Oracle System Overview-instances

An instance is an Oracle software in the running state, mainly composed of a memory structure and some processes. In Oracle, each database must have at least one corresponding instance.

An instance is an Oracle software in the running state, mainly composed of a memory structure and some processes. In Oracle, each database must have at least one corresponding instance.

1. Concept of an instance

An instance is an Oracle software in the running state, mainly composed of a memory structure and some processes. In Oracle, each database has at least one corresponding instance. A single instance can only open and use one database.

Related reading:

Oracle System Overview-Data Dictionary

Oracle System Overview-instances

Oracle System Overview-memory structure

Overview of Oracle Server Architecture

2. instance Composition

An Oracle instance consists of two parts:

* Memory Structure of the instance. It mainly includes two parts: SGA and PGA.

* Process structure of the instance. It mainly includes user processes, service processes, and background processes.

The Oracle instance structure is 1-1.

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3. How the instance works

An Oracle server is composed of databases and instances. Instances can access and control databases. When the instance for database management is one, the Oracle server becomes a single-node server. There is also a multi-node server, that is, more than one instance accesses and controls an Oracle database together, which can better provide the stability of the Oracle server and the capability of parallel processing. In Oracle 8i, it becomes OPS (Oracle Parallel Server), and in Oracle 9i it becomes RAC (Real Application Cluster ). In the multi-node server structure, two or more instances are on different servers, all Oracle databases are on the shared disk array, and instances on multiple servers can work at the same time, they communicate through an internal network. If one server cannot provide services, the other server will take over its work.

Single-node server and multi-node server structures 1-2 and-3

In general, we use a single-node server, that is, an instance corresponds to only one database. In the future, we will introduce the single-node server management.

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An Oracle instance of a Single-node server is composed of a memory structure and a process. All the memory structures are in the memory, and the process is a job or task in the memory. Whenever the server starts, Oracle first establishes an SGA and starts the Oracle process. When a user accesses a database, the user actually connects to the instance through the user process. The server process in the instance performs syntax analysis and execution on the user's commands, and then reads and writes the database through the background process, finally, return the result to the user.

The memory structure and process in the instance will be detailed later. See