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PURPOSE
-------

UNDERSTANDING AND CONFIGURATION OF SHARED DATABASE LINKS

 
SCOPE & APPLICATION
-------------------

Every application that references a remote server using a standard database link
establishes a connection between the local database and the remote database. Many
users running applications simultaneously can cause a high number of connections
between the local and remote databases.

Shared database links enable you to limit the number of network connections required
between the local server and the remote server. To use shared database links, the
local server must run in multi-threaded server (MTS) mode. The remote server can either
run in MTS mode, or it can run in dedicated server mode.


Shared Database Links
-----------------------------
 
Shared database links are different from standard database links in two ways:

Network connections made with shared database links can be shared among those, that use the
same database link schema object. When a user needs to make a connection to a remote
server from a particular database link. The user can reuse the 
connections already established to the remote server.

When you use a shared database link, a network connection is established directly out of
the shared server on the local server. For a non shared dblink, if the local server
was a multi-threaded server, this connection would have been established through
the local dispatcher requiring data to go through that dispatcher. 

A rule of thumb is to use shared dblinks when the number of users accessing a dblink
is expected to be much larger than the number of shared servers in the local
database.

Otherwise, if you have one user and ten shared servers that user can require up 
to 10 connections to the remote server. Each shared server may have established
a connection to the remote server, since each shared server may have been used by
that user. In this case, you would want to use a standard database link.
 

Configuration of Shared Database Links
---------------------------------------
To create a shared dblink you use the Shared parameter in the sql create database link 
command:

CREATE SHARED DATABASE LINK 
[CONNECT TO  IDENTIFIED BY ]|[CONNECT TO CURRENT_USER]
AUTHENTICATED BY  IDENTIFIED BY 
[USING ''];

Whenever you create a Shared Database Link, the clause AUTHENTICATED BY is 
required. There must be an account on the remote database with the specified
USERID/PASSWORD and CREATE SESSION privilege. No other privileges are
required.