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DBMS
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What is a database?
A collection of data that is managed by adatabase management system, DBMS.•^
Requirements of a Database System1.^
Allow users to create new databases, specify the logical structure ofdata (schema) using specialized language, data-definition language
2.^
Give users the ability to query the data, and modify the data, using alanguage called a query language or data-manipulation language.
3.^
Support storage of very large amounts of data (Gb >), over a longperiod of time, secure from accidents, and unauthorized access,allowing efficient access to the data.
4.^
Control concurrent access to the data, presenting “correct” views of thedata to each user, preventing accidental corruption of the data due tosimultaneous access patterns.
Types of Database Management Systems• Probably everybody’s first attempt was the
file system. Did this work?
- Examples
- Airline Reservation System– Banking Systems– Corporate Records (HR, payroll)
Examples
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Relations are tables, their columns are headed byattributes, describing the entries in the column.
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Below the attributes are the rows or the tuples
checking
savings
type
balance
accountNo
Smaller and Smaller, Bigger and Bigger Systems
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Very sophisticated, high-performance database systems can be run onregular desktop machines.
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The quantities of data needed to be stored and manipulated arecontinuously growing. The structure of the data stored is becomingmore and more complex.
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Terabytes (1000 gigabytes) is becoming more like a norm than anexception, petabytes (1000 terabytes) databases have surfaced.
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Secondary storage (disk arrays), tertiary storage (tapes, CD, DVD)have higher access times, but are cheaper.
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Parallel computing is also required due to both the nature of theapplications and the vast amounts of data accessed.
Components
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Storage Manager– Obtains the requested information from the data storage– Modifies the information if requested– Indexes are used (data structures that help us find data
items quickly given a part of their value). Advanceddata structures are used for the indexes (B-tree)
- Indexes are part of the data, their description, part of
the metadata.
- SM consists of buffer manager and file manager
Components (cont.)
- The file manager keeps track of the location of files on
the disk and obtains the block(s) containing therequested data.
- The buffer manager handles main memory. It manages
the memory blocks, obtaining disk blocks from the filemanager, trying to optimize the access to data.
- The more hits you get to blocks in memory, and the
more you minimize the number of I/O operations thefaster the entire system will be.
Components (cont.)
•^
Transaction Manager– A DBMS allows the user to group one or more queries
and/or modifications into a transaction (a group ofoperations that must appear to have been executedtogether sequentially, as a unit)
- ACID properties
- Atomicity – all operations to be executed, or none of it• Consistency – “consistent state” before and after a transaction• Isolation – when concurrent, transaction are isolated from e.o.• Durability – if commited, cannot be lost due to system failure.
Components (cont.)
- Techniques to enforce ACID
- Locking – important feature is the granularity of locks. Trade-
off. Complexity and high-concurrency versus simpler lockingmechanism and less granular access.
- Logging – a log is always written to nonvolatile storage
(medium that will survive a power failure). Transaction isusually kept in memory as well, for speed. Logging isextremely important to assure durability.
- Transaction Commitment – for durability and atomicity,
transactions are computed “tentatively”, recorded, but nochanges are made to the database until the transaction getscommited. Changes copied to the log, then copied to database.
