prof. chuan-ming liu computer science and information ...cmliu/np/ntut_np_f07u/slides/...prof....

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Mobile Computing & Software Engineering Lab

Concurrency in Clients

Prof. Chuan-Ming LiuComputer Science and Information Engineering

National Taipei University of TechnologyTaipei, TAIWAN

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IntroductionThis chapter considers the issue of concurrency in client softwareIt shows an example client that illustrates concurrent operation

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The Advantage of Concurrency

Servers use concurrency for two main reasons:

Concurrency can improve the observed response time (and therefore the overall throughput to all clients)Concurrency can eliminate potential deadlocks

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The Advantage of Concurrency (cont.)

Concurrency does have advantages in clients:

Concurrent implementations can be easier to program because they separate functionality into conceptually separate componentsConcurrent implementations can be easier to maintain and extend because they make the code modular

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Concurrent clients can contact several servers at the same time, either to compare response time or merge the results the servers returnConcurrency can allow the user to change parameters, inquire about the client status, or control processing dynamically

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This chapter will focus on the idea of interacting with multiple servers at the same timeThe key advantage of using concurrency in clients lies in asynchrony – allows a client to handle multiple tasks simultaneously without imposing a strict execution order on them

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The Motivation for Exercising Control

One possible use of asynchrony arises from the need to separate control functions from normal processingA user who invokes a client may have little or no idea how long it will take to receive a response or how large that response will be

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The Motivation for Exercising Control (cont.)

An appropriately designed concurrent client can permit the user to continue to interact with the client while the client waits for a responseThe user can find out whether any data has been received, choose to send a different request, or to terminate the communication gracefully

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The Motivation for Exercising Control (cont.)

For example, an concurrent implementation can read and process input from the user’s keyboard or mouse concurrently with database search

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Concurrent Contact with Multiple Servers

Concurrency can allow a single client to contact several servers at the same time and to report to the user as soon as it receives a response from any of themConsider how concurrency can enhance a client that uses ECHO to measure throughput

Because it makes all measurements at the same time, they are faster than a non-concurrent client, and are all affected equally by the loads on the CPU and the local network

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Implementing Concurrent Clients

Like concurrent servers, most concurrent client implementations follow one of the two basic approaches:

A multi-thread implementation that each thread handles one functionA single thread implementation that uses select to handle multiple I/O events asynchronously

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Implementing Concurrent Clients (cont.)

As Fig. 17.1 illustrates, multiple threads allow a connection-oriented client to separate I/O processing

An input thread reads from standard input, formulates requests, and sends them to the server over the TCP connectionA separate output thread receives responses from the server and writes them to the standard outputA control thread accepts commands from the user that control processing

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Figure 17.1

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Single-Thread Implementation

Fig. 17.2 shows the process structure used to provide apparent concurrency in a single thread, connection-oriented client (Algorithm 8.5, the examples in Ch 13 through 15)If the input descriptor becomes ready, the client reads the input and either stores it for later use or acts on it immediatelyIf a TCP socket becomes ready for output, the client prepares and sends a request across a TCP connection

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Figure 17.2

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Algorithm 8.5

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Single-Thread Implementation (cont.)

If a TCP socket becomes ready for input, the client reads the response that the server has sent and handles itThe client reads input or responses from the server at whatever rate they are generatedThe client will continue to read and honor control commands even if the server fails to respond

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Single-Thread Implementation (cont.)

A single thread client can become deadlocked if it invokes a system function that blocksThe programmer must be careful to ensure that the client does not block indefinitely waiting for an event that will not occur

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An Example Concurrent Client That Uses ECHO

Fig. 17.2a shows TCPtecho.c that uses the ECHO service described in Ch 7 to measure network throughput to a set of machines

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Execution of the Concurrent Client

Fig. 17.3 shows sample output from three separate executions of TCPtecho

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Figure 17.3

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Concurrency in the Example Code

A concurrent implementation of TCPtechoimproves the program in two ways

Obtain a more accurate measure of the time required for each connection because congestion affects all connection equallyA concurrent version makes TCPtecho more appealing to users (it offers faster response time than a sequential version

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SummaryConcurrent client implementation can offer faster response time and can avoid deadlock problemsConcurrency can help designers separate control and status processing from normal input and output

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Summary (cont.)We studied a TCP client that measures the time required to access the ECHO server one one or more machines

Avoid the differences in throughput caused by network congestion by making all measurements during the same time intervalAppeals to users because it overlaps the measurements

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