The CO4 protocol provides a way to build a consensual artefact through submission of part of it to a common description. This reference manual provides an in depth presentation of the CO4 protocol. It can be found on-line at

About the CO4 protocol

Groupwork (for individuals, firms or groups themselves) can be favoured through telecommunication and computer advances. Telecommunication allows for communication of and distributed access to the same artefact (a text, a contract, a product design, a knowledge base, etc.); computer science allows the manipulation of such an artefact through more and more sophisticated tools and is able to deal with many aspects of the cooperation process (coherence, broadcast and security). The development of software agents let us foresee a kind of groupwork addressed to both humans and programs.

Our aim is to enable people to find an agreement on the construction of a particular artefact such as a knowledge base, a good design or an article. One can imagine several laboratories and firms grouped together with the aim of building and maintaining an encyclopaedic knowledge server about a particular domain (whose contents can be text and images for instance). For that purpose this group can be implemented through one or several software agents aimed at collecting data and distributing it to those who are allowed to consult it.

In order to extend such an organisation towards the knowledge itself (i.e. the declarative and digital representation of executable knowledge), a computer environment called CO4 (for collaborative construction of consensual knowledge) is presented in [Rechenmann 1993, Euzenat 1995]. CO4 is dedicated to the incremental and concurrent building of a knowledge base organising a set of various annotations around formalised knowledge. Annotations can be text, bibliography, image, experimental data from which the knowledge originates.

The CO4 protocol allows the communication of pieces of the artefact between various repositories and their integration into consensual repositories. It implements a consensual decision policy (i.e. a policy in which a modification, in order to be accepted, must have been agreed by all the other members). The principles underlying CO4 are derived from those of peer-reviewed journals: before introducing it into a consensual repository, the knowledge must be submitted and accepted by the whole community. For that purpose, knowledge is submitted to the repository, reviewed by the other participants and accepted or amended according to their reactions. Thus finally, the knowledge stored in a consensual knowledge base is safe enough so that anybody can use it confidently and easily. This protocol works at several levels: the group repositories can be recursively grouped together into a more important group repository and so on, and so forth. For instance, a consensual representation could be achieved inside a particular firm before being submitted to the inter-institution repository. However, the behaviour of such a group repository is still subject to the consensual approbation of its subscribers.

CO4 has been designed independently from the kind of artefact under construction. Once accepted, a modification is applied to the concerned artefact through the usual manipulation software for that artefact, the only constraint being that the artefact must be described in a digital format in some kind of repository accessible with few primitives. The advantage brought by this generality is the ability to apply and experiment the protocol on various applications such as computer aided-design, collaborative hypertext documentation or the redaction of a corporate memory.

The consensual aspect of the protocol does not restrict its applicability: it should be quite easy to change the policy from "consensual" to "majority" or "intersection". However, the properties would not be the same.

The CO4 protocol is a result of the Sherpa project headed by François Rechenmann since 1988. The protocol has been designed by Jérôme Euzenat, model checked by Charles Pecheur and implemented by Loïc Tricand de La Goutte under the supervision of Jérôme Euzenat.

Beware: although other documents, anterior or posterior, can help to understand particular aspects of the system, this reference manual is the only trustworthy documentation of the CO4 protocol 1.0 made available from July 1998.

Organisation and notation

An interface to the structure manipulated by the protocol is provided through HTTP. Particular figures called Screendumps display this interface. They provide a picture of the HTTP browser screen together with the indications of the next screens when clicking on an active element (link or button). Some screens (mainly dialogues) are omitted since they do not carry a lot of information and other ones are reduced through white stripes across them. Screendump <number>: <Description of the screen function>. <Additional information>


The manual is made of four main chapters. The first one, Introduction to the CO4 protocol, describes the philosophy of the system and its general behaviour. The second one, User reference manual, is dedicated to the presentation of the actual behaviour of the system including various practical things not specified in the protocol. In particular it is the reference manual for the CO4 HTML interface. The third one, Understanding the CO4 behaviour, provides some insight of the internal behaviour of the protocol. The last chapter, Developer reference manual, describes how to embed CO4 in a particular application. This will be turned into a tutorial.

A set of appendices is added to these four main chapters. They concern:

The user is invited to read only the two main sections. The installer and/or newcomer should consult the Appendix 1 for a quick start. The programmer will read the two first sections and the last one. She may have to consult the third section only in case of deep modification of the protocol. The programmer will access directly the manual from the function index at the end of the manual or by browsing the on-line manual through a WWW client (see Appendix 1).

Montbonnot, le 1er juillet 1998