Components of SDI System 

To be operational, the SDI System must have the following components built into it:. 

i)  users represented by profiles 

ii)  database or document profile  

iii)  computer system (6aobwmre ) 

iv)  software 

v)  user interface 

i) User Profiles 

In any library, or information unit, the clientele of the organisation comprise the user community for whom the SDI service is to be rendered. In fact, one of the basic steps in designing SDI service is to identify the potential users and also their subject interests and compile what is technically referred to as "user profiles". It may be mentioned that a 'user profile', is a succinct statement of the information requirements of a user or group of users.    

ii) Database 

SDI service can be offered to the users on the basis of the document resources selected and acquired by the library or information unit. Under the conditions, a computer readable database of these documentary material acquired by the organisation has to be created on a computer readable storage media. The database thus created will be known as the local or in-house database. On the other hand, the library or the information unit may decide to subscribe for externally created and commercially available databases on subjects of interest to he organisation and search these locally with in-house computer facilities to provide SDI service to its user community. In this situation, all the documentary material listed in the databases searched may not be available locally for the use of the clientele. They will have to he procured when demanded for. In the present day context, the range of externally available databases is large, and many SDI services search more than one database and make use of cross-tile searching facility. 

iii) Computer System

In the earlier days when Luhn introduced SDI service, the computer technology itself was in its infancy; computer power was costly and was not available at affordable price for library and information units. Therefore, they tried to utilise the computer facility available elsewhere on a rental or sharing basis. The advent of microcomputers, has changed the situation and most of the libraries are now going for their own in-house dedicated systems. The computer system thus acquired are also made use of in the network mode in case such networks exist in the environment where the library or the information unit is located. In other word, it may be stated that libraries and information units may organise SDI service either through their own database and their own computer system or could become members of a network, which offer this facility and serve their clientele.

iv) Software 

Software is one of the essential components of the SDI system. Here again, alternative approaches are available. The first option is to develop in-house software tailored to the specific requirements of the particular library or information unit, and then offer SDI service, The second one is to identify commercially available package, which could be run on the in-house computer system and provide SDI service using it. The third option is to subscribe for external databases which have a SDI software component that can be used to provide SDI service by searching that database. In the present day context the third option is more viable and financially leash-tie. Since software development involves a lot of money and time consuming, the third option is more acceptable and pragmatic. Another significant aspect worth noting in this regard is the availability of many commercial databases in CD-ROM format. Each of these has its own Searchable software which may he used for SDI. But, the user profiles need to constructed based on such software separately.

v)  User and SDI System Interface 

Every SDI system should have a mechanism which permits interaction between the user and the system. This is essential to make the SDI service more purposeful and effective. It includes the following aspects: notification of recent literature, request to the users to furnish feed back on a regular basis. The feedback may be positive or negative depending upon the relevancy and usefulness of the items received by the user, and information intermediaries who interact with the users and are knowledgeable regarding databases, retrieval techniques, creation of user profiles, etc.