Connection to the database (beginner's question)

Question

I do not quite understand the relation between "@connect alias" and actual TNS name. The documentation tells that the syntax should be:
connect user/pass[@connect_alias] as sysdba
I have TNS names defined myora01 and global one myora01.home.net. Are one these two names suppose to be a "@connect_alias"? Well, it does not work this way:
connect user/pass@myora01 as sysdba;
ERROR:
ORA-12514: TNS:listener does not currently know of service requested in connect descriptor
Where is my mistake?
Regards
Leonid

Answer 1

dwarf7 wrote:
I do not quite understand the relation between "@connect alias" and actual TNS name. The documentation tells that the syntax should be:
connect user/pass[@connect_alias] as sysdba
I have TNS names defined myora01 and global one myora01.home.net. Are one these two names suppose to be a "@connect_alias"? Well, it does not work this way:
connect user/pass@myora01 as sysdba;
ERROR:
ORA-12514: TNS:listener does not currently know of service requested in connect descriptor
Where is my mistake?
Regards
LeonidIf you use the syntax "sqlplus scott/tiger" (or within sqlplus, "conn scott/tiger"), you are asking to connect to a local (on the same machine) database instance identified by the value of the environment variable ORACLE_SID. Tnsnames.ora is not involved.
If you use the syntax "sqlplus scott/tiger@mydb" (or within sqlplus "conn scott/tiger@mydb") you are asking to connect to the database (local or remote) identified by the "mydb' entry in your tnsnames.ora file.
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Assume you have the following in your tnsnames.ora:
{code}
larry =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = myhost)(PORT = 1521))
(CONNECT_DATA =
(SERVICE_NAME = curley)
{code}
Now, when you issue a connect, say like this:
{code}
$> sqlplus scott/tiger@larry
{code}
tns will look in your tnsnames.ora for an entry called 'larry'. Next, tns sends a request 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 request arrives at 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, you'll be connected.
A couple of important points.
First, the listener is a +server side only+ process. It's entire purpose in life is the 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, 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. That would be like the telephone company building a separate switchboard for each customer.
Second, the tnsnames.ora file is a +client side+ issue. It's purpose is for addressess resolution - the tns equivelent 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.
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"
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