Minimize coupling with REST processes

While integrating systems, implementing access or processes is typically achieved through man ordered list of steps, where one expects specific results from the server.

Expecting specific results means coupling your client to a servers behavior, something that we want to minimize.

In REST clients, and in business process modeled following REST practices, we can reach lesser decoupling factors. Imagine the following system (among others) that buys products at amazon.com:


When there is a required item
And there is a basket
But didnt desire anything
Then desire this item as first item

When there is a required item
And there is a basket
Then desire this item

When it is a basket
And there are still products to buy
Then start again

When there is a payment
Then release payment information


The above code works and can be seen running in the last video from this post. In this Rest modelled process, several types of evolutions can happen on the server, without breaking the client:

1. After adding a product to the basket, if the server renders recommendations instead of your basket, the client still achieve its goal.

2. If some products are not found, the client is still able to pay for it.

3. If the server does not have the products, but links to another system in a affiliated program, we as clients will never notice it, and buy the products.

4. If we want to achieve the same goal in other systems that understand the same media type that we do, simply change the entry point..

Note the benefits from backward compatibility, forward compatibility and we are still capable to achieve our goals without knowing the existence of some changes on the protocol. We can even compare prices without changing one line of code, everything using well known media types as Atom. RDFa support can also be implemented.

This evolution power is not achieved in traditional process modelling, and several times, compatibility is left aside. This is something that REST and hypermedia brought us.

Mapping our process in a adaptable way using hypermedia was described in an article recently for the WWW2010 proceedings.

Now let’s imagine the traditional modelling:

1. Search
2. Add
3. Search
4. Add
5. Pay

In a typical web services + business processing scenario, how would a server notify me to access another system without changing one line of code in my client? It would break compatibility.

If the server now supports a limited quantity of items in your basket, your client will also break, while the adaptable REST client adapts itself and buys what it is capable of.

If our produt or company uses those traditional paths and every change on the service implies in heavy change costs for clients, thats a hint that your project is too much coupled, and a REST client as mentioned might help.

Tradicional web services decouple a little bit more than traditional RPC, but still provide this amount of coupling that cost our companies more than they were expecting to pay.

The following presentation 6 minutes presentation shows how to avoid some of this coupling from your server.

The entire video cast on the 3rd part from Rest from scratch is here:

This code achieves its results using Restfulie 0.8.0, with many thanks to everyone who contributed maturing this release: Caue Guerra, Luis Cipriani, Éverton Ribeiro, George Guimarães, Paulo Ahagon, Fabio Akita, Fernando Meyer and others.

Thanks to all feedback from Mike Amundsen, Jim Webber and Ian Robinson.

Is your client api code already adapting itself to changes?

(update: changed some of the “Then” clauses from the dsl for easier spotting of document messages instead of commands)

Rest from scratch – Part 2

This 20 minutes video shows how to move from a basic REST api to one which makes use of linked resources and adds semantic values to those links.

Did your REST api do that already? Great.

Otherwise, it’s time to move ahead and decouple a little bit further your clients from your server.

Buying through REST: applying REST to the Enterprise

REST was a research result that left us with an open question, as its researcher suggested: it beautifully solves a lot of problems, but how to apply it on contemporary concerns that enterprise have?

REST Applied

After many talks, I have summed up a model, derived from REST constraints, that allows one to measure how his entire system (client and server) achieves a REST architecture.

The following video shows an example on how to start from a typical non restful architecture to adopting REST constraints and creating a buying process in any REST server.

So what is the power behing applied REST?

“Rest Applied” as I have exemplified, solves our contemporary concerns, filling the gap between Roy’s description and application’s usage, opening up a new world of possibilities.

The same way that REST ideas, although they were not called REST at that time, allowed web crawling to be an amazing client, “REST applied”, as described, can change the way our applications communicate with servers.

Why did we miss it? Because Roy’s description goes for with crawling examples, which benefit directly from content type negotiation. i.e. different languages, same resource and google post ranking it:

“In fact, the application details are hidden from the server by the generic connector interface, and thus a user agent could equally be an automated robot performing information retrieval for an indexing service, a personal agent looking for data that matches certain criteria, or a maintenance spider busy patrolling the information for broken references or modified content [39].”

