ReST (Representational State Transfer) Explained
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ReST (Representational State Transfer) Explained

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A long presentation on a variety of aspects of REST.

A long presentation on a variety of aspects of REST.

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ReST (Representational State Transfer) Explained ReST (Representational State Transfer) Explained Presentation Transcript

  • REST Explained Representational State Transfer Dhananjay Nene July 4, 2009 TechWeekend – Pune http://blog.dhananjaynene.com http://twitter.com/dnene
  • What REST is not !
  • REST is not a .. framework
  • REST is not a .. technology
  • REST is not a .. a standards specification
  • REST is an architecture style
  • .. as documented and described by Roy Fielding ..
  • .. which specifies a set of architecture constraints.
  • Fielding on Architecture Style ● An architecture style is a coordinated set of architectural constraints that restricts the roles and features of architectural elements, and the allowed relationships between those elements, within any architecture that conforms to that style ● A style can be applied to many architectures ● An architecture can consist of many styles
  • Architecture Constraint 1 Client - Server
  • Client Server Separates user interface concerns from data storage concerns
  • Client Server Improves portability of interface across multiple platforms
  • Client Server Improves scalability by simplifying server components
  • Client Server Allows the components to evolve independently
  • Architecture Constraint 2 Statelessness No Client State
  • Statelessness Each request from client to server must contain all of the information necessary to understand the request and cannot take any advantage of any stored context on the server. and Each request contains all of the information necessary for a connector to understand the request, independent of any requests that may have preceded it
  • Statelessness Session state is therefore kept entirely on the client
  • Statelessness Improved visibility since a monitoring system does not have to look beyond a single request
  • Statelessness Improved reliability due to easier recoverability from partial failures
  • Statelessness Improved scalability due to not having to allocate resources for storing state
  • Statelessness Server does not have to manage resource usage across requests
  • Statelessness Tradeoff : Reduced Network Performance
  • Statelessness Tradeoff : Reduced server control over application consistency
  • Statelessness is one of the most difficult to deal with constraints (but more on that later)
  • Architecture Constraint 3 Specified Cacheability
  • Specified Cacheability Data within a response to a request be implicitly or explicitly labeled as cacheable or non- cacheable
  • Specified Cacheability If a response is cacheable, then a client cache is given the right to reuse that response data for later, equivalent requests
  • Specified Cacheability Improves efficiency, scalability and user perceived performance
  • Specified Cacheability Tradeoff : Reduced Reliability
  • Architecture Constraint 4 Uniform Interface
  • Uniform Interface Overall system architecture is simplified and the visibility of interactions is improved
  • Uniform Interface Implementations are decoupled from the services they provide and encourage independent evolvability
  • Uniform Interface Tradeoff : Degrades efficiency since Information is transferred in a standardised form rather than one which is specific to application's needs
  • Uniform Interface Four interface constraints (more later .. we shall be spending the maximum time on this) ● Identification of resources ● Manipulation of resources through representations ● Self descriptive messages ● Hypermedia as the engine of application state (HATEOAS)
  • Architecture Constraint 5 Layered System
  • Layered System Places a bound on overall system complexity
  • Layered System Promotes substrate independence
  • Layered System Can be used to encapsulate legacy services or protect new services from legacy clients
  • Layered System Intermediaries can be used to improve system scalability by enabling load balancing
  • Layered System Tradeoff : Add overhead and latency and reduce user perceived performance
  • Layered System Placing shared caches at boundaries of organisational domain can result in significant benefits. Can also enforce security policies eg. firewalls
  • Layered System Intermediaries can actively transform message content since messages are self descriptive and their semantics are visible to the intermediaries
  • Architecture Constraint 5 Code on demand (is an optional constraint)
  • Code on demand Client functionality can be extended by downloading and executing code in the form of applets or scripts
  • Lets get back to .. and explore in far more detail ..
  • Interface constraints of ReST
  • Resources
  • Resources What are resources ?
  • Any information that can be named is a resource
  • A resource is a conceptual mapping to a set of entities not the entity itself. Such a mapping can change over time.
