To check the availability of the database, the application executes the spCheckDbAvailability stored procedure. This starts a transaction, inserts a row into the AvailabilityCheck table, flushes the data to the transaction log to ensure that the write is persisted to disk even if delayed durability is enabled, explicitly reads the inserted row, and then rolls back the transaction, to avoid accumulating unnecessary synthetic transaction data in the database. The database is available if the stored procedure completes successfully, and returns a single row with the value 1 in the single column.
Note that an execution of sp_flush_log procedure is scoped to the entire database. Executing this stored procedure will flush log buffers for all sessions that are currently writing to the database and have uncommitted transactions, or are running with delayed durability enabled and have committed transactions not yet flushed to storage. The assumption here is that the availability check is executed relatively infrequently, e.g. every 30-60 seconds, therefore the potential performance impact from an occasional extra log flush is minimal.
This ends up being much more useful than a simple “is the service on?” heartbeat, as it shows that the database is available, not just that the engine is running.