But, “Not surprisingly, this exactly matches the user interface of a hypermedia browser. “… the client adapts itself to its current representation – limited to the client’s cleverness.

REST Applied takes those ideas to solve our problems, as some examples from Rest in Practice and procurement through rest.

Frameworks/libraries used

Restfulie gives better HTTP support to http libraries and provides a REST frameworks while Mikyung allows you to create your REST clients. With both of them you are ready to apply REST to enterprise problems.

Mikyung stands for “beauty capital” in Korean, in a attempt to reproduce what a beautiful rest client could look like when following REST constraints.

REST maturity model

Not yet REST

How do we achieve REST? Leonard Richardson’s model was widely commented and Martin Fowler posted on “Rest in Practice” (a book I recommend reading). But what is left out from REST in Richardson’s model and why?

According to his model, level 3 adds hypermedia support, leveraging a system through the use of linked data – a requirement for a REST architecture. But HATEOAS alone does not imply in REST, as Roy stated back in 2008.

Remember how method invocation on distributed objects allowed you to navigate through objects and their states? The following sample exemplifies such situation:


orders = RemoteSystem.locate().orders();
System.out.println(orders.get(0).getProducts().get(0));
receipt = order.payment(payment_information);
System.out.println(receipt.getCode());


But what if the above code was an EJB invocation? If navigating through relations is REST, implementing EJB’s protocol through HTTP would also be REST because linked data is also present in EJB’s code – although lacking an uniform interface.

While Richardson’s model get close to REST on the server side, Rest in Practice goes all way to a REST example, describing the importance of semantics and media type importance. The rest of the post will explain what was left out of this “Rest services” model and why, proposing a model that encompasses REST, not REST under http; while the next post, with a video, describes how to create a REST system.

What is missing?

“The model application is therefore an engine that moves from one state to the next by examining and choosing from among the alternative state transitions in the current set of representations.“.

Did the previous code inspect the relations and state transitions and adapted accordingly?
It did not choose a state transition, it contains a fixed set of instructions to be followed, no matter which responses are given by your server. If the API in use is http and the server returns with a “Server too busy” response, a REST client would try again 10 minutes later, but what does the above code do? It fails.

We are missing the step where REST clients adapt themselves to the resource state. Interaction results are not expected as we used to in other architectures. REST client behavior was not modelled on Richardson model because the model only thought about server side behavior.

This is the reason why there should be no such a thing as “rest web services” or “rest services”. In order to benefit from a REST architecture, both client and server should stick to REST constraints.

Richardson’s server + http model

Semantic meaningful relations are understood by the client, and because of that we need a model which describes how to create a REST system, not a REST server.

An important point to note is that this model is pretty good to show a REST server maturity over HTTP, but limiting REST analysis both to server and http.

A REST architecture maturity model

For all those reasons, I propose a REST maturity model which is protocol independent and covers both consumer and provider aspects of a REST system:

Rest Architecture Maturity

Trying to achieve REST, the first step is to determine and use an uniform interface: a default set of actions that can be taken for each well defined resource. For instance, Richardson’s assumes HTTP and its verbs to define a uniform interface for a REST over HTTP architecture.

The second step is the use of linked data to allow a client navigate through a resource’s state and relations in a uniform way. In Richardson’s model, this is the usage of hypermedia as connectedness.

The third step is to add semantic value to those links. Relations defined as “related” might have a significant value for some protocols, but less value for others, “payment” might make sense for some resources, but not for others. The creation and adoption of meaningful media types allows but do not imply in client code being written in a adaptable way.

The fourth step is to create clients in a way that decisions are based only in a resource representation relations, plus its media type understanding.

All of the above steps allow servers to evolve independently of a client’s behavior.

The last step is implied client evolution. Code on demand teach clients how to behave in specific situations that were not foreseen, i.e. a new media type definition.

Note that no level mentions an specific protocol as HTTP because REST is protocol independent.
The following post will describe one example on how to create a REST system using the above maturity model as a guide.

REST is crawling: early binding and the web without hypermedia

The most frequently asked question about REST in any presentation: why hypermedia is so important to our machine to machine software?

Is not early binding through fixed URI’s and using http verbs, headers and response codes better than what we have been doing earlier?

