HETEROGENEITY IN THE DISTRIBUTED DATABASE MANAGEMENT SYSTEM SIRIUS-DELTA Arlette FERRIER, Christine STANGRET INRIA, B.P. 105, 78153 Le Chesnay Cedex, France Within the pilot project SIRIUS (ADI, INRIA), we user site, a query must be decomposed and trans- have studied the problems set by the cooperation lated into a set of PETAL sub-queries. On each of heterogeneous componentsin a DDBMS. sitewhere data are stored, a PETAL sub-query is Heterogeneity appears at many levels : computer, translated into the usual local manipulation lan- DBMS, data manipulation language, etc. The aims guage. of a heterogeneous DDBMS are : In section 3 and 4, we describe the different - to allow the users to manipulate the distribu- systema used in the heterogeneous SIRIUS-DELTA ted database like a unique database and with prototype and the interfaces we have implemented the language they use to practice, in order that they keep to the pivot concepts. - to make possible the integration of an existing An example of query processing is described in database in the DDBMS. an appendix. In the SIRIUS-DELTA heterogeneous prototype, com- 1. Heterogeneous distributed databases puters as well as DBMSs and languages are diffe- rent. We have designed a pivot system, that is to say a set of functions which have to be ensured A DDBMS relies on several databases managed by by every system in the DDBMS. These functions are several systems running on several machines. In implemented using the existing services of the most systems implemented up to now, all compo- nents are homogeneous, different systems. that is to say that all the functions are realized in the same way on We have studied a pivot model and a pivot langua- all the sites of the DDB (same computer and same ge and implemented it on the DDBMS SIRIUS-DELTA software). and two other systems : As soon asoneofthe functions is assumed in the - PHLOX, a navigational DBMS for micro-computers, whole DDdMS by components of different types, - MRDS, a relational DBMS which runs on the we can say that the system is heterogeneous for MULTICS operating system implemented on a H.B. this function. 68. Many components oan be heterogeneous in a DDBMS : Introduction the networks that interconnect the different sites (the system can use several local net- In the last few years, much attention has been works connected on a national network), placed on the study of distributed database mana- gement systems. Implementations have already the Data Description Languages (DDL) and the being made, using homogeneous networks of compu- Data Manipulation Languatges(DML), ters. A new problem is now merging : how to build - the DBMSs and each function they ensure : pro- heterogeneous DDBMSs, using existing database sys- tection, synchronization, resources allocation, tems ? transactions management, . In this paper, we first define the characteristics - data models (relational, network, hierarchical) of a heterogeneous DDBMS (section 1). Then we at the three levels CANS1751 : external, con- present the heterogeneous DDBMS we are implemen- ceptual and internal, ting. This prototype relies on the SIRIUS-DELTA - the operating systems on which the DBMSs are homogeneous DDBMS and two local DBMSs : MRDS running, (running on a very large computer) and PHLOX (a DBMS for micro-computers). The basis of the sys- - the computers. tem is a pivot system concept (section 2). The description of the distributed database schema Various heterogeneous systems can be imagined : refers to a pivot model which is the relational different computers with different operating sys- model. A pivot language, called PETAL, has been tems running the same DBMS (POREL [NEUH771), dif- designed for the transfer of queries. On each ferent DBMSs on identical computers, etc. Proceedings of the Eighth International Conference on Very Large Data Bases 45 Mexico City, September, 1982 In practice, a heterogeneous system will be use- users users ful if it proposes the following facilities : - to allow the users to manipulate a database @yi) Ft-f=J without having to worry about the distribution of data and the diversity of local systems, - to provide the facility to manipulate the dis- tributed database with different languages. So, each language can be more adapted for each usage, - to make possible the integration of an existing database in a DDBMS without any reorganization of this new local database and without any modi- fication in applications (keeping data and lan- guages). In order to build a system which corresponds to all these criteria, heterogeneity have to be mana- ged for each componant level. 2. -.-The SIRIUS.