Oracle standby databases are an integral part of Oracle failover, and standby database technology appears in several Oracle products.  Being the world's most flexible and robust database, standby database is available in several forms:

• Physical Standby Database (apply redo logs)
• Logical Standby Database (Oracle Data Guard with SQL Apply)
• Replicated Database (using snapshots)
• Streams standby database (using schema-level data capture)

Let's take a closer look at each approach:

• Physical Standby Database - This ancient method of transporting and applying redo logs has been around for over 15 years, well before it was incorporated into Oracle Data Guard. 
  
  
• Logical Standby Database - The Logical Standby Database uses Oracle Data Guard with SQL Apply to keep a consistent replication of the master database.  The main advantage of logical database is that the data structure of the replicated many be different than the master database.  This is perfect for reporting application where the DBA will create pre-joined, de-normalized table structures for super-fast queries.  For details, see the book "Oracle Data Guard Standby Database".
  
  
• Snapshot Replicated Standby Database - Oracle offers snapshot replication whereby update logs are created for all DML tables, and the snapshot logs are applied to the standby database at specified intervals (from "on commit" to any delayed time period).  For details, see the book "Oracle Replication".
  
  
• Streams standby database - Oracle Streams is a great solution in Oracle 10g (I do not recommend using Streams with 9i due to bugs), and Streams provides high-speed data replication with a minimum of set-up fuss.  Using schema-level data capture it is easy to define a replication strategy.  For more details, see the book "Oracle Streams" and its handy code depot.
  

Standby database Configuration nortes

tnsnames.ora on primary database
----------------------------


standby1 = (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)
           (PORT=1234)(HOST=myserver))
           (CONNECT_DATA=(SID=mysid)))


listener.ora for standby database:

STANDBY SITE
============
STDBY1_LISTENER = (ADDRESS_LIST=(ADDRESS=(PROTOCOL=tcp)
                         (PORT=5212)(HOST=standbyserver)))

SID_LIST_STDBY1_LISTENER =
  (SID_LIST=
    SID_DESC=
     (SID_NAME=mystandby)
     (ORACLE_HOME=/u01/app/oracle/mysid)))

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