1.4. SAP HANA HA/DR provider hooks

Current versions of SAP HANA provide an API in the form of hooks that allow the HANA instance to send notifications for certain events, for example the loss or establishment of the system replication. For each event, the HANA instance calls the configured hooks, also called HA/DR providers. Hooks are custom Python scripts which process the events that HANA sends and the scripts can trigger different actions based on the event information.

You must add the HA/DR provider definition to the HANA global configuration to enable the required functionality of triggering additional actions for certain events.

HanaSR for the srConnectionChanged() hook method

The HanaSR hook is required for processing the srConnectionChanged() hook method. This method is used by the primary HANA instance for a notification of any change in the HANA system replication status. The primary HANA instance calls the HanaSR HA/DR provider when a HANA system replication related event occurs. The hook script HanaSR.py then parses srConnectionChanged() events for the system replication status detail and as a result it updates the srHook cluster attribute. This attribute is used by the resource agents to evaluate the landscape health and make decisions. The value of the system replication or sync state defines if the cluster recovers a failed primary instance on the same node or if it triggers a takeover to the secondary. The takeover is only triggered when the system replication is fully in sync, which means the HANA data is consistent between the HANA sites.

ChkSrv for the srServiceStateChanged() hook method

When the HANA instance detects an issue with a HANA indexserver process it recovers from the problem by stopping and restarting the hdbindexserver service automatically through an internal mechanism.

However, especially for very large HANA instances, the hdbindexserver service can take a very long time for the stopping phase of this recovery process. Although HANA reports this service degradation not as an error in the HANA landscape, the situation poses a risk to the data consistency if anything else fails in the instance during that time. To improve the unpredictable service recovery time, you can configure the ChkSrv hook to stop or kill the entire affected HANA instance instead.

In a setup with automatic failover enabled (PREFER_SITE_TAKEOVER=true), the instance stop leads to a takeover if the secondary node is in a healthy state. Otherwise, instance recovery happens locally, but the enforced local instance restart accelerates the process.

The HANA instance calls the ChkSrv hook when an event occurs. The hook script ChkSrv.py processes the srServiceStateChanged() hook method and executes actions based on the results of the filters it applies to event details. This way the ChkSrv.py hook script can distinguish a HANA hdbindexserver process that is being stopped and restarted by HANA after a failure from the same process being stopped as part of an intended instance shutdown. When the hook script determines that the event is caused by a failure it triggers the configured action.

The ChkSrv.py hook script has multiple options to define what happens when an indexserver failure event is detected: