Oracle Cluster Synchronization Services Daemon (ocssd) is the component, which provides the synchronization services between the nodes. OCSSD provides the access to the node membership. It also enables basic cluster services including cluster group services and cluster locking. It can also run without integration with vendor clusterware. Failure of ocssd causes the machine to reboot to avoid split-brain situation. This is also required in a single instance configuration if Automatic Storage Management (ASM) is used. Automatic Storage management was a new feature in oracle database 10g. Ocssd runs as ‘oracle’ user. The following functionalities are covered by CSS daemon

• CSS provides basic ‘group services’ support. Group Services is a distributed group membership system that allows the applications to coordinate activities to archive a common result.
• ’Group services’ use vendor clusterware group services when vendor clusterware is available. But it is capable of working independently if there is no vendor clusterware group services available
• ‘Lock services’ is another service from the CSS daemon. Lock services provide the basic cluster wide serialization locking functions. It uses FIFO mechanism to manage locking
• Node Services is the third service produced by the CSSD. It uses OCR to store the data and updates the information during re configuration. It also manages the OCR data, which is static otherwise.