What is a TNS:listener in the Context of Oracle?

Question

Borderline ServerFault question, but figured I'd try here first since I've had luck with Oracle questions in the past.

I'm trying to connect to an oracle database from PHP, and I'm getting the following error.

ORA-12505: TNS:listener does not currently know of SID given in connect descriptor

This is the error that PHP reports, and the error that shows up in Oracle's listener.log.

My immediate problem is fixing this error. The larger question I'd like answered is how does Oracle connection model work?

This is in a development environment that's running on my local windows machine and has been working up until now. Unfortunately, the environment was handed to me (I didn't set it up) and the people who did set it up are unavailable to help me debug it.

If I was getting a similar error with MySQL or PostgreSQL (two systems I'm more familiar with), I'd check to ensure that a database process was running, and then attempt to connect manually to the database using the username/password/connection string. Unfortunately, I'm not familiar with the Oracle tools on windows (other than SQL Developer) and I don't know what a TNS:listener or SID are in the context of Oracle (I have vague ideas, but vague ideas rarely help when you're debugging something like this)

Any general advice would be appreciated.

Updates per Comments:

There's a number of entires in my tnsnames.ora file, the relevant entry being

OBS2 =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = steel-ae39650)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = OBS2)
    )
  )

This is not reflected in the list of instances when I run

    LSNRCTL> services

So I think my next question is, how do I attempt to manually start up the OBS2 instance?

Answer 1

A TNS name is like an alias to your service instance. The TNS listener service acts as a sort of lookup service for you in this regard. It will fail with that error message if the actual service you're trying to connect to via a TNS name isn't valid.

You can then test out to see if the TNS listener sees the service correctly using the command line tool:

%>lsnrctl services

Which should output something like the following:

Service "myservice" has 1 instance(s).
  Instance "myinstance", status READY, has 1 handler(s) for this service...
    Handler(s):
      "D000" established:0 refused:0 current:0 max:1002 state:ready
         DISPATCHER <machine: LOCALHOST, pid: 12345>
         (ADDRESS=(PROTOCOL=tcp)(HOST=LOCALHOST)(PORT=6789))

Can you please post the relevant TNS entry (in the tnsnames.ora file)? It is located in ORAHOME\client or db\ADMIN\NETWORK. If you have both client and server, make sure both copies of the tnsnames.ora file have correct values, just to be safe.

Here's an example of a proper TNS name definition in tnsnames.ora called 'mydb':

myDbAlias =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 12345)(QUEUESIZE = 100))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = myservice)
    )
  )

Answer 2

Mike Atlas' answer is fairly comprehensive, but note that you can connect to 10g (or later) DBs which don't have a published tnsname using [//]host_name[:port][/service_name]

HTH

C.