At the moment of designing a web application, we usually run into the problem of how to deal with logical connections among resources. These connections have important implications in the operations that we take on a certain resource and its representation, as we could verify in the design of the collaborative web application that we have developed, the Virtual Conference Centre. For those reasons, in this paper we analyze the relationships among resources, especially focused on collaborative web applications, and we propose some solutions and good practices for the difficulties that we have encountered

Barra Arias, Enrique

Mendo Hernández, Antonio

Prieto Ruiz, David

Quemada Vives, Juan

Tapiador del Dujo, Antonio

English

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Agile Web Development with Rails: Third Edition. Bookshelf.

Architectural Styles and the Design of Network-based Software Architectures. Doctoral dissertation.

The Object Primer: Agile Model-Driven Development with UML 2.0.

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Resource's Relationships in the Design of Collaborative Web Applications

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RESOURCE´S RELATIONSHIPS IN THE DESIGN OF 
COLLABORATIVE WEB APPLICATIONS 
 
Enrique Barra Arias                    Antonio Mendo Hernández                   David Prieto Ruiz           
ebarra@dit.upm.es                        amendo@dit.upm.es                           dprieto@dit.upm.es            
Antonio Tapiador del Dujo                                Juan Quemada Vives 
atapiador@dit.upm.es                                      jquemada@dit.upm.es 
 
ETSI de Telecomunicación - Universidad Politécnica de Madrid 
Avenida Complutense, 30 - 28040 - Madrid (Spain). Phone: +34 91 549 57 00 
ABSTRACT 
At the moment of designing a web application, we usually run into the problem of how to deal with logical connections 
among resources. These connections have important implications in the operations that we take on a certain resource and 
its representation, as we could verify in the design of the collaborative web application that we have developed, the 
Virtual Conference Centre. For those reasons, in this paper we analyze the relationships among resources, especially 
focused on collaborative web applications, and we propose some solutions and good practices for the difficulties that we 
have encountered. 
KEYWORDS: REST, resource design, collaborative web applications, relationships. 
1.     INTRODUCTION 
Web based collaboration tools are becoming one of the main ways for group and distance collaboration. 
Due to the easy accessibility to the Internet (e.g. via Wi-Fi) and the popularization of web browsers, users 
have a nearly ubiquitous means of collaboration. 
Every collaboration tool needs to manage resources and the relationships among them. As it is defined in 
RESTful Web Services book [1] "A resource is anything that's important enough (in your web application) 
to be referenced as a thing in itself..." and "... it has to have at least one URI". Also it is important to define 
relationship, understood as "a general term covering the specific types of logical connections found on class 
and object diagrams". 
The Representational State Transfer (REST) is an architectural style for hypermedia distributed systems 
based in these resources and relationships, introduced by Roy Fielding in 2000 [2]. It is defined by a set of 
constraints, and it is inspired by the features and principles that made World Wide Web successful. In fact, 
The Web is the best instance of a REST architecture style. 
In this paper we analyze, from the REST point of view, the relationships among resources as a key part of 
web collaboration tools architecture. We will see how these logical connections affect the representation of a 
resource or a collection of resources, how these cases arose in the design of the collaborative web application 
that we have developed, the Virtual Conference Centre (VCC) [3], and we will propose some good practices 
and solutions to solve the obstacles we encountered. 
 