  • This presentation is a resource
  • As is this presentation's latest version (if I am regularly backing it up to different files)
  • All available presentations on ReST is also a resource. A resource can be a collection of entities too.
  • Resource Identifiers
  • Every resource has a name that uniquely identifies it – the URI
  • Names don't change (at least not frequently)
  • Think of it like a primary key for each row in a database http://informationbase/locationdb/citiestable/pune
  • REST doesn't dictate URI choice. Leaves it to the application author.
  • The URI should generally carry no meaning to the client except as a resource locator
  • However don't let that encourage you to name URIs arbitrarily and confusingly
  • Good, clean, structured URIs are helpful for developers
  • If you are naming a specific single resource all the information to locate the resource should be in the URI itself and not through additional parameters
  • eg. choose http://informationbase/locationdb/citiestable/pune not http://informationbase/locator?type=city&name=pune
  • However optional parameters for identifying subsets of resources are conventionally acceptable
  • eg. http://ibase/cities?startswith=pu&start=11&count=10
  • Resources have Representations
  • A representation captures the current or intended state of a resource
  • Resources are transferred between the client and the server
  • Resources may include metadata describing themselves
  • A particular resource may have multiple representations
  • Commonly used representation formats are html, xml and json however they could also be pdf, png etc.
  • When multiple resource formats are supported by the server, the actual resource format returned is subject to content negotiation between the client and the server
  • This should ideally happen through control data i.e. By using HTTP “Accept” headers and not by appending additional information to the URL. Prefer Accept: text/xml;q=0.5, application/json http://infobase/cities/pune to http://infobase/cities/pune.json
  • REST doesn't dictate or constrain you to using particular representation formats. Use what suits the application context the best.
  • Interface constraint 3 Self descriptive messages
  • Requests and responses contain inband description about the schema it adopts
  • This is done by describing the XML Schema for the representation (or its units) using the same by declaring its appropriate XML namespace. Further clarity can be introduced by using a custom “application/vnd.*****” Content-Type header.
  • The entire schema does not need to be known upfront. Only the mandatory and relevant parts need to be known.
  • The schema can continue to be extended without client modifications if it is only adding optional elements or attributes.
  • Intermediate layers can both parse and transform messages intelligently
  • Metadata helps both page and form rendering and client side validations could be introduced based on an understanding of the schema and its semantics
  • Interface constraint 4 Hypermedia as the engine of application state (HATEOAS)
  • Hypermedia Hypermedia is used as a logical extension of the term hypertext in which graphics, audio, video, plain text and hyperlinks intertwine to create a generally non-linear medium of information. source : Wikipedia
  • HyperText Simultaneous presentation of information and controls such that the information becomes the affordance through which the user obtains choices and selects actions - Roy Fielding
  • Application State state that determines "where" the user is in the process of completing a task It is not the resource or state of the resource on the server
  • To understand application state, you need to visualise the pages / resources of the application as a wireframe model or a state machine and each page as a state
  • Each state allows for only a few valid triggers to allow it to navigate to another state
  • These possible navigations out of the state can be embedded in the resource representation overlying the state by using hypertext (links)
  • Since each state self describes the possible links given the context, the client can choose to select the appropriate link by examining the link metadata.
  • To put it differently Make hypermedia constrain client choices, and the client choice influences the application state
  • Therefore : Hypermedia as the engine of application state
  • Client needs to know only the starting URL
  • In addition client needs to be able to understand the mediatypes and semantics associated with the links (ie. What does a link with a particular “rel” type mean)
  • One more aspect of Uniform Interfaces
  • Uniform Operations
  • In case of database tables, these are Insert, Select, Update, Delete
  • In common parlance these are Create, Read, Update, Delete (CRUD)
  • In REST over HTTP these are POST, GET, PUT, DELETE
  • Those are the only verbs you need
  • Simplifies semantics
  • Simplifies client complexity
  • Simplifies application model
  • Clients interact with REST based systems by sequentially performing one of POST, GET, PUT, DELETE operations on self describing resources and by traversing the links offered by the server
  • For this clients need to understand resource representation schemas (xml schemas) and ...
