Relational Algebra & Client-Server Systems, CS263 Lectures 11 and 12.

Relational Algebra & Client-Server Systems, CS263 Lectures 11

and 12

Relational Algebra Relational algebra operations work on one or more relations to define another

relation leaving the original intact.

Both operands and results are relations, so output from one operation can become input to another operation.

Allows expressions to be nested, just as in arithmetic. This property is called closure.

5 basic operations in relational algebra: Selection, Projection, Cartesian product, Union, and Set Difference.

These perform most of the data retrieval operations needed.

Also have Join, Intersection, and Division operations, which can be expressed in terms of 5 basic operations.

Relational Algebra Operations

Selection (Restriction)predicate (R)

Works on a single relation R and defines a relation that contains only those tuples of R that satisfy the specified condition (predicate).

Example: List all staff with a salary greater than £10,000.

salary > 10000 (Staff)

Projectioncol1, . . . , coln(R)

Works on a single relation R and defines a relation that contains a vertical subset of R, extracting the values of specified attributes and eliminating duplicates.

Example: Produce a list of salaries for all staff, showing only their staffNo, fName, lName, and salary details.

staffNo, fName, lName, salary (Staff)

UnionR S

Union of two relations R and S defines a relation that contains all the tuples of R, or S, or both R and S, duplicate tuples being eliminated. R and S must be union-compatible (i.e. same attributes).

Example: Produce a list of all staff that work in either of two departments (each department has a separate database), showing only their staffNo, and date of birth.

staffNo, dob(Staff_DepA) staffNo, dob (Staff_DepB)

staffNo dobSL10 14-02-64SA51 21-11-82DS40 01-01-40

staffNo dobCC15 11-03-66SA51 21-11-82

staffNo dobSL10 14-02-64SA51 21-11-82DS40 01-01-40CC15 11-03-66

Staff_DepA Staff_DepB

Intersect

R SDefines a relation consisting of the set of all tuples that are in both R and S. R and S must be union-compatible.

Example: Produce a list of staff that work in both department A and department B, showing only their staffNo, and date of birth.

( staffNo, dob(Staff_DepA)) ( staffNo, dob (Staff_DepB))


staffNo dobCC15 11-03-66SA51 21-11-82

staffNo dobSA51 21-11-82


Set DifferenceR – S

Defines a relation consisting of the tuples that are in relation R, but not in S. R and S must be union-compatible.


staffNo dobCC15 11-03-66SA51 21-11-82

Example: Produce a list of all staff that only work in department A (each department has a separate database), showing only their staffNo, and date of birth.

staffNo, dob(Staff_DepA) staffNo, dob (Staff_DepB)

staffNo dobSL10 14-02-64DS40 01-01-40


Cartesian productR X S

Defines a relation that is the concatenation of every tuple of relation R with every tuple of relation S.

X

Example: Combine details of staff and the departments they work in.

staffNo, job, dept (Staff) dept, name (Dept)X

staffNo job deptSL10 Salesman 10SA51 Manager 20DS40 Clerk 20

dept name 10 Stratford20 Barking

XstaffNo job dept dept nameSL10 Salesman 10 10 StratfordSA51 Manager 20 10 StratfordDS40 Clerk 20 10 StratfordSL10 Salesman 10 20 BarkingSA51 Manager 20 20 BarkingDS40 Clerk 20 20 Barking

Staff Dept

Relational Algebra Operations

JoinR S

Defines a relation that results from a selection operation (with a join predicate) over the Cartesian product of relation R and relation S.

<join condition>

<join condition>

Example: Produce a list of staff and the departments they work in.

( staffNo, job, dept (Staff)) ( dept, name (Dept))



staffNo job dept dept nameSL10 Salesman 10 10 StratfordSA51 Manager 20 20 BarkingDS40 Clerk 20 20 Barking

Staff Dept

Staff.dept = Dept.dept

Because the predicate operator is an ‘=‘ this is known as an Equijoin

Natural JoinR S

This performs an Equijoin of the two relations R and S over all common attributes. One occurrence of each common attribute is eliminated from the result.

Example: Produce a list of staff and the departments they work in.

( staffNo, job, dept (Staff)) ( dept, name (Dept))



staffNo job dept nameSL10 Salesman 10 StratfordSA51 Manager 20 BarkingDS40 Clerk 20 Barking

Staff Dept

Left Outer JoinR S

Left outer join is a join in which tuples from R that do not have matching values in common columns of S are also included in the resulting relation.

