oracle rdbms что это
1 Introduction to Oracle Database
This chapter provides an overview of Oracle Database and contains the following sections:
About Relational Databases
Every organization has information that it must store and manage to meet its requirements. For example, a corporation must collect and maintain human resources records for its employees. This information must be available to those who need it. An information system is a formal system for storing and processing information.
An information system could be a set of cardboard boxes containing manila folders along with rules for how to store and retrieve the folders. However, most companies today use a database to automate their information systems. A database is an organized collection of information treated as a unit. The purpose of a database is to collect, store, and retrieve related information for use by database applications.
Database Management System (DBMS)
A database management system (DBMS) is software that controls the storage, organization, and retrieval of data. Typically, a DBMS has the following elements:
This code manages memory and storage for the DBMS.
Repository of metadata
This language enables applications to access the data.
A database application is a software program that interacts with a database to access and manipulate data.
The first generation of database management systems included the following types:
A hierarchical database organizes data in a tree structure. Each parent record has one or more child records, similar to the structure of a file system.
A network database is similar to a hierarchical database, except records have a many-to-many rather than a one-to-many relationship.
The preceding database management systems stored data in rigid, predetermined relationships. Because no data definition language existed, changing the structure of the data was difficult. Also, these systems lacked a simple query language, which hindered application development.
Relational Model
In his seminal 1970 paper «A Relational Model of Data for Large Shared Data Banks,» E. F. Codd defined a relational model based on mathematical set theory. Today, the most widely accepted database model is the relational model.
A relational database is a database that conforms to the relational model. The relational model has the following major aspects:
Well-defined objects store or access the data of a database.
Clearly defined actions enable applications to manipulate the data and structures of a database.
Integrity rules govern operations on the data and structures of a database.
A relational database stores data in a set of simple relations. A relation is a set of tuples. A tuple is an unordered set of attribute values.
A table is a two-dimensional representation of a relation in the form of rows (tuples) and columns (attributes). Each row in a table has the same set of columns. A relational database is a database that stores data in relations (tables). For example, a relational database could store information about company employees in an employee table, a department table, and a salary table.
Relational Database Management System (RDBMS)
In this case, an application specifies what content is required. For example, an application requests an employee name or adds an employee record to a table.
In this case, the RDBMS determines how things should be done and carries out the operation. For example, after an application queries a table, the database may use an index to find the requested rows, read the data into memory, and perform many other steps before returning a result to the user. The RDBMS stores and retrieves data so that physical operations are transparent to database applications.
Brief History of Oracle Database
The current version of Oracle Database is the result of over 30 years of innovative development. Highlights in the evolution of Oracle Database include the following:
Founding of Oracle
In 1977, Larry Ellison, Bob Miner, and Ed Oates started the consultancy Software Development Laboratories, which became Relational Software, Inc. (RSI). In 1983, RSI became Oracle Systems Corporation and then later Oracle Corporation.
First commercially available RDBMS
Portable version of Oracle Database
Oracle Version 3, released in 1983, was the first relational database to run on mainframes, minicomputers, and PCs. The database was written in C, enabling the database to be ported to multiple platforms.
Enhancements to concurrency control, data distribution, and scalability
PL/SQL stored program units
Objects and partitioning
Oracle8 was released in 1997 as the object-relational database, supporting many new data types. Additionally, Oracle8 supported partitioning of large tables.
Oracle8 i Database, released in 1999, provided native support for internet protocols and server-side support for Java. Oracle8 i was designed for internet computing, enabling the database to be deployed in a multitier environment.
Oracle Real Application Clusters (Oracle RAC)
Oracle9 i Database introduced Oracle RAC in 2001, enabling multiple instances to access a single database simultaneously. Additionally, Oracle XML Database (Oracle XML DB) introduced the ability to store and query XML.
Oracle Database 10 g introduced grid computing in 2003. This release enabled organizations to virtualize computing resources by building a grid infrastructure based on low-cost commodity servers. A key goal was to make the database self-managing and self-tuning. Oracle Automatic Storage Management (Oracle ASM) helped achieve this goal by virtualizing and simplifying database storage management.