An approach that makes real use of all http verbs, http headers and response codes already presents a set of benefits. But there is not only the Accept header, not only 404, 400, 200 and 201 response codes: real use means not forgetting important verbs as PATCH and OPTIONS and supporting conditional requests. Not implementing features as automatic 304 (as a conditional requests) parsing means not using http headers and response codes as they can be used, but just providing this information to your system.

But if such approach already provides so many benefits, why would someone require a machine-to-machine software to use hypermedia? Is not it good enough to write code without it?

The power of hypermedia is related to software evolution, and if you think about how your system works right now (its expected set of resources and allowed verbs), hypermedia content might not help. But as soon as it evolves and creates a new set of resources, building unforeseen relations between them and their states (thus allowed verbs), that early binding becomes a burden to be felt when requiring all your clients to update their code.

Google and web search engines are a powerful system that makes use of the web. They deal with URIs, http headers and result codes.

If google’s bot was a statically coded bot that was uncapable of handling hypermedia content, it would require a initial – coding time or hand-uploaded – set of URIs coded that tells where are the pages on the web so it retrieves and parses it. If any of those resources creates a new relationship to other ones (and so on), Google’s early binding, static URIs bot would never find out.

This bot that only works with one system, one specific domain application protocol, one static site. Google would
not be able to spider any other website but that original one, making it reasonably useless. Hypermedia is vital to any crawling or discovery related systems.

Creating consumer clients (such as google’s bot) with early binding to relations and transitions do not allow system evolution to occur in the same way that late binding does, and some of the most amazing machine-to-machine systems on the web up to date are based in its dynamic nature, parsing content through hyperlinks and its semantic meaning.

Although we have chosen to show Google and web search engines as examples, any other web systems that communicate with a set of unknown systems (“servers”) can benefit from hypermedia in the same way.

Your servers can only evolve their resources, relations and states without requiring client-rewrite if your code allows service-crawling.

REST systems are based in this premise, crawling your resources and being able to access its well understood transitions through links.

While important systems have noticed the semantic value and power of links to their businesses, most frameworks have not yet helped users accomplishing late binding following the above mentioned principles.

Restfulie 0.5, atom feeds, content negotiation and default controllers

ATOM FEED

Restfulie 0.5.0 is out and its major new feature is its support to Atom feeds with variable media types.

A feed can be easily rendered by invoking the to_atom method:


@hotels = Hotel.all
render :content_type => 'application/atom+xml',
:text => @hotels.to_atom(:title=>'Hotels', :controller => self)


A collection might contain entries with different media types and each one will be rendered in its own way. The client code works as any other usual client would, note that there is still content negotiation taking place:


hotels = Restfulie.at('http://localhost:3000/hotels').get
puts hotels[0].name
puts hotels[1].name


And using hypermedia to drive our business, deleting an entry will send a DELETE request to that entry’s self URI:


Restfulie.at('http://localhost:3000/hotels').get.each do |h|
h.delete if h.city=="Sao Paulo"
end


In future releases we expect to support atom feed generation (and consuming) through Ratom.

CONTENT TYPE

Another easy to use feature is content type negotiation, which also happens when rendering a single resource:


@hotel = Hotel.find(params[:id])
render_resource @hotel


Users can now extend Restfulie in order to create custom formats (not based on json/xml/xhtml related media types)

CLIENT SIDE

Entry points have been extended and now they can be reached even if there is no class representing the received information in the client side:


support Restfulie.at('http://localhost:3000/cities').create(city)
Restfulie.at('http://localhost:3000/cities/5').get


One can also access the web response:

response = Restfulie.at('http://localhost:3000/cities/5').get.web_response


And play with content negotiation:


Restfulie.at(uri).accepts('media-type').get
Restfulie.at(uri).as('media-type').create(something)


DEFAULT CONTROLLERS

In the server side, a generic controller has been created which supports show, delete and create by default.

There is also a new method called render_created that can be used in order to answer a request with a 201 Created response and its resource location.

WEBSITE

Restfulie got its own website and all Ruby docs have been migrated.

Thanks to all the team and collaborators!

If you look carefully, you might find out next week’s upcoming news.

