 Distributed Software Chapter 18 - Distributed Software1

Why Distributed Solutions?  Resource sharing  Processor, disk, printer  Resource Utilization – off hours CPU usage  Performance – multiple processor usage  Availability – widespread locations  Scalability – “out not up”  Fault tolerance – depends on implementation Chapter 18 - Distributed Software2

Challenges  Transparancy  Openness (use of standard protocols)  Security  Scalability  Quality of Service  Dependence on network bandwidth  Key issue for streaming media  Failure Management  Detection of failure  Redirection of service Chapter 18 - Distributed Software3

Security  Threats  Interception  Interrumption  Modification  Fabrication  Difficulty  Managing a policy over all components Chapter 18 - Distributed Software4

Model of Interaction Remote Procedure Call  RPC (Remote Procedure Call)  Client:  Calling program calls a stub for the function,  e.g. ComputeArea(width,height, area)  Stub packs request, parameters into data packet (marshalling) and sends to server  Server  Receives request, separates the data (unmarshalling), calls function with parameters  Packs resulting values, sends response to client system  Client  Stub receives response, unpacks data  Returns values to calling code  Example  CAD system Chapter 18 - Distributed Software5

Models of Interaction Messages  Messages  Not based on a stub waiting for a response.  HTTP  Client sends a “GET” message with multiple lines of parameters to server  Server sends data (web page to client)  Client sends “GET” messages to server to get images  Server sends responses to client  Author’s example - waiter to staff  Uses a standard format – XML (p 486) Chapter 18 - Distributed Software6

RPC vs Messages  RPC  Client calls functions that appear local  Functions developed at other end  Stubs at both ends make this appear seamless  Messages  Each component (client or server) must:  Formulate and send messages  Interpret the messages from other components Chapter 18 - Distributed Software7

Client-Server Where do you use it?  Browser – Web Server – provides web pages  Local system – DHCP server – Get IP address  Local System – DNS server – Translate web names ( to IP address ( )  Local System – Time Server – get current time  – SMTP Server – sends  – POP Server – receives Chapter 18 - Distributed Software8

Client-Server Where do you use it?  Local Application – DB server – read/write database  Local Application – File server – read/write files  Skype – Skype server – Connect to other user  FTP – FTP Server – store, retrieve files on server  I Tunes – Itunes Server – get audio, video, podcasts  Music Plugin – Media server – Streaming media (audio, video)  Log in to domain – Domain Server - authentication Chapter 18 - Distributed Software9

Distributed Architecture Patterns  Master- Slave  2 tier  Multi-Tier  Distributed  Peer-Peer Chapter 18 - Distributed Software10

Master-Slave  Intent – control  Master – drives all activities  Slave – responds to master Chapter 18 - Distributed Software11

(Master – Slave)  Examples  SCADA – Supervisory Control and Acquisition  Gas Piping Network  Power Plant  Power Grid  Chemical Plant  Network Management  SNMP – Simple Network Management Protocol Chapter 18 - Distributed Software12

2-Tier  Thin Client  Characteristics  Client software largely for display, interaction  Business processing done at server  Utilizes server resources, allows for slower, more limited clients  Client software seldom needs updates  Updating software on server makes upgrades easy  Examples  Web Browser (with server side scripting)  Citrix Server Chapter 18 - Distributed Software13

(2-Tier)  Fat client  Characteristics  Most processing occurs on client  Relies on server for data storage  Relies on power of client, offloads server  Updating the client software may require more work  Examples  Web broswer (pages implemented with JavaScript)  Custom applications accessing a database Chapter 18 - Distributed Software14

Multi-Tier  More and more common  2 Tier architecture does not have enough capability  Common  Client (Web Browser)  Server (Web Server)  Database (Database server)  Could be more  Web server could access a program that in turn accesses a database Chapter 18 - Distributed Software15

Distributed Architectures  Characteristics  Multiple systems cooperate to provide services  Not layered  Expandable by adding new nodes  Middleware that acts as a communicator Chapter 18 - Distributed Software16

Distributed Architectures  Examples  DNS – Domain Name system  Provides mapping of domain names (google.com) to IP ( )  Different servers responsible for partitions of the global namespace  Replication of data allows separate servers to provide the same information  Response to a request may involve several servers  The DNS system manages itself as a distributed architectures. Clients access one of the servers as a server  Often referred to as a “hierarchical distributed database” Chapter 18 - Distributed Software17

(Distributed Architectures)  Examples  Routers  Every PC on a network is configured to find addresses via the router (default gateway)  Configured to know how to find any IP address on the Internet  Configured to be responsible for some range of IP addresses  Communicates through routing protocols to other routers  Routing protocols configure neighboring routers  Database replication  Databases can be live backups of a database Chapter 18 - Distributed Software18

Distributed Systems  Disadvantages  Complex to design  No common middleware to help coordinate systems  Corba, JavaBeans,.net Chapter 18 - Distributed Software19

Peer-Peer Architecture  No server is in control – systems talk to each other  Generally for small, local networks  General areas:  Computationally intense, but distributable tasks  Local information sharing Chapter 18 - Distributed Software20

Peer-Peer Architecture  WINS - Window Internet Naming System  Microsoft naming system for small networks  Individual systems contact peers to discover system names  VOIP – Voice Over IP  Skype, Vonage, Cisco phone systems.  Systems access a server to set up a call  Server keeps list of participating systems  Ongoing communication is directly between two peers. Chapter 18 - Distributed Software21

Software as a Service  Characteristics  User does not buy software license  Software not installed on the user’s system  User subscribes to use the service over the Internet  Generally delivered on web pages and accessed via a browser  Can be based on other servers and clients (Citrix, Microsoft)  Server stores data for the user Chapter 18 - Distributed Software22

Software as a Service Examples  Voter Targeting  Google Apps  Gmail  Google Docs  Calendar  Ancestry.com Chapter 18 - Distributed Software23

Software as a Service  Advantages  Installation is not needed.  Setup is minimal  Updating software only needs to be done on the server  Users do not have to manage backups Chapter 18 - Distributed Software24

Software as a Service  Challenges  Performance with many users on a server  Problems can escalate with many users on system  Separation of data  Security  Scalability  Configuration per user  No custom software per user  Backup Chapter 18 - Distributed Software25