allocate a previously established connection at both sides
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Client-Server Programming

9. Interprocess Communication in basic HTTP
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Client-Server Programming

10. Event Synchronization
Interprocess communication requires that the two processes synchronize their operations: one side sends, then the other receives until all data has been sent and received.
The synchronization requires system support.
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Client-Server Programming

11. Synchronous vs. asynchronous Communication
The IPC operations may provide the synchronization necessary using blocking. A blocking operation issued by a process will block further processing of the process until the operation is fulfilled.
Alternatively, IPC operations may be asynchronous or nonblocking. An asynchronous operation issued by a process will not block further processing of the process. Instead, the process is free to proceed with its processing, and may optionally be notified by the system when the operation is fulfilled.
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Client-Server Programming

12. Synchronous send and receive
Event diagram
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Client-Server Programming

13. Asynchronous send and synchronous receive
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Client-Server Programming

14. Synchronous send and async. receive - 1
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Client-Server Programming

15. Synchronous send and async. receive - 1
The data requested by the receive operation has already arrived at the time the receive operation is issued. In this case, the data is delivered to process 2 immediately, and an acknowledgement from host 2’s IPC facility will unblock process 1.
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Client-Server Programming

16. Synchronous send and async. receive - 2
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Client-Server Programming

17. Synchronous send and async. receive - 2
The data requested by the receive operation has not yet arrived. Process 1 is blocked indefinitely until process 2 reissue a receive request and an acknowledgement arrives from host 2’s IPC facility.
It is common to use a loop to repeatedly issue a receive operation until the awaited data is received.
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Client-Server Programming

18. Synchronous send and async. receive - 3
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Client-Server Programming

19. Synchronous send and async. receive - 3
The data requested by the receive operation has not yet arrived. Host 2’ IPC facility will notify process 2 when the data arrives.
This scenarios requires process 2 provide a listener that can be invoked by the IPC facility to notify the process of the arrival of the data.
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Client-Server Programming

20. Deadlocks and Timeouts
Operations can result in indefinite blocking
For example, a blocking connect request can result in the requesting process to be suspended indefinitely if the connection is unfulfilled or cannot be fulfilled, perhaps as a result of a breakdown in the network .
It is generally unacceptable for a requesting process to “hang” indefinitely. Indefinite blocking can be avoided by using timeout.
Indefinite blocking may also be caused by a deadlock
22-Apr-19
Client-Server Programming

21. Client-Server Programming
Example
The daytime service is the simplest network service, whereby a client process obtains a timestamp (the time of day on the server host) from the server process.
Is it possible for a daytime client to be blocked indefinitely? Explain your answer.
yes, if after issuing a block receive operation, server does not respond and there is no time out defined.
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Client-Server Programming

22. Indefinite blocking due to a deadlock
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Client-Server Programming

23. Client-Server Programming
Event diagram
Event diagram can be used to document the detailed sequence of events and blocking during the of a protocol.
The execution of each process with respect to time is represented using a vertical line.
A solid line interval represents a period that the process is active.
A broken line interval represents the process is blocked.
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Client-Server Programming

24. In class exercises Question1
Q1:Process A sends a single message to process B using connectionless IPC. To do so, A issues a send operation (specifying the message as argument) sometime during its execution, and B issues a receive operation. Suppose the send operation is blocking and the receive operation is non-blocking. Draw an event diagram for each of the following scenario:
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Client-Server Programming

25. In class exercises Question 1 cont.
Process A issues its send operation prior to process B issues its receive operation.
Process B issues its receive operation prior to process A issues its send operation.
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Client-Server Programming

26. Client-Server Programming
Question 1 answer
The data requested by the receive operation has already arrived, the data is delivered to process B and an acknowledgement from host 2’s IPC facility will unblock process A subsequently.
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Client-Server Programming

27. Client-Server Programming
Question 1 answer
The data requested by the receive operation has not yet arrived, no data is delivered to the process. It is the receiving process’ responsibility to ascertain that it has indeed received the data and, if necessary, repeat the receive operation until the data has arrived. Process 1 is blocked indefinitely until process 2 reissues a receive request and an acknowledgement eventually arrives from host 2’s IPC facility.
22-Apr-19
Client-Server Programming

28. Client-Server Programming
Question 1 answer
The data requested by the receive operation has not yet arrived. The IPC facility of host 2 will notify process 2 when the data it requested has arrived, at which point process 2 may proceed to process the data. This scenario requires that process 2 provides a listener or event handler which can be invoked by the IPC facility to notify the process of the arrival of the requested data.
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Client-Server Programming

