61A Lecture 33 - inst.eecs.berkeley.edu › ~cs61a › fa13 › slides › 33-Distributed_… · •No lecture on Wednesday 11/27 or Friday 11/29 •No discussion section Wednesday

61A Lecture 33

Monday, November 25

Announcements

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Announcements

• Homework 10 due Tuesday 11/26 @ 11:59pm

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Announcements


• No lecture on Wednesday 11/27 or Friday 11/29

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Announcements



• No discussion section Wednesday 11/27 through Friday 11/29

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Announcements




!Lab will be held on Wednesday 11/27

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Announcements





• Recursive art contest entries due Monday 12/2 @ 11:59pm

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Announcements






• Guerrilla section about logic programming coming soon...

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Announcements






• Guerrilla section about logic programming coming soon...

• Homework 11 due Thursday 12/5 @ 11:59pm

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Addition in Logic

(Demo)

Distributed Computing


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A distributed computing application consists of multiple programs running on multiple computers that together coordinate to perform some task.

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• Computation is performed in parallel by many computers.

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• Information can be restricted to certain computers.

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• Redundancy and geographic diversity improve reliability.

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Characteristics of distributed computing:

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• Computers are independent — they do not share memory.

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• Coordination is enabled by messages passed across a network.

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• Individual programs have differentiating roles.

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Distributed computing for large-scale data processing:

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• Databases respond to queries over a network.

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• Databases respond to queries over a network.

• Data sets can be partitioned across multiple machines (next lecture).

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Network Messages

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Network Messages

Computers communicate via messages: sequences of bytes transmitted over a network.

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Network Messages


Messages can serve many purposes:

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Network Messages



• Send data to another computer

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Network Messages




• Request data from another computer

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Network Messages





• Instruct a program to call a function on some arguments.

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Network Messages






• Transfer a program to be executed by another computer.

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Network Messages







Messages conform to a message protocol adopted by both the sender (to encode the message) & receiver (to interpret the message).

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Network Messages








• For example, bits at fixed positions may have fixed meanings.

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Network Messages









• Components of a message may be separated by delimiters.

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Network Messages









• Components of a message may be separated by delimiters.

• Protocols are designed to be implemented by many different programming languages on many different types of machines.

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Internet Protocol

The Internet Protocol

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The Internet Protocol (IP) specifies how to transfer packets of data among networks.

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• Networks are inherently unreliable at any point.

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• The structure of a network is dynamic, not fixed.

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• No system exists to monitor or track communications.

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http://en.wikipedia.org/wiki/IPv4






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IPv4

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IPv4

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All machines know IPv4







Where to send the packet

IPv4

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Where to send error reports

IPv4

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IPv4

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E.g.,192.168.1.1










The packet knows its sizeIPv4

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E.g.,192.168.1.1











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Max length: 216 = 65,536

E.g.,192.168.1.1










Packets can't survive forever


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Max length: 216 = 65,536

E.g.,192.168.1.1












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Max length: 216 = 65,536

E.g.,192.168.1.1


Decremented on forwarding







Packets are forwarded toward their destination on a best effort basis.

Programs that use IP typically need a policy for handling lost packets.





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Max length: 216 = 65,536

E.g.,192.168.1.1


Decremented on forwarding

Transmission Control Protocol

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The design of the Internet Protocol (IPv4) imposes constraints:

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• Packets are limited to 65,535 bytes each.

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• Packets may arrive in a different order than they were sent.

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• Packets may be duplicated or lost.

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The Transmission Control Protocol (TCP) improves reliability:

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• Ordered, reliable transmission of arbitrary byte streams.

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• Implemented using the IP. Every TCP connection involves sending IP packets.

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• Each packet in a TCP session has a sequence number:

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• Each packet in a TCP session has a sequence number:!The receiver can correctly order packets that arrive out of order.

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• Each packet in a TCP session has a sequence number:!The receiver can correctly order packets that arrive out of order.!The receiver can ignore duplicate packets.

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• All received packets are acknowledged; both parties know that transmission succeeded.

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• Packets that aren't acknowledged are sent repeatedly.

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• Packets that aren't acknowledged are sent repeatedly.

The socket module in Python implements the TCP.

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TCP Handshakes

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TCP Handshakes

All TCP connections begin with a sequence of messages called a "handshake" which verifies that communication is possible.

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TCP Handshakes


"Can you hear me now?" Let's design a handshake protocol.

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TCP Handshakes



Handshake Goals:

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TCP Handshakes



Handshake Goals:

• Computer A knows that it can send data to and receive data from Computer B.

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TCP Handshakes



Handshake Goals:


• Computer B knows that it can send data to and receive data from Computer A.

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TCP Handshakes



Handshake Goals:



• Lots of separate connections can exist without any confusion.

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TCP Handshakes



Handshake Goals:




• The number of required messages is minimized.

