Tuesday, 23 September 2008

Part 7 - Uniform Interface Constraint

Its been a couple of weeks since the last BORED post. Something has gone wrong with my very old Thinkpad. It now only boots in safemode with 640x480 resolution; not the nicest environment to work. I'm holding out for another few weeks before hopefully getting hold of a new Mac. I'm still not sure if it will be an air, book, pro. The posts here will probably slow down until they're out.

For now, back to BORED. Today's post is probably the most interesting of the posts and highlights the real issue that BORED, Argot and every other protocol is really trying to solve. That is the movement of information and knowledge between client and server. This is very different to the simple task of moving data (ie bits and bytes). The problem of moving knowledge between applications is the central aspects of what draws me to this otherwise dull area of computer science.

I look at the current browsers, programming languages and enterprise systems and see a single underlying problem; we have very little understanding of how to move knowledge between systems. Solving this problem can lead to more fluidity of data with less work by programmers. This should also lead to better useability for the applications we build. There's a lot of work to do and probably a few books to be written in this area before it will be solved. BORED is an excersize in breaking out of the mold and seeing if a better approach can be found. Without further ado, lets get back to BORED!

The BORED protocol has now been tested against some of the challenging REST constraints. The next and probably the most difficult constraint to be tested is the Uniform Interface Constraint. This is the point where the request message data structures hit the target object and the mismatch between a hypermedia system and other types of interactions with servers is most obvious. As the aim of the BORED protocol is to bring find some alignment between REST and Object orientated systems, this is where things should get interesting.

Uniform Interface

The Uniform Interface constraint is one of the more interesting constraints of REST. It reduces all operations to a small set of file like operations, e.g. GET, POST, PUT, DELETE, HEAD, etc. In the case of BORED, however, I'm trying to bring together the concept of an Object Orientated system with that of a Hypermedia system in a sensible way. At this point it is a good time to review the BORED architectural model:

client --[request]-->Server-->Container-->Object ReceiverObject

client<--[response]--Server<--Container<--Object ReceiverObject

The BORED Remote Message Call(RMC) model encompasses all interface request data into the message data portion of the request. This is delivered to the Object Receiver, which uses this information to interact with the target Object. These interactions could involve any one of the following:

Object Receiver -------> Document/File
Object Receiver -------> Object Instance with public methods
Object Receiver -------> Data Collection
Object Receiver -------> Proxy Interface
Object Receiver -------> Etc...

It is also worth reviewing what Fielding has to say about the Uniform Interface Constraint:

"The central feature that distinguishes the REST architectural style from other network based styles is its emphasis on a uniform interface between components (Figure 5-6). By applying the software engineering principle of generality to the component interface, the overall system architecture is simplified and the visibility of interactions is improved. Implementations are decoupled from the services they provide, which encourages independent evolvability. The trade-off, though, is that a uniform interface degrades efficiency, since information is transferred in a standardized form rather than one which is specific to an application’s needs. The REST interface is designed to be efficient for large grain hypermedia data transfer, optimizing for the common case of the Web, but resulting in an interface that is not optimal for other forms of architectural interaction."

As stated, it is the Uniform Interface constraint that really sets the REST approach apart from many other systems. It is the simplicity of the uniform interface that makes the interactions between browser and web server so powerful.

Fielding continues with:

"In order to obtain a uniform interface, multiple architectural constraints are needed to guide the behaviour of components. REST is defined by four interface constraints: identification of resources; manipulation of resources through representations; self descriptive messages; and, hypermedia as the engine of application state. These constraints will be discussed in Section 5.2."

The Uniform Interface constraint therefore has multiple sub-constraints. Any diversion from these constraints will cause BORED to diverge from the REST approach. However, Fielding also states that the Uniform Interface constraint is a trade-off between degrading efficiency and providing an:

"efficient interface for large grain hypermedia data transfer, optimizing for the common case of the Web, but resulting in an interface that is not optimal for other forms of architectural interaction."

