Distributed Processing, Client/Server and Clusters

1. Distributed Processing, Client/Serverand Clusters Chapter 16

2. Client/Server Computing • Client machines: single-user PCs or workstations that provide a highly user-friendly interfaceto the end user • Each server provides a set of shared user services to the clients • The server enables many clients to share access to the same database and enables the use of a high-performance computer system to manage the database

3. Client/Server Applications • Client and server platforms/OS may be different • These lower-level differences are irrelevant as long as a client and server share the same communications protocols (ex: TCP/IP) and support the same applications • Actual functions performed by the application can be split up between client and server in a way that optimizes the use of resources • Optimize the ability of users to perform various tasks and to cooperate with one another using shared resources • Heavy emphasis on providing a user-friendly Graphical User Interface (GUI) on the client side (presentation services layer)

4. Generic Client/Server Architecture

5. Database Applications • One of the most common families of client/server applications • The server is a database server responsible for maintaining the database • Interaction between client and server is in the form of transactions • the client makes a database request and receives a database response • A variety of different client applications can use the same database server; all using the same interface/protocol

6. Client/Server Architecture for Database Applications

7. Client/Server Database Usage Example: think of an application in which we need to compute the mean of the ages of a certain population and the search criteria returns 300K records  heavy network traffic To optimize performance: server can be equipped with application logic for performing data analysis (computation of mean).  Split-up the application logic 10

8. Classes of Client/Server Applications • Host-based (dumb terminal) • not true client/server computing • traditional mainframe environment • Server-based (thin client) • server does all the processing • User(client) workstation provides a graphical user interface Fat client models Takes advantage of desktop power and can serve large number of clients • Cooperative • application processing is performed in an optimized fashion • complex to set up and maintain but • greater user productivity gains and greater network efficiency • Client-based • Most common client/server model • all application processing done at the client • data validation routines and other database logic function are done at the server

9. Three-Tier Client/Server Architecture • Application software distributed among three types of machines • User machine • thin client • Middle-tier server • Gateway • Converts protocols • Map from one type of database query to another • Merge/integrate results from different data sources • Assumes both roles: server & client • Backend server • Legacy applications

10. File Cache Consistency • File caches hold recently accessed file records • Cache consistency problem: • Caches are consistent when they contain exact copies for remote data • Simple solution: File-locking prevents simultaneous access to a file • Complicated approach: allow multiple read but one write access; when there is a write, mark the file as non-cacheable

11. Middleware • Lack of standards for client/server models makes it difficult to implement an integrated, multivendor, enterprise-wide client/server configuration • Middleware: Set of tools that provide a uniform means and style of access to system resources across different platforms. Goal: to enable an application or user at a client to access a variety of services on servers without being concerned about differences among them • Provides standard programming interfaces/protocols that sit between the application above and the communications software+OS below. • Capability to hide the complexities and disparities of different network protocols and OS • Enable programmers to build applications that look and feel the same with little effort • Enable programmers to use the same method to access data

12. LOGICAL VIEW OF MIDDLEWARE Middleware which cuts across all client and server platforms, is responsible for routing client requests to the appropriate server.

13. Distributed Message Passing • Middleware products are typically based on one of two underlying mechanisms: Message-passing or RPC (Remote procedure calls) • Send and receive messages as used in a single system OR Remote procedure calls

14. Message-passing schemes • Reliable • Guarantees delivery if possible - Not necessary to let the sending process know that the message was delivered • Unreliable • Send the message out into the communication network without reporting success or failure - Reduces complexity and overhead • Blocking • Send does not return control to the sending process until the message has been transmitted • OR does not return control until an acknowledgment is received • Receive does not return until a message has been placed in the allocated buffer • Nonblocking • Process is not suspended as a result of a Send or a Receive • Efficient and flexible • Difficult to debug

15. Clusters • Alternative to symmetric multiprocessing (SMP) • Group of interconnected, whole computers working together as a unified computing resource • illusion is one machine • system can run on its own Clusters Compared to SMP • SMP is easier to manage and configure • SMP takes up less space and draws less power • Clusters are better for incremental and absolute scalability • Add new systems in small increments • Can have dozens of machines each of which is a multiprocessor • Clusters are superior in terms of availability • Failure of one node does not mean loss of service • Clusters have superior price/performance

16. Beowulf and Linux Clusters • Mass market commodity components (No custom components) • A dedicated, private network (LAN or WAN or internetworked combination) • Easy replication from multiple vendors • Scalable I/O • A freely available software base • Returning the design and improvements to the community

17. Issues on Clusters • Failure management • Highly available vs. fault-tolerant clusters • Highly available clusters offers a high probability that all resources will be in service • Fault-tolerant cluster ensures that all resources are always available (use of redundant disks/processors etc.) • Load balancing • When a new computer is added to the cluster, the load-balancing facility should automatically include this computer in scheduling applications • Parallelizing Computation • Parallelizing compiler • Parallelized application