Differences between Oracle SID / Instance/ Listener / Service or Service_Na

Question

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?
Care to advice? Thanks gurus. Deeply appreciated.

Answer 1

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 ?No it is not possible, the instance is associated to only one database. The oracle instance is created based on the rdbms installed version, and the rdbms version (including patchset) should match the database version, otherwise, the operation dba should perform is the upgrade process.
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?Yes, you can first have as much installed RDBMS as your computer can have, obviously each rdbms installed on different Oracle Homes (restriction started since 8.1.5), and you can start as many instances as the host is able to handle depending on the resources and kernel parameter the host is configured with.
3) let say i have 2 instance each accessing a
database of its own, how many listener do i need ?The minimum number of listeners is 1, this is enough to handle all connections, if you are planning to setup more than two listeneres, it is up to you, each listener must have its own port and you should beware of the self-instance registration issues at the default 1521 port.
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?SID is the way Oracle used to configure tns connect alias up to version 8.0 it is just considered for backward compatibility, but its use should be deprecated. Using services is the way to configure tns connect alias starting from 8i. A service is a way to refer to an instance. Administering by means of services is a helpful topic when administering RAC environments. You can declare certain instances to belong to a service and other to another service name, this way you can better distribuite workloads during peak hours and off time, you can better schedule tasks and avoid bottlenecks by redirecting jobs to specific services and using features such as resource manager.
~ Madrid