rest inf 123 – software architecture [email protected] 1
TRANSCRIPT
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Outline
• SOAP and REST• REST constraints and gains• REST guidelines
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SOAP AND REST
Simple Object Access ProtocolRepresentational State Transfer
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From RPC to SOAP: RPC
• Pass remote procedure name and arguments• Expect a return value• Procedure signature is implicit
– If you change the signature, people have no way to know why it’s not working anymore
• It’s about (function) names– Whereas REST is about standard verbs like GET,
POST, etc.
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From RPC to SOAP: SOAP
• 1998: Simple Object Access Protocol• 2000: Web Service Description Language
(WSDL) uses SOAP as underlying protocol, cf SOA slides
• SOAP = XML RPC done “right”• Long/verbose structured XML messages
– Verbosity != metadata
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SOAP message
POST /InStock HTTP/1.1Host: www.example.orgContent-Type: application/soap+xml; charset=utf-8Content-Length: 299SOAPAction: "http://www.w3.org/2003/05/soap-envelope" <?xml version="1.0"?><soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope"> <soap:Header></soap:Header> <soap:Body> <m:GetStockPrice xmlns:m="http://www.example.org/stock"> <m:StockName>IBM</m:StockName> </m:GetStockPrice> </soap:Body></soap:Envelope>
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Metadata vs verbosity
There are two kinds of pain. The sort of pain that makes you strong, or useless pain that is only suffering.
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Metadata vs verbosity
Pay attention to the fine print metadata. It’s far more important than the selling price data itself.
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Amazon uses both REST and SOAP
• Amazon has both SOAP and REST interfaces to their web services, and 85% of their usage is of the REST interface (2003)
• http://oreilly.com/pub/wlg/3005
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REST and SOAP
• They happened concurrently– SOAP: 1998– REST: 2000
• SOAP = envelope, REST = postcard• SOAP derived from RPC• REST is not a move against SOAP• REST is very complex
– But it looks simpler than SOAP
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REST History
• Hypertext– 1945 memex (Vannevar Bush)– 1967 hypertext (Project Xanadu)
• Internet– 1969 ARPANET (army)
• 1992: WWW = Internet + hypertext• 2000: REST = reverse-engineer/document the
WWW architectural style• HTTP is not mandatory for REST, but it helps
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SOAP vs REST
• SOAP is verbose: large overhead of metadata and boilerplate text
• SOAP
• REST
<?xml version="1.0"?> <soap:Envelope xmlns:soap="http://www.w3.org/2001/12/soap-envelope" soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding"> <soap:body pb="http://www.acme.com/phonebook"> <pb:GetUserDetails> <pb:UserID>12345</pb:UserID> </pb:GetUserDetails> </soap:Body> </soap:Envelope>
GET http://www.acme.com/phonebook/UserDetails/12345
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Solutions to SOAP’s verbosity
• MTOM: Message Transmission Optimization Mechanism– Encode/compress XML into binary
• XOP: XML-binary Optimized Packaging– To encode/decode MTOM
• TLDR: Binary-encoded XML over HTTP• But HTTP = hypertext transfer protocol
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REST CONSTRAINTSRepresentational State Transfer
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Reminder: Architectural style
• Set of constraints• Constraints induce properties
– Desirable or undesirable– Design trade-offs
• REST = architectural style of the WWW
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Resource = information, dataWe have great resources at our disposal.
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REST is made of several styles
• Client-Server• Stateless• Cache• Layered• Code on demand• Uniform Interface
• These styles are not always inter-compatible• But they are in the case of the WWW
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Deriving REST from other styles
http://roy.gbiv.com/talks/200804_REST_ApacheCon.pdf
No style
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1 Client-server
• Cf week 2• Separation of concerns: client display vs server
logic– Display is client-side: clients can have different UIs
• Each website has a server• Same client (browser) can access multiple
servers
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2 Stateless interactions
• AKA context-free interactions• Stateless interaction does not mean “no data
in the server”• The server does not store any client-specific
information between two requests• State is client-side or in a database
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Stateful example: SMTPtagus: crista$ telnet smtp.ics.uci.edu 25Trying 128.195.1.219...Connected to smtp.ics.uci.edu.Escape character is '^]'.220 david-tennant-v0.ics.uci.edu ESMTP mailer ready at Mon, 5 Apr 2010 17:15:01 -0700'HELO smtp.ics.uci.edu250 david-tennant-v0.ics.uci.edu Hello barbara-wright.ics.uci.edu [128.195.1.137], pleased to meet youMAIL FROM:<[email protected]>250 2.1.0 <[email protected]>... Sender okRCPT TO:<[email protected]>250 2.1.5 <[email protected]>... Recipient okDATA354 Enter mail, end with "." on a line by itselftest.250 2.0.0 o360F1Mo029280 Message accepted for deliveryQUIT221 2.0.0 david-tennant-v0.ics.uci.edu closing connectionConnection closed by foreign host.
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Gains from statelessness
• Immune to server restart/migration– Server restart = lose all data– But stateless = no data to lose!