Manageability, diagnosability, and availability
Schema Objects
Schema objects are user-created structures that directly refer to the data in the database. The database supports many types of schema objects, the most important of which are tables and indexes.
Tables
A table is a set of rows. A column identifies an attribute of the entity described by the table, whereas a row identifies an instance of the entity. For example, attributes of the employees entity correspond to columns for employee ID and last name. A row identifies a specific employee.
Indexes
An index is an optional data structure that you can create on one or more columns of a table. Indexes can increase the performance of data retrieval. When processing a request, the database can use available indexes to locate the requested rows efficiently. Indexes are useful when applications often query a specific row or range of rows.
Indexes are logically and physically independent of the data. Thus, you can drop and create indexes with no effect on the tables or other indexes. All applications continue to function after you drop an index.
Data Access
A general requirement for a DBMS is to adhere to accepted industry standards for a data access language.
Structured Query Language (SQL)
SQL is a set-based declarative language that provides an interface to an RDBMS such as Oracle Database. In contrast to procedural languages such as C, which describe how things should be done, SQL is nonprocedural and describes what should be done. Users specify the result that they want (for example, the names of current employees), not how to derive it. SQL is the ANSI standard language for relational databases.
All operations on the data in an Oracle database are performed using SQL statements. For example, you use SQL to create tables and query and modify data in tables. A SQL statement can be thought of as a very simple, but powerful, computer program or instruction. A SQL statement is a string of SQL text such as the following:
SQL statements enable you to perform the following tasks:
Insert, update, and delete rows in a table
Create, replace, alter, and drop objects
Control access to the database and its objects
Guarantee database consistency and integrity
SQL unifies the preceding tasks in one consistent language. Oracle SQL is an implementation of the ANSI standard. Oracle SQL supports numerous features that extend beyond standard SQL.
PL/SQL and Java
PL/SQL is a procedural extension to Oracle SQL. PL/SQL is integrated with Oracle Database, enabling you to use all of the Oracle Database SQL statements, functions, and data types. You can use PL/SQL to control the flow of a SQL program, use variables, and write error-handling procedures.
A primary benefit of PL/SQL is the ability to store application logic in the database itself. A procedure or function is a schema object that consists of a set of SQL statements and other PL/SQL constructs, grouped together, stored in the database, and run as a unit to solve a specific problem or to perform a set of related tasks. The principal benefit of server-side programming is that built-in functionality can be deployed anywhere.
Oracle Database can also store program units written in Java. A Java stored procedure is a Java method published to SQL and stored in the database for general use. You can call existing PL/SQL programs from Java and Java programs from PL/SQL.
Transaction Management
Oracle Database is designed as a multiuser database. The database must ensure that multiple users can work concurrently without corrupting one another’s data.
Transactions
An illustration of the need for transactions is a funds transfer from a savings account to a checking account. The transfer consists of the following separate operations:
Decrease the savings account.
Increase the checking account.
Record the transaction in the transaction journal.
Oracle Database guarantees that all three operations succeed or fail as a unit. For example, if a hardware failure prevents a statement in the transaction from executing, then the other statements must be rolled back.
Transactions are one of the features that sets Oracle Database apart from a file system. If you perform an atomic operation that updates several files, and if the system fails halfway through, then the files will not be consistent. In contrast, a transaction moves an Oracle database from one consistent state to another. The basic principle of a transaction is «all or nothing»: an atomic operation succeeds or fails as a whole.
Data Concurrency
If multiple users access the same data, then one way of managing concurrency is to make users wait. However, the goal of a DBMS is to reduce wait time so it is either nonexistent or negligible. All SQL statements that modify data must proceed with as little interference as possible. Destructive interactions, which are interactions that incorrectly update data or alter underlying data structures, must be avoided.
Oracle Database uses locks to control concurrent access to data. A lock is a mechanism that prevents destructive interaction between transactions accessing a shared resource. Locks help ensure data integrity while allowing maximum concurrent access to data.
Data Consistency
In Oracle Database, each user must see a consistent view of the data, including visible changes made by a user’s own transactions and committed transactions of other users. For example, the database must prevent dirty reads, which occur when one transaction sees uncommitted changes made by another concurrent transaction.
