Understanding the Role of Media Types in RESTful Applications. Jan Algermissen GOTO Copenhagen 2011

Transcription

1 Understanding the Role of Media Types in RESTful Applications Jan Algermissen GOTO Copenhagen 2011

2 Jan Algermissen Applying the Web to Enterprise NORD Software Consulting

3 Application

4 Application User

5 Application Use Case

6 Application Application

7 Monolithic

8 User Monolithic

9 Monolithic Use Case

10 Monolithic Application

11 Monolithic Components

12 Monolithic Connectors

13 Unix Command Line

14 User Unix Command Line

15 Unix Command Line Use Case

16 Unix Command Line Application

17 Unix Command Line Components

18 Unix Command Line Connectors

19 Network-Based (ODBC)

20 User Network-Based (ODBC)

21 Network-Based (ODBC) Use Case

22 Network-Based (ODBC) Application

23 Network-Based (ODBC) Components

24 Network-Based (ODBC) Connector

25 Network-Based (ODBC)

26 Network-Based (ODBC) User

27 Network-Based (ODBC) Use Case

28 Network-Based (ODBC) Application

29 Network-Based (ODBC) Components

30 Network-Based (ODBC) Connector

31 Network-Based (HTTP)

32 User Network-Based (HTTP)

33 Network-Based (HTTP) Use Case

34 Network-Based (HTTP) Application

35 Network-Based (HTTP) User Agent Component Intermediary Component Server Component

36 Network-Based (HTTP) Connectors

37 Network-Based (HTTP)

38 User Network-Based (HTTP)

39 Network-Based (HTTP) Use Case

40 Network-Based (HTTP) Application

41 Network-Based (HTTP) User Agent Component Intermediary Component Server Component

42 Network-Based (HTTP) Connectors

43 Network-Based (HTTP) Administrative Domain

44

45 Issues of network-based, decentralized systems

46 Network Issues of network-based, decentralized systems

47 Network Unpredictable Use Issues of network-based, decentralized systems

48 Network Unpredictable Use Issues of network-based, decentralized systems Complexity

49 Network Unpredictable Use Issues of network-based, decentralized systems Trust Complexity

50 Network Federated Change Unpredictable Use Issues of network-based, decentralized systems Trust Complexity

51 Performance Network Federated Change Unpredictable Use Issues of network-based, decentralized systems Trust Complexity

52 Performance Network Scalability Federated Change Unpredictable Use Issues of network-based, decentralized systems Trust Complexity

53 Performance Network Scalability Federated Change Unpredictable Use Issues of network-based, decentralized systems Trust Complexity Simplicity

54 Performance Network Scalability Federated Change Unpredictable Use Issues of network-based, decentralized systems Trust Visibility Complexity Simplicity

55 Performance Network Evolvability Federated Change Scalability Unpredictable Use Issues of network-based, decentralized systems Trust Visibility Complexity Simplicity

56 Performance Scalability Simplicity Visibility Evolvability

57 Properties of Key Interest Performance Scalability Simplicity Visibility Evolvability

58 Evolvability

59 GOAL Server owner can implement any change without asking existing clients.

60 Client Contract Server

61 Contract Client Server

62 Contract Media Type Client Server

63 Media Type

64 Media Type 1.What is a media type?

65 Media Type 1.What is a media type? 2.How do I design one?

66 Media Type 1.What is a media type? 2.How do I design one? 3.How do media types enable evolvability?

67 1. What is a media type?

68 1. What is a media type? A name (identifier)

69 1. What is a media type? A name (identifier) A family of compatible schemas

70 1. What is a media type? A name (identifier) A family of compatible schemas Processing rules

71 1. What is a media type? A name (identifier) A family of compatible schemas Processing rules Extension rules

72 1. What is a media type? A name (identifier) A family of compatible schemas Processing rules Extension rules Hypermedia semantics

73 2. How do I design a media type?

74 User Automatically check prices and place order if price is below threshold Shop A User Agent (Crawler) Shop B

75 UC 01: Automatically check prices and... Primary Actor: User (Buyer) Precondition: Crawler knows catalog entry URIs Main Success Scenario: 1. User starts crawler 2. Crawler requests catalogs of known shops 3. Crawler compares prices of items to threshold 4. Crawler selects item if price below threshold 5. Crawler creates order for item 6. Crawler submits order to order-processor 7. Crawler archives order response 8. User reviews archived order responses

76 UC 01: Automatically check prices and... Primary Actor: User (Buyer) Precondition: Crawler knows catalog entry URIs Main Success Scenario: 1. User starts crawler 2. Crawler requests catalogs of known shops 3. Crawler compares prices of items to threshold 4. Crawler selects item if price below threshold 5. Crawler creates order for item 6. Crawler submits order to order-processor 7. Crawler archives order response 8. User reviews archived order responses

77 Elements of Example Media Type Data: Catalog (with items) Order (with order-items) Order Response Hypermedia controls: Order-Processor (Catalog)

78 Catalog Example <catalog> <title>shoe Catalog</title> <order-processor href="/orders"/> <item id= 123 href= /items/23 > <name>shiny Boots</name> <price currency="eur" amount="27.99"> </item> <item id= 456 href= /items/388 > <name>fancy Sneakers</name> <price currency="eur" amount="12.95"> </item> </catalog>

79 Order Example <order> <buyer>...</buyer> <line-items> <line-item id= 123 > <price currency= EUR >27.99</price> <quantity>4</quantity> </line-item> <line-item id= 456 > <price currency= EUR >12.95</price> <quantity>4</quantity> </line-item> </line-items> </order>

80 application/procurement Three schemas <catalog>, <order> and <order-response> No mandatory elements Extensible Specified as version 1 Processing Rules If an <order-processor> element is present an order can be POSTed to that resource. Such an oder accepting resource SHOULD accept <order>-documents....

