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Differences between Oracle SID / Instance/ Listener / Service or Service_Name / Database [message #302142] Sun, 24 February 2008 01:43 Go to next message
Messages: 26
Registered: January 2008
Junior Member
Hi guys, i am confuse about the following terms above and how they relate to one another when connecting to a database..

This is what i know

- SID is the identifier of an Oracle Instance.
- An Oracle Instance is a group of programs and memory allocated for accessing the database.
- A database is a group of data files, control files etc.
- A database can be accessed by different instances at the same time.
- A instance can only access 1 database at a time.

From what i know, this are my questions below

1) An instance can only access 1 database(9i) at a time, but does that mean an instance can also access another database of different version(10g) at another time ? If it is possible, what parameter file should i look into ?

2) Can i run two or more instances in the same computer if each instance is assign to access different database of the same version or different version?

3) let say i have 2 instance each accessing a database of its own, how many listener do i need ?

4) when i look into tnsnames.ora, some of the connect_data contains SID, whereas some only contains service_name.
Can I know what is Service_Name ?
What is the difference between SID and service_name when use in accessing the database?

Any advices gurus? thanks deeply appreciated.

Re: Differences between Oracle SID / Instance/ Listener / Service or Service_Name / Database [message #302144 is a reply to message #302142] Sun, 24 February 2008 02:41 Go to previous message
Michel Cadot
Messages: 65154
Registered: March 2007
Location: Nanterre, France, http://...
Senior Member
Account Moderator
1) Before it is started, an instance is just a name. Depending on your ORACLE_HOME it can be associated to a version or another one, a database or another one.

2) Yes, with instance of different names

3) One, preferably (but not mandatory) of the latest version

4) Service_name is part of the SERVICE_NAMES parameter of your instance in init.ora or spfile.
They are 2 different ways to connect to the instance, see Database Net Services Reference, Chapter 6 Local Naming Parameters (tnsnames.ora), section Local Naming Parameters, subsection Connect Data Section.

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