Recovery

Published Date: 02 Nov 2017

Recovery is the process of ensuring that a database can achieve a consistent state in the event of failure. The basic unit of recovery in a database system is the transaction. The module of the DBMS tasked with handling recovery must ensure that transactions display the properties of atomicity and durability. This process is complicated by the fact that the management of update activities against a database is not a single-step process. Frequently the writing of changes to a database will involve the steps of reading data from disk into a database buffer, updating data in main memory and writing the database buffer back to disk. The process of transferring data from buffers in main memory to secondary storage is known as flushing. Flushing may occur when triggered by a command such as a commit, may occur automatically when the buffer becomes full or may be set to occur at periodic intervals.

Recovery issues

Backup and recovery is one of the most important aspects of database administration. If a database crashed and there was no way to recover it, a business could face devastating results including lost data, lost revenue and customer dissatisfaction. Companies operating single or multiple databases, storing hundreds of gigabytes or even terabytes of data, all share one common factor - the need for a plan to back up important data and protect themselves from disaster.

The backup of database and there recovery help you to establish an optimized database protection plan and develop a strong backup strategy. These can be done through:

While RDBMSs provide many features to protect your data, a solid backup and recovery strategy is irreplaceable for protecting your data from corruptions, hardware failures and data failure.

Concurrency

A DBMS must enable many users to share data in a database - to access data concurrently. The DBMS must ensure that if two transactions are accessing the same data then they do not leave the database in an inconsistent state.

Concurrency issues

Concurrency refers to the sharing of resources by multiple interactive users or application programs at the same time. The supports concurrent transactions, enabling an application to establish several distinct connections to the same database. When developing such an application, take care to prevent undesirable effects, such as:

1. Your application executes a query.

2. Another application inserts or updates data that satisfies your application's query criteria.

3. Your application repeats the query from step 1 (within the same unit of work), but the result set is different because it includes additional "phantom" rows inserted or updated by the other application.

Security

One of the major ways of ensuring the integrity of a database is by restricting access - in other words, securing the database. The main way this is done in contemporary database systems is by defining in some detail a set of authorized users of the whole, or more usually parts, of the database.

Security issues

Data security actually is an important function of a database management system whether it is centralized or distributed. Data security controls protects the data from unauthorized access and unwanted changes. Data security controls have two major aspects:

1. Data protection

2. Authorization control

Data protections make sure that no any unauthorized person can understand the physical contents of data. File systems in centralized and distributed operating systems are used to provide this type of security. Mostly used approach for providing this protection is data encryption. Data encryption encodes the data such that nobody can understand the actual data contents. His encryption not only useful to secure the data stored on disks but also for exchanging the information over a network. This encoded data can be decoded (decrypted) only by than authorized users that know what the code is. Authorization security control ensures that only privileged user can manipulate the data in the way they are allowed to do. The database management system must determine that which users are allowed to perform which functions and which data portion is accessible by them. Authorization controls are different in a centralized database to the distributed database environment. Authorization control definition in a distributed database system is derived from that in centralized system but in the context of distributed system some additional complexity is also considered.

Optimisation

This function determines the optimal strategy for executing any queries expressed against a database

Optimization issues

A DBMS often has a choice about the access path for retrieving data. For example, the DBMS can use an index (fast lookup for specific entries) or scan the entire table to retrieve the appropriate rows. In addition, in statements in which two tables are joined, the DBMS can choose which table to examine first (join order) and how to join the tables (join strategy). Optimization means that DBMS makes the best (optimal) choice of access paths, join order, and join strategy. True query optimization means that the DBMS will usually make a good choice regardless of how the query is written. The optimizer does not necessarily make the best choice, just a good one. Derby can use indexes to improve the performance of DML (data manipulation language) statements such as queries, updates, and deletes. The query optimizer can make decisions about whether to use an index for a particular table (access path) and also makes decisions about join order, type of join, and a few other matters.