Wednesday, January 15, 2014

Failover Cluster Terminology

Failover Clustering Terminology

Node – A node is an individual server in the cluster.

Cluster Resource - A hardware or software component in the cluster such as disk, Virtual Name and IP Address.

Resource group – Cluster resources are contained within a cluster in a single unit and this is also called application and service group. 

Active Node – An active node is one that cluster is currently running, A resource or resource group is only be active on one node at a time.

Passive Node – A passive node is one that cluster is currently not running on it.

Cluster Access point – A cluster access point is the term used in windows server 2008 R2 failover clusters that represents the combination of network name and associated IP address resource. 

Virtual Server – Virtual server consists of network name and IP address to which clients are connect. This helps the client to connect the service which hosted in the cluster environment without knowing the details of server nodes.

Witness disk / file share – The cluster witness or the witness file share are used to store the cluster configuration information and help to determined the state of the cluster when some, if not all, of the cluster nodes cannot be contacted.

Cluster Quorum – The cluster quorum maintains the definitive cluster configuration data and the current state of each node, each services and application group and each resource network in the cluster.

Private storage – Local disks are referred as private storage; There will be one private disk for OS and SQL binaries.

Shared disk – Each server needs to be attached to external storage, in non-clustered environments data is stored under local disks whereas in clustered environment data is stored under shared disk, then only the nodes which are in cluster can access the shared disk.

Public network / private network – Each node needs two network cards, one is the public network and the other is private network between the nodes. The private network is used internal communication of network called as heartbeat. Public network is connected to LAN / WAN.
Heartbeat – It is a health check mechanism of the cluster, A single UDP packet is sent to all nodes in the cluster via private network to confirm whether all nodes in the cluster is online or not, By default cluster service will wait for 5 seconds (one hearbeat sent in every second) before considering the cluster node is unreachable.

LooksAlive – It’s a basic resource (SQL Service) health check to verify whether the service is running fine or not, to perform this cluster service queries the windows service control manager to check the status of the service. By default lookalive check will happen in every 5 seconds

IsAlive – An exhaustive check to verify that a resource is running properly. If this check fails the resource is moved offline and failover process is triggered, during the IsAlive check the cluster service connects to SQL Instance and executes @@servername. It will check only the SQL instance availability and doesnot check the availability of user databases.

Possible Owners – These are the server nodes of cluster group

Preferred Owners – This is the best suited for running application group. 

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