Oracle Database Architecture
A database server is the key to information management. In general, a server reliably manages a large amount of data in a multiuser environment so that users can concurrently access the same data. A database server also prevents unauthorized access and provides efficient solutions for failure recovery.
Database and Instance
An Oracle database server consists of a database and at least one database instance (commonly referred to as simply an instance ). Because an instance and a database are so closely connected, the term Oracle database is sometimes used to refer to both instance and database. In the strictest sense the terms have the following meanings:
A database is a set of files, located on disk, that store data. These files can exist independently of a database instance.
Figure 1-1 Oracle Instance and Database
Description of «Figure 1-1 Oracle Instance and Database»
A database can be considered from both a physical and logical perspective. Physical data is data viewable at the operating system level. For example, operating system utilities such as the Linux ls and ps can list database files and processes. Logical data such as a table is meaningful only for the database. A SQL statement can list the tables in an Oracle database, but an operating system utility cannot.
Database Storage Structures
An essential task of a relational database is data storage. This section briefly describes the physical and logical storage structures used by Oracle Database.
Physical Storage Structures
Online redo log files
Many other files are important for the functioning of an Oracle database server. These files include parameter files and diagnostic files. Backup files and archived redo log files are offline files important for backup and recovery.
Logical Storage Structures
This section discusses logical storage structures. The following logical storage structures enable Oracle Database to have fine-grained control of disk space use:
At the finest level of granularity, Oracle Database data is stored in data blocks. One data block corresponds to a specific number of bytes on disk.
An extent is a specific number of logically contiguous data blocks, obtained in a single allocation, used to store a specific type of information.
Database Instance Structures
An Oracle database uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the RDBMS.
When applications connect to an Oracle database, they are connected to a database instance. The instance services applications by allocating other memory areas in addition to the SGA, and starting other processes in addition to background processes.
Oracle Database Processes
These processes are created and maintained to run the software code of an application program or an Oracle tool. Most environments have separate computers for client processes.
These processes consolidate functions that would otherwise be handled by multiple Oracle Database programs running for each client process. Background processes asynchronously perform I/O and monitor other Oracle Database processes to provide increased parallelism for better performance and reliability.
These processes communicate with client processes and interact with Oracle Database to fulfill requests.
Oracle processes include server processes and background processes. In most environments, Oracle processes and client processes run on separate computers.
Instance Memory Structures
Oracle Database creates and uses memory structures for purposes such as memory for program code, data shared among users, and private data areas for each connected user. The following memory structures are associated with an instance:
System Global Area (SGA)
The SGA is a group of shared memory structures that contain data and control information for one database instance. Examples of SGA components include cached data blocks and shared SQL areas.
Program Global Areas (PGA)
A PGA is a memory region that contain data and control information for a server or background process. Access to the PGA is exclusive to the process. Each server process and background process has its own PGA.
Application and Networking Architecture
To take full advantage of a given computer system or network, Oracle Database enables processing to be split between the database server and the client programs. The computer running the RDBMS handles the database server responsibilities while the computers running the applications handle the interpretation and display of data.
Application Architecture
The application architecture refers to the computing environment in which a database application connects to an Oracle database. The two most common database architectures are client/server and multitier.
Oracle Database can act as a Web service provider in a traditional multitier or SOA environment.
Oracle XML DB Developer’s Guide for more information about using Web services with the database
Networking Architecture
Oracle Net Services is the interface between the database and the network communication protocols that facilitate distributed processing and distributed databases. Communication protocols define the way that data is transmitted and received on a network. Oracle Net Services supports communications on all major network protocols, including TCP/IP, HTTP, FTP, and WebDAV.
An important component of Net Services is the Oracle Net Listener (called the listener ), which is a separate process that runs on the database server or elsewhere in the network. Client applications can send connection requests to the listener, which manages the traffic of these requests to the database server. When a connection is established, the client and database communicate directly.
The most common ways to configure an Oracle database to service client requests are:
Dedicated server architecture
Each client process connects to a dedicated server process. The server process is not shared by any other client for the duration of the client’s session. Each new session is assigned a dedicated server process.
