TLS switchover: Ensure Sentinel restarts only one unit at a time

[reneradoi]

For enabling or disabling client TLS, it is possible to reload the TLS configuration in Valkey without restarting. For Sentinel, this is not the case, it has to be restarted to load an updated TLS configuration (and currently also TLS files). To avoid service unavailability due to slow restarts of the Sentinel service, we have to ensure that only one Sentinel restarts at any given time.

For more information, see the discussion in the following link: #18 (comment)