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Lesson 19

Oracle Instance Architecture Conclusion

This is the end of the module on Oracle processes. This module discussed the purpose of the Oracle significant background processes.
You should understand the difference between a database and an instance, and you should also be able to do the following:
  1. Explain the difference between a database and an instance
  2. Identify the major background processes of an Oracle instance, and explain their purpose
  3. Display a list of the background processes that are currently running
  4. Identify the instances running on your system based on a listing of process names

Connecting to the Database with SQL*Plus

Oracle Database includes the following components:
  1. The Oracle Database instance, which is a collection of processes and memory
  2. A set of disk files that contain user data and system data

When you connect with SQL*Plus, you are connecting to the Oracle instance. Each instance has an instance ID, also known as a system ID (SID). Because there can be more than one Oracle instance on a host computer, each with its own set of data files, you must identify the instance to which you want to connect. For a local connection, you identify the instance by setting operating system environment variables. For a remote connection, you identify the instance by specifying a network address and a database service name. For both local and remote connections, you must set environment variables to help the operating system find the SQL*Plus executable and to provide the executable with a path to its support files and scripts. To connect to an Oracle instance with SQL*Plus, therefore, you must complete the following steps:
  1. Step 1: Open a Command Window
  2. Step 2: Set Operating System Environment Variables
  3. Step 3: Start SQL*Plus
  4. Step 4: Submit the SQL*Plus CONNECT Statement

Instance Architecture Glossary

This module introduced you to the following terms:
  1. asynchronously:When applied to processes, Oracle processes in particular, to execute asynchronously means to execute at the same time as other processes, without having to take turns
  2. buffer I/O: A logical I/O, also known as a buffer I/O, refers to reads and writes of buffers in the buffer cache.
  3. crash recovery: The process of redoing changes that were lost during a system crash because the affected data blocks were contained only in memory. During crash recovery, the redo log files are read, and the changes reapplied to the datafiles
  4. dirty read: A transaction reads data that has been written by another transaction that has not been committed yet.
  5. grepping: The process, used under UNIX, of filtering the output of one command through the grep command in order to restrict output to specific items of interest.
  6. head of the log: A pointer to the oldest redo log entry, in the redo log buffer, that has not yet been written to the redo log files. The log writer process always writes from the head-end of the log.
  7. Least Recently Used List: A list of pointers to database buffers that is maintained by Oracle in the SGA. The pointers in the LRU list are arranged in order based on the time interval since a buffer was last accessed.
  8. LRU List: Least Recently Used List. A list of pointers to database buffers that is maintained by Oracle in the SGA. The pointers in the LRU list are arranged in order based on the time interval since a buffer was last accessed.
  9. (MTS)multi-threaded server: The (MTS) multi-threaded server has been replaced by the Oracle shared server since Oracle 9i.
  10. recovery: The process of reading entries from the redo log files (including the archived redo log files) and using that information to replay changes that have been made to the database. Recovery is often used to bring a database up-to-date after it has been restored from a backup.
  11. redo log entry: chronological records of database actions used during database recoveries.
  12. redo log buffer: The redo log buffer is an area of memory in the System Global Area (SGA) of an Oracle database that stores information about changes made to the database. This information is stored in redo entries, which are used to reconstruct the database in the event of a crash or other failure.
  13. SGA (System Global Area): The System Global Area (SGA) is a shared memory area that is used by all Oracle processes in a database instance. It contains data and control information that is.shared by all users and processes.
  14. SID:The system identifier (SID) is a unique name for an Oracle database instance on a specific host.
  15. tail of the log: The Log Writer writes from the other end, the tail, so that redo log entries are written in the order in which they are created.
  16. temporary segments: In Oracle RDBMS, a temporary segment is a segment that is used to store temporary data during the execution of a SQL statement. This data can include intermediate results of a query, temporary tables, and sort buffers.
  17. thread:A thread is an independent execution path within a program.

Oracle Instance Architecture - Exercise

Click the Exercise link below to answer some essay questions and submit them .
Oracle Instance Architecture - Exercise
The next module is about memory architecture. It is a short module, and it's the last of the architecture modules.
After you are finished with that, we will move on to doing some more interesting things, like creating a database from scratch.

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