Introduction to Database Systems

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Why databases are important

• Because we use databases all the time!!– Google– Youtube– Facebook– E-shopping– Banking

• Almost all private and official organisations use databasers, one way or another

Databases!

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Why databases are important

• What is it, actually…?– A database is just a well-

structured collection of data, where data can be easily stored and retrieved

• Why is that useful…?– Data becomes information– Information becomes knowledge– Knowledge becomes wisdom

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Types of Databases

• There are several types of database-systems:– Hierachical databases– Network-databases– Object-oriented databases– Relational databases

• Relational databases is by far the most common type, we will concentrate on that type in the following

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Types of Databases

• Relational databases are (still) the most common type of database…

• …but does not fit perfectly with the object-oriented paradigm

• Mapping from object model to relational model is a very common task

• Why is this so…? Inertia, probably…

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A database system is often called DBMS

• Data

• Base

• Management

• System

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What is a DBMS…?

• A DBMS is the database itself, plus a number of programs used for interaction and maintenance of the database– Backup, restore– Performance monitoring– User interface– Interfaces to other programs– Etc.

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Computer

What is a DBMS…?

Database

Maintenance

Performance

User Interface

Driver

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ComputerComputer

What is a DBMS…?

Database

Maintenance

Performance

User Interface

Driver

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Computer

ComputerComputer

What is a DBMS…?

Database

Maintenance

Performance

User Interface

Driver

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What is a DBMS…?

• The exact physical configuration of database and programs is not as such important – it is a matter of setup

• No difference in functionality

• It is more relevant to think of such a system in terms of layers

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What is a DBMS…?

User Interface

Maintenance

PerformanceDriver Database

Business LogicData processing

User inter-face layer

Business logic layer

DBMS layer

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What is a DBMS…?

• A multi-layer (or multi-tier) structure is well suited for the Internet!

• Database on a central server, user interface through an Internet browser

• We use that every day!– Facebook– Net-banking– World of Warcraft…

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DBMS – a closer look

• In many environments, it is absolutely critical that the DBMS functions perfectly!– Financial systems– Emergency systems– Traffic

• Perfectly – no persistent errors must become visible to the outside world

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DBMS – a closer look

• Many functions in a DBMS handle error management and prevention– Transaction support– Concurrency management– Recovery services– Authorisation services– Integrity services

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DBMS – Transactions

• A transaction is a set of changes to the state of the database

• Before the transaction, the database is in a valid and consistent state

• After the transaction, the database is in a valid and consistent state

• Example: Bank transfer

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DBMS – Transactions

Account A:20000

Account B:10000

Transfers history:

Transfer 5000From A to B

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DBMS – Transactions

Account A:20000

Account B:10000

Transfers history:

Step 1: Deduct 5000 from A

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DBMS – Transactions

Account A:15000

Account B:10000

Transfers history:

Step 2: Deposit 5000 to B

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DBMS – Transactions

Account A:15000

Account B:15000

Transfers history:

Step 3: Update transfers history

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DBMS – Transactions

Account A:15000

Account B:15000

Transfers history:Transferred 5000 from A to B

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DBMS – Transactions

• What if…– Power goes out between Step 1 and 2– A disk error occurs– …

• In any case, the database could be left in an inconsistent state!

• For a transaction, either none or all of the steps must be completed

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DBMS – Transactions

• In general, transactions must be ACID:– Atomic – either all steps or none– Consistent – leaves the database in a

consistent state– Isolated – other processes cannot see a

transaction ”in progress”– Durable – the change is permanent (but the

affected data may of course be updated later)

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DBMS – Concurrency

• Concurrency management is somewhat related to transaction

• The DBMS must be able to allow multiple clients concurrent access to the database

• No problem when reading data

• Becomes tricky when clients are writing data…

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DBMS – Concurrency

• The DBMS must ”lock” certain parts of the data in the database, when a uses wants to update it

• May cause requests to queue up…

• Conflicting goals:– Ensure consistency (lock enough)– Ensure efficiency (lock only what is needed)

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DBMS – Recovery

• In case of failure (of any kind), the DBMS must be able to recover the database

• Recover: When DBMS is back online, the database must be in a consistent state

• Often achieved by using duplicate databases, either online or offline

• Online duplicate enables immediate recovery

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DBMS – Authorisation

• Many databases contain sensitive data

• We may wish to limit the actions a specific user can perform on the database

• Such user rights can be defined on several levels

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DBMS – Authorisation

• User rights– Only read data, or read and write– Only work with certain parts of data– Only do certain updates to data– Only allowed to do maintenance– And so on…

• User rights are often defined in terms of defining user roles

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DBMS – Authorisation

Role

Actions

Admini-strator

Mainte-nance

Super-user Manager Empolyee

Add/delete

tables Yes No No No No

Make Backup Yes Yes No No No

Write dataYes No Yes No No

Write data

(predefined) Yes No Yes Yes No

Read dataYes No Yes Yes Yes

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DBMS – Integrity

• Data can be incorrect on multiple levels

• Each piece of data can be correct as such, but a collection of data might be inconsistent

• Can be handled using constraints on data

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DBMS – Integrity

• A constraint can specify cross-field consistency rules

• ”Sum of fields 2- 10 may not exceed value in field 1”

• ”Field 2 must at most be half of field 1”

• Updates that will break a constraint are not allowed by the DBMS