Postgres instance manager

CloudNativePG does not rely on an external tool for failover management. It simply relies on the Kubernetes API server and a native key component called: the Postgres instance manager.

The instance manager takes care of the entire lifecycle of the PostgreSQL leading process (also known as postmaster).

When you create a new cluster, the operator makes a Pod per instance. The field .spec.instances specifies how many instances to create.

Each Pod will start the instance manager as the parent process (PID 1) for the main container, which in turn runs the PostgreSQL instance. During the lifetime of the Pod, the instance manager acts as a backend to handle the liveness and readiness probes.

Liveness and readiness probes

The liveness probe relies on pg_isready, while the readiness probe checks if the database is up and able to accept connections using the superuser credentials. The readiness probe is positive when the Pod is ready to accept traffic. The liveness probe controls when to restart the container.

The two probes will report a failure if the probe command fails 3 times with a 10 seconds interval between each check.

For now, the operator doesn't configure a startupProbe on the Pods, since startup probes have been introduced only in Kubernetes 1.17.

The liveness probe is used to detect if the PostgreSQL instance is in a broken state and needs to be restarted. The value in startDelay is used to delay the probe's execution, which is used to prevent an instance with a long startup time from being restarted.

The number of seconds after the Pod has started before the liveness probe starts working is expressed in the .spec.startDelay parameter, which defaults to 30 seconds. The correct value for your cluster is related to the time needed by PostgreSQL to start.

If .spec.startDelay is too low, the liveness probe will start working before the PostgreSQL startup, and the Pod could be restarted inappropriately.

Shutdown control

When a Pod running Postgres is deleted, either manually or by Kubernetes following a node drain operation, the kubelet will send a termination signal to the instance manager, and the instance manager will take care of shutting down PostgreSQL in an appropriate way. The .spec.stopDelay, expressed in seconds, is the amount of time given to PostgreSQL to shut down. The value defaults to 30 seconds.

The shutdown procedure is composed of two steps:

1. The instance manager requests a smart shut down, disallowing any new connection to PostgreSQL. This step will last for half of the time set in .spec.stopDelay.

2. If PostgreSQL is still up, the instance manager requests a fast shut down, terminating any existing connection and exiting promptly. If the instance is archiving and/or streaming WAL files, the process will wait for up to the remaining half of the time set in .spec.stopDelay to complete the operation and then forcibly shut down.

Shutdown of the primary during a switchover

During a switchover, the shutdown procedure is slightly different from the general case. Indeed, the operator requires the former primary to issue a fast shut down before the selected new primary can be promoted, in order to ensure that all the data are available on the new primary.

For this reason, the .spec.switchoverDelay, expressed in seconds, controls the time given to the former primary to shut down gracefully and archive all the WAL files. During this time frame, the primary instance does not accept connections. The value defaults is greater than one year in seconds, big enough to simulate an infinite delay and therefore preserve data durability.

Failover

In case of primary pod failure, the cluster will go into failover mode. Please refer to the "Failover" section for details.