INF 123 SW Arch, dist sys & interop Lecture 10

INF 123 SW Arch, dist sys & interopLecture 10 Prof. Crista Lopes

Objectives • Understanding of external data representations • why • well-known ones

Distributed System • No shared memory • Different processors may represent data differently • “endianness”: little endian, big endian … Component n Component n Component n Component1 Component1 Component1 Network Network OS Network OS Network OS Hardware Hardware Hardware Host 2 Host 1 Host 3

Endianness • Big endian • Little endian

Coping • IP defines a standard byte order (big endian) • Standard socket implementations provide converters • htonl, htons (host to network – 32 and 16 bits) • ntohl, htohs (network to host – 32 and 16 bits) • Problem solved by standardization at this level

Binary vs. text data • Choice in protocol design: • number 132: 1000 0100 • text “132”: 0011 0001 0011 0011 0011 0010 • Tradeoff: compactness vs. readability • What would HTTP look like in binary??? • Many above-transport protocols choose latter

Next: payload data struct Folder { UUID parentID; UUID owner; short type; short version; string name; UUID id; } ? too many options! obj host 1 host 2

Well-known data representations • eXternal Data Representation – XDR (IETF 1995) • eXtensible Markup Language – XML (W3C 2008) • long history, back to 60’s • JavaScript Object Notation – JSON (IETF 2006)

XDR • Binary data (i.e. hard to parse for humans) • Defines several basic data types • Does not include type information with data • receiver is assumed to know what types are being sent

XDR example Data to be transmitted const MAXUSERNAME = 32; /* max length of a user name */ const MAXFILELEN = 65535; /* max length of a file */ const MAXNAMELEN = 255; /* max length of a file name */ /* * Types of files: */ enumfilekind { TEXT = 0, /* ascii data */ DATA = 1, /* raw data */ EXEC = 2 /* executable */ }; /* * File information, per kind of file: */ union filetypeswitch (filekind kind) { case TEXT: void; /* no extra information */ case DATA: string creator<MAXNAMELEN>; /* data creator / case EXEC: string interpretor<MAXNAMELEN>; /* interpreter */ }; /* * A complete file: */ structfile { /* name of file */ string filename<MAXNAMELEN>; /* info about file */ filetype type; /* owner of file */ string owner<MAXUSERNAME>; /* file data */ opaque data<MAXFILELEN>; };

XDR example OFFSET HEX BYTES ASCII COMMENTS ------ --------- ----- -------- 0 00 00 00 09 .... -- length of filename = 9 4 73 69 6c 6c sill -- filename characters 8 79 70 72 6f ypro -- ... and more characters ... 12 67 00 00 00 g... -- ... and 3 zero-bytes of fill 16 00 00 00 02 .... -- filekind is EXEC = 2 20 00 00 00 04 .... -- length of interpretor = 4 24 6c 69 73 70 lisp -- interpretor characters 28 00 00 00 04 .... -- length of owner = 4 32 6a 6f 68 6e john -- owner characters 36 00 00 00 06 .... -- length of file data = 6 40 28 71 75 69 (qui -- file data bytes ... 44 74 29 00 00 t).. -- ... and 2 zero-bytes of fill

XML • Textual data (i.e. humans can easily parse it) • User-defined data • No predefined types, just syntax • Includes type information with data • Receiver is assumed to know what types mean

XML example <?xml version="1.0" encoding='UTF-8'?> <bookstore> <book category="COOKING"> <title lang="en">Everyday Italian</title> <author>Giada De Laurentiis</author> <year>2005</year> <price>30.00</price> </book> <book category="CHILDREN"> <title lang="en">Harry Potter</title> <author>J K. Rowling</author> <year>2005</year> <price>29.99</price> </book> </bookstore> XML Declaration Tag (start) Attribute Content Markup … … Tag (end) Element

XML Namespaces (XMLNS) • Used to disambiguate tags • Not used by parser – info for humans go here and see what’s there <root> <h:tablexmlns:h="http://www.w3.org/TR/html4/"> <h:tr> <h:td>Apples</h:td> <h:td>Bananas</h:td> </h:tr> </h:table> <f:tablexmlns:f="http://www.w3schools.com/furniture"> <f:name>African Coffee Table</f:name> <f:width>80</f:width> <f:length>120</f:length> </f:table> </root>

XSL and XSLT • eXtensibleStylesheet Language • eXtensibleStylesheet Language Transformations • Markups for browsers – how to display XML data • most browsers support this • usually irrelevant for non-browser clients • Example

JSON • Same spirit as XML, more concise and much simpler • Textual data (i.e. humans can easily parse it) • Syntactically-valid JavaScript – object literals • User-defined data • Some predefined types • Includes type information with data

JSON simple syntax • object: {} | {members} • members: pair | pair, members • pair: name : value • name: string • array: [] | [elements] • elements: value | value , elements • value: string | number | object | array | true | false | null

JSON example { "Success":true, "Items": [ { "Version":12, "ChildCount":12, "ID":"cbee00bb-1f26-414a-a0aa-9c5a7156fe64", "ParentID":"00000000-0000-0000-0000-000000000000", "OwnerID":"9ffd5a95-b8bd-4d91-bbed-ded4c80ba151", "Name":"Library", "ContentType":"application/vnd.ll.folder”, "CreationDate":1261042614, "Type":"Folder" },{ "Version":0, "ChildCount":0, "ID":"a2bd28c4-9243-418f-887d-4aa219b0046e", "ParentID":"cbee00bb-1f26-414a-a0aa-9c5a7156fe64", "OwnerID":"9ffd5a95-b8bd-4d91-bbed-ded4c80ba151", "Name":"Animations", "ContentType":"application/vnd.ll.animation”, "CreationDate":1261042614, "Type":"Folder" } ] } object object array object

JSON in JavaScript object literal, i.e. textual representation varmyJSONObject = {"bindings": [ {"ircEvent": "PRIVMSG", "method": "newURI", "regex": "^http://.*"}, {"ircEvent": "PRIVMSG", "method": "deleteURI", "regex": "^delete.*"}, {"ircEvent": "PRIVMSG", "method": "randomURI", "regex": "^random.*"} ] }; myJSONObject.bindings[0].method // "newURI” varmyObject = eval('(' + myJSONtext + ')'); varmyJSONText = JSON.stringify(myObject); actual object back to literal

Summary • Understanding of external data representations • why – because heterogeneity happens and we want to interoperate • well-known ones – XDR, XML and JSON • btw, all of this applies to files too! bits  ASCII  syntax  semantics

INF 123 SW Arch, dist sys & interop Lecture 10