This trade-off is clearly shown in AJAX based applications. Application designers are forced to use the REST approach for all aspects of the client-server interactions. An AJAX based application downloads Javascript which often makes remote calls back to the web server. These AJAX calls are better suited to a solution which allows a program centric interaction with the server (note I'm being careful not to use the term RPC). The client may be attempting to return document fragments or even simple single string responses. In these situations the uniform interface constraint creates additional work for the developer and designer. These AJAX/Web 2.0 interactions would benefit from a stronger binding between client and server.

The AJAX/Web 2.0 example shows the trade-off that Fielding discusses in regards to REST. However, the trade-off has obviously served the Web Hypermedia system well to this point. Take for example the simplicity of:


By entering a URL into a browser we imply the GET request, and the image is retrieved. The web's power is driven through this simplicity.

At this point it is worth doing a small detour into the realm of data contracts. Understanding the different types of data contracts that client/server systems use will provide a better set of tests to base the BORED protocol.

Data Contracts

The topic of data contracts is probably the most interesting aspect of distributed computing. This is where there is some agreement between client and server that after sending a specific set of data to a location will result in an agreed set of other data being returned. The contract can range from being implied, to being rigidly defined using procedure call semantics (as is the case in CORBA IDL). This philosophy around the area of data contracts changes with each new technology and fad.

The reason for this constant flux is that what is required changes for different purposes. If a user is involved then human cognition is the most important part of the contract. If the communication is purely between code on both client and server then as long as the client matches the server the contract can be implied. If the clients are many and varied and are using a 3rd party service then consistancy and an Interface Definition Language is desierable. If the client wishes to discover new interfaces then discoverability and associated IDL is a requirement. Finally, in some cases an IDL does not go far enough and a full and independent textual specification (eg RFC) is required.

Each of the methods of creating data contracts implies different requirements for the BORED protocol. The following is a simple breakdown of different contracts and some implications for the BORED protocol. There has probably been better and more thorough analysis of data contracts been done before; if you're aware of any, please let me know via comments.

Human Cognition Data Contract

The URL is probably the best example of providing human cognition to a data contract. By reading a URL a user is able to have a fairly good idea of what information will be returned. There is obviously skill in defining a good URL structure for any web site. However, the current URL also include request parameters which can modify the result of a particular page.

Take for example the following hyperthetical request:


To an experienced web user the parameters of this request are obvious:

s=books (search the term “books”)
author=ryan (find books by the
author “ryan”)
page=2 (return the second page of results)

The contract between client and server regarding these parameters are loose and do not provide a formal contract between client and server. However, the need for this contract is reduced for most web pages as Javascript is usually used to construct the required URL. This reduces the usability of the server application for purposes outside the scope of the web page that uses it. To fix this issue many web sites that wish to offer services to external sites use additional web services based around SOAP technologies.

As part of the BORED protocol we've already stated a location requirement. At the sametime this ability for a user to modify a request URL parameters provides flexibility and gives the user a greater control over the information requested. To solve this in BORED a mechanism for the data contract for a URL to be made available to the browser. The user can then be presented with an interface to allow them to modify request parameters in a formal way.

Hypermedia Data Contract

The hypermedia data contract is associated with the REST design philosophy. The idea that each object in a system is identified using a simple URL. Links and references between objects are provided through hyperlinks. For instance instead of saying that a customer order contains product id A1234, a hyperlink is provided which gives a direct link to the component.

The final solution in a hypermedia system consists of many URLs with the interaction of each object in the system reduced to the REST uniform interface constraints. The client may request to be provided with different formats in responses depending on its requirements. Examples include HTML for user interaction, XML for programming language interaction and PDF for printing purposes. The REST approach requires that a client can specify the type of data it wishes to receive through its request parameters.

The actual data contract is deferred to the data representation returned by the specific request. In the case of XML, the contract can be specified using XML Schema or DTD. In a way, REST splits the data contract into two halves; The data representation using mime and the location of information through the publishing of URLs. REST does not currently have a method of bringing this information together in a way to make the data discoverable.

This type of solution puts minimal requirements on the BORED protocol. Infact, the hypermedia/REST method does its best to avoid the data contract issue by reducing all calls to a uniform interface and mime-types. The BORED protocol must allow the client to request the type of data it wishes to have returned in the GET verb. The response must support the ability to provide at least a mime type associated with the data returned.