- answer to heterogeneity in DDBMSs PHLOX The first aim of the SIRIUS pilot project has ci7 ” been to develop a prototype of a homogeneous DDBMS called SIRIUS-DELTA rLEBI811. From this prototype, we have experimented the heterogeneity,; that for, we have connected heterogeneous compu- users ters and DBhlSs through a local network in order Fig. 1 : Heterogeneous DDBMS : SIRIUS-DELTA + to realize a prototype of heterogeneous DDBMS. - PHLOX + MRDS 2.1. Survey of the heterogeneous prototype The DDB global level : SIRIUS-DELTA has been built with three mini- It is at this level that the DDB is considered computers connected, in a first step, through as a whole, as a unique base, therefore, the point to point links. Each of these computers DDB conceptual schema and all the associated has the functions of data base management and external schemata (as for a classical data distributed data base interrogation. A large com- base) are included at this level IANSI751, puter supporting the MRDS relational DBMS and a CTSIC771. micro-computer supporting the PHLOX network DBMS ([PHL079],[PHL080]) are connected to these three The particularity due to the fact that the data computers through the DANUBE local network deve- base is distributed appears in the global inter- loped at INRIA by the KAYAK pilot project. In nal schema. The global internal schema contains the heterogeneous DDBMS prototype, these two last the description of the mappings between global computers only have the data management function and local data as well as the necessary loca- (see figure 1). lization and duplication rules. Global data are described as the result of a sequence of ooera- 2.2. The prototype architecture tions applied to local data. The global inter- nal schema gives the way to rebuild global data The prototype architecture is considered with from local data. It contains the description of regard to : sets of tiocal data belonging to each site, that compose the DDB (the example in appendix shows - firstly data base, the content of a GIS). - secondly fwnctionalities. The DDB local level : 2.2.1. The data base architecture ---_--me--- ------ ------ This level contains the main components of the DDB : the data bases called local DBs. The data This architecture is shown through the schemata are stored in the local D&s and the global describing the DDB components. level takes the part of a user for each of them. Therefore, in the DDB context, the local It is mainly made up of two levels : the global level, corresponding to the whole DDB and the level includes an external schema, a conceptual local level corresponding to the DBs which com- schema and an internal schema per local base (see figure 2). pose it. Proceedings of the Eighth International Conference 46 on Very Large Data Bases Mexico City, September, 1982 Local data are expected to contribute to the SCORE layer : function of distributed control DDB, but not all local data do, and from the (data base consistency and reliability). DDB point of view, local data may look hetero- SER layer : function of distributed execution geneous. (remote process activation, data transfer bet- The local external schemata are provided so ween processes). that heterogeneous local data are presented and handled as homogeneous at the DDB level. -.3.3 Definition of the pivot system A local external schema allows the mapping bet- In order to co-operate with the heterogeneous ween the two descriptions of local data contai- DDBMS, each system must have a minimal set of ned.res ectively in the global internal schema functions. andln a Yocal conceptual schema. If a local base is also used directly (in ano- This minimal set of functions is called pivot ther way than via the DDBMS) corresponding system. It is a virtual system as close a possi- external schema or schemata must be added (e.g. ble to the main existing systems. LES1, in figure 2). The use of a common reference, the pivot system, has two advantages : it makes possible to realize a unique DDBMS at the global level -in expressing all the operations according to the pivot systen- and to limit the number of translators as shown by the following figure (figure 3). ES:external schema CS:conceptual sche- I ma I IS:internal schema N(N-1) translators N translators 2 Fig. 3 : Advantage of the pivot system The definition of the pivot system has a large impact over the system in its whole. If it is much more powerful than most of the existing sys- tems, the missing functions will have to be deve- loped on each system in order that it reachs the "pivot level". If it is not much powerful, the expression possi- Fig. 2 : DDB architecture according to the bilities of the global level should be loosed. schemata. This can lead to reduce the performances of local systems or the general functionalities. For 2.2.2. Functional architecture -------------------- example, a pivot system with only navigational manipulations would imply a very large number In the heterogeneous prototype, four layers are of messages, unacceptable over a network. constructed on top of a transport service as defined by the IS0 reference model of open sys- In our prototype, the pivot system consists of tem architecture CISO791.