Shared server architecture
Oracle XML DB Developer’s Guide for information about using WebDAV with the database
Oracle Database Documentation Roadmap
This section explains how this manual should be read and where it fits into the Oracle Database documentation set as a whole.
To a new user, the Oracle Database documentation library can seem daunting. Not only are there are over 175 manuals, but many of these manuals are several hundred pages long. However, the documentation is designed with specific access paths to ensure that users are able to find the information they need as efficiently as possible.
Basic Group
Technical users who are new to Oracle Database begin by reading one or more manuals in the basic group from cover to cover. Each manual in this group is designed to be read in two days. In addition to this manual, the basic group includes:
This manual is a task-based DBA quick start that teaches you how to perform day-to-day database administrative tasks. It teaches you how to perform all common administrative tasks needed to keep the database operational, including how to perform basic troubleshooting and performance monitoring activities.
This manual is a task-based database developer quick start guide that explains how to use the basic features of Oracle Database through SQL and PL/SQL.
The manuals in the basic group are closely related, which is reflected in the number of cross-references. For example, Oracle Database Concepts frequently sends users to a 2 Day manual to learn how to perform a task based on a concept. The 2 Day manuals frequently references Oracle Database Concepts for conceptual background about a task.
Intermediate Group
The next step up from the basic group is the intermediate group. The manuals in this group are prefixed with the word 2 Day + because they expand on and assume information contained in the 2 Day manuals. These manuals cover topics in more depth than was possible in the basic manuals, or cover topics of special interest. As shown in Table 1-1, the 2 Day + manuals are divided into manuals for DBAs and developers.
Table 1-1 Intermediate Group: 2 Day + Guides
Oracle rdbms что это
This chapter provides an overview of Oracle Database.
This chapter contains the following topics:
About Relational Databases
Every organization has information that it must store and manage to meet its requirements. For example, a corporation must collect and maintain human resources records for its employees. This information must be available to those who need it.
An information system is a formal system for storing and processing information. An information system could be a set of cardboard boxes containing manila folders along with rules for how to store and retrieve the folders. However, most companies today use a database to automate their information systems. A database is an organized collection of information treated as a unit. The purpose of a database is to collect, store, and retrieve related information for use by database applications.
Database Management System (DBMS)
A database management system (DBMS) is software that controls the storage, organization, and retrieval of data.
Typically, a DBMS has the following elements:
This code manages memory and storage for the DBMS.
Repository of metadata
This language enables applications to access the data.
A database application is a software program that interacts with a database to access and manipulate data.
The first generation of database management systems included the following types:
A hierarchical database organizes data in a tree structure. Each parent record has one or more child records, similar to the structure of a file system.
A network database is similar to a hierarchical database, except records have a many-to-many rather than a one-to-many relationship.
The preceding database management systems stored data in rigid, predetermined relationships. Because no data definition language existed, changing the structure of the data was difficult. Also, these systems lacked a simple query language, which hindered application development.
Relational Model
In his seminal 1970 paper «A Relational Model of Data for Large Shared Data Banks,» E. F. Codd defined a relational model based on mathematical set theory. Today, the most widely accepted database model is the relational model.
A relational database is a database that conforms to the relational model. The relational model has the following major aspects:
Well-defined objects store or access the data of a database.
Clearly defined actions enable applications to manipulate the data and structures of a database.
Integrity rules govern operations on the data and structures of a database.
A relational database stores data in a set of simple relations. A relation is a set of tuples. A tuple is an unordered set of attribute values.
A table is a two-dimensional representation of a relation in the form of rows (tuples) and columns (attributes). Each row in a table has the same set of columns. A relational database is a database that stores data in relations (tables). For example, a relational database could store information about company employees in an employee table, a department table, and a salary table.
Relational Database Management System (RDBMS)
An RDBMS distinguishes between the following types of operations:
In this case, an application specifies what content is required. For example, an application requests an employee name or adds an employee record to a table.
In this case, the RDBMS determines how things should be done and carries out the operation. For example, after an application queries a table, the database may use an index to find the requested rows, read the data into memory, and perform many other steps before returning a result to the user. The RDBMS stores and retrieves data so that physical operations are transparent to database applications.
Brief History of Oracle Database
The current version of Oracle Database is the result of over 35 years of innovative development.