Transactions do not exist in a Restful world…

Due to the last posts on infoq related to Restfulie, my work at Caelum Objects involved a presentation at one client, “Beginning a REST initiative” (based on Ian’s work) and the question came up: “but how do I control transactions without a custom software stack to help me?”

The answer was, “you do not need to”.

Restwiki has an old entry on how to implement transaction support through http using some non-standard http headers.

The idea was not new, as Roy Fielding mentioned on an old mail that this extra http header could be a solution and later seemed to change his mind about it, according to an infoq news.

In practice most ideas are based on a transaction being a resource named “Transaction”: an idea heavily based on HTTP and URIs, but forgetting about HATEOAS – again.

In the human web, how does one buys some products? Every product is added to the shopping basket, which then generates the order. Does the user creates a transaction before processing his order?

The human being behing the computer did not create a transaction: the browser is even unaware of that concept, but hyperlinks given by the server guided the client through this “transaction”. In this case, where the typical “REST” solution would create a “Transaction” resource and use the non-standard header to support it, a Restfulie one creates a shopping basket:

	Typical “REST” approach	Restfulie
sequence	POST /transaction POST /product * POST /product *	POST /basket POST /basket/:id/product POST /basket/:id/product
commit	POST /transaction/commit *	PUT /basket/:id/payment
rollback	DELETE /transaction/ *	DELETE /basket/:id

* with non-standard http header

The standard way of thinking about transactions is to not use HATEOAS and believe that transactions are resources by themselves. Transactions are not resources, but a tool to implement ACID in your (i.e.) databases, not in a web system.

In our example, an order creation maps to internal transactions. In a bank example, a Transfer resource would map to the internal transaction.

By renaming the “transaction” to the real objective of that transaction, one can better map meaningful URI’s to resources.

Note that these are only the advantages of valuing the use of URIs over non-standard http headers (manifest hint?): there is no loss of visibility to layers between the client and the server.

But now one might argue that there are too many entry points. Actually, both implementations contain the same number of “entry” points if there is no hypermedia support: 4. Too many entry points should not be called “entry” points. (entry-hell pattern?)

But do we, in the human web, type in URIs as we go further with our online “transaction”? Do we type in URIs as we do a two-step flight and hotel booking process?

If the entry point POST /basket answers with a:


Header
Location: http://caelumobjects.com/basket/5
Content
<basket>
<link rel="products" href="http://caelumobjects.com/basket/5/products" />
<link rel="coupon" href="http://caelumobjects.com/basket/5/coupon" />
<link rel="pay" href="http://caelumobjects.com/basket/5/payment" />
<link rel="cancel" href="http://caelumobjects.com/basket/5" />
</basket>


Note that our basket – our transaction’s meaning – contains hints on how to operate with it and its relations pretty much in the same way that it would do in the human web: dynamically generated links that allows the server to guide the client throughout the process, eliminating the need to extra “entry-points”.

In a hotel and flight booking system, the booking POST result could be represented as:


Header
Location: http://caelumobjects.com/booking/5
Content
<booking>
<link rel="flights" href="http://caelumobjects.com/booking/5/flights" />
<link rel="hotels" href="http://caelumobjects.com/booking/5/hotels" />
<link rel="pay" href="http://caelumobjects.com/booking/5/payment" />
<link rel="cancel" href="http://caelumobjects.com/booking/5" />
</booking>


Note how the first idea on implementing transactions evolved. From a custom header which interferes with visibility and creates the need for custom built clients and layers to understand this instruction, with no server guidance at all to a system where there is no need to customize your client api or layers and the server guides the user flow through hypermedia, maturing your system.

Transactions should not be called “transactions”. The basket or transfer resource are examples of that: they are typical server side implemented transactions that should be actual resources.

Our basket (and thus transfer) seems to match Roy’s comment at that time:

• “As far as the client is concerned, it is only interacting with one resource at a time even when those interactions overlap asynchronously.”: the basket or the transfer
• “There is no “transaction protocol” aside from whatever agreement mechanism is implemented in the back-end in accordance with the resource semantics (in a separate architecture that we don’t care about here).”: you add products to the list of products form that basket, add some coupons and so on
• “There is no commit protocol other than the presentation of various options to the client at any given point in the application.”: hateoas
• “There is no need for client-side agreement with the transaction protocol because the client is only capable of choosing from the choices provided by the server.”: transaction protocol? no transaction protocol here, just a simple resource handling

Restfulie – as many other rest frameworks -already support the first step (running away from the custom header), but goes further when being “hypermedia centric”, it allows the developer to implement it without any effort.