29. Client-Server Programming
Question 2
Q2: Repeat the last question. This time both operations (send, receive) are blocking.
Process A issues its send operation prior to process B issues its receive operation
Process B issues its receive operation prior to process A issues its send operation.
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30. Client-Server Programming
Question 2 answer
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31. Client-Server Programming
Question 2 answer
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32. Client-Server Programming
Question 3
Three processes P1, P2, P3 are engaged in interprocess
At time 1, P3 issues a receive from P2.
At time 2, P1 sends m1 to P2.
At time 3, P2 issues a receive from P1.
At time 4, P2 receives m1.
At time 5, P2 sends message m1 to P3.
At time 6, P3 receives m1; P1 issues receive from P2.
At time 7, P2 issues a receive from P3.
At time 8, P3 sends m2 to P2.
At time 9, P2 receives m2.
At time 10, P2 sends m2 to P1.
At time 11, P1 receives m2.
Draw a time event diagram for P1, P2 and P3 to show the sequence of events. All operations are blocking.
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33. Client-Server Programming
Question 3 answer
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34. Client-Server Programming
Notes
In a system environment such as Java, which supports multithreading, the blocking receive has no disadvantages as it can issue a separate thread to handle the blocking receive, and the other threads remain active.
Non-blocking receive appears to be more efficient, but it requires extra complexity in the receiving process.
Many today’s systems do not generally provide the non-blocking receive operation.
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Client-Server Programming

35. Data Representation and Data Marshalling
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Client-Server Programming

36. External data representation and marshalling
Information stored in a running program is represented as data structures.
Data transmitted on the network is sequence of bytes.
Therefore data structures must be flattened, i.e. convert to sequence of bytes. Before transmission and rebuilt on arrival.
22-Apr-19
Client-Server Programming

37. External data representation and marshalling
Different computers may store primitive values in different formats.
To enable any two computers to exchange binary values (e.g. to support RPC):
The values are converted to an agreed external format before transmission and converted to the local form on arrival
The values are sent in the sender’s format, and receiver converts the value is necessary.
External representation: an agreed standard for the representation of data structures and primitive values.
22-Apr-19
Client-Server Programming

38. External data representation and marshalling
Marshalling is the process of taking a collection of data items and put them into a form suitable for transmission in a message. Unmarshalling is the process of disassembling them on arrival to produce an equivalent collection of data items at the destination.
Marshalling consists of the translation of structured data items and primitive values into an external data representation. Unmarshalling consists of the generation of primitive values from their external data representation and the rebuild of the data structure.
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Client-Server Programming

39. Examples of external data representation
CORBA’s common data representation
Java’s object serialization
XML a text-based markup language that is fast becoming the standard for data interchange on the Web.
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Client-Server Programming

40. Client-Server Programming
Sample XML file
XML has syntax similar to HTML.
Unlike HTML, XML tags tell you what the data means
Example:
<message>
<subject> What is XML </subject>
<text> XML is … </text>
</message>
Middleware to map to local machine’s representation.
22-Apr-19
Client-Server Programming

41. Client-server communication
The IPC form of communication is designed to support roles and message exchanges in typical client-server interaction, where there is a request-reply communication.
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Client-Server Programming

42. Client-Server Programming
Protocol
In a distributed application, two processes perform interprocess communication in a mutually agreed upon protocol.
The specification of a protocol should include
(i) the sequence of data exchange, which can be described using a time event diagram.
(ii) the specification of the format of the data exchanged.
22-Apr-19
Client-Server Programming

43. Request-reply protocols
An important type of protocol is request-reply protocol
One side issues a request and awaits a response from the other side.
The protocl proceeds in an iteration of request-response until the task is complete.
Popular network protocols, including FTP (File Transfer Protocol) and HTTP, are request-response protocols.
22-Apr-19
Client-Server Programming

44. HTTP: A sample protocol
The Hypertext Transfer Protocol is a protocol for a process (the browser) to obtain a document from a web server process.
It is a request-response protocol: a browser sends a request to a web server process, which replies with a response.
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Client-Server Programming

45. Client-Server Programming
HTTP session
In its basic form, HTTP is a text-based, request-response protocol that calls for only one round of exchange of messages.
A web server process constantly listens for incoming request.
A web browser process makes a connection to the web server process, then issues a request.
The server processes the request and dispatches a response include the document requested by the browser.
The browser process then parses the response and displays the document.
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Client-Server Programming

46. Client-Server Programming
Question 4
In the context of IPC, what is meant by data marshaling? (There are two components to data marshalling; name and describe each.) Why is it necessary?
Answer:
The two components of data marshaling are:
Data structures used in programming need to be flattened or serialized, since data transmitted over the network must be serialized.
The data needs to be converted to an external representation so that it may be interpreted correctly by the receiver, which then maps the external representation to its local representation.
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47. Client-Server Programming
Review questions
On completion of block 2, you should be able to
Know the operations in IPC
understand event synchronisation
draw event diagrams to illustrate the events
understand the concept of deadlock and timeout
understand data marshalling
22-Apr-19
Client-Server Programming