Highlights in the evolution of Oracle Database include the following:
Founding of Oracle Corporation
In 1977, Larry Ellison, Bob Miner, and Ed Oates started the consultancy Software Development Laboratories, which became Relational Software, Inc. (RSI). In 1983, RSI became Oracle Systems Corporation and then later Oracle Corporation.
First commercially available RDBMS
Portable version of Oracle Database
Oracle Version 3, released in 1983, was the first relational database to run on mainframes, minicomputers, and personal computers. The database was written in C, enabling the database to be ported to multiple platforms.
Enhancements to concurrency control, data distribution, and scalability
PL/SQL stored program units
Oracle7, released in 1992, introduced PL/SQL stored procedures and triggers.
Objects and partitioning
Oracle8 was released in 1997 as the object-relational database, supporting many new data types. Additionally, Oracle8 supported partitioning of large tables.
Oracle8i Database, released in 1999, provided native support for internet protocols and server-side support for Java. Oracle8i was designed for internet computing, enabling the database to be deployed in a multitier environment.
Oracle Real Application Clusters (Oracle RAC)
Oracle9i Database introduced Oracle RAC in 2001, enabling multiple instances to access a single database simultaneously. Additionally, Oracle XML Database ( Oracle XML DB ) introduced the ability to store and query XML.
Oracle Database 10g introduced grid computing in 2003. This release enabled organizations to virtualize computing resources by building a grid infrastructure based on low-cost commodity servers. A key goal was to make the database self-managing and self-tuning. Oracle Automatic Storage Management (Oracle ASM) helped achieve this goal by virtualizing and simplifying database storage management.
Manageability, diagnosability, and availability
Plugging In to the Cloud
Integration and memory performance
Oracle Database 18c simplified integration with directory services such as Microsoft Active Directory. It also introduced functionality to exploit memory not only for columnar data models but also for high speed row access.
Oracle Database 19c is the long-support version of the Oracle Database 12c (Release 12.2) family of products. A major focus of this release was stability. Oracle Database 19c also introduces several small but significant improvements to features such as JSON and Active Data Guard.
Schema Objects
One characteristic of an RDBMS is the independence of physical data storage from logical data structures.
Schema objects are user-created structures that directly refer to the data in the database. The database supports many types of schema objects, the most important of which are tables and indexes.
«Introduction to Schema Objects» to learn more about schema object types, storage, and dependencies
Tables
A table describes an entity such as employees.
A table is a set of rows. A column identifies an attribute of the entity described by the table, whereas a row identifies an instance of the entity. For example, attributes of the employees entity correspond to columns for employee ID and last name. A row identifies a specific employee.
«Overview of Tables» to learn about columns and rows, data types, table storage, and table compression
«Data Integrity» to learn about the possible types and states of constraints
Indexes
An index is an optional data structure that you can create on one or more columns of a table. Indexes can increase the performance of data retrieval.
When processing a request, the database can use available indexes to locate the requested rows efficiently. Indexes are useful when applications often query a specific row or range of rows.
Indexes are logically and physically independent of the data. Thus, you can drop and create indexes with no effect on the tables or other indexes. All applications continue to function after you drop an index.
«Introduction to Indexes» to learn about the purpose and types of indexes
Data Access
A general requirement for a DBMS is to adhere to accepted industry standards for a data access language.
Structured Query Language (SQL)
SQL is a set-based declarative language that provides an interface to an RDBMS such as Oracle Database.
Procedural languages such as C describe how things should be done. SQL is nonprocedural and describes what should be done.
SQL is the ANSI standard language for relational databases. All operations on the data in an Oracle database are performed using SQL statements. For example, you use SQL to create tables and query and modify data in tables.
A SQL statement can be thought of as a very simple, but powerful, computer program or instruction. Users specify the result that they want (for example, the names of employees), not how to derive it. A SQL statement is a string of SQL text such as the following:
SQL statements enable you to perform the following tasks:
Insert, update, and delete rows in a table
Create, replace, alter, and drop objects
Control access to the database and its objects
Guarantee database consistency and integrity
SQL unifies the preceding tasks in one consistent language. Oracle SQL is an implementation of the ANSI standard. Oracle SQL supports numerous features that extend beyond standard SQL.