Implied Data Contract

This form of contract is when a client is built specifically for a server. This is the case with most Web 2.0/AJAX based applications today. The data contract is not specified in any formal way and the specific data sent and received is at the full discretion of the developer. The developer must be aware of any changes with the data sent or received between client and server and ensure each section of the software is updated.

A large Web 2.0/AJAX based application is likely to have associated documentation that is put together by the team developing the product. This documentation is unlikely to provide a formal description of the data. Any changes to the product would require a separate task to update the associated documentation.

An important aspect of this type of communication is that the data format sent to/from the server is at the total discretion of the developer. A developer may return a simple string, some preformated HTML, some JSON data, or a new script.

This type of communication puts a requirement that the protocol should be open to any data type the developer wishes to sent between the client and server. To support this, the message data should allow at the very least a mime-type to be specified with the data. It may also be beneficial that additional meta data be provided separate to the data being supplied.

Strongly Typed Data Contract

This form of contract is defined using an Interface Definition Language(IDL) or Web Services Definition Language(WSDL). This is the most formal method of defining a contract between client and server. Associated with this type of formal contract is a complete communications stack (I.e. stubs and skeletons) which provides the middleware for the formal agreement.

In examples such as CORBA and Web Services there is a complete framework which surrounds strongly typed data contracts. As has already been shown numerous times over, a single framework is unlikely to provide all the facilities required by developers. Multiple Data Contract Languages (DCL's) may be required with more specific purposes to meet a developers requirements.

To support strongly typed data contracts the BORED protocol should support allow an interface to be associated with a specific location. In these cases the default GET uniform interface may provide an objects current state, however more specific interfaces can also be provided. The Data Contract Language associated with the object can be made available via a META call to the location.

Uniform Interface Constraint Reviewed

After reviewing the various types of contracts it is clear that the Uniform Interface constraint is just one of the many ways of building a data contract between client and server. However, the Uniform Interface constraint is an important element of the REST design; this is because as already suggested, its power is in its simplicity. eg


This URL links to a Web 2.0 application that uses heavy JavaScript and has many dynamic elements. In these cases the Uniform Interface constraint is defining a common naming mechanism that defines a link to a web application. The URL in effect is a bootstrap for the application.

The calls to the server after the application is loaded are hidden away from the user. If the user went hunting through the code for these AJAX calls they would be of little use alone. They are likely to require very specific parameters and return information that only the client application can interpret.

This concept of identifying the URL as a bootstrap has helped frame a few conclusions about the Uniform Interface constraint. In particular the requirement for the uniform interface constraint is only important for those locations that require direct user interaction or return a simple data representation. However, any data that needs to be transferred between the client application and server can use any other means that is suitable to the application. A developer may choose a mixture of implied and strongly typed interface contracts. It depends on the needs of the interactions that will occur between client and server.

Uniform Meta Interface

The discussion above points to the need for a uniform meta interface layer to be created. The purpose of this layer is to provide a mechanism to discover the meta data associated with the object being interacted. This meta data can describe the type of interactions that the object receiver will resposne. This layer should be flexible enough to cater for any type of data be sent and received. It should allow the interactions with the object to be described using meta data that is most appropriate for the service being delivered.

The danger of providing a single meta interface layer which could describe any number of interfaces and protocols is that the REST uniform interface is lost. I've already stated numerous times that the power of the REST architectural style is in the uniform interface. For this reason, a reduced number of verbs needs to be defined which allow the object to publish a basic set of known operations. This supports the main aim of the REST architectural style, but still allows flexibility in catering for other application protocols.

To support this concept of a uniform meta presentation layer, the object receiver must at a minimum respond to a request with a META verb. The response to this verb must be a description of the interfaces supported by the object. The meta data returned may include a set of REST style verbs such as GET, HEAD and DELETE.

This implies that for many interactions the client must make a call to the server to retrieve the meta data and then a second call to perform an operation. Additional calls may also be required to retrieve and bind interfaces depending on the method described in the meta data. However, it will be assumed that most locations will support atleast the GET verb with no parameters. This support will allow published URLs to be accessed without making multiple calls to the service.

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