A light pen is a light-sensitive computer input device, basically a stylus, that is used to select text, draw pictures and interact with user interface elements on a computer screen or monitor. The light pen works well with CRT monitors because of the way such monitors scan the screen, which is one pixel at a time, giving the computer a way to...
A database guarantees the following four properties to ensure database reliability, as follows:
Atomicity: A database follows the all or nothing rule, i.e., the database considers all transaction operations as one whole unit or atom. Thus, when a database processes a transaction, it is either fully completed or not executed at all.
Consistency: Ensures that only valid data following all rules and constraints is written in the database. When a transaction results in invalid data, the database reverts to its previous state, which abides by all customary rules and constraints.
Isolation: Ensures that transactions are securely and independently processed at the same time without interference, but it does not ensure the order of transactions. For example, user A withdraws $100 and user B withdraws $250 from user Z’s account, which has a balance of $1000. Since both A and B draw from Z’s account, one of the users is required to wait until the other user transaction is completed, avoiding inconsistent data. If B is required to wait, then B must wait until A’s transaction is completed, and Z’s account balance changes to $900. Now, B can withdraw $250 from this $900 balance.
Durability: In the above example, user B may withdraw $100 only after user A’s transaction is completed and is updated in the database. If the system fails before A’s transaction is logged in the database, A cannot withdraw any money, and Z’s account returns to its previous consistent state.
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