Implications of high availability group policy settings

All of the settings that are specified for a policy affect how the high availability manager governs a high availability group associated with that policy. Some of the policy settings are policy type specific, while others apply to all policy types. It is important to understand the implications for all of the associated high availability groups before you change the settings of an existing policy.

 

Implications of the Policy type setting

The policy type determines which members of a high availability group are automatically made active when the servers containing these members start. You can not directly change the policy type of an existing high availability group policy. If you need to change the policy type, create a new policy with a different policy type and give it a match criteria that makes the high availability manager select a new policy instead of the original one to associate with the high availability group.

Before creating a new policy with a different policy type, determine which components are using the high availability groups that are governed by the original policy and make sure that those components support the new policy type. For example, the service integration bus (SIB) component might require a One of N policy for its high availability group, because it only wants one group member active at a given time. If you change the policy that is associated with the service integration bus high availability group to be an All Active policy, the service integration bus high availability support might not function properly and data corruption can occur.

You can select one of the following policy types when you create a new policy:

All active policy

When this policy is selected, all of the members of the high availability group are made active.

M of N policy

When this policy is selected for a high availability group with N members, M of them are made active. The number that M represents is configurable in the policy settings. Use the Preferred servers setting to designate the preference order in which members of the high availability group are made active.

No operation policy

When this policy is selected, none of the high availability group members are made active. Use the administrative console to manually activate specific group members.

One of N policy

When this policy is selected for a high availability group with N members, only one member of the group is made active. Use the Preferred servers setting to designate the preference order in which members of the high availability group are made active.

Static policy

When this policy is selected, only the members specified in the Static group servers setting are made active.

 

Implications of the Preferred servers setting

With the One of N and M of N policy types, you can set up a list of preferred servers as part of the policy settings. The preferred server list enables an administrator to indicate a preference as to which high availability group member is made active. If no preferred server list is specified, any of the available high availability group members can be selected as the member to activate. If a preferred server list is specified, then the member to activate is selected from this list, in order of preference. The most preferred server is the first one on the list. The following example demonstrates how a policy uses of the preferred server list.

Example:

A high availability group has three members that are located on application servers named ServerA, ServerB, and ServerC. This group is governed by a One of N policy, under which only one of the three members can be active at a given time. When all three members are running and available at the time that the policy is enforced:

The two other policy settings that directly affect how the preferred server list is used are the Failback and Preferred servers only settings.

 

Implications of the Failback setting

The Failback setting is used to specify what happens to the high availability group member on the most preferred server when it is restarted following a failure. The affect of the Failback setting on a member is best demonstrated with two examples.

During startup example:

A high availability group has three members that are located on application servers named ServerA, ServerB, and ServerC. This group is governed by a One of N policy, under which only one of the three members can be active at a given time. The server named ServerB is the only server on the preferred server list. In this example, none of the servers are started.

When ServerA starts, the One of N policy dictates that the high availability manager makes a member active. Because this application server is the only server running, the member on ServerA is activated. When we start ServerB, which is the only server on the preferred server list, one of two things happens:

Following a failure example:

A high availability group has three members that are located on application servers named ServerA, ServerB, and ServerC. This group is governed by a One of N policy, under which only one of the three members can be active at a given time. ServerB is the only server on the preferred server list and is the only member that is currently active. If ServerB fails, the high availability manager activates one of the remaining members to replace that member. The Failback setting determines what happens after ServerB is repaired and restarted.

 

Implications of the Preferred servers only setting

The Preferred Servers Only setting is used to instruct the policy to activate members on preferred servers only. With this setting enabled, only members running on the servers that are specified in the preferred servers list are activated. If no preferred servers are specified, or no preferred servers are currently available, then no members are activated.

During startup example:

A high availability group has three members that are located on application servers named ServerA, ServerB, and ServerC. This group is governed by a One of N policy, under which only one of the three members can be active at a given time. ServerB the only server on the preferred server list. In this example, none of the servers are started.

When ServerA starts, the One of N policy dictates that the high availability manager activates a member. Because ServerA is the only server running, the member on ServerA is activated. Because it is the only server on the preferred server list, when ServerB starts, one of two things happens:

Following a failure example:

A high availability group has three members that are located on application servers named ServerA, ServerB, and ServerC. This group is governed by a One of N policy, under which only one of the three members can be active at a given time. ServerB the only server on the preferred server list. The member located on ServerB is the active member. If ServerB fails, one of two things happens:

 

Implications of the Static group servers setting

You can specify a list of static group servers as part of the configuration settings for a static policy type. When a high availability group is governed by a static policy type, the static group server list defines which group members are activated if it is possible to do so.

 

Implications of the Is alive timer setting

The Is alive timer setting controls how frequently the high availability manager checks the health of the active group members that are governed by a given policy. The high availability manager can detect two fundamentally different kinds of failures:

The administrator has the ability to specify the Is alive timer at the policy level, where it applies to all the members that are governed by this policy, at the process level where it applies to all members running in a particular process. The administrator can also disable this type of failure detection at either of these levels.

 

Implications of the Quorum setting

Quorum is a mechanism that use to protect resources that, in the event of a failure, are shared across members of a high availability group. When enabled, the policy does not activate any group members until quorum is achieved. A high availability group does not achieve quorum until a majority of the members are running. For example, if there are n members in a group, (n/2) + 1 servers must be online to achieve quorum.

Quorum is an advanced function that is designed to work with clusters, specialized component code, and a hardware control facility. Currently, none of the high availability groups supporting WAS components use the quorum mechanism. Therefore, do not enabled the Quorum setting.



Related concepts
High availability group policy selection process High availability group policies

 



 

 

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