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     Overview of Oracle's topology solution

Oracle SQL*Net software provides the ability to define all of the components of a distributed network, including all database servers, clients, listeners and database links.  This global definition is called a “topology”.  To support the topology, SQL*Net, later re-named Net8, allows Oracle database to connect with each other in a seamless fashion.  Connectivity is first established in Oracle by defining "service names" in a special file. 

The service name is translated by Oracle to get all of the information it needs to connect to the remote database.   The next step is to create "database links" that take the service name and add a remote USER ID and password.  Once defined by the Remote DBA, these remote databases can participate in queries and updates from within any Oracle application.  For example, a database in London and Paris can be defined to the Denver system with the following SQL extension:

 

CREATE PUBLIC DATABASE LINK london
      CONNECT TO user_id INEDTIFIED BY secret_password
            USING 'london_unix';
 
CREATE PUBLIC DATABASE LINK paris
      CONNECT TO user_id identified by secret_password
            USING 'paris_vms';

 

Note that the USING clause of the database link specifies a TNS service name.  This service name will be looked-up in the tnsnames.ora file to get the protocol, IP address and the database system ID (SID) name.

We can now include any tables from these remote sites by qualifying their remote site name in the SQL query.  This example joins three tables; a local ORDER table in Denver, a CUSTOMER table in Paris, and a ORDERLINE table in London.

SELECT
   customer.customer_name,
   order.order_date,
   orderline.quantity_ordered
FROM
   customer@london,
   order,
  orderline@paris
WHERE
      customer.cust_number = order.customer_number
      AND
      order.order_number = orderline.order_number;

Here we see that the SQL query joins three tables, the local ORDER table and two remote tables.  The remote table CUSTOMER will access the customer table at London, and the ORDER table in Paris will also be included.

You can also hide the fact that the CUSTOMER and ORDER tables are remote by creating a synonym that hides the database link name:

 

Create public synonym customer for customer@london;
Create public synonym order for order@paris;

 

The SQL could now be written as if the tables were local to the database.

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