Developing web apps using Erlang-Web

1. Server-side web development Using ErlangWeb Stefan Comanescu

2. A brief history of Erlang Initially developed by Joe Armstrong in 1986, as a proprietary language within Ericsson for usage in telephony applications

3. Named after the Danish mathematician Agner Krarup Erlang

4. First versions were running on a Prolog interpreter, a C emulator and a compiler were later written, for reasons concerning performance

5. In 1997 bit syntax and binaries were added (for protocol programming) and in the same year work started on OTP

6. Released as open source in 1998

7. SMP support was added in 2006

8. Why Erlang ? Concurrency Actor model (no threads, no locks, no shared memory, only asynchronous message passing) Fault Tolerance Process linking, supervising trees, distribution and event managers help in keeping the system going when processes crash. Distribution Abstracts the difference between local and distributed message passing

9. Runs on Yaws or Inets web server

10. MVC (Model-View-Controller) paradigm Model – data (stored in mnesia/dets tables) manipulation

11. Templates – XHTML files and templating engine

12. Controller – Erlang functions called on dispatching rules

13. Dispatcher – Request routing based on regexp

14. Request dictionary Reusable components (“ecomponent”)

15. Caching system

16. Annotations

17. Directory structure For creating the directory tree of a new application, ./bin/add is executed which generates the directories:

18. config –dispatch.conf, errors.conf, projects.conf, server specific .conf

19. docroot –static files and folders (except html templates), e.g : js, css, img

20. lib –framework applications and applications created by the user

21. log –logs created while running in embedded mode and server specific log

22. pipes –OS pipes for cummunicating with the shell while in embeded mode

23. priv –project specific data (usually static html)

24. release –separated subdirectories for each release version

25. templates –all the template files templates/cache – cached files

26. Generator script (bin/generate.erl) Module responsible for generating the boilerplate code

27. General format: ./bin/generate.erl TYPE -argument_name -argument_value

28. Generating a controller: ./bin/generate.erl controller --app app_name --name name_of_the_generated_model --functions (fun1, fun2)

29. ./bin/generate.erl model --app app_name --name name_of_the_generated_model --hrl name_of_the_header_file Generates a file named wtype_name where 'name' is the given arg

30. Generates basic CRUD functionality for the given header

31. Dispatcher Dispatching types Static dispatching: serving static content, like xml pages, media or javascript. The controller is not accessed. The server first searches for the file in the “templates” dir, afterward in the “docroot” dir. For faster retrieval of static content, they should be placed in the docroot dir and enoent should be placed instead of File {static, Regexp, File} | {static, Regexp, enoent} Dynamic dispatching: the controller will handle the URL requests allowing specific content to be created or retrieved on demand {dynamic, Regexp, {Module, Function}} Alias dispatching: if the URL matches the Regexp, the rule of NewURL will be triggered {alias, Regexp, NewURL} Delegates: passing the URL to another dispatch config file, only helps in structuring the application {dynamic, delegate, Regexp, File}

32. Dispatcher evaluation order Docroot content is served (files that point to 'enoent')

33. Dynamic entries are matched with their URLs

34. Static routes are matched

35. If no patterns are matched, a 404 error page is displayed

36. Request dictionary Temporary storage – created for each HTTP request, lives as long as the request lives

37. Provides an easy to use API for setting and getting values from the dictionary

38. Used for retrieving GET/POST request data from the query string and also used by the expanding templates

39. API: wpart:fget(“post:user_address”).

40. wpart:fget(“get:page_number”).

41. wpart:fset(“user”,[[{“name”,”bogdan”},{age,21}],[{“name”,”marian”},{age,18}]]).

