ORACLE CLUSTER START SEQUENCE

 Brief explanation of the startup sequence.

Image Credit: Oracle

Once the Operating system starts and finish the boot scrap process it reads /etc/init.d file via the initialisation daemon called init or init.d. The init tab file is the one it triggers oracle high availability service daemon.

1. When a node of an Oracle Clusterware cluster starts, OHASD is started by platform-specific means like init.d in Linux. OHASD is the root for bringing up Oracle Clusterware. OHASD has access to the OLR (Oracle Local Registry) stored on the local file system. OLR provides needed data to complete OHASD initialization.
2. OHASD brings up GPNPD and CSSD ( Cluster synchronization Service Daemon ). CSSD has access to the GPNP Profile stored on the local file system. This profile contains the following vital bootstrap data:
   a. ASM Diskgroup Discovery String
   b. ASM SPFILE location (Diskgroup name)
   c. Name of the ASM Diskgroup containing the Voting Files
3. The Voting Files locations on ASM Disks are accessed by CSSD with well-known pointers in the ASM Disk headers and CSSD is able to complete initialization and start or join an existing cluster.
4. OHASD starts an ASM instance and ASM can now operate with CSSD initialized and operating. The ASM instance uses special code to locate the contents of the ASM SPFILE, assuming it is stored in a Diskgroup.
5. With an ASM instance operating and its Diskgroups mounted, access to Clusterware’s OCR is available to CRSD.
6. OHASD starts CRSD with access to the OCR in an ASM Diskgroup.
7. Clusterware completes initialization and brings up other services under its control.

As Per Oracle doc below are the high level steps for clusterware initialization.

INIT spawns init.ohasd (with respawn) which in turn starts the OHASD process (Oracle High Availability Services Daemon). This daemon spawns 4 processes.

Level 1: OHASD Spawns:
• cssdagent – Agent responsible for spawning CSSD.
• orarootagent – Agent responsible for managing all root owned ohasd resources.
• oraagent – Agent responsible for managing all oracle owned ohasd resources.
• cssdmonitor – Monitors CSSD and node health (along wth the cssdagent).

Level 2: OHASD rootagent spawns:
• CRSD – Primary daemon responsible for managing cluster resources.
• CTSSD – Cluster Time Synchronization Services Daemon
• Diskmon
• ACFS (ASM Cluster File System) Drivers

Level 3: OHASD oraagent spawns:
• MDNSD – Used for DNS lookup
• GIPCD – Used for inter-process and inter-node communication
• GPNPD – Grid Plug & Play Profile Daemon
• EVMD – Event Monitor Daemon
• ASM – Resource for monitoring ASM instances

Level 4: CRSD spawns:
• orarootagent – Agent responsible for managing all root owned crsd resources.
• oraagent – Agent responsible for managing all oracle owned crsd resources.
Level 4: CRSD rootagent spawns:
• Network resource – To monitor the public network
• SCAN VIP(s) – Single Client Access Name Virtual IPs
• Node VIPs – One per node
• ACFS Registery – For mounting ASM Cluster File System
• GNS VIP (optional) – VIP for GNS

Level 5: CRSD oraagent spawns:
• ASM Resource – ASM Instance(s) resource
• Diskgroup – Used for managing/monitoring ASM diskgroups.
• DB Resource – Used for monitoring and managing the DB and instances
• SCAN Listener – Listener for single client access name, listening on SCAN VIP
• Listener – Node listener listening on the Node VIP
• Services – Used for monitoring and managing services
• ONS – Oracle Notification Service
• eONS – Enhanced Oracle Notification Service
• GSD – For 9i backward compatibility
• GNS (optional) – Grid Naming Service – Performs name resolution