«SQL» to learn more about SQL standards and the main types of SQL statements
PL/SQL and Java
PL/SQL is a procedural extension to Oracle SQL.
PL/SQL is integrated with Oracle Database, enabling you to use all of the Oracle Database SQL statements, functions, and data types. You can use PL/SQL to control the flow of a SQL program, use variables, and write error-handling procedures.
A primary benefit of PL/SQL is the ability to store application logic in the database itself. A PL/SQL procedure or function is a schema object that consists of a set of SQL statements and other PL/SQL constructs, grouped together, stored in the database, and run as a unit to solve a specific problem or to perform a set of related tasks. The principal benefit of server-side programming is that built-in functionality can be deployed anywhere.
Oracle Database can also store program units written in Java. A Java stored procedure is a Java method published to SQL and stored in the database for general use. You can call existing PL/SQL programs from Java and Java programs from PL/SQL.
Transaction Management
Oracle Database is designed as a multiuser database. The database must ensure that multiple users can work concurrently without corrupting one another’s data.
Transactions
A transaction is a logical, atomic unit of work that contains one or more SQL statements.
An RDBMS must be able to group SQL statements so that they are either all committed, which means they are applied to the database, or all rolled back, which means they are undone.
An illustration of the need for transactions is a funds transfer from a savings account to a checking account. The transfer consists of the following separate operations:
Decrease the savings account.
Increase the checking account.
Record the transaction in the transaction journal.
Oracle Database guarantees that all three operations succeed or fail as a unit. For example, if a hardware failure prevents a statement in the transaction from executing, then the other statements must be rolled back.
Transactions are one feature that set Oracle Database apart from a file system. If you perform an atomic operation that updates several files, and if the system fails halfway through, then the files will not be consistent. In contrast, a transaction moves an Oracle database from one consistent state to another. The basic principle of a transaction is «all or nothing»: an atomic operation succeeds or fails as a whole.
«Transactions» to learn about the definition of a transaction, statement-level atomicity, and transaction control
Data Concurrency
A requirement of a multiuser RDBMS is the control of data concurrency, which is the simultaneous access of the same data by multiple users.
If multiple users access the same data, then one way of managing concurrency is to make users wait. However, the goal of a DBMS is to reduce wait time so it is either nonexistent or negligible. All SQL statements that modify data must proceed with as little interference as possible. Destructive interactions, which are interactions that incorrectly update data or alter underlying data structures, must be avoided.
Oracle Database uses locks to control concurrent access to data. A lock is a mechanism that prevents destructive interaction between transactions accessing a shared resource. Locks help ensure data integrity while allowing maximum concurrent access to data.
Data Consistency
In Oracle Database, each user must see a consistent view of the data, including visible changes made by a user’s own transactions and committed transactions of other users.
For example, the database must prevent the dirty read problem, which occurs when one transaction sees uncommitted changes made by another concurrent transaction.
The database can also provide read consistency to all queries in a transaction, known as transaction-level read consistency. In this case, each statement in a transaction sees data from the same point in time, which is the time at which the transaction began.
Oracle Database Development Guide to learn about Oracle Flashback Query
Oracle Database Architecture
A database server is the key to information management.
In general, a server reliably manages a large amount of data in a multiuser environment so that users can concurrently access the same data. A database server also prevents unauthorized access and provides efficient solutions for failure recovery.
Database and Instance
An Oracle database server consists of a database and at least one database instance, commonly referred to as simply an instance.
Because an instance and a database are so closely connected, the term Oracle database is sometimes used to refer to both instance and database. In the strictest sense the terms have the following meanings:
A database is a set of files, located on disk, that store data. These files can exist independently of a database instance.
Figure 1-1 Oracle Instance and Database
Description of «Figure 1-1 Oracle Instance and Database»
Although in the strict sense an Oracle database is a set of physical structures (files and memory structures), applications can interact with multiple logical databases inside a single physical database, or a single logical database distributed across multiple physical databases.