81 3.How does a media type enable evolvability?

82 3.How does a media type enable evolvability? Contract Media Type Client Server

83 public String makeprocurementcatalog() { } return "<catalog/>";

84 Catalog Example <catalog> <title>shoe Catalog</title> <order-processor href="/orders"/> <item id= 123 href= /items/23 > <name>shiny Boots</name> <price currency="eur" amount="27.99"> </item> <item id= 456 href= /items/388 > <name>fancy Sneakers</name> <price currency="eur" amount="12.95"> </item> </catalog>

85 Shop public Catalog makeprocurementcatalog() { Catalog catalog = new Catalog( Shoe Catalog ); catalog.setorderprocessor("/orders"); catalog.additem("/items/23","shiny Boots","27.99"); catalog.additem("/items/388","fancy Sneakers","12.95"); } return catalog;

86 Server Development Principle Design by purpose

87 Crawler Implementation public void handlecatalog(cataloguri) { } WebResource resource = getclient().resource(cataloguri); ClientResponse response = resource.get(clientresponse.class); Catalog catalog = response.getentity(catalog.class); Item item = catalog.getitembyid( 123 ); if(item!= null && item.getprice()!= null && item.getprice().getamount( EUR ) < THRESHOLD) { placeorder(catalog,item); }

88 Crawler Impl. (cont.) public void placeorder(catalog catalog, Item item) { } String orderprocessoruri = catalog.getorderprocessoruri(); Order order = new Order(item); WebResource resource = getclient().resource(orderprocessoruri); ClientResponse response = resource.post(clientresponse.class,order); // handle order placement response

89 Client Development Principle Design by intent

90 Observation REST limits the possible contract changes to just two kinds: Adding data or controls and removing data and controls from a message.

91 New Requirement Add in-stock quantity

92 Extended Catalog Example GET /catalog 200 Ok Content-Type: application/procurement <catalog> <title>shoe Catalog</title> <order-processor href="/orders"/> <item id= 123 href= /items/23 > <name>shiny Boots</name> <price currency="eur" amount="27.99"> <in-stock>5</in-stock> </item> <item id= 456 href= /items/388 > <name>fancy Sneakers</name> <price currency="eur" amount="12.95"> <in-stock>2</in-stock> </item> </catalog>

93 Media Type Principle Extensible schemas

94 Client Development Principle Must Ignore

95 Share Useful Extensions Feed extensions back into media type, increase schema version.

96 Shop Server Change No order processor available

97 Catalog Example GET /catalog 200 Ok Content-Type: application/procurement <catalog> <title>shoe Catalog</title> <item id= 123 href= /items/23 > <name>shiny Boots</name> <price currency="eur" amount="27.99"> </item> <item id= 456 href= /items/388 > <name>fancy Sneakers</name> <price currency="eur" amount="12.95"> </item> </catalog>

98 Crawler Impl. (fails) public void placeorder(catalog catalog, Item item) { } String orderprocessoruri = catalog.getorderprocessoruri(); Order order = new Order(item); WebResource resource = getclient().resource(orderprocessoruri); ClientResponse response = resource.post(clientresponse.class,order); // handle order placement response

99 Crawler Impl. (improved) public void placeorder(client Response response, Catalog catalog, Item item) { } String orderprocessoruri = catalog.getorderprocessoruri(); if(orderprocessoruri == null) { throw new FailedExpectationException( Expected order processor not found,response); } Order order = new Order(item); WebResource resource = getclient().resource(orderprocessoruri); ClientResponse response = resource.post(clientresponse.class,order); // handle order placement response

100 Crawler Impl. (improved) try { handlecatalog(cataloguri); } catch(failedexpectationexception e) { String message = e.getmessage(); ClientResponse response = e.getclientresponse(); LOG.error(... ); reportincident( Failed expectation when processing catalog + cataloguri, message, response, PRIORITY_CRITICAL); } //...

101 Client Development Principle What if programming style

102 Media Type Principle Avoid mandatory elements

103 Server Development Principle Be responsible

104 If you are a media type designer...

105 If you are a media type designer... Define extensible schemas

106 If you are a media type designer... Define extensible schemas Avoid mandatory elements

107 If you are a server developer...

108 If you are a server developer... Use from the media type what suits your possibilities

109 If you are a server developer... Use from the media type what suits your possibilities Be responsible and maintain things

110 If you are a client developer...

111 If you are a client developer... Use from the media type what suits your intended application

112 If you are a client developer... Use from the media type what suits your intended application Ignore unknown data and controls

113 If you are a client developer... Use from the media type what suits your intended application Ignore unknown data and controls What if programming style

114 Server does not own the contract

115 Server does not own the contract

116 Flexible Contract

117 Source of information

118 Q & A