  • Client need to understand semantics of the relationship types (<link rel=”...”>) offered by the server
  • REST is the DBMS of the internet
  • With a slight caveat
  • It doesn't break encapsulation
  • It exposes resource representations and not resources themselves
  • Thats like a parallel set of tables / views that clients can access and which have triggers which in turn appropriately update the real tables
  • Its often much easier and quicker to understand table schemas than it is to understand stored procedure semantics
  • This is an important distinction compared to RPC/ SOA based architectures which in case of this analogy would represent stored procedures
  • Which is why a client is likely to be far quicker off the starting block if given a set of schemas and standard SQL semantics to work with rather than a list of stored procedures describing each procedure, its parameters and the interrelationships between the procedures. Thats what makes ReST so much easier for its clients and users
  • Sample ReST request
  • Sample ReST response
  • ReST simplifies
  • ● Hypertext is standardised. Fewer UIs ● Identification is standardised. Lesser communication ● Exchange protocols are standardised. Fewer Integrations ● Interactions are standardised. Fewer semantics ● Data formats are standardised. Fewer translations - Roy Fielding
  • No IDLs, WADLs, WSDLs
  • No static compilations required
  • No methods and what each method means
  • No inter method sequencing
  • No registries
  • When dealing with complex stuff, you always feel, you can use some rest.
  • When you use ReST, things are simpler
  • Benefits of REST - Roy Fielding
  • Uniform resources having identifiers increases reuse potential
  • Uniform interface hides implementation details supporting low coupling
  • Hypertext allows for late binding leading to reduction in attempted inappropriate accesses and resultant errors
  • Server failures don't befuddle client state leading, while shared state is easily recoverable leading to improved fault tolerance
  • Supports gradual and fragmented change across organisations.
  • Services can be layered, clustered and cached leading to improved scalability
  • ReST extends the very capabilities that made WWW successful into application design and architecture
  • What are these characteristics of static W W W and ReST?
  • You can connect to any web server if you know the home page URL You can connect to ReST application if you know the starting URI
  • On the home page you can view the content along with the appropriate hyperlinks which suggest appropriate paths for you to traverse The response will provide you important initial content along with hyperlinks which describe their nature to navigate to other resources
  • You can navigate to the next page by clicking on the hyperlink You can conduct an operation by performing a POST/GET/PUT/DELETE on one of the suggested URIs
  • You can save the hyperlink URL, bookmark it or email it to you boss or tweet it to your friends A ReST client can store a URI for future use or embed it as a foreign key in other resources that it maintains
  • They will not need to repeat your sequence of steps. They will be able to directly access the page given the URL. The receiving ReST client will be able to directly access the earlier stored URI without going through a sequence of pages
  • You can save the contents of any page by saving its HTML representation You can save the representation of any resource into a XML / Document database
  • You can modify the contents of the web pages by entering data in forms (and even full page content in blogs, Wikis etc.) and POSTing them. You can perform PUT, POST and DELETE operations on resources to modify them
  • You can upload new files by browsing for the file on your desktop and submitting the button on appropriately configured pages (PUT file) You can add new resources by conducting the POST operation
  • The server retains no information about the pages you've traversed The server retains no information about you or the pages you've traversed
  • The server can send you different media types (eg. HTML, PDF, Videos etc.) by describing these media types in the headers The server sends the metadata describing the resource representation inband with the representation
  • Did you notice there is no global internet registry for website discovery ? There is no registry required for ReST applications
  • Yahoo tried, as does Open Directory but it just doesn't work And it may not for many other architectures requiring registries
  • Since the content depends on basic HTML tags and each URL is uniquely addressable, it is easy for search engines to index its content and allow users to find the required pages Representations for URIs can be browsed, indexed and eventually searched through
  • These are all characteristics that made static www simple to use, deploy and leverage making it popular and eventually omnipresent These are also characteristics of ReST contributing to its simplicity and ease of leveraging accounting for its popularity
  • Designing ReSTful applications
  • Using a ReST supportive framework does not make your application ReSTful
  • You need to model your application interfaces as a set of resources
  • And basic CRUD operations on these resources
  • Since controllers in traditional web frameworks drive the interface, we shall focus on these
  • When the interface is meant for browsers, there are some limitations. Hence browser oriented interfaces are a little different than POST, GET, PUT, DELETE
  • Assuming each controller represents a lifecycle manager for a particular resource type, it needs a few basic methods. And the same methods get reproduced across all such controllers
  • Resource URI HTTP Controller Description Method Method /cities GET index Get list of cities (optional params) /cities POST create Create a new city /cities/Pune GET show Show pune resource representation /cities/Pune PUT update Modify pune resource /cities/Pune DELETE destroy Delete pune resource /cities/new GET new Initiate a new city resource creation /cities/Pune;edit GET edit Initiate a new city modification
  • No more actions like city.expand (CityExpansion.create) , city.holdElections (CityElection.create) etc.
