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COMP 150-IDS: Internet Scale Distributed Systems (Spring 2017)

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Presentation on theme: "COMP 150-IDS: Internet Scale Distributed Systems (Spring 2017)"— Presentation transcript:

1 COMP 150-IDS: Internet Scale Distributed Systems (Spring 2017)
HTTP & REST Noah Mendelsohn Tufts University Web: Copyright 2012, 2103, 2015 & 2017

2 Architecting a universal Web
Identification: URIs Interaction: HTTP Data formats: HTML, JPEG, GIF, etc.

3 Goals Learn why traditional architectures would not have worked for the Web Learn the basics of HTTP (review) Explore some interesting characteristics of HTTP Learn the REST distributed computing architecture

4 Question: What would happen if we built the Web from RPC?

5 Why not use RPC for the Web?
No uniform interface You’d need an interface definition to follow a link No global naming Most RPC data types poorly tuned for documents Int, float, struct not HTML, XML, mixed-content, etc. We’ll revisit the RPC/Web question later and add some more

6 HTTP in Action (review from first week)

7 What Happens When We Browse a Web Page?

8 The user clicks on a link
URI is URI is

9 The http “scheme” tells client to send HTTP GET msg
URI is URI is HTTP GET

10 The server is identified by DNS name in the URI
URI is URI is HTTP GET Host: webarch.noahdemo.com

11 The client sends an HTTP GET
URI is HTTP GET demo1/test.html GET /demo1/test.html HTTP/1.0 Host: webarch.noahdemo.com User-Agent: Noahs Demo HttpClient v1.0 Accept: */* Accept-language: en-us Host: webarch.noahdemo.com

12 The client sends an HTTP GET
URI is HTTP GET …but it can carry binary entity bodies such as image/jpeg Note that HTTP is a text-based protocol demo1/test.html GET /demo1/test.html HTTP/1.0 Host: webarch.noahdemo.com User-Agent: Noahs Demo HttpClient v1.0 Accept: */* Accept-language: en-us Host: webarch.noahdemo.com

13 The server sends an HTTP Response
HTTP/ OK Date: Tue, 28 Aug :49:33 GMT Server: Apache Transfer-Encoding: chunked Content-Type: text/html <html> <head> <title>Demo #1</title> </head> <body> <h1>A very simple Web page</h1> </body> </html> The server sends an HTTP Response HTTP GET HTTP Status Code 200 Means Success! demo1/test.html Host: webarch.noahdemo.com HTTP RESPONSE

14 The server sends an HTTP Response
HTTP/ OK Date: Tue, 28 Aug :49:33 GMT Server: Apache Transfer-Encoding: chunked Content-Type: text/html <html> <head> <title>Demo #1</title> </head> <body> <h1>A very simple Web page</h1> </body> </html> The server sends an HTTP Response HTTP GET demo1/test.html The “representation” returned is an HTML document Host: webarch.noahdemo.com HTTP RESPONSE

15 The server sends an HTTP Response
HTTP/ OK Date: Tue, 28 Aug :49:33 GMT Server: Apache Transfer-Encoding: chunked Content-Type: text/html <html> <head> <title>Demo #1</title> </head> <body> <h1>A very simple Web page</h1> </body> </html> The server sends an HTTP Response HTTP GET demo1/test.html Host: webarch.noahdemo.com The HTML for the page. HTTP RESPONSE

16 HTTP Methods, Headers & Extensibility

17 HTTP Requests always have a method
URI is HTTP REQUEST Host: webarch.noahdemo.com GET GET /demo1.html HTTP/1.0 Host: webarch.noahdemo.com User-Agent: Noahs Demo HttpClient v1.0 Accept: */* Accept-language: en-us

18 HTTP Headers GET /demo1.html HTTP/1.0 Host: webarch.noahdemo.com
User-Agent: Noahs Demo HttpClient v1.0 Accept: */* Accept-language: en-us Request headers HTTP/ OK Date: Tue, 28 Aug :49:33 GMT Server: Apache Transfer-Encoding: chunked Content-Type: text/html <!DOCTYPE html> <html> <head> <title>Demo #1</title> </head> <body> <h1>A very simple Web page</h1> </body> </html> Response headers

19 HTTP Extension Points HTTP Technologies used by HTTP
New headers (quite common) New methods (rare) New versions of HTTP itself (e.g. HTTP 1.1) (very rare) New status codes (very rare) Technologies used by HTTP Media types (text/html, image/jpec, application/…something new?... Languages (e.g. en-us) Transfer encodings (e.g. gzip)

20 HTTP Protocol for a Discoverable Web

21 HTTP: the protocol for a discoverable Web
HTTP provides a uniform interface to all resources You don’t have to know in advance what a resource is like to access it with HTTP…so, you or your software can explore the Web in ad-hoc ways Self-describing: when a response comes back, you can figure out what it means Using HTTP to access a resource doesn’t damage the resource For all this to work, you must use HTTP…and you must use it properly!

