Before you can talk about differences between clustering and load balancing, and there are more than a few, you’ve got to get the definitions straight. Clustering is often understood to mean the capability of some software to provide load balancing services, and load balancing is often used as a synonym for a hardware- or third-party-software-based solution.
In practice, clustering is usually used with application servers like IBM WebSphere, BEA WebLogic and Oracle AS (10g). Also being used in that environment are load balancing features found in Application Delivery Controllers (ADC) like BIG-IP. (For simplicity, we will talk about clustering versus ADC approaches.)

Scalability, horizontally speaking
There are hardware load balancers, of course, but there we talk about pools or farms, the server groupings where application requests get distributed. It is in the software world that the term cluster is applied to that same group.

Clustering will typically convert one instance of an application server to a master controller, then process/distribute requests to multiple instances using such industry standard algorithms as round robin, weighted round robin or least connections. Clustering is similar to load balancing in that it has horizontal scalability, a nearly transparent way to add additional instances of application servers for increased capacity or response time performance. To ensure that an instance is actually availab Continue reading »

In Computer Science Clustering is a concept where a set of components provide scalable data at low overheads. Clustering enables high-end applications to flow stored and manipulated information on a network.

This particular concept conflicts with Database Mirroring as apparently this two provide the same facility of partner failover to the database. But there are some differences as the SQL Server Melbourne group has revealed that in a meeting.

After going through the presentation I have intercepted some points which are noted below:

1. Mirroring provides automatic failover and failback between servers, and doesn’t require SAN / NAS hardware from Microsoft’s approved hardware list. Each Mirror server can use entirely separate hardware. Where as Clusters use shared hard disks (SAN / NAS) with the servers, creating a single point of failure. Clustering in 2000 required Enterprise edition of SQL server and windows advanced server. No word from MS yet about what licensing will be required for Mirroring or clustering in 2005.
2. I don’t think you would use Mirroring and Clustering together, unless you had two clusters being mirrored. Otherwise you would have a single point of failure.
3. You could snapshot the Server ‘B’ (the non active server) and report from the snapshot.
4. Replication allows a greater distance between servers, ideal if DBA wants to have highly responsive servers in local / international offices. The Continue reading »