Understand terms used in Oracle RAC

Understand terms used in Oracle RAC

Combination of Servers work together is known as cluster

Oracle Clustware
Oracle Clusterware enables servers to communicate with each other, so that they appear to function as a collective unit.
Oracle Clusterware is portable cluster software that provides comprehensive multi-tiered high availability and resource management for consolidated environments.

Oracle Flex Cluster
Oracle Clusterware 12c release 2 (12.2), all clusters are configured as Oracle Flex
Clusters, meaning that a cluster is configured with one or more Hub Nodes and can support a large number of Leaf Nodes.
Hub Nodes have direct access to memory. Leave node connected with hub nodes.

Voting files
Voting files which record node membership information. Oracle Clusterware uses voting files to determine which nodes are members of a
cluster. Configure voting files on Oracle ASM or on shared storage.
Note: In Shared storage you should create 3 minimum voting files in separate storage.
Oracle recommends that you do not use more than five voting files

Oracle Cluster Registry
Oracle Cluster Registry (OCR) which records cluster configuration information.
such as Oracle RAC databases, listeners, virtual IP addresses (VIPs), and services and any applications.
OCR stores configuration information in a series of key-value pairs in a tree structure.

Grid Naming Service (GNS)
Oracle Clusterware uses Grid Naming Service (GNS) for address resolution in a
single-cluster or multi-cluster environment. In RAC, all client need to resolve the VIP and SCAN name to connect with the database.
This problem is solved with Grid Naming Service (GNS).

Single Client Access Name (SCAN)
It used for dynamic VIP address configuration, removing the need to perform manual server configuration.
SCAN is a domain name registered to at least one and up to three IP addresses configured either in DNS or GNS.
Node VIP and three scan VIPS are obtained from DHCP server using GNS. If new node join the cluster then cluster-ware get VIP address from DHCP server
and update the cluster resource and make server accessible though GNS.
Note: You have the option to manually configure addresses, instead of using GNS and DHCP for dynamic configuration.

Oracle Clusterware consists of two separate technology
Upper stack: Cluster Ready Services (CRS) daemon (CRSD)
Lower stack: Oracle High Availability Services daemon(OHASD)

Cluster Ready Services
Following are the some process involved in cluster ready services:

Cluster Ready Services (CRS)
CRSD used for managing high availability operations in a cluster.
CRSD manages cluster resources based on the configuration information that is stored in OCR.
This includes start, stop, monitor, and fail-over operations.
The CRSD process generates events when the status of a resource changes.
The CRSD process monitors the Oracle database instance, listener, and so on, and automatically restarts when a failure occurs.

Cluster Synchronization Services (CSS)
Manages the cluster configuration by controlling which nodes are members of the cluster and by notifying members when a node joins or leaves the cluster.
cssdagent process monitors the cluster and provides I/O fencing. Oracle Process Monitor Daemon (oprocd) or OraFenceService on Windows
A cssdagent failure may result in Oracle Clusterware restarting the node.

Oracle ASM
Provide disk management for Oracle Clustware & database.

Cluster Time Synchronization Service (CTSS)
Provides time management in a cluster for Oracle Clusterware.

Event Management(EVM)
A background process that publishes events that Oracle Clusterware creates.

Grid Naming Service (GNS)
Handles requests sent by external DNS servers, performing name resolution for names defined by the cluster.

Oracle Agent (oraagent)
Extends clusterware to support Oracle-specific requirements and complex resources.
This process runs server callout scripts when FAN events occur.

Oracle Notification Service (ONS)
A publish and subscribe service for communicating Fast Application Notification (FAN) events.

Oracle Root Agent(orarootagent)
specialized oraagent process that helps the CRSD manage resources owned by root, such as the network, and the Grid virtual IP address.

Oracle High Availability Services daemon(OHASD)
Following list describes the processes in the Oracle High Availability Services:

Protects any resources of the application resource type used in previous versions of Oracle Clusterware.

Cluster Logger Service (ologgerd)
Receives information from all the nodes in the cluster and persists in an Oracle Grid Infrastructure Management Repository.
This service runs on only two nodes in a cluster

Grid Interprocess Communication (GIPC)
A support daemon that enables Redundant Interconnect Usage.

Grid Plug and Play (GPNPD)
Provides access to the Grid Plug and Play profile, and coordinates updates to the profile among the nodes of the cluster to ensure
that all of the nodes have the most recent profile.

Multicast Domain Name Service (mDNS)
Used by Grid Plug and Play to locate profiles in the cluster, and by GNS to perform name resolution. The mDNS
process is a background process on Linux and UNIX and on Windows.

Oracle Agent (oraagent)
Extends clusterware to support Oracle-specific requirements and complex resources.
This process manages daemons that run as the Oracle Clusterware owner, like the GIPC, GPNPD, and GIPC daemons.

Oracle Root Agent (orarootagent)
A specialized oraagent process that helps the CRSD manage resources owned by root, such as the Cluster Health Monitor(CHM).

Protects resources of resource types other than application when using shell or batch scripts to protect an application.

System Monitor Service (osysmond)
The monitoring and operating system metric collection service that sends the data to the cluster logger service.
This service runs on every node in a cluster.

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