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Database failover

August 3, 2009
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A database fail over means, In a network model database there are so many data servers be there, if any frailer occur in the network the failover tool monitor will be automatically detect the failure and automatically redirect the connection to the backup server. This known as failover.

A database system facilitates quick failover. The database system includes a primary node; a standby node and a fast interconnect between the primary node and the standby node. The primary node includes a primary buffer pool, which stores pages of the database information, and a log storage, which receives a log of updates for the pages within the primary buffer pool. When a page is to be updated by a primary node, the primary node sends a copy of a before update image of the page to the standby node. When the primary log writes logs of updates to the log storage, the primary node also forwards the logs of updates to the standby node

Failover ensures that if your primary server is ever un-available, your users are automatically directed to a backup server.

The concept really is quite simple: Failover continuously monitors your servers to find out which are up and which are down, and then it dynamically updates your DNS records accordingly so that your domain name always points to a functional server.

It works with web-servers (HTTP), mail-servers (SMTP, IMAP, and POP3), FTP-servers, and practically any other TCP/IP based server type.DNS records are updated directly via the DNS protocol (optionally using encrypted signatures for security), via WMI, via HTTP based dynamic IP updates, or via custom scripting.

Failover can also send you e-mail notifications when problems are detected, and it can be extended through scripting to provide custom notifications, logging, and data capture.

The best failover is the one that no one notices. Unfortunately, even though Oracle has been architected to recover very quickly, failures can severely disrupt users by dropping connections to the database. There various kinds of user operations against the database, INSERT, UPDATES, DELETS and SELECTS. Off these various types of operations, by implementing TAF with out making any application changes, only SELECT statements are failed over. Well this is good for a data warehouse operations, were most of the operations are SELECT queries. However in the real world, operations against the database are not limited to just SELECT queries, DML operations (INSERT, UPDATE and DELETE) are critical and failover without moving such operations transparently does not provide a true user friendly environment.

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