Lỗi ora 12504 connection was refused with error ora-12504 năm 2024

To properly resolve this error and connect to the appropriate Oracle database, we’ll need to expound a bit on how Oracle behaves and, therefore, what is causing this issue in the first place.

SIDs vs SERVICE_NAMES

It is important to understand the (slight) difference between what Oracle defines as a SID compared to a SERVICE_NAME, as we’ll use this information to create a proper connection string later on.

In Oracle, the system identifier (or ) is a local identifier of up to eight characters in length that is used to identify a particular database and differentiate it from other databases on the system.

Often the SID is the prefix word or DB_UNIQUE_NAME that precedes the

username/password@NET_SERVICE_NAME

0. For example, the SID of our

username/password@NET_SERVICE_NAME

2 database, as seen in in the full

username/password@NET_SERVICE_NAME

3 of

username/password@NET_SERVICE_NAME

4.

username/password@NET_SERVICE_NAME

5, on the other hand, represent the names by which database instances can be connected to. A SERVICE_NAME will typically follow the format of the SID followed by the database domain, like so:

username/password@NET_SERVICE_NAME

8

The TNS Listener

When a client is attempting to connect to an Oracle database, rather than connecting to the database directly, there is a broker service that intervenes and handles the connection request for the client.

This broker application is known as the and it performs the task of listening for incoming client requests. When a request is received, the

username/password@NET_SERVICE_NAME

9 processes and forwards that request onto the appropriate Oracle database server using a , which just acts as the connection between the

username/password@NET_SERVICE_NAME

9 and the database server.

TNS Names and Configuration

When connecting to an Oracle database, typically your database server will have

john/Hunter2@myDatabaseNetService

3, which is a configuration file that informs the server about

john/Hunter2@myDatabaseNetService

4 which are valid database connections. By default, this file is located at

john/Hunter2@myDatabaseNetService

5.

For example, a

john/Hunter2@myDatabaseNetService

6 descriptor in

john/Hunter2@myDatabaseNetService

3 may be formatted like this:

myDatabaseNetService =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = tcp)(HOST = localhost)(PORT = 1521)(QUEUESIZE = 100))
    (CONNECT_DATA =
      (SERVICE_NAME = bookstore.company.com)
    )
  )

This would define a

john/Hunter2@myDatabaseNetService

6 using the SERVICE_NAME we discussed earlier (

username/password@NET_SERVICE_NAME

4. and connecting to

username/password@host:port/SERVICE_NAME

1 through port

username/password@host:port/SERVICE_NAME

2.

Connection String

With a bit more knowledge about how Oracle actually connects to databases, we can now look at how

username/password@host:port/SERVICE_NAME

3 are formatted.

Connect via TNS Name/NET_SERVICE_NAME

When connecting through a

john/Hunter2@myDatabaseNetService

6 as specified in your

john/Hunter2@myDatabaseNetService

3 config file, you must use the

username/password@host:port/SERVICE_NAME

6,

username/password@host:port/SERVICE_NAME

7, and then append the

john/Hunter2@myDatabaseNetService

6 with the

username/password@host:port/SERVICE_NAME

9 symbol, like so:

username/password@NET_SERVICE_NAME

Thus, for our previous

john/Hunter2@myDatabaseNetService

6 descriptor above, the actual

john/Hunter2@myDatabaseNetService

6 we defined was

username/password@host:port:SID

2, so our connection string might look something like this:

john/Hunter2@myDatabaseNetService

Connect via SERVICE_NAME

When connecting through a SERVICE_NAME, you’ll also need to add the

username/password@host:port:SID

4 and

username/password@host:port:SID

5, along with the

username/password@host:port:SID

6 symbol preceding the SERVICE_NAME itself:

username/password@host:port/SERVICE_NAME

Connect via SID

Finally, if connecting without a configured

john/Hunter2@myDatabaseNetService

6 or even SERVICE_NAME, you can do so directly through the SID by using the `SID`1 symbol instead of the

username/password@host:port:SID

6 symbol as with the SERVICE_NAME connection string: