A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES1 Chapter 1. Distributed systems: Definitions, design and example of distributed services INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES2 Content Distributed systems: Definition Distributed systems characteristics Distributed system design Types of distributed systems Client-server Environment File servers Databases servers Transactions Servers Groupeware Servers News Servers Objects Servers Examples

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES3 Distributed systems: Definition A distributed system is made up of: A set of personal computers Linked together by a network Equiped with distributed softwares Distributed softwares are operating system and applications that enable computers to coordinate their processing activities and share the network ressources: equipments, softwares and data. Computers have the following characteristics: They run independent processes in parallel They don’t share a global clock Their failures are independent

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES4 Distributed systems characteristics Some characteristics clearly show how important are distributed systems : Ressource sharing Openess Multi Threading Transparency Faults Tolerance Scalability

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES5 Distributed systems characteristics(2) Ressources sharing: Peripheral devices (drivers, printers, …) softwares: files, Data bases, Programming langages,… Every network ressource is managed by a ressource manager (e.g DBMS, …) Costs Reduction. Openess: New softwares and hardwares can be easily added within a distributed system. MultiThreading: Many programs running in parallel on the same CPU. It is important to keep a control on this processing behavior. Transparency: Consist of using a distributed system to hide components details to the End user and programmer in a way that they see the system as a black box that offers services than as a collection of independant components

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES6 Distributed systems characteristics(3) Faults tolerance : To resume a distributed system running process after a failure has occured it is important for permanent data states(files and any hardware data stored in memory) to be saved after any failure detection. Scalability : A distributed system should efficiently work at different scales: –Few desktops with a file server –Local network with a hundreds of computers and a file server –Many Local networks linked together ( Internet).

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES7 Distributed system design The following are some problems directly tied to the distribution state of an information system: Naming Communication Software structure Tasks dispatching Keeping the system Coherent Application Programmer Interfaces (API’s) design

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES8 Distributed system design (2) Naming: Namespace, Domain, Domain name service (DNS), Name Resolution Transparency,… Communication: Data transfer from the sender process environment to the receiver process environment. In some cases reception must be synchronized with sending. Communication through ports Synchronous or asynchronous Mode client/server architecture

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES9 Distributed system design (3) Software structure : Applications Support for operating system core : Memories management Processes management RAM (Random access memory i.e working space) management Inter process Communication (interwork) Communication tools (RPC, Multicast,…) Tasks dispatching: Optimize distrubution of tasks to CPUs in relation with their processing capabilities and the distance.

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES10 Distributed system design (4) Keeping the system coherent: Appears when many running processes are updating their data in parallel. Updates should be atomic tasks. A set of changes made by a process should seem instantaneous for the other running processe. To make sure that long lasting data are saved within all the components, failure recovery routines are necessary in order to resume the running process to the state befor the failure. APIs (interfaces) design: Text, GUI (Graphical User Interface), hypertexte, hypermedia, multimedia,...

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES11 Types of distributed systems Centralized system (no more in use) Client-Server Client-Multiple servers (Google) Client-Proxy-Server Colleagues (peer-to-peer P2P) (Client + code mobile) – server Mobile agents Spontaneous network ressources discovery: e.g DHCP Infrared network Blue tooth

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES12 Client-server Environment Moving from a centrilazed environment to a decentralized one. Downsizing

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES13 Client-server Environment (2) Communication is achived through messages exchanged between two peer to peer (P2P) processes. A process is the client, and the other one is the server. The processes represent cooperative or colleagues systems. This cooperation results in an exchange of data: where the client receives final results from the server.

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES14 Client-server Environment (3) Example of servers: File servers Databases servers Transactions servers Groupware servers News servers Objects servers Example of clients: Browser (Netscape, Internet explorer) Cellular phone softwares

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES15 File servers It handle call on the network, related to writings and readings in files. No filtering nor changes are made on data exchanged.

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES16 Databases servers The data received by the client application are results of SQL request processing on the DB-server. The DB-server here is also called « applications server ».

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES17 Transactions Servers Used to implements transaction processing systems : OLPT: On line Transaction Processing Client Call a remote procedure that implement a transaction. Transaction server (or transaction monitor: MT) runs this procedure. New monitor are trying to integrate other features : communication, distribution,...

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES18 Groupeware Servers The groupware server (or collectware) is used to share or convey documents (multimedia ) within a company The Push technique is often used.

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES19 News Servers Those servers provide End users with documents (HTML, images,…). E.g : HTTP, FTP, … servers They can use intermediaries (eg. CGI programs, proxy’s) to improve funtionnalities.

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES20 Objects Servers This service aim to let communication between objects through Object Broker (ORB: Object Request Broker). It is a paradigm that tend to replace client/server model : objects call other objects (local or remote) methods. Objects are located by the broker ORB.

A.Obaid - Wilfried Probst - Rufin Soh INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES21 Examples Distributed systems : Internet with firewall and services like , video conference, … Train or plane tickets reservation Softwares: Operating systems (platform) CORBA, DCOM, RMI (Integration layer or Middlewares) Instant Messenger, Chat (Applications) Failures : Bad response Synchronization Power Omission Autodetection