6.033 Spring 2018Lecture #1

• Complexity• Modularity and abstraction• Enforced modularity via client/server models

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what is a system? a set of interconnected components that has an

expected behavior observed at the interface with its environment

what makes building systemsdifficult?

complexity

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source: http://www.informationisbeautiful.net/visualizations/million-lines-of-code/

Today’s Systems are Incredibly Complex

© source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use.

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complexity limits what we can build and causes a number of unforeseen issues

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how do we mitigate complexity?

with design principles such as modularity and abstraction

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how do we enforce modularity?

one way is to use the client/server model

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Class Server(on machine 2) Class Browser

(on machine 1)

requestdef main(): def server_load_url(): html = browser_load_url(URL) ...

... return html

reply

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Stub Clients and RPCs Class Browser

(on machine 1)

def main(): def server_load_url(): html = browser_load_url(URL) ...

... return html

Class Server(on machine 2)

def browser_load_url(url): msg = url # could reformat send request wait for reply html = reply # could reformat

stub

return html

request

reply

def handle_server_load_url(url): wait for request url = request html = server_load_url(URL) reply = htmlsend reply stub

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Challenges with RPCs

Client Server

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Challenges with RPCs

Client Serverinternet

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Challenges with RPCs

Client Serverinternet

load(“view.html?item”)

X

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Challenges with RPCs

Client Serverinternet

load(“view.html?item”)

Xload(“view.html?item”)

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Challenges with RPCs

Client Serverinternet

load(“buy.html?item&ccNo=xxx”)

X

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Challenges with RPCs

Client Serverinternet

load(“buy.html?item&ccNo=xxx”)

Xload(“buy.html?item&ccNo=xxx”)

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Challenges with RPCs

Client Serverinternet

load(“buy.html?item&ccNo=xxx”)

Xload(“buy.html?item&ccNo=xxx”)

problem: just bought the same thing twice6.033 | spring 2018 | Katrina LaCurts

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Challenges with RPCs

Client Serverinternet

load(“buy.html?UID”)

Xload(“buy.html?UID”)

client | UID | reply

state on server

replay results from table instead of reprocessing

order

problem: server can still fail

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What else might we want?

Client Serverinternet

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What else might we want?

Serverinternet

scalability

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What else might we want?

internet

scalability fault-tolerance/reliability

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What else might we want?

internet

scalability fault-tolerance/reliability

!

!!

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What else might we want?

internet

scalability fault-tolerance/reliability

!

"

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security

"

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• Complexity limits what we can build, but can bemitigated with modularity and abstraction

• One way to enforce modularity is with a client/servermodel, where the two modules reside on differentmachines and communicate with RPCs; network/serverfailures are still an issue

next lecture: naming, which allows modules to communicate

coming up: operating systems, which enforce modularity on a single machine

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6.033 Computer System EngineeringSpring 2018

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