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TCP Handshakes



Handshake Goals:





Communication Rules:

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TCP Handshakes



Handshake Goals:






• Computer A can send an initial message to Computer B requesting a new connection.

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TCP Handshakes



Handshake Goals:







• Computer B can respond to messages from Computer A.

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TCP Handshakes



Handshake Goals:







• Computer B can respond to messages from Computer A.

• Computer A can respond to messages from Computer B.

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Message Sequence of a TCP Connection

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Computer A

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Computer A Computer B

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Synchronization request

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Acknowledgement & synchronization request

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Acknowledgement

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Acknowledgement

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Acknowledgement

Establishes packet numbering system

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Acknowledgement


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..

Data message from A to B

Data message from B to A

..Acknowledgement

Acknowledgement

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Acknowledgement

Termination signal


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..



..Acknowledgement

Acknowledgement

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Acknowledgement

Termination signalAcknowledgement & termination signal


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..



..Acknowledgement

Acknowledgement

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Acknowledgement


Acknowledgement


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..



..Acknowledgement

Acknowledgement

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Acknowledgement


Acknowledgement


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..



..Acknowledgement

Acknowledgement

..

Client/Server Architecture

The Client/Server Architecture

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One server provides information to multiple clients through request and response messages.

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Server role: Respond to service requests with requested information.

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Client role: Request information and make use of the response.

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Abstraction: The client knows what service a server provides, but not how it is provided.

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Abstraction: The client knows what service a server provides, but not how it is provided.

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Client/Server Example: The World Wide Web

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The client is a web browser (e.g., Firefox):

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• Request content for a location.

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• Interpret the content for the user.

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The server is a web server:

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• Interpret requests and respond with content.

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Web browser Web server

TCP Initialization Handshake

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HTTP GET request of content



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HTTP response with content



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HTTP response with content

Follow-up requests for auxiliary content

...



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The Hypertext Transfer Protocol

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The Hypertext Transfer Protocol (HTTP) is a protocol designed to implement a Client/Server architecture.

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Uniform resource locator (URL)

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Browser issues a GET request to a server at www.nytimes.com for the content (resource) at location "pages/todayspaper".


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Server response contains more than just the resource itself:

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• Status code, e.g. 200 OK, 404 Not Found, 403 Forbidden, etc.

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• Date of response; type of server responding

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• Last-modified time of the resource

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• Last-modified time of the resource

• Type of content and length of content

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Properties of a Client/Server Architecture

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Benefits:

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Benefits:

• Creates a separation of concerns among components.

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Benefits:


• Enforces an abstraction barrier between client and server.

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Benefits:



• A centralized server can reuse computation across clients.

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Benefits:




Liabilities:

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Benefits:




Liabilities:

• A single point of failure: the server.

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Benefits:




Liabilities:


• Computing resources become scarce when demand increases.

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Benefits:




Liabilities:



Common use cases:

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Benefits:




Liabilities:



Common use cases:

• Databases — The database serves responses to query requests.

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Benefits:




Liabilities:



Common use cases:


• Open Graphics Library (OpenGL) — A graphics processing unit (GPU) serves images to a central processing unit (CPU).

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Benefits:




Liabilities:



Common use cases:


• Open Graphics Library (OpenGL) — A graphics processing unit (GPU) serves images to a central processing unit (CPU).

• Internet file and resource transfer: HTTP, FTP, email, etc.

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Peer-to-Peer Architecture

The Peer-to-Peer Architecture

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All participants in a distributed application contribute computational resources: processing, storage, and network capacity.

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Messages are relayed through a network of participants.

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Each participant has only partial knowledge of the network.

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Each participant has only partial knowledge of the network.

http://en.wikipedia.org/wiki/File:P2P-network.svg 19

Network Structure Concerns



Some data transfers on the Internet are faster than others.




The time required to transfer a message through a peer-to-peer network depends on the route chosen.






















Example: Skype

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Example: Skype

Skype is a Voice Over IP (VOIP) system that uses a hybrid peer-to-peer architecture.

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Example: Skype


Login & contacts are handled via a centralized server.

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Example: Skype



Conversations between two computers that cannot send messages to each other directly are relayed through supernodes.

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Example: Skype




Any Skype client with its own IP address may be a supernode.

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Example: Skype





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Client A

Example: Skype





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Client A Client B

Example: Skype





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Client A Client B

Clients behind firewalls cannot

communicate directly

Example: Skype





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Client A Client B

Client CClients behind

firewalls cannot communicate directly

Example: Skype





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Client A Client B

Client CA client not behind a firewall may be used

as a supernode



Example: Skype





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Client A Client B


as a supernode



Example: Skype





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Client A Client B


as a supernode



Documents

61A Lecture 33 - inst.eecs.berkeley.edu › ~cs61a › fa13 › slides › 33-Distributed_… · •No lecture on Wednesday 11/27 or Friday 11/29 •No discussion section Wednesday