Oracle DB Architecture

Oracle structures a database, both physically and logically.  An Oracle Database logical structure is the set of tables in the database . A database physical is the set of operating system files that store the bits and bytes of database information on disk. It is important to understand the physical and logical components in an Oracle database.

1.1. Physical Structure

Physical structure of a database comprises of the following files.

• Data Files
• Redolog Files
• Control Files
• Archive Log  Files
• Parameter File of Server Parameter File
• Alert Log & Trace Files

Data Files: An Oracle database has one or more physical ‘Data’ files that hold the actual data of all logical structures like tables, indexes, etc. A Data file can be associated with only one database and only one Tablespace.

Redolog Files: The primary function of Redologs is to record all the changes made to the database before they are written to the data files.  These files can be mirrored and are used in performing recovery operations

Control Files: The control files record control information of all files within the database.  They are used to maintain internal consistency and play a vital role in recovery operations.  These files can also be mirrored.  Oracle automatically modifies control files, which users cannot edit.  They are used to maintain internal consistency and guide during Database recovery.  It is divided into five part :

• Information about the database. total no. of  Datafiles, Redologs and threads that are enabled (Parallel Server).
• Information about each log group and current log group that LGWR is writing.
• Redologs: Each member of log group, the size, path, full name, log sequence number etc.,
• Datafile: Their size, name, path, status etc.,
• Log history of database.

1.2. Logical Structure

Logical structure comprises of Tablespaces, Schema objects like tables, indexes, views etc.,

Tablespace: it is a logical area of storage in a database that directly corresponds to one or more physical Data files.

Schema Objects: Schema is a logical collection of database objects of a user. Eg: Tables, Views, synonyms, Sequences, Indexes, Clusters, Database Triggers, Procedures, Functions, Packages, Database links etc.

The relationship between a Database. Tablespace and Datafile can be stated as :

• Each database is logically divided into one or more tablespaces.
• One or more datafiles are explicitly created for each tablespace to physically store the data of all logical structures in a tablespace.
• The combined size of a tablespace datafiles is the total storage capacity of the tablespace.
• The combined storage capacity of a database tablespaces is the total storage capacity of the database.