Each session in your database has a limited number of database links that can be opened at the same time.

This number is determined by a database-wide initialization parameter: OPEN_LINKS.

The default value is set to 4.

If it is set to 0, distributed transactions are not allowed.

If you are expecting your transactions to have a maximum of 3 database links open concurrently, set this parameter to 3 or higher.

Do not set it too high, it is better for the application to close database links when no longer in use than to change the parameter to a high number.

OPEN_LINKS cannot be modified in real-time, so you will have to change it in the spfile or pfile and bounce the database.

SQL> alter system set open_links=10 scope=both;
alter system set open_links=0 scope=both
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter
cannot be modified

SQL> alter system set open_links=10 scope=spfile;

System altered.

If you are not sure how many database links are opened up concurrently by your session’s database application, you can query v$dblink.

SQL> select in_transaction, count(*) from v$dblink
     group by in_transaction;

IN_   COUNT(*)
--- ----------
YES          1

Recommended reading: