Archive for the ‘restful’ Category
Expecting specific results means coupling your client to a servers behavior, something that we want to minimize.
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.
Now let’s imagine the traditional modelling:
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 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.
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 is not only about http and the web, but an architectural style derived from the web and others.
In this 20 minutes video, we will see how the web has been capable of scaling, providing different services and being a more effective system than other human distributed systems alike (i.e. water distribution and electricity).
We move on to describing the basic characteristics of a rest architecture and how it leverages your system.
This is the first video on the Rest from Scratch – Theory. You can watch the other Rest From Scratch – Practice videos online here.
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.
On this 10 minutes video you will learn how it is possible to make several representations of one resource available on the server side (xml, json, atom and so on) and one line of code access and parse it on the client side.
This video shows how to access those basic REST api’s as Twitters, CouchDB which are based on http verbs and multiple URIs. The following videos shall demonstrate how to access more advanced REST API’s.
If you have any questions, join us at our mailing list.
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?
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 .”
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.
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.
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();
receipt = order.payment(payment_information);
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?
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:
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.