Chapter 7: Client/Server Computing Business Data Communications, 5e What is Client/Server? • • • • Client Server Network How is client/server different from other distributed computing? – Heavy reliance on user-friendly applications – Emphasis on centralizing databases and management functions – Commitment to openness/modularity – Networking fundamental to operation Client-Server Environment Why is Client-Server Different? • Emphasis on user-friendly client applications • Focus on access to centralized databases • Commitment to open and modular applications • Networking is fundamental to the organization Client/Server Applications • Emphasis on GUI for users • Database Example – Database on server, applications for access on client, “glue” (like SQL) enables requests) – Application logic can be client-only, or split between client and server Classes of Client-Server Applications • • • • • Host-based processing Server-based processing Client-based processing Cooperative processing “Fat client” vs “fat server” 3-Tier Client/Server Architecture Middleware • Standardized interfaces and protocols between clients and back-end databases • Hides complexity of data sources from the end-user • Compatible with a range of client and server options • All applications operate over a uniform applications programming interface (API) Middleware Architecture Logical View of Middleware Basic Message Passing Primitives Message Passing Issues • Reliability vs Unreliability – Reliable facilities guarantee delivery, provide errorchecking, retransmission, etc – Alternatively, the message can be sent without success/failure; reduces complexity and overhead, passes responsibility for confirmation to application • Blocking vs Nonblocking – Non-blocking more efficient, but difficult to test and debug programs – Blocking (synchronous) retains control until acknowledgment is received Remote Procedure Call Mechanism Client/Server Binding • Nonpersistent binding – Does not maintain state information, connections re-established as necessary – Inappropriate for RPCs used frequently by same caller • Persistent binding – Connection sustained until procedure return – Useful for applications making repeated calls to remote procedures Object-Oriented Mechanisms • Clients and servers ship messages between objects • May rely on an underlying message or RPC structure or be developed directly on top of object-oriented capabilities in the operating system • Success depends on standardization of the object mechanism, but competing models exist – COM, OLE, CORBA Intranets • Implementation of internet-based client/server technology within an organization, rather than for global connectivity • Immensely successful in corporate computing contexts Advantages of Intranets • Rapid prototyping • Scales effectively • Little training required • Can be implemented on variety of systems • Open architecture allows interaction across platforms • Supports a range of distributed servers • Allows integration of legacy systems on client and server side • Supports a range of media types • Inexpensive to implement The Intranet Web • Web Content – The web can be used to effectively distribute content in a way that requires no new training for end-users • Web/Database Connectivity – Multiple tools exist to serve as middleware between web servers and data sources • Electronic Mail • Network News Web/Database Connectivity • Advantages – – – – Ease of administration Deployment Development speed Flexible information presentation • Disadvantages – Limited functionality – Stateless operation makes tracking difficult Intranet Disadvantages • Long development cycles • Difficulty in partitioning applications, and modifying based on user feedback • Effort in distributing upgrades to clients • Difficult in scaling servers to respond to increased load • Continuous requirement for more powerful desktop machines Other Intranet Technologies • Electronic Mail – Closed internal mail systems (delivery verification, etc) – Internal mailing lists • Network news (USENET) – Can be adopted for internal intranet uses The Extranet Web • Extends the intranet concept to provide information and services to selected outside populations, such as customers and suppliers • Enables the sharing of information between companies • A TCP/IP enabled form of EDI Advantages of Extranets • • • • • • Reduced costs More marketable products Increased productivity Enhanced profits Reduced inventories Faster time to market Methods for Converting Intranets to Extranets • Long-distance dial-up access • Internet access to intranet with security • Internet access to an external server that duplicates some of a company’s intranet data • Internet access to an external server that originates database queries to internal servers • Virtual private network ... centralized databases • Commitment to open and modular applications • Networking is fundamental to the organization Client/Server Applications • Emphasis on GUI for users • Database Example – Database... training for end-users • Web/Database Connectivity – Multiple tools exist to serve as middleware between web servers and data sources • Electronic Mail • Network News Web/Database Connectivity • Advantages... Middleware • Standardized interfaces and protocols between clients and back-end databases • Hides complexity of data sources from the end-user • Compatible with a range of client and server options