22 HTTP Requests always have a method
URI is HTTP REQUEST Host: webarch.noahdemo.com GET GET /demo1.html HTTP/1.0 Host: webarch.noahdemo.com User-Agent: Noahs Demo HttpClient v1.0 Accept: */* Accept-language: en-us

23 Method Meaning GET POST HEAD PUT DELETE HTTP Methods
GET is very carefully defined to make the Web scalable and discoverable. HTTP Methods Method Meaning GET Retrieve a representation of the resource but do not change anything! POST Update the resource or a dependent resource base on supplied representation HEAD Optimized GET – retrieves only headers, used for checking caches, etc. PUT Create or completely replace the resource base on supplied representation DELETE Delete the resource identified by the URI

24 Method Meaning GET POST HEAD PUT DELETE HTTP Methods
GET is very carefully defined to make the Web scalable and discoverable. HTTP Methods Method Meaning GET Retrieve a representation of the resource but do not change anything! POST Update the resource or a dependent resource base on supplied representation HEAD Optimized GET – retrieves only headers, used for checking caches, etc. PUT Create or completely replace the resource base on supplied representation DELETE Delete the resource identified by the URI

25 HTTP GET Has A Very Particular Meaning
URI is HTTP GET It’s always safe to try a GET on any URI. In fact, that’s why search engines like Google can safely crawl the Web! GET /demo1.html HTTP/1.0 Host: webarch.noahdemo.com User-Agent: Noahs Demo HttpClient v1.0 Accept: */* Accept-language: en-us Host: webarch.noahdemo.com GET

26 HTTP GET Has A Very Particular Meaning
It’s always safe to try a GET on any URI. In fact, that’s why search engines like Google can safely crawl the Web! But GET is so convenient that some people use it in the wrong places…if you’re changing the state of a resource you must use PUT or POST!

27 Demonstration #3: http://webarch.noahdemo.com/MisuseGet/

28 Using HTTP Properly If GET is used instead of POST, your applications may update accidentally If too many applications misuse GET, then it will become impossible to explore the web If you click a link, you might break something..you would have to ask about each link before clicking! Search crawlers couldn’t be used…there would be no Google! Read TAG Finding “URIs, Addressability, and the use of HTTP GET and POST”

29 Web Interaction: Highlights
HTTP: a uniform way to interact with resources Promotes interoperability & discoverablility Use HTTP properly: GET is special! HTTP has features like content negotiation and headers that let you adapt content for your users

30 The Self-Describing Web

31 The Self-Describing Web
Everything you need to understand a response is in the response… …to find the instructions, read RFC 3989… …and follow your nose!

32 “The Self-Describing Web”
Follow your nose The Web community’s term for: Going step by step through a Web response and the pertinent specifications… …to figure out what the response means All the specifications you need are found (transitively) from RFC 3986! Thus there is a quite rigorous sense in which the Web is browsable and explorable Read TAG Finding “The Self-Describing Web”

33 The server sends an HTTP Response
HTTP/ OK Date: Tue, 28 Aug :49:33 GMT Server: Apache Transfer-Encoding: chunked Content-Type: text/html <html> <head> <title>Demo #1</title> </head> <body> <h1>A very simple Web page</h1> </body> </html> The server sends an HTTP Response HTTP GET demo1/test.html The “representation” returned is an HTML document Host: webarch.noahdemo.com HTTP RESPONSE

34 Stateful and Stateless Protocols

35 Stateful and Stateless Protocols
Stateful: server knows which step (state) has been reached Stateless: Client remembers the state, sends to server each time Server processes each request independently Can vary with level Many systems like Web run stateless protocols (e.g. HTTP) over streams…at the packet level, TCP streams are stateful HTTP itself is mostly stateless, but many HTTP requests (typically POSTs) update persistent state at the server

36 Advantages of stateless protocols
Protocol usually simpler Server processes each request independently Load balancing and restart easier Typically easier to scale and make fault-tolerant Visibility: individual requests more self-describing

37 Advantages of stateful protocols
Individual messages carry less data Server does not have to re-establish context each time There’s usually some changing state at the server at some level, except for completely static publishing systems

