5.3 Dependency Management.

Owned by Copy Page Tree
2018/07/18
7 min read

When a group is brought online or taken offline, the resources in the group should be defined the order in which the resources are brought online or taken offline and these are related to each other.
For an instance, an IP address cannot be assigned properly without network interface card and a database service cannot be online properly without mounted disk volume.

In MCCS, dependency is defined as a hierarchy relationship among resources. MCCS can define this antecedent-consequent logical relationship as the hierarchy involving parents and children. A resource that depends on other resources is a parent which is located above, whereas a resource on which the other resources depend is a child which is located below.

These parent/child resources are linked with each other by rule. MCCS brings the parent/child resource online or takes it offline when one of the linked resources faults.

When MCCS brings online a group, a child resource to be online before any parent resource can be brought online. Of course, when the group is taken offline, it will be the opposite way around.

Table of Contents

[Figure] Online/Offline Sequence of Dependent Resources


Dependency Configuration

To define this dependency among resources, configure this link in the "Resource Dependency" view from detailed information panel of MCCS console.

 

[Figure] Dependency relationship


You can simply configure the dependency link of these resources in a tree form.You can see the dependency among resources intuitively from the view above.
The IP address resource as a parent depends on the NIC resource as its child. The application resource as a parent depends on the IP address resource as its child.
Therefore, some resources as like IP address of above example can be both child and parent relatively.
As described before, When a single resource is failed MCCS failover all of the resources in a group. 
Therefore, when these are linked to one another as dependency rule and tied up together as a group, they can provide proper application service.
When you configure parent-child dependency link between two resources, there are 2 levels and the maximum level can be checked on value of  "MaxDependencyLevel" of group attribute.


[Figure] Setting Link


The above screen shows how to set the dependency link between the process resource (parent) and the IP address resource (child). The process resource (parent) depends on the IP address resource (parent).

  1. In the Group tab of the configuration tree of MCCS console, select the Group –> the 'Resource dependency' tab from detailed information panel.
  2. On the right palette screen, select "Create Dependency".
  3. Select the parent resource (e.g. App) first and then select the child resource (e.g. IP address), then dependency link will be set.

For linking process, first selected resource is parent resource and the one selected later is the child resource.


[Figure] Setting Unlink


  1. When you unlink the dependency, select "Select" icon from the "Palette".
  2. Select a dependency link to delete.
  3. On the selected link, right click with your mouse button and select the "Unlink" button  from pop up menu and click "OK" button.


Dependency settings

[Figure] Edit DependancyType


You can review and edit a dependency attribute when you select a dependency among resources.
There are "OnlineDependency" and "OfflineDependency" in the dependency attribute.
This is applied when you bring online and take offline a resource within "Restartlimit". (For more details about Restartlimit, Please refer to "Chapter 7 Resource" in this manual.)

AttributeDescriptionTypeDefaultEditNote
OnlineDependency

This indicates that the child is also brought online or not when a resource online is initiated.
Types are STRONG and WEAK.
Child is not brought online when a resource online is initiated, if this set as WEAK. 

STRINGSTRONGO

OfflineDependency

This indicates that the parent is also taken offline or not when a resource offline is initiated.
Types are STRONG and WEAK.
Parent is not taken offline when a resource offline is initiated, if this set as WEAK.

STRING

STRONG O


Basic Dependency Cases among resources

Following are the general cases of dependency.


Network Interface card(NIC) Resource, IP address Resource

A NIC must be online first to assign an IP address. It is impossible to assign an IP address without a NIC.  Accordingly, these two resources are dependent on each other with the network interface card being the child and the IP being the parent. 

When dependency is configured between both resources, MCCS checks NIC resource is online state before it bring online an IP address resource. 


Disk Resource, IP address Resource, Application Resource (e.g. DB)

Most applications may stores their data on the disks (storage). Accordingly, applications are dependent on disks.

For an instance, database service like MS-SQL or Oracle is not running properly without disk which contains data records.   

Therefore, both resources are dependent on each other with the database service being the parent and the disk becoming the child resource.

If a client needs an IP address for accessing database, both resources are dependent on each other with the database service being the parent and the IP address being the child.

On the basis of the dependency rule, MCCS will  bring online the database service after checking whether the IP address and disk  resources were brought online properly when the group is brought online.

Accordingly, database service must depend on both IP address and disk resources at the same time to provide proper service.

If dependency is configured according to the relationship explained above, following diagram can be obtained.



[Figure] Dependency of database application 




Actions due to dependency

Online/Offline of the group is performed from bottom to top according to the dependency in case of online, and from top to bottom in case of offline.
Following are a few examples how MCCS manages the resources after dependency link among the resources when a resource is failed. Attributes and state of resources are defined as the figure below.


[Figure] Resource State Information Charts

 

Action by dependency attribute

  • OnlineDependency

[Figure] Example of bringing online of resource r1

OnlineDependency attribute of D1 between r1 and r2 is STRONG, OnlineDependency attribute of D2 between r1 and r3 is WEAK.

  1. MCCS checks the Online Dependency attribute linked to r1.
  2. When you initiate resource online of r1, Child resources, r4 -> r2 -> r1, are brought online through the hierarchy as the attribute of D1 is STRONG.
    (r3 isn't brought online as the attribute of D2 is WEAK.)


  • OfflineDependency


[Figure] Example of take offline of resource r4

D1의 OfflineDependency attribute of D1 between r2 and r4 is STRONG, OfflineDependency attribute of D2 between r3 and r4 is WEAK. 

  1. MCCS checks OfflineDependency attribute linked to r4.
  2. When you initiate resource online of r4, Parents resources, r1 -> r2 -> r4, are taken offline through the hierarchy as the attribute of D1 is STRONG.
    (r3 isn't taken offline as the attribute of D2 is WEAK.)


Resource Critical Attributes

  • Failure occurs from a resource on which critical resource depend.

[Figure] Example of Failure Occur 1

  1. As a failure is occurred at resource r2, resource r1 will be taken offline. (Since resource r1 depends on resource r2 which is failed, resource r1 may cannot be online properly.)
  2. Since resource r2 has critical attribute, resource r3 and r4 will be taken offline in order to failover the group.
  3. As a result, resource r2 is considered as failure and all resources in the group will be taken offline


  • Non-critical Resources

[Figure] Example of Failure Occur 2

  1. Resource r2 which is not critical is online status.
  2. When resource r2 get failed, resource r1 will be taken offline because it dependent on resource r2.
  3. As r2 resource is not critical, group failover will be not performed. Therefore, the group will be partial online status.

RestartLimit Attribute

RestartLimit is an attribute of the resource type. This value determines how many times the recovery will be attempted until it is finally confirmed that it is failure. 
(Please refer to "Chapter 7. Resource" for more details.)

[Figure] Example of Failure Occur 3

  1. On the assumption that the value of RestartLimit is 1 for resource r2, the first failure has occurred.
  2. MCCS will bring online resource r2 again as the value of RestartLimit. 
    At this time, resource r1 which depends on resource r2 will be taken offline.
  3. resource r2 restart.
  4. resource r1 online.
  5. When another failure occurs on resource r2, all of the resource will be taken offline from resource r1 to r3 by order.
  6. Resource r3 will be taken offline.
  7. As a result, resource r2 is considered as failure and all resources in the group is offline status in the node.



When it restart bring online of a group in a node due to failure, parent resources are taken offline first based on OfflineDependency attribute in the failure node. And then, bring online resources through the hierarchy.