A cluster is a group of computers used as one resource.

Cluster farms can be used to scale out (with parallel processing), distribute data (with multiple databases if necessary), and separate services/tiers (i.e. distribute processing).

There are multiple means to do this, but for now I shall talk about cluster farms and the MSCS (Microsoft Cluster Server), formerly known as "Wolfpack".

MSCS uses pairs of machines, where one machine actively serves while the other is passive (i.e. ready to replace the active server if necessary). Which services are active or passive on which machine is contained in a "quorom disk" which has contains cluster-scope information.

  • An Active-Passive pair has all of the services active on one machine.
  • An Active-Active pair has some services active on one machine, but other services active on the other machine.

An Active-Passive pair can be used to make services on one tier fault tolerant. Note that the second machine is basically idle unless the first goes down. EG:

Machine Status Service Tier
1 Active SQL Server Data
2 Passive SQL Server Data
Results 1 SQL Server w/BU.

An Active-Active pair can be used to make services across two tiers fault tolerant. Note that the second machine is not idle anymore.

Machine Status Service Tier
1 Active SQL Server Data
Passive MTS Business
2 Passive SQL Server Data
Active MTS Business
Results 1 SQL Server w/BU.
1 MTS w/BU.

An additional machine could be added to the Business tier:

Machine Status Service Tier
1 Active SQL Server Data
Passive MTS Business
2 Passive SQL Server Data
Active MTS Business
3
Active MTS Business
Results 1 SQL Server w/BU.
1 MTS w/BU &
1 MTS parallel.

The third machine could be Active-Active paired with a fourth machine in the Presentation tier and even more machines could be added to the Presentation tier:

Machine Status Service Tier
1 Active SQL Server Data
Passive MTS Business
2 Passive SQL Server Data
Active MTS Business
3 Active MTS Business
Passive IIS Presentation
4 Passive MTS Business
Active IIS Presentation
5 Active IIS Presentation
6 Active IIS Presentation
7 Active IIS Presentation
Results 1 SQL Server w/BU.
2 MTS w/BU &
1 MTS parallel.

1 IIS w/BU &
3 IIS parallel.

It should be apparent that a centralized data source can service a large number of applications.

The COM objects or DLLs would have to be distributed across the machines as well.



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