Being opiniated and forcing the adoption of hypermedia as a way to guide or clients through out our processes might be one step ahead into more web (rest in this case?) friendly world as Ryan Riley pointed out.

HATEOAS, HTTP and URIs allow you to eliminate the concept of transaction management (and web transaction managers) from your systems as we usually think of them. There are two steps to follow:

1. there are no transactions
2. let the server guide you, do not try to guide him with multiple entry points

Hypermedia and dynamic contracts: let my bandwidth rest!

“Break it” to scale!

Many systems contain webpages that are very similar to user “custom pages”, where they can configure what they want to see, and every piece is aggregated from different sources into one single page.

In some cases, those are widget based frameworks as wicket and gwt that can be added to my custom page; in other cases you have aggregating portals.

An example of this kind of application (even though its not configurable) is a retail website containing four sections in its home page: the top 10, my orders, random items, and weird items.

In this case, all information come from the same source, but every part has a different probable validity if it is going to be cached. If the page is served as one big chunck of information, it will always be stale due to the random items section. “My orders” is stale only when I place a new order and, in the same way, the top 10 is only stale if any item is bought and surpasses the number of times the 10th item was bought so far.

One of the main issues with this type of pages which aggregate information from one or many sources with different expire-expectations is that cached versions in proxies and clients become stale faster than it should for some elements: once one of this providing sources publishes new information or is updated, the entire representation becomes stale..

Martin Fowler described once a well spread approach to allow those pages to be partially cached within local proxies and clients, thus sharing requested representations between multiple users.

The approach

Given the coffee scenario, one would create different json representations:

• http://restbucks.com/top_sellers
• http://restbucks.com/my_orders
• http://restbucks.com/weird_items
• http://restbucks.com/random_item

And finally an aggregating page:

<html>
<a class="lazy_load" href="http://restbucks.com/top_sellers">Top sellers</a>
<a class="lazy_load" href="http://restbucks.com/my_orders">My orders</a>
<a class="lazy_load" href="http://restbucks.com/random_items">Random items</a>
<a class="lazy_load" href="http://restbucks.com/weird_items">Weird items</a>

And then, for each lazy_load link, we create a div with its content:

<script>
$('.lazy_load').each(function(link) {
  uri = link.attr('href'); 
  div = $('
').load(uri); // cache hits!
  link.after(div);
});
</script>
</html>

This allows our proxies to cache each component in our page apart from the page itself: whenever one page’s content becomes stale in a proxy, only part of that page needs update.

In a web were most data can be cached and does not become stale so fast, this technique should usually lessen the amount of data being transfered between client and server.

All one needs to do is properly use the http headers for caching.

Remember that if your client supports either parallel requests to the server and/or keep-alive connection, the results might be even better.

Distributed systems? Linked resources?

Roy Fielding mentions that in the data view in REST systems, “small or medium-grain messages are used for control semantics, but the bulk of application work is accomplished via large-grain messages containing a complete resource representation.”

Pretty much in the same way as with the human web, a distributed system using the web as its infrastructure will gain the same cache benefits as long as they implement correct caching policies through http headers (and correct http verbs).

When your server provides a resource representation linking to a series of other related resources the client and proxies staying on the way will be allowed to cache each and every other resource on its own.

This approach results, again, in changes applied to one resource not affecting cached representations of other resources. An stale representation will not affect those accessing other resources within the same context.

Sometimes the decision whether to change latency for scalability might depend on how you think your clients will use your resources: in the human web mentioned above, the developer knew exactly how its clients would access it.

In distributed systems using REST, guessing how resources will be used can be dangerous as it allows you to tight couple yourself to this behaviour while published resources can and would be used in unforeseen ways.

Roy’s dissertation seems to apply here to balance things: “a protocol that requires multiple interactions per user action, in order to do things like negotiate feature capabilities prior to sending a content response, will be perceptively slower than a protocol that sends whatever is most likely to be optimal first and then provides a list of alternatives for the client to retrieve if the first response is unsatisfactory”.