This section contains the following topics:
Multitenant Architecture
A non-CDB is a traditional Oracle database that cannot contain PDBs. A CDB is a single physical database that contains zero, one, or many user-created pluggable databases. A pluggable database (PDB) is a portable collection of schemas, schema objects, and nonschema objects that appears to an Oracle Net client as a non-CDB.
CDBs and non-CDBs have architectural differences. This manual assumes the architecture of a non-CDB unless otherwise indicated.
Benefits of the Multitenant Architecture
The multitenant architecture solves a number of problems posed by the traditional non-CDB architecture.
Large enterprises may use hundreds or thousands of databases, often running on different platforms on multiple physical servers. Modern servers are able to handle heavier workloads than before. A database may use only a fraction of the server hardware capacity. This approach wastes both hardware and human resources.
By consolidating multiple physical databases on separate computers into a single database on a single computer, the multitenant architecture provides the following benefits:
Cost reduction for hardware
Easier and more rapid movement of data and code
Easier management and monitoring of the physical database
Separation of data and code
Benefits for manageability include:
Easier upgrade of data and code by unplugging and plugging in PDBs
Easier testing by using PDBs for development before plugging them in to the production CDB
Ability to flash back an individual PDB to a previous SCN
Ability to set performance limits for memory and I/O at the PDB level
Oracle Multitenant Administrator’s Guide to learn more about the benefits of the multitenant architecture
Database Consolidation
Database consolidation is the general process of moving data from one or more non-CDBs into a CDB.
Description of «Figure 1-2 Non-CDBs»
Figure 1-3 Consolidation of Non-CDBs into a CDB
Description of «Figure 1-3 Consolidation of Non-CDBs into a CDB»
Physically, MYCDB is an Oracle database. MYCDB has one database instance, although multiple instances are possible in Oracle Real Application Clusters, and one set of database files, just like a non-CDB.
Oracle Multitenant Administrator’s Guide to learn how to consolidate non-CDBs into a CDB
Application Containers
Starting in Oracle Database 12c Release 2 (12.2), an application container is an optional, user-created container that stores data and metadata for one or more application models.
In some ways, an application container functions as an application-specific CDB within a CDB. An application container, like the CDB itself, can include multiple application PDBs, and enables these PDBs to share metadata and data.
For example, a SaaS deployment can use multiple application PDBs, each for a separate customer, which share application metadata and data. For example, in the following figure, sales_app is the application model in the application root. The application PDB named cust1_pdb contains sales data only for customer 1, whereas the application PDB named cust2_pdb contains sales data only for customer 2. Plugging, unplugging, cloning, and other PDB-level operations are available for individual customer PDBs.
Figure 1-4 SaaS Use Case
Description of «Figure 1-4 SaaS Use Case»
Oracle Multitenant Administrator’s Guide for an overview of application containers
Sharding Architecture
Oracle Sharding is a database scaling technique based on horizontal partitioning of data across multiple Oracle databases. Applications perceive the pool of databases as a single logical database.
Key benefits of sharding for OLTP applications include linear scalability, fault containment, and geographical data distribution. Sharding is well suited to deployment in the Oracle Cloud. Unlike NoSQL data stores that implement sharding, Oracle Sharding provides the benefits of sharding without sacrificing the capabilities of an enterprise RDBMS.
Horizontal partitioning involves splitting a database table across shards so that each shard contains the table with the same columns but a different subset of rows. A table split up in this manner is also known as a sharded table.
The following figure shows a sharded table horizontally partitioned across three shards.
Figure 1-5 Horizontal Partitioning of a Table Across Shards
Description of «Figure 1-5 Horizontal Partitioning of a Table Across Shards»
A use case is distributing customer account data across multiple physical databases. For example, a customer with ID 28459361 may look up his records. The following figure shows a possible architecture. The customer request is routed through a connection pool, where sharding directors (network listeners) direct the request to the appropriate shard, which contains all the customer rows.
Figure 1-6 Oracle Sharding Architecture
Description of «Figure 1-6 Oracle Sharding Architecture»
Using Oracle Sharding for an overview of Oracle Sharding
Database Storage Structures
A database can be considered from both a physical and logical perspective.
