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Developing Distributed Web Applications, Where does REST fit in?

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  • 1. Developing Distributed Web Applications, Where does REST fit in?
    Dr. Srinath Perera
    Senior Software Architect, WSO2 Inc.
    Visiting Faculty, University of Moratuwa
  • 2. What is a Distributed Web Application?
    Users access a one server (or he thinks he does) while really the app is composed of many machines.
    Why “Distributed”?
    for scaling. Single machine can not handle the load
    High availability
    photo by arvindgrover on Flickr, http://www.fotopedia.com/items/flickr-3154212659
  • 3. Google as a Distributed Web Application
    Google crawls the Web and stores all (well most of ) the Web
    It builds indexes for included words and incoming links for each document
    When a user sends a query, it applies the Page Rank Algorithm and returns the most relevant results.
    It is a distributed application that built using few 100k servers
  • 4. Programming Distributed Web Applications
    Essentially (Web) applications deal with state (data) and operations (executions or processing units)
    Users talk to applications and either retrieve data or make changes to the current state.
    There are two primary architectural models
    SOA (Service Oriented Architectures) : based on processing units (verbs)
    ROA (Resource Oriented Architecture): based on data(nouns)
    photo by QolePejorian on Flickr, http://www.fotopedia.com/items/flickr-184160623
  • 5. SOA (Service Oriented Architecture)
    There are many stateless processing units (often grouped as services)
    Users provide data to processing units, and they either return data after processing or change the system wide state (e.g. stored in the database).
    A SOA based architecture includes a main storage (e.g. database) and many stateless processing units, and an application combines and composes those processing units together.
    E.g. Google has its indexes etc., and one main operation “search” that accepts a search query as a input and returns matching documents.
    Also implementation has many internal services
    photo by L. Marie on Flickr, http://www.flickr.com/photos/lenore-m/312319615/
  • 6. ROA (Resource Oriented Architecture)
    There is a resource structure
    Each resource supports four operations, PUT, GET, POST, DELETE
    Users retrieve information through GET and change system state through POST, DELETE, and PUT
    The architecture is based on resources (nouns) and processing is represented through four verbs on the resource.
    photo by Svadilfari on Flickr, http://www.flickr.com/photos/22280677@N07/2435832518/ (CC)
  • 7. ROA vs. SOA
    There are lots of fights, which I am not going to discuss in detail.
    Argument is Web uses ROA, and it works.
    SOA is much closer to the programing models we used to with languages like C or Java. So that is natural to us.
    I would say, they are two useful styles, and we have to know both and use them where it naturally fits.
    I will focus more on ROA for this discussion.
    photo by hans s on Flickr, http://www.flickr.com/photos/archeon/2359334908/
  • 8. REST (Representational State Transfer)
    ROA is realized with REST services or HTTP services (Good intro to REST, http://www.infoq.com/articles/rest-introduction)
    Main principles of REST
    Give everything an ID (URI)
    Things (resources) refer to each other
    All interactions are done through four verbs (PUT, GET, DELETE, POST) on resources
    Photo by paraflyer on Flickr, http://www.fotopedia.com/items/flickr-386529128
    Resources can return multiple formats (choosed based on content negotiation)
    All communication are stateless (each request has all the information need to serve the request)
  • 9. Example: Web App to Manage a Network
    Assume we want to build a Web app to manage networks. Users can view, query, and change the network configurations through this Web App.
    Lets see how this would look with both SOA and REST
    Photo by sylviaduckworth on Flickr, http://www.geograph.org.uk/photo/956353
  • 10. With SOA
    Architecture will include a database to hold network data and services supporting following operations
    ListNetworks()
    NetworkDataGetNetworkData()
    ApplyNetworkData(NetworkDataquearyParams)
    SearchNetworks(String queryParams)
    Notice Each operation is a Verb, users combine those verbs to do what he needs
    Message formats are defined through a WSDL and each service supports one message format.
  • 11. With REST
    Give everything a ID (URI)
    http://uom.lk/Networks
    http://uom.lk/Networks/ENT
    http://uom.lk/Networks/Civil
    Things (resources) refer to each other
    Retrieving a network will return something like
    <Networks>
    <cse ref=“http://uom.lk/Networks/CSE”/>
    ….
    </Networks>
    All interactions are done through four verbs (PUT, GET, DELETE, POST) on resources
    GET on http://uom.lk/Networks/CSE : Getting network data
    POST on http://uom.lk/Networks/CSE: update the network config.
    DELETE on http://uom.lk/Networks/CSE can delete the network etc.
  • 12. With REST (Contd.)
    Resources can return multiple formats
    Results can return multiple formats, XML, Jason, Binary or several XML formats etc.
    Clients send the header Accept: text/json, text/xml+foo defining supported formats, and the server returns result in a format client supports. This means a resource can support multiple formats at the same time. Also we can use this to support new formats later.
    All communication is stateless – this is same for SOA and ROA
    System can use HTTP style caching
  • 13. Implementing REST Services
    Define the resources structure
    Define many URLs for different types of resources
    Use a REST engine to attach code to each URL patterns, they support four verbs (e.g. see http://www.sinatrarb.com/intro)
    Code segment is for URL, verb, content type
    Example
    http://…/Networks:
    On GET text/xml do
    ….
    end
    Photo by Kevin Dooley on Flickr, http://www.flickr.com/photos/pagedooley/2201791390/
  • 14. REST Anti-patterns
    Verbs in URLs
    http://uom.lk/SearchNetworks?pattern=foo
    This is doing SOA using HTTP, not REST
    Ignoring status codes on responses
    Not including references in responses
    <Networks>
    <cse>
    <nodeCount>..</nodeCount>
    <subnetmask> ..</subnetmask>
    <cse>
    </Networks>
    Use references whenever that makes sense, do not copy data in to response. Remember the underline linked resource structure.
  • 15. REST, is it the Architecture to solve them all?
    Does the REST a silver bullet for all problems
    This is the source for most fights, the answer looks “No”. Some examples.
    It does not have a contract (Lack a WSDL)
    Messaging – does not talk about eventing/ messaging aspects. May be need a SUBSCRIBE verb added.
    Asynchronous invocations and reliability
    What about composition and transactions etc. mostly things come from WS-*
    Does error messages are enough?
    Photo by Playaduraon Flickr, http://www.flickr.com/photos/playadura/1401756881
  • 16. So when to use it?
    Use when REST is the natural choice
    It is very good at representing resources
    Not that good with processes/ executions
    Same for SOA
    Is it OK to mix it?
    I would say yes, but not too much
    WSO2 use REST for registry API, while SOA for many other places.
    Photo by QUEEN YUNA on Flickr, http://www.flickr.com/photos/queenyuna/5710802864/ (CC)
  • 17. Conclusion
    What is a Distributed Application?
    SOA and ROA as solutions
    Detailed looked at ROA with a Example
    It is the architecture that solves everything? “Look like we need both”
  • 18. For more information
    Stefan Tilkov: A Brief Introduction to REST, http://www.infoq.com/articles/rest-introduction
    Stefan Tilkov: REST Anti-Patterns, http://www.infoq.com/articles/rest-anti-patterns
    More articles here http://en.wikipedia.org/wiki/Representational_State_Transfer
    SanjivaWeerawarana: WS-* vs. REST: Mashing up the Truth from Facts, Myths and Lies, http://wso2.org/library/2818
  • 19. Questions?

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