RAC

Cluster :

* A cluster is a group of interconnected nodes that acts like a single large server capable of growing and shrinking on demand.

* In other words, clustering can be viewed logically as a method for enabling multiple standalone servers to work as a coordinate unit called a cluster.

You can implement clustering technology at:

a> Database Layer.

b> Application server Layer.

c>Os Layer.

d>Hardware Network load balance Layer.

Types of Clustering Technologies:

>>Active Passive Clustering Technology.

>>Active Active Clustering Technology.

Active Passive Clustering Technology :

 * The best example for this type of example is Dataguard.

* In this type of clustering, when active node comes down, passive node will be up and running.

Disadvantages:

> Expensive solution to customer.
>No Load balancing.
>Poor Scalability.
>No better  performance. 

Active Active Clustering Technology:

Ex: Oracle RAC

* In this type of clustering , when one of the node comes down , all the transactions on the failed nodes are transferred to other running nodes without any user inconvience.

Advantages :

1. High Availability.

2. Load Balancing.

3. Better Performance.

4. Not an expensive solution to customer with respect to hardware.

5. Scalability.

6. cost affective solution to customer.

    *******************************Oracle RAC************************************



  An architecture that combines the Oracle Database with Oracle RAC is inherently a highly available system. Unlike a traditional monolithic database server that is expensive and is not flexible to changing capacity & resource demands, Oracle RAC combines the processing power of multiple interconnected computers to provide system redundancy, scalability, and high availability.
The clusters that are typical of Oracle RAC environments can provide continuous service for both planned and unplanned outages. Oracle RAC builds higher levels of availability on top of the standard Oracle features. All single instance high availability features, such as the Flashback technologies and online reorganization, also apply to Oracle RAC. Applications scale in an Oracle RAC environment to meet increasing data processing demands without changing the application code. In addition, allowing maintenance operations to occur on a subset of components in the cluster while the application continues to run on the rest of the cluster can reduce planned downtime.

  Oracle RAC exploits the redundancy that is provided by clustering to deliver availability with n - 1 node failures in an n-node cluster. Unlike the cold cluster model where one node is completely idle, all instances and nodes can be active to scale your application.

  The Oracle Database with Oracle RAC architecture provides the following benefits over a traditional monolithic database server and the cold failover cluster model:

• Scalability across database instances
  
• Flexibility to increase processing capacity using commodity hardware without downtime or changes to the application
  
• Ability to tolerate and quickly recover from computer and instance failures (measured in seconds)
  
• Rolling upgrades for system and hardware changes
  
• Rolling patch upgrades for some interim patches
  
• Fast, automatic, and intelligent connection and service relocation and failover
  
• Load balancing advisory and runtime connection load balancing
  
• Comprehensive manageability integrating database and cluster features
  

History of RAC:

* The first clustering product  "ARCnet"  was developed by Datapoint in 1977.

* ARCnet was not a commercial success and clustering didn't really take off until DIGITAL EQUIPMENT CORPORATION (DEC) released its VAX cluster product for VAX/VMS Os.

*Oracle's cluster database was introduced with Oracle 6 for the Digital VAX  cluster product.

*Oracle created a Lock Manager for VAX/VMS clusters from digital was not very scalable for database applications, the database requires fine-grained locking mechanism at block level.

* Oracle 6.2 gave birth to Oracle Parallel Server with its own DML(Distributed Lock Manager) and worked well with all VAX/VMS clusters.

*In Oracle 8, Oracle lock manager is integrated with oracle code with an additional layer called OSD(Operating System Dependent) and became IDML(Integrated Distributed Lock Manager).

*Oracle provided its own clusterware for linux and windows from oracle 9i and for all os from oracle 10g databases.

*In Oracle 11g, It introduced server pools and additional API's to manage third party applications.

*In Oracle 9i, DLM has been renamed as Global cache services(GCS) and Global enqueue Services(GES).