Physical data is data viewable at the operating system level. For example, operating system utilities such as the Linux ls and ps can list database files and processes. Logical data such as a table is meaningful only for the database. A SQL statement can list the tables in an Oracle database, but an operating system utility cannot.
The database has physical structures and logical structures. Because the physical and logical structures are separate, you can manage the physical storage of data without affecting access to logical storage structures. For example, renaming a physical database file does not rename the tables whose data is stored in this file.
Physical Storage Structures
The physical database structures are the files that store the data.
When you execute a CREATE DATABASE statement, the following files are created:
Every Oracle database has one or more physical data files, which contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the data files.
Online redo log files
Many other files are important for the functioning of an Oracle database server. These include parameter files and networking files. Backup files and archived redo log files are offline files important for backup and recovery.
Logical Storage Structures
Logical storage structures enable Oracle Database to have fine-grained control of disk space use.
This topic discusses logical storage structures:
At the finest level of granularity, Oracle Database data is stored in data blocks. One data block corresponds to a specific number of bytes on disk.
An extent is a specific number of logically contiguous data blocks, obtained in a single allocation, used to store a specific type of information.
A database is divided into logical storage units called tablespaces. A tablespace is the logical container for segments. Each tablespace consists of at least one data file.
Database Instance Structures
An Oracle database uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the RDBMS.
Oracle Database Processes
For the purposes of this topic, a thread is equivalent to a process. An Oracle database instance has the following types of processes:
These processes are created and maintained to run the software code of an application program or an Oracle tool. Most environments have separate computers for client processes.
These processes consolidate functions that would otherwise be handled by multiple Oracle Database programs running for each client process. Background processes asynchronously perform I/O and monitor other Oracle Database processes to provide increased parallelism for better performance and reliability.
These processes communicate with client processes and interact with Oracle Database to fulfill requests.
Oracle processes include server processes and background processes. In most environments, Oracle processes and client processes run on separate computers.
Instance Memory Structures
Oracle Database creates and uses memory structures for program code, data shared among users, and private data areas for each connected user.
The following memory structures are associated with a database instance:
System Global Area (SGA)
Program Global Areas (PGA)
A PGA is a memory region that contains data and control information for a server or background process. Access to the PGA is exclusive to the process. Each server process and background process has its own PGA.
Application and Networking Architecture
To take full advantage of a given computer system or network, Oracle Database enables processing to be split between the database server and the client programs. The computer running the RDBMS handles the database server responsibilities while the computers running the applications handle the interpretation and display of data.
Application Architecture
The application architecture is the computing environment in which a database application connects to an Oracle database. The two most common database architectures are client/server and multitier.
A service-oriented architecture (SOA) is a multitier architecture in which application functionality is encapsulated in services. SOA services are usually implemented as Web services. Web services are accessible through HTTP and are based on XML-based standards such as Web Services Description Language (WSDL) and SOAP. Oracle Database can act as a Web service provider in a traditional multitier or SOA environment.
Simple Oracle Document Access (SODA) is an adaption of SOA that enables you to access to data stored in the database. SODA is designed for schemaless application development without knowledge of relational database features or languages such as SQL and PL/SQL. You can create and store collections of documents in Oracle Database, retrieve them, and query them, without needing to know how the documents are stored. SODA for REST uses the representational state transfer (REST) architectural style to implement SODA.
Oracle XML DB Developer’s Guide for more information about using Web services with the database
Oracle Net Services Architecture
Oracle Net Services is the interface between the database and the network communication protocols that facilitate distributed processing and distributed databases.
Communication protocols define the way that data is transmitted and received on a network. Oracle Net Services supports communications on all major network protocols, including TCP/IP, HTTP, FTP, and WebDAV.
An important component of Net Services is the Oracle Net Listener (called the listener ), which is a process that runs on the database or elsewhere in the network. Client applications send connection requests to the listener, which manages the traffic of these requests to the database. When a connection is established, the client and database communicate directly.
The most common ways to configure an Oracle database to service client requests are:
Dedicated server architecture
Each client process connects to a dedicated server process. The server process is not shared by any other client for the duration of the client’s session. Each new session is assigned a dedicated server process.
Shared server architecture
Oracle XML DB Developer’s Guide for information about using WebDAV with the database