Availability modes

In Always On availability groups, the

availability mode

is a replica property that determines

whether a given availability replica can run in synchronous-commit mode. For each availability

replica, the availability mode must be configured for either synchronous-commit mode,

asynchronous-commit, or configuration only mode.

If the primary replica is configured for

asynchronous-commit mode

, it doesn’t wait for any

secondary replica to write incoming transaction log records to disk (to

harden the log

).

If a given secondary replica is configured for asynchronous-commit mode, the primary replica

doesn’t wait for that secondary replica to harden the log. If both the primary replica and a

given secondary replica are both configured for

synchronous-commit mode

, the primary replica

waits for the secondary replica to confirm that it has hardened the log (unless the secondary

replica fails to ping the primary replica within the primary’s

session-timeout period

).

Always On availability groups supports three availability modes:

Asynchronous-commit mode

Synchronous-commit mode

Configuration only mode

Asynchronous-commit mode

is a disaster-recovery solution that works well when the availability

replicas are distributed over considerable distances. If every secondary replica is running under

asynchronous-commit mode, the primary replica doesn’t wait for any of the secondary replicas

to harden the log. Rather, immediately after writing the log record to the local log file, the

７

Note

If a synchronous-commit secondary replica exceeds the primary replica’s session-timeout

period (default is

), the primary replica temporarily marks the synchronization

state of every database on this secondary replica as

and the replica

state as. When the secondary replica reconnects with the primary replica,

they resume synchronous-commit mode.

NOT SYNCHRONIZING
NOT_HEALTHY