Giving information that will help most cases is fine and providing links to further resources details allow you to balance between latency and scalability (due to caching) as you wish.

Dynamic contracts

This is only possible because we have signed dynamic contracts with our clients. They expect us to follow some formal format definition (defined in xhtml) and processes. How our processes are presented within our representations is the dynamic part of the contract.

While the fixed part can be validated with the use of schema validators, the dynamic part – the process – which is guided by our server needs to be validated through testing the behaviour of our applications: asserting that hypermedia guided transitions should be reflected in our application state.

Nowadays

On the other hand, many contemporary systems use the POST verb receiving a response including many representations at once or the GET verb without any cache related headers: thus not profiting from the web infrastructure at all. This could changed with one (or both) of the following:

• use the GET verb with cache headers
• use hypermedia and micro formats to describe relations between resources

Using it might present similar results as hypermedia+GET+cache headers in the human web – and some styles might already be providing support for it, although not being a constraint.

Note that in this case hypermedia is not driving the application state, but helping with scalability issues.

Progressive enhancement

Martin notes that this is a kind of progressive enhancement: although its definition is related to accessibility, its control over bandwidth benefits are similar to the approach mentioned ones.

Any other systems that use hyperlinks to “break” representations and scale?

Hypermedia: making it easier to create dynamic contracts

The human web and christmas gifts

You have been buying books at amazon.com for 5 years now: typing http://www.amazon.com in your browser, searching for your book, adding it to the cart and entering your credit card information.

But this year, on December 15th 2009 something new happens. Amazon has launched an entire new “christmas discount program” and in their front page there is a huge ad notifying their clients about this new item.

How do you react?

“Contract violated! I am not buying anything today.”

The key issue in loosely coupled systems is the ability to evolve one side without implying in any modifications on the other part.

As some Rest guys agree, hypermedia content was the factor which allowed such situations to happen in the human web without clients screaming “i don’t know what to do now that there is a black friday clearance!” or “there is a new link in this page, let me email the ‘webmaster’ and complain about it“.

In the human web, some contracts are agreed upon and validated through end-to-end tests. Some companies will use tools as selenium-rc, webdriver or cucumber to drive their tests and ensure that expected behaviour by their clients does not break with a new release of their software.

Those tests do not validate all content, though, giving space for what is called forward-compatibility: the system is free to create new functionalities without breaking previous expected behaviour.

But my rest-client is not human

In the non-human web, the most well known media type used is xml, although not hypermedia-capable. There are a couple of ways to create forward or backward-compatible schemas that check xml structures, but – unfortunately – usually
fixed schemas will not invest part of its contract in order to making it forward-compatible: its an optional feature.

One option is to create “polymorphic” types through xsd schemas, which will get nasty if your system evolves continuously – not once every year – and you find yourself in a schema-hell situation.

One easy solution is to accept anything in too many places, which seems odd.

What are we missing then? According to Subbu Allamaraju, in RESTful applications, “only a part of the contract can be described statically, and the rest is dynamic and contextual”: you tell your client that they can believe you will not break the statically contract – you might use some schema validation to do that – and it’s up to you on the server side to not do it on the dynamic part.

Some might think it sounds too loose… let’s recall the human web again:

• xhtml allows you to validate your system’s fixed contract
• it’s up to you not to remove an important form used throughout the buying process

So, what are the dynamic parts of my “contract”?

In a RESTful application the contract depends on its context, which is highly affected by three distinct components:

1. your resource’s state

If a person had his application denied to open an account, your resource representation will not offer a “create_loan” request. A denied application is an information regarding its state.

While your company and application evolves, its common to find ourselves in a position where new states appear.

2. your resource’s relations

In a book store (i.e. amazon a few years ago), a book might have a category associated with it so you can access other similar books:

<book>
<name>Rest if you do not want to get tired</name>
<link rel="category" href="http://www.caelumobjects.com/categories/self-help" />
</book>


A couple years later, your system might add extra relations, as "clients which bought this book also recommend"

<book>
 <name>Rest if you do not want to get tired</name>
 <link rel="category" href="http://www.caelumobjects.com/categories/self-help" />
 <link rel="recommendation" href="http://www.caelumobjects.com/books/take-a-shower-with-a-good-soap-if-you-need-to-rest" />
</book>

When your company and application evolves, its common to find ourselves in a position where new relations appear.

