Database Systems
This section covers some basic concepts related to Database Systems their usage.
A database (DB) is a collection of data that lives for a long time. Many systems fit this definition, for example, a paper-based file system, a notebook, or even a string with knobs for counting.
A Database Management System (DBMS) is a system (software) that provides an interface to the database for information storage and retrieval. We are more interested in software systems rather than manual systems because they can do the job more efficiently. The common features of a DBMS include
capacity for a large amount of data
an easy to use interface language (SQL-structured query language)
efficient retrieval mechanisms
multi-user support
security management
concurrency and transaction control
persistent storage with backup and recovery for reliability
The users of a database assume different roles such as
end-user - application programmers that use the DB as a storage subsystem
designer - application programmers and/or business analysts who design the layout of the DB
administrator - operators who maintain the health and efficiency of the DB
implementor - programmers who maintain and develop the DBMS
The key concepts of the database include
schema - the structure and the constraints of data
data - the actual content of the DB representing information
data definition language - used to specify the schema
data manipulation and query language - used to change the data and query them
Schema is a meta-data that describe data. Such meta-data can describe the structure of the data which ranges from strictly enforced structure (relational) to semi-structured (XML) and free-structured data (text files). Before we define the schema we must decide on a model of the data - a metaphor. For relational database n-ary relation is used to model data.
ACID Properties
A transaction is a collection of instructions. To maintain the integrity of a database, all transactions must obey ACID properties. ACID is an acronym for atomicity, consistency, isolation, and durability. Let’s go over each of these properties.
Atomicity - A transaction is an atomic unit; hence, all the instructions within a transaction will successfully execute, or none of them will execute.
Consistency - A database is initially in a consistent state, and it should remain consistent after every transaction.
Isolation - If the multiple transactions are running concurrently, they should not be affected by each other; i.e., the result should be the same as the result obtained if the transactions were running sequentially.
Durability - Changes that have been committed to the database should remain even in the case of software and system failure.
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