A couple of important points.
First, the listener is a server side only process. It’s entire purpose in life is to receive requests for connections to databases and set up those connections. Once the connection is established, the listener is out of the picture. It creates the connection. It doesn’t sustain the connection. One listener, with the default name of LISTENER, running from one oracle home, listening on a single port, will serve multiple database instances of multiple versions running from multiple homes. It is an unnecessary complexity to try to have multiple listeners or to name the listener as if it belongs to a particular database. That would be like the telephone company building a separate switchboard for each customer.
Additional notes on the listener: One listener is capable of listening on multiple ports. But please notice that it is the listener using these ports, not the database instance. You can’t bind a specific listener port to a specific db instance. Similarly, one listener is capable of listnening on multiple IP addresses (in the case of a server with multiple NICs) But just like the port, you can’t bind a specific ip address to a specific db instance.
Second, the tnsnames.ora file is a client side issue. (but beware that a ‘client’ process, such as exp, can run on the server machine. It is still a ‘client’) It’s purpose is for address resolution – the tns equivalent of the ‘hosts’ file further down the network stack. The only reason it exists on a host machine is because that machine can also run client processes.
Assume you have the following in your tnsnames.ora:
larry =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = myhost)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = curley)
)
)
Now, when you issue a connect, say like this:
$> sqlplus scott/tiger@larry
tns will look in your tnsnames.ora for an entry called ‘larry’. Finding it, tns sends a request through the normal network stack to (PORT = 1521) on (HOST = myhost) using (PROTOCOL = TCP), asking for a connection to (SERVICE_NAME = curley).
Where is (HOST = myhost) on the network? When the request gets passed from tns to the next layer in the network stack, the name ‘myhost’ will get resolved to an IP address, either via a local ‘hosts’ file, via DNS, or possibly other less used mechanisms. You can also hard-code the ip address (HOST = 123.456.789.101) in the tnsnames.ora.
Next, the standard networking process delivers the message to port 1521 on myhost. Hopefully, there is a listener on myhost configured to listen on port 1521, and that listener knows about SERVICE_NAME = curley. If so, the listener will spawn a server process to act as the intermediary between your client and the database instance. Communication to the server process will be on a randomly selected available port. At that point the listener is out of the process and continues to user port 1521 to await other connection requests.
What can go wrong?
First, there may not be an entry for ‘larry’ in your tnsnames. In that case you get “ORA-12154: TNS:could not resolve the connect identifier specified” No need to go looking for a problem on the host, with the listener, etc. If you can’t place a telephone call because you don’t know the number (can’t find your telephone directory (tnsnames.ora) or can’t find the party you are looking for listed in it (no entry for larry)) you don’t look for problems at the telephone switchboard.
Maybe the entry for larry was found, but myhost couldn’t be resolved to an IP address (say there was no entry for myhost in the local hosts file). This will result in “ORA-12545: Connect failed because target host or object does not exist”
Maybe there was an entry for myserver in the local hosts file, but it specified a bad IP address. This will result in “ORA-12545: Connect failed because target host or object does not exist”
Maybe the IP was good, but there is no listener running: “ORA-12541: TNS:no listener”
Maybe the IP was good, there is a listener at myhost, but it is listening on a different port. “ORA-12560: TNS:protocol adapter error”
Maybe the IP was good, there is a listener at myhost, it is listening on the specified port, but doesn’t know about SERVICE_NAME = curley. “ORA-12514: TNS:listener does not currently know of service requested in connect descriptor”
Third: If the client is on the same machine as the db instance, it is possible to connect without referencing tnsnames and without going through the listener.
Now, when you issue a connect, say like this:
$> sqlplus scott/tiger
tns will attempt to establish an IPC connection to the db instance. How does it know the name of the instance? It uses the current value of the enviornment variable ORACLE_SID. So…
$> export ORACLE_SID=fred
$> sqlplus scott/tiger
It will attempt to connect to the instance known as “fred”. If there is no such instance, it will, of course, fail. Also, if there is no value set for ORACLE_SID, the connect will fail.
check executing instances to get the SID
[oracle@vmlnx01 ~]$ ps -ef|grep pmon|grep -v grep
oracle 4236 1 0 10:30 ? 00:00:00 ora_pmon_vlnxora1
set ORACLE_SID appropriately, and connect
[oracle@vmlnx01 ~]$ export ORACLE_SID='vlnxora1
[oracle@vmlnx01 ~]$ sqlplus scott/tiger
SQL*Plus: Release 10.2.0.4.0 - Production on Wed Sep 22 10:42:37 2010
Copyright (c) 1982, 2007, Oracle. All Rights Reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Now set ORACLE_SID to a bogus value, and try to connect
SQL> exit
[oracle@vmlnx01 ~]$ export ORACLE_SID=FUBAR
[oracle@vmlnx01 ~]$ sqlplus scott/tiger
SQL*Plus: Release 10.2.0.4.0 - Production on Wed Sep 22 10:42:57 2010
Copyright (c) 1982, 2007, Oracle. All Rights Reserved.
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux Error: 2: No such file or directory
Enter user-name:
Now set ORACLE_SID to null, and try to connect
[oracle@vmlnx01 ~]$ export ORACLE_SID=
[oracle@vmlnx01 ~]$ sqlplus /scott/tiger
SQL*Plus: Release 10.2.0.4.0 - Production on Wed Sep 22 10:43:24 2010
Copyright (c) 1982, 2007, Oracle. All Rights Reserved.
ERROR:
ORA-12162: TNS:net service name is incorrectly specified
Ok, that is how we get from the client connection request to the listener. What about the listener’s part of all this?
The listener is very simple. It’s job is to listen for connection requests and make the connection (server process) between the client and the database instance. Once that connection is made, the listener is out of the picture. If you were to kill the listener, all existing connections would continue. The listener is configured with the listener.ora file, but if that file doesn’t exist, the listener is quite capable of starting up with all default values. One common mistake with the listner configuration is to specify “HOST=localhost” or “HOST=127.0.01”. This is a NONROUTABLE ip address. LOCALHOST and ip address 127.0.0.1 always mean “this machine on which I am sitting”. So, all computers are known as “localhost” or “127.0.0.1”. If you specify this address, the listener will only be capable of receiving requests from the machine on which it is running. If you specified that address in your tnsnames file – on a remote client machine – the request would be routed to the machine on which the requesting client resides. Probably not what you want.
source: http://forums.oracle.com/forums/thread.jspa?threadID=2155354&tstart=0
computer science *consciousness 