Understand Distributed Database in Oracle

A distributed database system lets applications and users access data from both local and remote databases. In a homogeneous environment, all databases are Oracle. In a heterogeneous environment, at least one database can be different, such as SQL Server, MySQL, PostgreSQL, or DB2. Distributed databases use a client/server architecture.

Following are terms used in Distributed Database

Database Link
A database link is a connection between two physical database servers that allows a client to access them as one logical database.

Transaction
A transaction is logical unit of consist of one or more SQL queries. A transaction begin and end with rollback or committed statements.
A remote transaction contains only statements that access a single remote node.
A distributed transaction contains statements that access multiple nodes.

Distributed database
A set of databases in a distributed system that appear logical as single unit.

Distributed processing
Operating done by application from distributed environment by accessing different computer in a network.

OPEN_LINKS Parameter
You can limit the number of connections from a user process to remote databases using the static initialization parameter OPEN_LINKS. default value for OPEN_LINKS is 4. If OPEN_LINKS is set to 0, then no distributed transactions are allowed.

Distributed database is used to create the database link in the oracle database for accessing the remote databases for transactions.
Application user will seems one logical database.

Check DB Link Present in Oracle Database

COL OWNER FORMAT a10
COL USERNAME FORMAT A8 HEADING "USER"
COL DB_LINK FORMAT A30
COL HOST FORMAT A7 HEADING "SERVICE"
SELECT * FROM DBA_DB_LINKS;

For More info: Understand distributed Transaction & Two Phase Commit Process