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Replication

What is Replication?

According to Webster's Online, the definition we mean is:

 3 a : COPY, REPRODUCTION b : the action or process of reproducing 

Replication is simply copying data from one database to another. This could be a subset of the data or a complete copy.

I'm going to use insert for my examples but any DML can be replicated.

If you create an insert trigger on a table and push data across a database link every time a record gets inserted, you've performed replication. Call that the poor man's SYNCHRONOUS replication. If in that trigger, you populate a local queue for a later push or pull to a different database, that's ASYNCHRONOUS replication.

The down side to synchronous replication is that the act of inserting a record is delayed until the data can be moved to all replicated databases. If a replicated database is down, the transaction cannot complete. The pro for synchronous is guaranteeing the transaction's delivery to all databases and the elimination of conflicts (more about that later). The downside to asynchronous replication is that depending on database availability, network bandwidth and other factors, the delivery of the transaction may be delayed. This delay could impact reporting or other functional areas. It also increases the likelihood of conflicts in an active/active environment.

A master database is a database actively taking DML. Active/active means that multiple databases are running and accepting transactions and are replicating that data to each other. Oracle calls this multi-master replication. I'll talk about that below.

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Topic: Oracle Replication Postgres Replication MySQL Replication

Contact: Lewis Cunningham
lewisc@databasewisdom.com

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