Basic Two-Way Active/Passive Clustering
Even today, when most people think of "clustering" they envisage basic two-way, active/passive application fail-over, as shown below.
Two Way Clusters
Advantages
- Simple to undertstand - Two-host active/passive fail-over has been in use for many years and is well understood within the IT industry.
- Often lowest software cost - As simple fail-over is a mature technology, many vendors offer such software products at an attractive price point.
- Easiest to implement - The skills to implement basic fail-over systems are relatively widely available today.
Disadvantages
- Highest total cost overhead - This architecture requires an overhead of 100% to implement - each active server has an identical server sitting idle until there is a failure.
- Greatest management overhead as more hosts are clustered - Each clustered pair requires monitoring & management, so this cost increases linearly with the number of hosts or applications that are clustered.
As availability becomes increasingly important to larger enterprises, many are managing their costs with N+1 Clustering.
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