11g Cluster Architecture

1-Cluster Architecture:

The oracle cluster-ware is started by OS init daemon as shown below:

During the installation of Oracle Clusterware, the init.ohasd startup script is copied to /etc/init.d. This script is responsible for setting up environment variables and then starting the Oracle Clusterware daemons and processes.

The Oracle High Availability Services daemon (ohasd) is responsible for starting in proper order, monitoring, and restarting other local Oracle daemons, up through the crsd daemon,which manages clusterwide resources. When init starts ohasd on Clusterware startup,ohasd starts orarootagent, cssdagent, and oraagent. These processes then carry out the following tasks:

• orarootagent starts crsd.

- crsd starts another orarootagent process responsible for root-owned CRS

resources including the SCAN VIPS.

• cssdagent starts cssd (ocssd).

• oraagent starts mdnsd, evmd, ASM, ctssd, and gpnpd. oraagent also starts

gsd, Oracle Notification Service (ONS), and the listeners.

Some of the high availability daemons will be running under the root user with real-time

priority, and others will be running under the Clusterware owner with user-mode priorities

after they are started. When a command is used to stop Oracle Clusterware, the daemons will

be stopped, but the ohasd process will remain running.

2-Cluster Processes:

Oracle Clusterware comprises several processes that facilitate cluster operations. The Cluster

Ready Service (CRS), Cluster Synchronization Service (CSS), Event Management (EVM),

and Oracle Notification Service (ONS) components communicate with other cluster

component layers in the same cluster database environment. These components are also the

main communication links between Oracle Database, applications, and the Oracle Clusterware

high availability components. In addition, these background processes monitor and manage

database operations. The following list describes some major Oracle Clusterware background

processes. The list includes components that are processes on Linux and UNIX, or services on

Windows.

• Cluster Ready Service (CRS): Is the primary program for managing high availability

operations in a cluster. The CRS process manages two types of CRS resources:

- Cluster resources: A cluster resource is an Oracle Clusterware resource. Cluster

resources are viewed, added, modified, or removed using the crsctl command.

- Local resources: A local resource runs on every node in the cluster (no failover)

and can be, for example, a listener, ASM, a disk group, or Oracle Notification

Service (ONS).

• The CRS daemon (crsd) manages cluster resources based on configuration information

that is stored in Oracle Cluster Registry (OCR) for each resource. This includes start,

stop, monitor, and failover operations. The crsd process generates events when the

status of a resource changes.

When you have Oracle RAC installed, the crsd process monitors the Oracle database

instance, listener, and so on, and automatically restarts these components when a failure

occurs. When a CRS resource fails, the CRS daemon attempts to restart it, if the resource

is so configured. CRS fails the resource over to another node (again, if it is configured to

do so) after exhausting restart attempts.

• Cluster Synchronization Service (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. If you are using certified third-party clusterware, then

CSS processes interfaces with your clusterware to manage node membership

information. CSS has three separate processes: the CSS daemon (ocssd), the CSS

Agent (cssdagent), and the CSS Monitor (cssdmonitor). The cssdagent

process monitors the cluster and provides input/output fencing. This service formerly

was provided by Oracle Process Monitor daemon (oprocd), also known as

OraFenceService on Windows. A cssdagent failure results in Oracle Clusterware

restarting the node.

• Disk Monitor daemon (diskmon): Monitors and performs input/output fencing for

Oracle Exadata Storage Server. As Exadata storage can be added to any Oracle RAC

node at any point in time, the diskmon daemon is always started when ocssd is

started.

• Event Manager (EVM): Is a background process that publishes Oracle Clusterware

events

• Multicast domain name service (mDNS): Allows DNS requests. The mDNS process is

a background process on Linux and UNIX, and a service on Windows.

• Oracle Grid Naming Service (GNS): Is a gateway between the cluster mDNS and

external DNS servers. The GNS process performs name resolution within the cluster.

• Oracle Notification Service (ONS): Is a publish-and-subscribe service for

communicating Fast Application Notification (FAN) events

• oraagent: Extends clusterware to support Oracle-specific requirements and complex

resources. It runs server callout scripts when FAN events occur. This process was known

as RACG in Oracle Clusterware 11g Release 1 (11.1).

• Oracle root agent (orarootagent): Is a specialized oraagent process that helps

CRSD manage resources owned by root, such as the network, and the Grid virtual IP

address

• Cluster kill daemon (oclskd): Handles instance/node evictions requests that have

been escalated to CSS

• Grid IPC daemon (gipcd): Is a helper daemon for the communications infrastructure

Component	Processes	Owner
Cluster Ready Service (CRS)	crsd	Root
Cluster Synchronization Service (CSS)	ocssd,cssdmonitor, cssdagent	grid owner,root,root
Event Manager (EVM)	evmd, evmlogger	grid owner
Cluster Time Synchronization Service (CTSS)	octssd	root
Oracle Notification Service (ONS)	ons, eons	grid owner
Oracle Agent	oraagent	grid owner
Oracle Root Agent	orarootagent	root
Grid Naming Service (GNS)	gnsd	root
Grid Plug and Play (GPnP)	gpnpd	grid owner
Multicast domain name service (mDNS)	mdnsd	grid owner