3. your resource’s operations

In a REST application, your resource operation’s are represented by HTTP verbs: supporting a new one will not affect clients which use all other available verbs so far.

In the RPC/Webservices world, new operations would be implemented creating new remote procedures or services.

But how can my clients be sure that I will not break the dynamic contract?

Pretty much in the same way that you do in the human web: it’s your word.

In the human web, how do we guarantee that we will not remove or break some functionality the user expects to be there? We end-to-end automatically test its behaviour.
Our word (our tests) is the only reason to rest without worries that we will not break our client’s expectations. The same holds on the non-human web.

The dynamic contract should be throughly tested in order to not break our client’s expectations.

There are other approaches (as client-aware contracts) which might add some extra coupling between both sides.

HTTP+XML+ATOM gives us the possibility to work with both the fixed (schema validated) and dynamic (test validated) contract.

As Bill Burke pointed in a comment, “you can design your XML schemas to be both flexible and backward compatible ” and “companies, users, developers desire this contract”.

That’s the good points of using schemas, but its not everyone that use them in a flexible and backward compatible way. Even those who use might have a little bit of hard time to support it, i.e. having to maintain more than one entry point for each version of their schemas.

That’s when we can use the good points of the schema validation, as Bill pointed out, with the easy evolution advantages of a dynamic contract: as we do in the human web.

By using dynamic contracts as xml+atom following the Must Ignore rules, forward and backward compatibility is gained by default, independent on what the user does – assuming that tests are a must in any solution.

Dynamic contracts also give hints for frameworks, as they guide you on what your user can and can not do or access, but maybe not for tools, in a different fashion of what fixed contracts do: with a fixed schema I would be able to pre-generate my classes, while with dynamic schemas I the framework inject methods.

That’s why we try to take an approach which force programmers to adopt xml+atom. The entry point on the Restfulie framework is loosely evolution.

Its first example, the documentation and its examples do not focus on how easy it is to use nice URIs and the 4 most famous http verbs, but how easy it is to evolve your system using hypermedia and http: uri’s come soon afterwards.

And it seems to be working fine to far, the first developers using it in live systems have already supported hypermedia content as a way to guide clients through their systems.

Restfulie support in dynamic contracts

Matt pulver’s extension to Rails allows one to instantiate types with regards to their active record relations and attributes, but it requires every xml element to be present (strong coupling to the data structure presented by the server).

Using Jeokkarak (korean hashis), Restfulie instantiate objects matching your local data structure, supporting fields defined in your attributes and inserting extra fields for those elements unknown to your model.

For example, if you have a model as:

class Bill
  attr_accessor :value, :to_date
end

And the following xml:

<bill>
  <value>100</value>
  <to-date>10/10/2010</to-date>
  <taxes>0.07</taxes>
</bill>

The result is a dynamic object capable of answering to:

bill = Bill.from_web uri
puts bill.value 
puts bill.to_date
puts bill.taxes

If your model was ready to accept such xml, Restfulie will do the job, whilst if it doesn’t recognize the attribute, it will still be available to you.

That’s the default Restfulie behaviour: to allow the other part to evolve their dynamic contract (and even parts of the fixed one) by default, without any extra effort from your side.

Restfulie Java: quit pretending, start using the web for real

Its time to release Restfulie Java, offering the same power encountered in its ruby release, through the use of dynamic bytecode generation and request interception using VRaptor.

Serialization framework

Restfulie adopts XStream by default. Its simple usage and configuration gets even easier due to vraptor’s serialization extension built upon XStream – but it allows the usage of other serializers.

The following code will serialize the order object including its children items (much similar to rails to_xml only option):

serializer.from(order).include("items").serialize();

Connected: Hypermedia content creation

In order to guide the client from one application state to another, the server needs to create and dispatch links that can be interpreted by the client machine, thus the need for generating hypermedia aware serialization tools and consumer apis.

Its the basic usage of the web in a software-to-software communication level.