• No server affinity– Client requests can be processed by ANY server, not just one
particular server• Scalability
– Server never knows if/when client sends its next request– So stateful servers timeout the sessions of clients with long
inter-request times– Stateless servers don’t have any memory management issues
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Losses from statelessness
• Client is less efficient• Server needs to pull data for every request
– Pulling data is straightforward when this data is a static web page (most of the WWW in the 90s)
• How do you authenticate users?– Cookies (not good?)– External auth and directory services
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3 Caching
• Optional• Store data locally so I don’t have to retrieve it• In clients, in servers, or in intermediaries (cf
layered constraint)
• Reduces latency• Improves efficiency and scalability• But degrades reliability (stale data)
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4 Layered
• Intermediaries between client and server– Proxies, such as nginx– Caches– Content Delivery Network, such as Akamai– Web accelerator, such as CloudFlare
• Pros: ability to balance load (improves scalability), can reduce latency (when cache hits)
• Cons: can add latency (when cache misses)
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Do you know what I like about people intermediaries? They stack so neatly.
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Proxy for load balancing: nginx
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Intermezzo: ISPs
• Internet Service Providers (ISP): COX, Comcast Verizon, UCI
• When a client requests content, it goes through:– The client’s ISP– Intermediary ISP 1– Intermediary ISP 2– The server’s ISP– The server delivers the content
• Many hops = lots of delay
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tracert/traceroute
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Content Delivery Network: Akamai
• Akamai pays ISPs to host their servers within a few hops of many clients – Many clients = urban areas
• I pay Akamai to deliver my content• Now, when a client requests my content:
– Client ISP– Akamai server delivers my content!
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5 Code on demand
• Optional• Fetch JavaScript when a web page asks for it<html><head><script src=‘blabla.js’></script>• Pros: thinner clients, improves extensibility• Cons: reduces visibility
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I won't be a slave to anybody or anything you can order with a toll free number any static code.
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6 Uniform interface
• The hardest constraint to get right• Uniform identification of resources• Manipulation of resources via representations• Hypermedia as the engine of app state
(HATEOAS)
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REST Data ElementsUniform Interfaces
(The following slides are from Crista Lopes)
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Resources and their identifiers
• Resource = Abstraction of information, any information that can be named– A document, a temporal service (“today’s
weather”), collection of other resources– Some are static, some are dynamic
• Identifiers: Universal Resource Identifiers (URIs)
Uniform Interfaces
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Representations
• Server returns representations of resources, not the resources themselves.– E.g. HTML, XML
• Server response contains all metadata for client to interpret the representation
Uniform Interfaces
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HATEOAS
• Hypermedia As The Engine Of Application State• Idea: the application is a state machine
Uniform Interfaces
LoggedOut
CreateAccount
LoggedIn
User
ChangeAccount
LoggedIn
Admin
SearchUsers …
Question is:Where is the clients’ state stored?
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HATEOAS
• Non-REST– Clients’ state kept on the server– Server is both state machine and holder of state
• REST– State machine on the server– At any step, client is sent a complete “picture” of
where it can go next, ie its state and transitions
LoggedOut
LoggedIn
User
ChangeAccount
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HATEOAS
• Server sends representation of the client’s state back to the client– Hence, REpresentional State Transfer
• Server does not “hold on” to client’s state• Possible next state transitions of the client are
encoded in Hypermedia– Anchors, forms, scripted actions, …
LoggedOut
LoggedIn
User
ChangeAccount
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MORE REST GUIDELINES
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HTTP Operations
• GET• PUT• DELETE• HEAD• OPTIONS• TRACE• POST• CONNECT
Idempotent methods: the side effects of many invocations are exactly the same as the side effects of one invocation
PS: remember main and subroutines?
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Example: Paypal’s API
• https://developer.paypal.com/docs/api/
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RESTful Design Guidelines
• Embrace hypermedia– Name your resources/features with URIs– Design your namespace carefully
• Hide mechanisms– Bad: http://example.com/cgi-bin/users.pl?name=John– Good: http://example.com/users/John
• Serve POST, GET, PUT, DELETE on those resources– Nearly equivalent to CRUD (Create, Retrieve, Update, Delete)
• Don’t hold on to state– Serve and forget (functional programming-y)
• Consider serving multiple representations– HTML, XML, JSON
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RESTful Design Guidelines
• URIs are nouns• The 8 HTTP operations are verbs
• Very different from CGI-inspired web programming:
• Many/most web frameworks promote URIs as verbs and query data as nouns – old CGI model.
https://eee.uci.edu/toolbox/dropbox/index.php?op=createdropboxform
http://us.mc510.mail.yahoo.com/mc/welcome?.gx=1&.tm=1271634041& .rand=9anflcttvlh7n#_pg=showFolder&fid=Inbox&order=down&tt=237&pSize=100& .rand=952814729&.jsrand=4316826
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Choosing money CGI over power REST is a mistake almost everyone makes.
They just don’t know …
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RESTful Design Guidelines
Canonicalexample
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REST vs Linked Data
• Linked data – A data model– Proposed by Berners-Lee
• REST – An interaction model– Proposed by Fielding
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Taylor’s REST principles
• Any information is a resource, named by an URL. (uniform interface)
• Resource representation is accompanied by metadata about the representation. (uniform interface, code on demand)
• Interactions are context-free. (stateless)• Small set of methods. Each method can be applied to any
resource. The result of a method is a representation.• Idempotent operations help caching. (cache)• Intermediaries use metadata from requests or responses to
filter, redirect, or modify representations. This is transparent to client and server. (layered, cache)
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For you to read/watch
• http://www.infoq.com/articles/rest-introduction
• http://rest.elkstein.org/• https://
groups.yahoo.com/neo/groups/rest-discuss/conversations/topics/5841
• https://www.youtube.com/watch?v=e2PyeXRwhCE
• https://www.cloudflare.com/overview