  • You will need to create new controllers which represent new nouns representing the action
  • Not all controllers will implement all methods. But they should not implement any more methods.
  • As you move from an action oriented design towards resource oriented design, thinking of everything as nouns is one of the early challenges to overcome
  • Transaction.approve becomes TransactionApproval Account.pay become AccountPayment.create etc. etc.
  • For each resource you need to document the XML Schema and define a mime type (application/vnd.***) especially when the consumer is a machine
  • For each resource representation you need to list what are the appropriate URIs (application state transitions) to be returned along with the representations and implement introduction of these in the controller actions as well
  • REST and Security
  • This is one area where I choose to be non ReSTful
  • Sometimes the deliberate requirements of opaqueness of security and transparency of ReST don't cooperate well
  • Cookies Cookie interaction fails to match REST's model of application state, often resulting in confusion for the typical browser application. - Roy Fielding
  • I agree with that .. but ...
  • Cookies can help in user identification (other options being Basic HTTP authentication)
  • Basic HTTP Authentication is weak
  • Computes a hash which can be intercepted and reused later
  • If you do use Basic HTTP authentication at the minimum use HTTPS
  • But I prefer cookies when they are strictly used for user identification only
  • But cookies break the statelessness model
  • Yes they do. I prefer to store only the data thats expensive to compute but can be recomputed in case of loss in the session against the cookie. No storage of conversational state in the session
  • That is hard to ensure .. and thats another self imposed architecture constraint
  • But I think it is more practical for secure applications
  • Even though it takes away their ability of being called 100% ReSTful
  • What about alternative architecture styles (SOA) ?
  • They are an extension of the RPC construct not the www construct
  • They simply do not have most of the benefits I just referred to
  • And the hype-engine is really struggling to compete with the wide successes of ReST
  • Experience has shown when sites offered both SOA and ReST interfaces, clients quickly ended up choosing ReST - sounds intituitive enough to me but do not recollect the source.
  • Rest is not SOA
  • They both attempt to solve a similar set of problems ....
  • .... differently!
  • ● ReST requires you to think resources not actions or services ● ReST requires you to lay a greater emphasis on documentation of your schema and practically none on the actions ● ReST requires you to provide in band metadata ● ReST works very nicely with layered architectures ● Another way to describe ReST is ROA : Resource Oriented Architecture
  • The clear distinctions between ROA and SOA are being blurred for non technical reasons. Be aware when you read content debating ReST/SOA (including this presentation)
  • SOA is the evolution of RPC semantics ReST / ROA is the evolution of www semantics
  • A look forward to increasing ReST popularity
  • ReST already is starting to dominate the internet space and there's a good likelihood it could dominate enterprise architectures as well.
  • References and Sources ● Roy Fielding's Dissertation on ReST ● A little REST and Relaxation : presentation by Roy Fielding ● Pragmatic Intro to REST and SOA, REST and the Web: presentations by Stephan Tilkov ● Pragmatic REST And RESTful Web Apps: presentations by Subbu Allamaraju ● Describing RESTful applications : Article by Subbu Allamaraju at InfoQ. ● RESTful Best Practices : presentation by calamitas ● The REST architectural style : presentation by Robert Wilson
  • Thank You !