38 HTTP is Stateless (usually)
HTTP is fundamentally stateless…response determined entirely from GET request & URI (unless resource itself changes!!) In practice: Cookies stored in browser tied to session state and login status OK: determine access rights from cookie NOT OK: return different content based on cookie WHY?: On the Web, we identify with URIs, not cookies! Of course, the state of resources does change: E.g. when we add something to a shopping cart

39 REST

40 REST – REpresenational State Transfer
What is REST? A distributed system architecture In practice: implemented using HTTP Key features States transferred using representations (HTTP requests/responses) Server is stateless URIs (and request data if provided) convey state to application History Described by Roy Fielding in his PhD thesis Came after Tim BL invented the Web, but… …Roy was very influential in development of formal specifications of and details of HTTP and URIs Fielding’s PhD Thesis:

41 Building a REST application
All data communicated using HTTP GET / POST Unit of interaction is document Formats like JSON, XML, RDF, N3 used to convey data State of navigation typically captured in URIs: URI to start application URI for 2nd step URI for 3rd step Captures client parameters & input in URIs (e.g. search parms, flight numbers) States of a REST application are linkable, bookmarkable Examples: Google search Google maps (AJAX example: uses REST recursively)

42 A Complex REST Application Google Maps
Warning: some details of Google’s implementation may have changed since this presentation was prepared

43 AJAX Application – Google Maps
Images retrieved in segments using ordinary Web HTTP Requests

44 AJAX Application – Google Maps
JavaScript at client tracks mouse and moves images for smooth panning… asynchronously requests new image tiles in background

45 AJAX Application – Google Maps
The Web is used to retrieve an ordinary file listing points of interest…. (used to be XML but could use JSON, other?) <?xml version="1.0" ?> <page> <title>hotels in hawthorne</title> <query>pizza in atlanta</query> <center lat=" " lng=" " /> <info> <title>Wellesley Inn</title> <address> <line>540 Saw Mill River Rd.</line> <line>Elmsford, NY 10523</line> </address> </page>

46 AJAX Application – Google Maps
…Data used to drive formatting of points of interest <?xml version="1.0" ?> <page> <title>hotels in hawthorne</title> <query>pizza in atlanta</query> <center lat=" " lng=" " /> <info> <title>Wellesley Inn</title> <address> <line>540 Saw Mill River Rd.</line> <line>Elmsford, NY 10523</line> </address> </page>

47 AJAX Application – Google Maps
…and XSLT in the browser converts that to HTML <?xml version="1.0" ?> <page> <title>hotels in hawthorne</title> <query>pizza in atlanta</query> <center lat=" " lng=" " /> <info> <title>Wellesley Inn</title> <address> <line>540 Saw Mill River Rd.</line> <line>Elmsford, NY 10523</line> </address> </page>

48 AJAX Application – Google Maps

49 Question: What would happen if we built the Web from RPC? --- Revisited!

50 Why not use RPC for the Web?
No uniform interface You’d need an interface definition to follow a link No global naming Most RPC data types poorly tuned for documents

51 Why not use RPC for the Web?
No uniform interface You’d need an interface definition to follow a link No global naming Most RPC data types poorly tuned for documents No safe methods

52 Why not use RPC for the Web?
No uniform interface You’d need an interface definition to follow a link No global naming Most RPC data types poorly tuned for documents No safe methods No content negotiation

53 HTTP Content Negotiation

54 Content negotiation for language
URI is HTTP GET I prefer English, please! Host: webarch.noahdemo.com GET /demo1/test.html HTTP/1.0 Host: webarch.noahdemo.com User-Agent: Noahs Demo HttpClient v1.0 Accept: */* Accept-language: en-us Accept-language: en-us But: it is OK to also provide individual URIs for each language version

55 Content negotiation for device
URI is HTTP GET GET /demo1/test.html HTTP/1.0 Host: webarch.noahdemo.com User-Agent: Noahs Demo HttpClient v1.0 Accept: */* Accept-language: en-us User-Agent: Mozilla/4.0 Firefox Host: webarch.noahdemo.com

56 Content negotiation for device
URI is HTTP GET Cell phone browser Host: webarch.noahdemo.com GET /demo1/test.html HTTP/1.0 Host: webarch.noahdemo.com User-Agent: Noahs Demo HttpClient v1.0 Accept: */* Accept-language: en-us User-Agent: Cell Phone Browser V1.1

57 Even phone numbers are on the Web <a href=“tel:18005551212”>
Device independence The same URI works across devices… Even phone numbers are on the Web <a href=“tel: ”> …you can see my reservation from your cell phone

58 Summary

59 Summary HTTP and the REST model are designed to meet the unique needs of the Web Document-oriented Discoverable Stateless Extensible Global scale Supports unique features like content negotiation

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