Dependency rigidity

The type of dependency defines the rigidity of the link between parent and child groups. A soft dependency means minimum constraints, whereas a hard dependency means maximum constraints.

Table: Dependency rigidity shows dependency rigidity and associated constraints.

Table: Dependency rigidity

Dependency rigidity

Constraints between parent and child service groups

Soft dependency

Specifies the minimum constraints while bringing parent and child groups online. The only constraint is that the child group must be online before the parent group is brought online.

For example, in an online local soft dependency, an instance of the child group must be online on the same system before the parent group can come online.

Soft dependency provides the following flexibility:

  • If the child group faults, VCS does not immediately take the parent offline. If the child group cannot fail over, the parent remains online.

  • When both groups are online, either group, child or parent, may be taken offline while the other remains online.

  • If the parent group faults, the child group remains online.

  • When the link is created, the child group need not be online if the parent is online. However, when both groups are online, their online state must not conflict with the type of link.

Firm dependency

Imposes more constraints when VCS brings the parent or child groups online or takes them offline. In addition to the constraint that the child group must be online before the parent group is brought online, the constraints include:

  • If the child group faults, the parent is taken offline. If the parent is frozen at the time of the fault, the parent remains in its original state. If the child cannot fail over to another system, the parent remains offline.

  • If the parent group faults, the child group may remain online.

  • The child group cannot be taken offline if the parent group is online. The parent group can be taken offline while the child is online.

  • When the link is created, the parent group must be offline. However, if both groups are online, their online state must not conflict with the type of link.

Hard dependency

Imposes the maximum constraints when VCS brings the parent of child service groups online or takes them offline. For example:

  • If a child group faults, the parent is taken offline before the child group is taken offline. If the child group fails over, the parent fails over to another system (or the same system for a local dependency). If the child group cannot fail over, the parent group remains offline.

  • If the parent faults, the child is taken offline. If the child fails over, the parent fails over. If the child group cannot fail over, the parent group remains offline.

Note:

When the child faults, if the parent group is frozen, the parent remains online. The faulted child does not fail over.

The following restrictions apply when configuring a hard dependency:

  • Only online local hard dependencies are supported.

  • Only a single-level, parent-child relationship can be configured as a hard dependency.

  • A child group can have only one online hard parent group. Likewise, a parent group can have only one online hard child group.

  • Bringing the child group online does not automatically bring the parent online.

  • Taking the parent group offline does not automatically take the child offline.

  • Bringing the parent online is prohibited if the child is offline.

  • Hard dependencies are allowed only at the bottommost level of dependency tree (leaf level).