42. Request dictionary Sample browser request {<0.3110.0>,[ {"get",[]}, {"post",[ {"action_type","add"},{"id",[]},{"type","user"},{"user_password","nuitizic"}, {"user_phone","07273447823"},{"user_cnp","1901231231723"}, {"user_lastname","alex"},{"user_firstname","popescu"} ] }, {"__https",false}, {"__cookies",[{"eptic_cookie","nonode@nohost-7985065742948673537"}]}, {"session",[{"user_id","1111111111111"}, {"groups",["admin"]}]}, {"__path","admin/add"}, {"__cookie_key","nonode@nohost-7985065742948673537"},{"__ip",{127,0,0,1}}, {"__controller",controller} ] }

43. Controller Responsible for handling the user's requests

44. Has the arity 1, getting a proplist (list of key-value pairs)

45. Handles user input, model manipulation, etc.

46. Must return a tuple, which tells the template what to do: {redirect, URL} | {content,html,Data} | {content,text,Data} | {json,Data} | {template,Template} | {error,Code} | {headers, Headers, RetVal} where : Headers is a list of headers, of the form {cookie, CookieName, CookieValue, CookiePath, CookieExpDate}

47. Retval is one of the previous touples

48. Templating Engine The view is represented by a regular XHTML file with a few namespaces defined.

49. The most commonly used namespace, wpart, is always expanded by the framework . When the parser meets <wpart:lookup … /> for example, it will call the function wpart_lookup:handle_call/1, with the whole xmerl parsed tag as an argument.

50. Templating Engine Wpart tags wpart:choose provides “if” functionality; it contains the tags: <wpart:when test=”...”> … </wpart:when>

51. <wpart:otherwise> Something else </wpart:otherwise>

52. The test is an expression of the form test=”1 + 1 eq 2”, where the operators can be: eq, neq, lt, le, gt, ge (for =:=, =/=, <, >, =<, >=)

53. Templating Engine Wpart tags wpart:list provides “for” functionality; it contain the attributes: select, specified what part of the list we want, attributes: map – renders every element in the list

54. head – renders only the first element

55. tail – renders the last element of the list

56. filter – has an attribute named “pred” for filtering the list

57. find – the first element of the list where “pred” is evaled to true

58. sort – sorts according to the “pred” attribute as, every element of the list from inside the wpart:list will be known as the value of this attribute

59. Templating Engine Wpart tags list, specifies the list from the request dictionary that we want to traverse

60. pred, predicate used for some of the select tags, it's optional

61. e.g. : <wpart:list select=”map” list=”students” as=”student”> <span class=”name”><wpart:lookup value=”student:name”/></span> <span class=”group”><wpart:lookup value=”student:group”/></span> </wpart:list>

62. Templating Engine Wpart tags wpart:lookup provides access to a value from inside the request dictionary, it contains the attributes: key, the key from the dictionary that we are looking for

63. format, optional formatters

64. type, can have the values “text” (default) or “html”, setting “html” turns off html-escaping and can be dangerous

65. e.g : <wpart:lookup key=”name”/> <wpart:lookup key=”student:name”/>

66. Templating Engine (wtpl) Tool for faster building of html pages, by expanding or replacing content from other template files

67. Allows building of websites from small chunks of html

68. It helps with : Code re-usage

69. Not writing the same thing multiple times

70. Updating the common part of the pages only one time

71. Templating Engine (wtpl) Wtpl tags <wtpl:include name=Name /> - the location where the wtpl:content with the corresponding name will be placed

72. <wtpl:parent path=Path /> - specifies which template will be filled with the content provided by the wtpl:content content

73. <wtpl:content name=Name/> - content that will fill the wtpl:include tag which has the same name as the tag 'name' attribute, in the file specified by wtpl:parent

74. Templating Engine wpart_gen Enables building of HTML code directly from within Erlang modules.

75. wpart_gen:load_tpl(Namespace, Name, Path) – loads the html snippet specified by Path, and saves it in the memory under the key {Namespace,Name}

76. Tpl = wpart_gen:tpl_get(Namespace,Name) – gets the snippet from memory

77. wpart_gen:build_html(Tpl,Values) – builds the template file with the values from the list Values

78. Templating Engine wpart_gen – Template Format Anonymous slots:  <% slot %>  Named slots:  <% name_of_the_slot %>

79. Templating Engine wpart_gen – Passed Values Anonymous slots: [Value | … ] All fields must be filled. Named slots: [{ “slot_name”, Value} | … ] The unfilled slots will remain empty.

