CS352 Lecture Database System Architectures last revised 11 26 08 I Introduction A Most large databases require support for accesing the database by multiple users often at multiple physical locations sites There are a variety of overall system architectures that can be used to accomplish this B Historically early database systems were based on a CENTRALIZED MODEL in which the database resides on a single computer system that allows access by remote terminals This is still the model used by many systems today 1 Basic characteristics a All data resides on a single computer system b All computations using the data are performed by this system c Remote access is provided by dumb terminals or PC s running terminal emulation software Example Gordon s old administrative computer system used this model since it was first installed in 1979 The administrative database resided on a single computer system originally a PDP 11 70 then a VAX 11 780 then a cluster of three Alphas Campus offices accessed this database via DEC VTxxx terminals or PC s running terminal emulation software or later via web based applications accessing the database through a web server running on the same system 2 The chief advantage of this approach is simplicity for installing and maintaing software performing backups and managing issues such as concurrency and recovery 3 However the centralized model has a number of disadvantages a Typically centralized storage and manipulation of data goes hand in hand with centralized CONTROL of data users have limited autonomy b Dumb terminals support only very unsophisticater user interfaces basically just ASCII text and text based menus c A centralized system is totally vulnerable to failure of the centralized site Thus for example a power failure at the site where the computer is located can shut down all access to the database even at remote sites unaffected by the failure d As the volume of transactions handled by the system grows it can become increasingly difficult for a single system to handle the demand C These problems especially the latter have motivated the development of alternative system architectures which move away from reliance on a single computer to perform all processing 1 The Client Server Model 2 Use of Parallel Processing 3 Distributed Databases II The Client Server Model A The centralized model originated prior to the development of the microprocessor which made PC s possible With the replacement of dumb terminals by powerful and inexpensive PC s it becomes possible to shift some of the burden of computation from the computer system storing the database to the individual computers serving individual users This led to the development of the client server model which we have discussed earlier in the course B A typical client server application can be thought of as a system comprised of several layers 1 The structure discussed in the text A front end that manages interaction with the user and a back end that manages the database 2 A three layer architecture a The user interface layer typically a GUI b The business layer performs the actual processing specific to the application c The database layer C Actually there are a couple of ways to structure the database layer that we didn t discuss when we discussed client server architecture earlier in the course because we had not yet discussed the overall structure of a DBMS 1 One model is called the TRANSACTION SERVER model the database server executes database transactions on behalf of the client but need not incorporate any awareness of what purpose the transactions are actually serving as far as the user s application is concerned a As noted in the text when SQL is used as the medium of communication between the client and the server it becomes very possible for the client and server software to be produced by two different vendors and indeed for one server to service applications written using many different software packages and as well for one client to access different servers running different DBMS s b This shifts the load of application specific computation from the server to the client while still leaving the server responsible for all query processing related computation query parsing strategy selection performing joins etc In particular no application specific code need reside on the server unless stored procedures are used as might be done to support embedded SQL and no database specific code needs to reside on the client Example this is the model used by Gordon s new administrative software All applications use a common database server running on a single system Some are PC based applications that access the database directly via ODBC others access the database through one of several application servers supporting different adminstrative applications 2 It is also possible to use a DATA SERVER model in which the server delivers physical database pages to the client which then performs computation using them This shifts some of the database layer load from the server to the client but of course requires that the client software know more about the structure of the database and incorporate some of the software typically considered part of the DBMS D In none of the client server variants we have considered do we do away with a central system containing the database we simply reduce the amount of computation for which it is responsible Further all disk accesses needed are performed on the server s disk s that hold the database E When the phrase CLIENT SERVER is used without further qualification it typically refers to to the Transaction Server model with SQL providing the interface between the two layers often through the use of ODBC or JDBC F In addition to removing some of the computational load from the central system the client server model has a number of other advantages 1 The possibility of much more sophisticated user interfaces 2 The possibility of integrating database access with other non database applications on the client e g doing analysis of data obtained from a database using a spreadsheet or incorporating it into a document using a word processor 3 The ability to develop needed applications quickly without having to rely on the programming staff associated with the central database and using a variety of development environments possibly from a vendor other than the supplier of the database III Use of Parallel Processing A The client server model was motivated in part by a desire to