Sent: Thursday, July 03, 2003 10:49
AM
Subject: Re: service name, sid .
Hi!
There's actually more:
db_name - identifies database name, has to be the
same what is stated in controlfile (using create database or create
controlfile). You can have several databases with same db_name in one server,
there is no restriction.
SID - When we actually want to open and use the
database, we have to start an instance which will be servicing the
database. When starting instance, whe have to specify SID (system identifier)
for it. This is actually just a operating system name for instance (or should
I say SGA shared memory segments). As you know, SID is specified using
ORACLE_SID OS environment variable and is only used by listener, when spawning
new processes or when attaching directly to SGA using bequeath protocol.
You can not have instances with same SID in one server, even if you use
different oracle homes.
instance_name - An Oracle parameter for
specifying Oracle instance name. Seems that operating system doesn't know
anything about it. For example, if you use SID_LIST parameters in your
listener.ora, then listener always knows how to spawn processes for given SID,
because (almost) all it takes to start another server process, is the location
of oracle executable and SID value. But if you want to connect using instance
name (not SID itself) then Oracle instance actually has to register itself
with listener before any server processes can be spawned.
Note that ORACLE_SID and instance_name variables
do not have to match. (Tested on 9.2.0.1 on W2K). If my SID is ORCL for
example and instance_name is TEST, then I can connect using both SID=ORCL and
SID=TEST in CONNECT_DATA section of TNSNAMES (again, the instance name has to
be registered with listener). V$INSTANCE still shows SID (ORCL) in instance
name fielt.
As we know, in OPS and RAC environment you could
have several instances servicing one database, this is one of common
reasons why db_name and instance_name would be different. (ORCL for db_name,
ORC0, ORC1, ORC2, etc.. for instance names for example). Btw, does anyone know
if I can have the same instance name in all RAC nodes? RAC shouldn't care
much, because it uses instance numbers anyway?
service_names - this is an additional layer
for logically spreading work across nodes in RAC (and possibly other load
balanced environments such replicated databases and even logical standby). For
example, if you have 4 node RAC and have defined two "services" such OLTP and
REPORTING, then you could assign OLTP for nodes and REPORTING for one, but
during month end reporting you could still keep OLTP on 3 nodes, 1 node purely
for REPORTING and add REPORTING to one or more OLTPs as well. So, when user's
are connecting, they don't request a specific node or instance, they just
request a service. And in load balancing environments then users are directed
to instances, where appropriate service is defined. It gives somewhat
flexibility, but I've never used it myself (thus my explanation here might
wrong ;)
Cheers,
Tanel.
----- Original Message -----
Sent: Wednesday, July 02, 2003 9:49
PM
Subject: service name, sid .
I am reading oracle network admin guide and
getting confused abt service name, instance name , db name , sid ..
why service name is not same as db name .
Earlier service name and sid used to be same thing .. isn't it ( ? )
.
Can some one clarify with some examples .
TIA
-ak