80. Controlling cache persistent – data cached this way is not removed until the server is restarted or the cache is invalidated

81. timeout – every T minutes the cache is traversed and expired content is removed, each URL can have a different timeout.

82. no_cache

83. Controlling cache The caching configuration is made directly from the dispatch configuration file. E.g. : {dynamic, “^/index/?$”,{main,home},[{cache,persistent}]}.

84. {static, “^/faq$”,”doc/faq.html”,[{cache,normal}]}.

85. {dynamic, “^/posts/?$”,[{cache,{timeout,30}}]}.

86. {dynamic, “^/the_time/?$”,[{cache,no_cache}]}.

87. Controlling cache For controlling cache based on aspects other than URL, the tuple {is_cachable_mod, my_mod} should be placed in the configuration(config/project.conf) file.

88. Before processing any request, the function my_mod:is_cachable() will be called

89. The function is_cachable() should return true if the request should be cached under the URL key

90. {true,NewId} , if the request should be cached under NewId

91. false , if we don't want to check the cache

92. Annotations Available since version 1.3

93. Allows separation of the target function from different types of functions, non-domain specific wrappers.

94. e.g.: before making a post to a blog you should be logged in and your message should be checked for “bad words” maybe; after posting the message the cache should be invalidated and your post-count could be modified.

95. Usual dataflow mechanisms can be used but it could make the code harder to read and repetitive.

96. Annotations Controller Syntax % keeps the stored macros -include_lib(“blog/include/utils_annotations.hrl”). % before posting ?AUTHENTICATE([“administrator”]). ?CHECK(blog_post). % after posting ?INVALIDATE(posts). ?UPDATE(user). post(_Args) -> ...

97. Annotations Annotation Module Syntax ?BEFORE. %% ?AFTER. annotation_name(AnnotationArg, TargetModule, TargetFunction, TargetFunctionArgs | TargetFunctionResult) -> Result Result :: {proceed, NewFunctionArgs | NewResult} | {skip, NewResult} | {error, {ErrorMod, ErrorFun, ErrorArgs}}

98. Annotations Annotation Module Example ?AFTER. check (blog_post, _Mod, _Fun, _Args) -> case is_bad(wpart:fget(“post:msg”)) of true-> {skip, {redirect, “http://correctiveschool.org”}}; _-> {proceed, _Args} end.

99. e_components e_components is a mechanism for extending the capabilities of the framework

100. Erlang-Web comes with e_components for authentication, mnesia backup and for generating rss feeds

101. To search for an e_component we can run ./bin/e_component search component_name, command that will list all available components matching the name and their description

102. To install an e_component we run ./bin/e_component install component_name, the e_component is now copied in the lib folder of our application. ./bin/compile is now required, in order to compile the component's source files.

103. e_components One more thing is required in order to use the component in our project: inserting a new tuple in the configuration file: {ecomponents, [{e_auth, []}, {e_auth_dets, []} ]}. After restarting the server we can verify that the components are running, by executing the function that lists all running applications : application:which_applications()

104. e_components e_components is a mechanism for extending the capabilities of the framework

105. Erlang-Web comes with e_components for authentication, mnesia backup and for generating rss feeds

106. To search for an e_component we can run ./bin/e_component search component_name, command that will list all available components matching the name and their description

107. To install an e_component we run ./bin/e_component install component_name, the e_component is now copied in the lib folder of our application. ./bin/compile is now required, in order to compile the component's source files.

Developing web apps using Erlang-Web

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