( dept, name (Dept)) ( staffNo, job, dept (Staff))

dept name staffNo job 10 Stratford SL10 Salesman20 Barking SA51 Manager20 Barking DS40 Clerk30 Watford

Example: Produce a list of all departments and associated staff that work in them.


Staffdept name 10 Stratford20 Barking30 Watford

Dept

Intersect

R SDefines a relation consisting of the set of all tuples that are in both R and S. R and S must be union-compatible.

Example: Produce a list of staff that work in both department A and department B, showing only their staffNo, and date of birth.

( staffNo, dob(Staff_DepA)) ( staffNo, dob (Staff_DepB))


staffNo dobCC15 11-03-66SA51 21-11-82

staffNo dobSA51 21-11-82


Division

R SDefines a relation over common attributes C that consists of set of tuples from R that match a combination of every tuple in S.

Example: Show all staff that use all the company’s programming languages.

Staff_Prog Prog languageCOBOLBASIC

staffNo languageSL10 COBOLSA51 BASICSA51 COBOLSE14 BASICSE18 BASIC

staffNoSA51

Staff_Prog Prog

CS263 Lec. 12: Client/Server systems

• Operate in a networked environment• Processing of an application distributed between front-end

clients and back-end servers• Generally the client process requires some resource, which the

server provides to the client• Clients and servers can reside in the same computer, or they

can be on different computers that are networked together, usually:

• Client – Workstation (usually a PC) that requests and uses a service

• Server – Computer (PC/mini/mainframe) that provides a service. For DBMS, server is a database server

Three components of application logic

1. Input – output or presentation logic component – responsible for formatting and presenting data on the user’s screen (or other output device) and managing user input from keyboard (or other input device)

2. Processing component logic – handles data processing logic (validation and identification of processing errors), business rules logic, and data management logic (identifies the data necessary for processing the transaction or query)

3. Storage component logic – responsible for data storage and retrieval from the physical storage devices – DBMS activities occur here

Client/Server architectures

File Server Architecture

Database Server Architecture

Three-tier Architecture

Client does extensive processing

Client does little processing

File server architecture

The first client/server architectures developed All processing is done at the PC that requested the data, I.e. the

client handles the presentation logic, the processing logic and much of the storage logic

A file server is a device that manages file operations and is shared by each of the client PCs attached to the LAN

Each file server acts as an additional hard disk for each of the client PCs

Each PC may be called a FAT CLIENT (most processing occurs on the client)

Entire files are transferred from the server to the client for processing.

Three problems with file server architecture

1. Huge amount of data transfer on network - when client wants to access data whole table(s) transferred to PC – so server is doing very little work

2. Each client authorised to use DBMS when DB application program runs on that PC - one database but many concurrently running copies of DBMS (one on each PC) – heavy resource demand on clients

3. DBMS copy in each client must manage shared database integrity - must recognize shared locks, integrity checks, etc - programmers must recognise various conditions that can arise in this environment and understand concurrency, recovery and security controls

File Server Architecture

FAT FAT CLIENTCLIENT

Database server (2-tier) architectures

Client responsible for managing user interface, I/O processing logic, data processing logic and some business rules logic (front-end programs)

Database server performs data storage and access processing (back-end functions) – DBMS only on server

Clients do not have to be as powerful, and server can be tuned to optimise data processing performance

Greatly reduces data traffic on the network, as only records (rather than tables) that match request transmitted to client

Improved data integrity as all processed centrally

Stored proceduresModules of code implementing application logic – included on

the database server. Advantages: Performance improves for compiled SQL statements Reduced network traffic as processing moves from the

client to the server Improved security if stored procedure is accessed rather

than data and code being moved to server Improved data integrity - multiple applications access same

stored procedure Thinner clients (and a fatter database server)Disadvantages: Writing stored procedures takes more time than using e.g.

VB + proprietary nature reduces portability + performance degrades as number of on-line users increases

Database server architecture

ThinnThinner er clientclientss

DBMS DBMS only on only on serverserver

3-tier architectures

In general, these include another server layer in addition to the client and database server

This additional server may be used for different purposes Often application programs reside on the additional server (the application server) Or additional server may hold a local database whilst another server holds the

enterprise database Often a thin client - PC just for user interface and a little application processing.

