rest (representational state transfer) explained
DESCRIPTION
A long presentation on a variety of aspects of REST.TRANSCRIPT
REST ExplainedRepresentational State Transfer
Dhananjay NeneJuly 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
StatelessnessNo 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
● Identification of resources● Manipulation of resources through
representations● Self descriptive messages● Hypermedia as the engine of application state
(HATEOAS)
Four interface constraints(more later .. we shall be spending the maximum time on this)
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. choosehttp://informationbase/locationdb/citiestable/pune
nothttp://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.
PreferAccept: 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 areInsert, Select, Update, Delete
In common parlance these areCreate, Read, Update, Delete (CRUD)
In REST over HTTP these arePOST, 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 HTTPMethod
ControllerMethod
Description
/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 !