Oracle Database Architecture

Oracle Database Architecture



Oracle Database is an object-relational database management system developed and marketed by Oracle Corporation. Oracle Database is commonly referred to as Oracle RDBMS or simply Oracle.

Database and Instance

An Oracle Database consists of a database and at least one instance.

An instance, or database instance, is the combination of memory and processes that are a part of a running installation and a database is a set of files that store data.

The following picture illustrates the Oracle Database server architecture.

Sometimes, a database instance is referred to as an entire running database. However, it is important to understand the distinctions between the two.

First, you can start a database instance without having it accessing any database files. This is how you create a database, starting an instance first and creating the database from within the instance.

Second, an instance can access only one database at a time. When you start an instance, the next step is to mount that instance to a database. And an instance can mount only one database at a single point in time.

Third, multiple database instances can access the same database. In a clustering environment, many instances on several servers can access a central database to enable high availability and scalability.

Finally, a database can exist without an instance. However, it would be unusable because it is just a set of files.

Oracle Database

One of the essential tasks of the Oracle Database is to store data. The following section briefly describes the physical and logical storage structure of an Oracle Database.

Physical storage structures

The physical storage structures are simply files that store data. When you execute a CREATE DATABASE statement to create a new database, Oracle creates the following files:

  • Data files: data files contain real data, e.g., sales order and customer data. The data of logical database structures such as tables and indexes are physically stored in the data files.
  • Control files: every database has a control file that contains metadata. The metadata describes the physical structure of the database including the database name and the locations of data files.
  • Online redo log files: every database has an online redo log that consists of two or more online redo log files. An online redo log is made up of redo entries that record all changes made to the data.

Besides these files, an Oracle database includes other important files such as parameter files, network files, backup files, and archived redo log files for backup and recovery.


Logical Storage Structures

Oracle Database uses a logical storage structure for fine-grained control of disk space usage. The following are logical storage structures in an Oracle Database:

  • Data blocks: a data block corresponds to a number of bytes on the disk. Oracle stores data in data blocks. Data blocks are also referred to as logical blocks, Oracle blocks or pages.
  • Extents: An extent is a specific number of logically contiguous data blocks used to store the particular type of information.
  • Segments: a segment is a set of extents allocated for storing database objects, e.g., a table or an index.
  • Table spaces: a database is divided into logical storage units called table spaces. A table space is a logical container for a segment. Each table space consists of at least one data file.

The following picture illustrates segments, extents and data blocks within a table space:


And the next figure shows the relationship between logical and physical storage structures:


Database Instance

A Database Instance is an interface between client applications (users) and the database. An Oracle instance consists of three main parts: System Global Area (SGA), Program Global Area (PGA), and background processes.


The SGA is a shared memory structure allocated when the instance started up and released when it is shut down. The SGA is a group of shared memory structures that contain data and control information for one database instance.

Different from the SGA, which is available to all processes, PGA is a private memory area allocated to each session when the session started and released when the session ends.

Major Oracle Database’s background processes

The following are the major background processes of an Oracle instance:

  • PMON is the process monitor that regulates all other processes. PMON cleans up abnormally connected database connections and automatically registers a database instance with the listener process. PMON is a process that must be alive in an Oracle database.
  • SMON is the system monitor process that performs system-level clean-up operation. It has two primary responsibilities including automatically instance recovery in the event of a failed instance, e.g., power failure and cleaning up of temporary files.
  • DBWn is the database writer. Oracle performs every operation in memory instead of the disk because processing in memory is faster and more efficient than on disk. The DBWn process reads data from disk and writes it back to the disk. An Oracle instance has many database writers DBW0, DBW1, DBW2, and so on.
  • CKPT is the checkpoint process. In Oracle, data that is on disk is called block and the data which in memory is called buffer. When a block is written to the buffer and changed, the buffer becomes dirty, and it needs to be written down to the disk. The CKPT process updates the control and data file headers with checkpoint information and signals writing of dirty buffers to disk. Note that Oracle 12c allows both full and incremental checkpoints.
  • LGWR is the log writer process which is the key to the recoverability architecture. Every change occurs in the database is written out to a file called redo log for recovery purposes. And these changes are written and logged by LGWR process. The LGWR process first writes the changes to memory and then disk as redo logs which then can be used for recovery.
  • ARCn is the archiver process that copies the content of redo logs to archive redo log files. The archiver process can have multiple processes such as ARC0, ARC1, and ARC3, which allow the archiver to write to various destinations such as D: drive, E drive or other storage.
  • MMON is the manageability monitor process that gathers performance metrics.
  • MMAN is the memory manager that automatically manages memory in an Oracle database.
  • LREG is the listener registration process that registers information on the database instance and dispatcher processes with the Oracle Net Listener.

Now, you should have a good overview of the Oracle Database architecture and its components.


Comments

Post a Comment

Popular posts from this blog

Oracle Database Server Architecture: Overview

Image
  Memory Structures There are three major structures in Oracle Database server architecture: memory structures, process structures, and storage structures. A basic Oracle database system consists of an Oracle database and a database instance. The database consists of both physical structures and logical structures. Because the physical and logical structures are separate, the physical storage of data can be managed without affecting access to logical storage structures. The instance consists of memory structures and background processes associated with that instance. Every time an instance is started, a shared memory area called the System Global Area (SGA) is allocated and the background processes are started. Processes are jobs that work in the memory of computers. A process is defined as a “thread of control” or a mechanism in an operating system that can run a series of steps. After starting a database instance, the Oracle software associates the instance with ...
Read more

Oracle E-Business Suite (EBS) - Introduction

Image
       Oracle E-Business Suite is one of Oracle Corp.'s major product lines. Also known as Oracle EBS, it is  an integrated set of business applications for automating customer relationship management (CRM), enterprise resource planning (ERP) and supply chain management (SCM) processes within organizations .   Oracle E-business Suite (EBS)  refers to a set of combined business applications that are provided by Oracle. It’s often referred to as, Oracle EBS, Oracle ERP or E-Biz. It consists of multiple Oracle enterprise applications that help businesses with various processes to do daily business operations. These applications focus on AR (Accounts Receivable), AP (Accounts Payable), financials, order entry, HR, supply chain management, enterprise resource planning, customer relationship management and many others.   The product was developed over 2 decades ago incorporating high-level Oracle technologies and industry standards. The various elements...
Read more

Why enterprises must not ignore Azure DevOps Server

                   Why enterprises must not ignore Azure DevOps Server                     Azure DevOps is the successor to Team Foundation Server (TFS) and is said to be its advanced version. It is a complete suite of software development tools that are being used on-premises. Azure DevOps Server integrates with currently integrated development environments (IDEs) and helps teams to develop and deploy cross-functional software. It provides a set of tools and services with which you can easily manage the planning and development of your software projects through testing and deployment.                Azure DevOps practices enable IT departments to augment quality, decrease cycle times, and optimize the use of resources to improve the way software is built, delivered and operated. It increases agility and enables better software d...
Read more