Limited or no data storage (sometimes no hard drive)

Three-tier architecture

Thinnest Thinnest clientsclients

Business rules on Business rules on separate serverseparate server

DBMS only DBMS only on DB serveron DB server

Advantages Scalability – middle tier can be used to reduce load on

database sever by using a transaction processing monitor to reduce number of connections to server, and additional application servers can be added to distribute processing

Technological flexibility – easier to change DBMS engines – middle tier can be moved to different platform. Easier to implement new interfaces

Cost reduction – use of off-the-shelf components/services in the middle tier - also substitution of modules within application rather than whole application

Improved customer service – multiple interfaces on different clients can access the same business process

Competitive advantage – ability to react to business changes quickly by changing small modules of code rather than entire applications

Challenges

High short-term costs – presentation component must be split from process component – this requires more programming

Tools, training and experience– currently lack of development tools and training programmes, and people experienced in the technology

Incompatible standards – few standards yet proposed Lack of compatible end-user tools – many end-user tools such

as spreadsheets and report generators do not yet work through middle-tier services (see later discussion on middleware)

Middleware

Software which allows an application to interoperateinteroperate with other software, without requiring the user to understand and code the low-level operations required to achieve interoperability

With Synchronous systems, the requesting system waits for a response to the request in real time

Asynchronous systems send a request but do not wait for a response in real time – the response is accepted whenever it is received .

6 Types of Middleware ->

1. Asynchronous Remote Procedure Calls (RPC) - client makes calls to procedures running on remote computers but does not wait for a response. If connection lost, must re-establish the connection and send again. High scalability but low recovery

2. Synchronous RPC – distributed program using this calls services available on different computers – possible to achieve this without undertaking detailed coding (e.g. RMI in Java)

3. Publish/Subscribe (push technology) - server monitors activity and sends information to client when available - asynchronous, clients (subscribers) perform other activities between notifications from server.

4. Message-Oriented Middleware – asynchronous, sends messages that are collected and stored until acted upon - client continues with other processing.

5. Object Request Broker (ORB) – tracks location of each object and routes requests

6. SQL-oriented Data Access - translate generic SQL into the SQL specific to the database

Database middleware

ODBC – Open Database Connectivity - most DB vendors support this

OLE-DB - Microsoft enhancement of ODBC JDBC – Java Database Connectivity - Special Java classes

that allow Java applications/applets to connect to databases CORBA – Common Object Request Broker Architecture –

specification of object-oriented middleware DCOM – Microsoft’s version of CORBA – not as robust

as CORBA over multiple platforms

Client/Server security

Network environment has complex security issues. Networks susceptible to breaches of security through eavesdropping, unauthorised connections or unauthorised retrieval of packets of information flowing round the network. Specific security issues include:

System-level password security – user names and passwords for allowing access to the system. Password management utilities

Database-level password security - for determining access privileges to tables; read/update/insert/delete privileges

Secure client/server communication - via encryption – but encryption can negatively affect performance

DB access from clients Partitioning to create 2, 3 or n-tier architecture - decisions

must be made about the placement of the processing logic Storage logic (the database engine) handled by server, and

presentation logic handled by client Part a) of Fig. depicts possible 2-tier systems, placing

processing logic on client (fat client), on server (thin client) or partitioned across both (distributed environment)

Part b) depicts typical 3 and n -tier architectures Some processing logic placed on the client if desired Typical client in a Web enabled environment will be a thin

client, using a browser for its presentation logic Middle tiers are typically coded in a portable language such

as Java

Processing logic distributions

a) 2-tier

b) 3 and n-tier

Processing logic could be at client, server, or both

Processing logic will be at application server or Web server

Open Database Connectivity (ODBC)

An API providing common language for application programs to access/process SQL databases - independent of particular RDBMS

Required parameters: ODBC driver needed, Back-end server name, Database name, User id and password

Fig. Shows generic ODBC architecture Client application requests connection established with data source Driver manager identifies appropriate ODBC driver Driver selected processes requests from the client and submits

queries to RDBMS in required version of SQL Java Database Connectivity (JDBC) similar to ODBC – built

specifically for Java applications

ODBC Architecture

Each DBMS has its own ODBC-compliant driver

Client does not need to know anything about the DBMS

Application Program Interface (API) provides common interface to all DBMSs

Relational Algebra & Client-Server Systems, CS263 Lectures 11 and 12.

Documents

Transcript of Relational Algebra & Client-Server Systems, CS263 Lectures 11 and 12.