2.     DESIGN OF RESOURCE´S RELATIONSHIPS IN 
COLLABORATIVE TOOLS 
Web resources are related among them, they share connections and references. According to the UML [4] 
instance level relationships there are four kinds of relationships: aggregation, external links, composition and 
association. 
In next sections we will discuss some good practices to be taken into account when designing your web 
based collaborative applications, and we will show how implement them by using real examples taken from 
the VCC. We believe it will help developers design better applications, avoiding well known problems and 
promoting readability. 
The VCC is a Web application developed by the Universidad Politécnica de Madrid (UPM) within 
GLOBAL project, a research project supported by the European Commission’s seventh framework 
programme. It offers a complete environment for web collaboration; besides other classic tools like versioned 
document repository or dialog forums, the VCC offers users a service for creating, sharing, recording and 
publishing distributed events, which may vary from small meetings to big conferences. The VCC and 
specially the "VCC events" help project members to get closer and to collaborate as if they were office 
partners. 
2.1     URI Design 
URIs are one of the key building blocks of REST architectures like the WWW. They are used to reference 
resources and they are part of a resource’s representation; in fact, every resource must be referenced by at 
least one URI. That way we can link one resource to another (inside or outside our application). 
Although it is not mandatory, URIs should be human friendly and meaningful. Resources in web 
collaboration tools are usually linked in a big variety of media like emails, web pages and also in meetings 
and presentations. Providing meaningful URIs facilitate collaboration. Besides, resources are better indexed 
by web crawlers when their URIs are descriptive. Google provides webmasters [5] with advices in this 
direction. 
Composition relationships, such as the container-content one, are usually reflected on URIs. The 
hierarchical order provides a natural way to nest resources in the URI. They have this appearance: 
/container/contents. If we use an example from the VCC like /spaces/global/events/monthly_meeting, we 
realize there is a composition relationship between the event and the space (notice that in the VCC, group 
collaboration is organized into spaces). This URI clearly shows where you are going if you click on it, and 
so, it will influence on the user’s decision to follow it and, finally, collaborate. 
Sometimes, when contents have a lot of nesting, we end up using too long URIs: e.g. 
/spaces/global/events/meeting/attachments/session-1.odp. They provoke visual noise and they lack 
persistency, as relationships among resources can change. That is why we do not need to always keep all the 
containers in the path, e.g. when the content's identifier is unique per application scope. We can decouple 
some of the resources and make the URI shorter and more persistent. We need to find a compromise between 
URIs being descriptive and being persistent. 
2.2     Resource Representations  
Another key point in a RESTful design is the representation of a resource. Typically resource 
representations are divided in two groups: collections and elements. 
On one hand, forums, blogs or photo albums are just a few examples of collections that appear in most of 
the web collaboration tools. In a generic case, the relationship between the collection and its container can be 
found in the URI: /spaces/global/attachments or /spaces/global/events/monthly_meeting/attachments. In both 
URIs a collection of attachments is shown. The first one is the representation of the space document 
repository, containing all the attachments that belong to the “global” space; and the second one only returns 
those that belong to the event called “monthly meeting” of the “global” space. Furthermore, the first 
collection will be shown, in the HTML representation, inside the context of the space (surrounded with 
information and links related to the space), while the second one will be shown inside the event context. 
On the other hand, element representations show the state of a single resource, like a picture, a comment 
in a blog or a video. Usually the relationships of a resource with other resources are included at the resource 
representation, and means of communication are provided through links. 
These relationships have also several consequences in resource manipulation operations. More 
specifically, we will see how WRITE operations get affected in these scenarios, and how we applied it to the 
VCC. 
There are three ways of creating resources that are related to some other resource: 
1. Providing a reference to the container in the representation sent to the server: used in the cases 
that those relations were optional, e.g. posts that are related to events. 
2. Using a URI that includes the container: Using POST requests to a URI that contains the space 
in the path is our preferred way for the creation of resources that need a space to exist. 
3. Including the contents in the container representation when manipulating the container: we send 
nested resource representations in some cases, such as creating posts with several attachments. 
The web development framework tool that we have used, Ruby on Rails [6], provides some 
facilities like the one called “nested resources” which, along with the use of JavaScript to 
include several file inputs in the HTML form, make this method very convenient. 
Finally, update and delete methods are independent of container relations, beyond the representation sent 
to the server when updating resources. These methods do not need descriptive URIs, neither the full URL 
with all the nested resources. 
3.  CONCLUSIONS 
REST gives us some useful guidelines that can be used in the design of collaborative web applications. It 
is focused on resources and their representations, and we have learnt that the key for a successful design is 
the resource identification and definition. 
In this paper, in order to face the design and implementation of the VCC, we have classified the 
relationships present in collaborative web applications’ resources, according to the UML instance level 
relationships. Taking into account the compromise between URI persistency and URI description, we have 
analyzed how and when these relationships should be reflected on URIs. Clear and meaningful URIs 
facilitate collaboration, so it is important bringing the most relevant relations into URIs. We have also studied 
how relations affect the resource’s representation, documenting three different ways of establishing 
relationships. Finally, we have supported our study and conclusions with a real web service design. 
4.  REFERENCES  
[1] Leonard Richardson, Sam Ruby. May 2007. RESTful Web Services. O’Reilly. 
[2] Fielding, Roy Thomas. 2000. Architectural Styles and the Design of Network-based Software Architectures. Doctoral 
dissertation. University of California, Irvine. 
[3] The Virtual Conference Centre. http://www.globalplaza.org 
[4] Scott W. Ambler. 2004. The Object Primer: Agile Model-Driven Development with UML 2.0. Cambridge University 
Press, New York, USA. 
[5] URL structure, Webmaster Tools Help, 
http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=76329 
[6] Sam Ruby et al, March 2009. Agile Web Development with Rails: Third Edition. Pragmatic Bookshelf. 
 


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