Pretty much like Restfulie’s ruby implementation, by implementing the getFollowingTransitions method, restfulie will serialize your
resource, generating its representation with hypermedia links:

public List getFollowingTransitions(Restfulie control) {
  if (status.equals("unpaid")) {
    control.transition(OrderingController.class).cancel(this);
    control.transition(OrderingController.class).pay(this,null);
  }
  if(status.equals("paid")) {
    control.transition(OrderingController.class).retrieve(this);
  }
  return control.getTransitions();
}

Controller interception

Restfulie for Java goes further, intercepting transition invocations and checking for its status. The following example will only be executed if order is in a valid state for paying:

@Post @Path("/order/{order.id}/pay")
@Consumes("application/xml")
@Transition
public void pay(Order order, Payment payment) {
	order = database.getOrder(order.getId());
	order.pay(payment);
	result.use(xml()).from(order.getReceipt()).serialize();
}

Why?

Restfulie does not provide a bloated solution with huge api’s, trying to solve every other problem in the world. According to Richardson Maturity Model, systems are called RESTFul when they support this kind of state flow transition through hypermedia content contained within resources representations:

<order>
 <product>basic rails course</product>
 <product>RESTful training</product>
 <atom:link rel="refresh" href="http://www.caelum.com.br/orders/1" xmlns:atom="http://www.w3.org/2005/Atom"/>
 <atom:link rel="pay" href="http://www.caelum.com.br/orders/1/pay" xmlns:atom="http://www.w3.org/2005/Atom"/>
 <atom:link rel="cancel" href="http://www.caelum.com.br/orders/1" xmlns:atom="http://www.w3.org/2005/Atom"/>
</order>

Stateless state

In order to profit even more from the web infrastructure, Restfulie for Java provides a (client state) stateless api.

Addressability

VRaptor’s controller api allows you to specify custom URI’s (and http verbs) to identify resources (and transitions) in your system.

Addressability + client cache stateless state server allows one to achieve REST’s idea on cache usage and its related layered systems advantages by allowing other layers to be added between the client and the server.

Unknown usage of my resources

Addressability + hypermedia content allows clients to use your resources pretty much in a way that was not tought of at first. Addresses (in our case, URI’s) can be passed around from one application to another, to and from a client’s internal database (as simple as a browser favorites, or google gears).

Building your system upon such basis, it become unaware of its resources usage (resource representation’s interpretation) patterns, allowing clients to create such previously unknown systems.

Less and simpler code, more results

Both on the server and client side, restfulie tries to achieve results based on conventions, therefore one can easily access its entry api to insert a resource in a system, i.e.:

  Movie solino = new Movie();
  resource("http://www.caelum.com.br/movies").include("metadata").post(solino);

And after creating a resource, one can actually navigate through your resource’s connections:

  Movie solino = resource("http://www.caelum.com.br/movies/solino").get();
  Comments comments = resource(solino).getTransition("comments").executeAndRetrieve();
  // print all comments

And navigate through your resource’s states:

  Comments comments = resource(solino).getTransition("comments").executeAndRetrieve();

  Comment comment = new Comment("nice movie on generations of immigrants and the hard times that they face when moving to a new place");
  resource(comments).getTransition("add").execute(comment);

A lot of good practices should be put into place in order to avoid creating Leonard Richardson’s defined REST-RPC alike systems.

Those good practices involve simple steps as avoiding anemic models on the client side . Restfulie+Vraptor already tries to avoid this on the server side andwe will discuss about such practices in following posts.

Download and example applications

Beginners could start by downloading restfulie’s client and server example application, ready to run in a eclipse wtp enviroment or pure eclipse installation.

Users are encouraged to use either the java or ruby mailing lists.

Since Restfulie was born in Ruby…

Since we released restfulie for ruby (on rails), which can be found at its github page, it was commented by Jim Webbers both on his blog and in person during QCon in San Francisco, where both him and Ian Robinson held a tutorial on restful systems and the web and will hold another round, more hands-on, at QCon London 2010.

Meanwhile, Restfulie was commented at ruby5’s podcast, commented here, at infoq, and in portuguese by Juliana and by Anderson Leite – a fellow Caelum developer.

As some comments were out about restfulie’s ruby implementation, restfulie: it’s more than easy.