Availability modes

There are two availability modes, synchronous commit and asynchronous commit. Selecting a mode is equivalent to selecting whether you want to favor data protection or transaction performance. Both availability modes follow the same work flow, with one small yet critical difference.

With synchronous commit mode, the application does not receive confirmation that the transaction committed until after the log records are hardened (step 5) on all synchronous secondary replicas. This is how AGs can guarantee zero data loss. Any transactions which were not hardened before the primary failed would be rolled back and an appropriate error would be bubbled up to the application for it to alert the user or perform its own error handling.

With asynchronous commit mode, the application receives confirmation that the transaction committed after the last log record is flushed (step 1) to the primary replica’s log file. This improves performance because the application does not have to wait for the log records to be transmitted but it opens up the AG to the potential of data loss. If the primary replica fails before the secondary replicas harden the log records, then the application will believe a transaction was committed but a failover would result in the loss of that data.

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 does not wait for any of the secondary replicas to harden the log. Rather, immediately after writing the log record to the local log file, the primary replica sends the transaction confirmation to the client. The primary replica runs with minimum transaction latency in relation to a secondary replica that is configured for asynchronous-commit mode. If the current primary is configured for asynchronous commit availability mode, it will commit transactions asynchronously for all secondary replicas regardless of their individual availability mode settings.

Synchronous-commit mode emphasizes high availability over performance, at the cost of increased transaction latency. Under synchronous-commit mode, transactions wait to send the transaction confirmation to the client until the secondary replica has hardened the log to disk. When data synchronization begins on a secondary database, the secondary replica begins applying incoming log records from the corresponding primary database. As soon as every log record has been hardened, the secondary database enters the SYNCHRONIZED state. Thereafter, every new transaction is hardened by the secondary replica before the log record is written to the local log file. When all the secondary databases of a given secondary replica are synchronized, synchronous-commit mode supports manual failover and, optionally, automatic failover.

How Synchronization Works on a Secondary Replica

Under the synchronous-commit mode, after a secondary replica joins the availability group and establishes a session with the primary replica, the secondary replica writes incoming log records to disk (hardens the log) and sends a confirmation message to the primary replica. Once the hardened log on the secondary database has caught up the end of log on the primary database, the state of the secondary database is set to SYNCHRONIZED. The time required for synchronization depends essentially on how far the secondary database was behind the primary database at the start of the session (measured by the number of log records initially received from the primary replica), the work load on the primary database, and the speed of the computer of the server instance that hosts the secondary replica.

Synchronous operation is maintained in the following manner:

  1. On receiving a transaction from a client, the primary replica writes the log for the transaction to the transaction log and concurrently sends the log record to the secondary replicas.
  2. Once a log record is written to the transaction log of the primary database, the transaction can be undone only if there is a failover at this point to a secondary that did not receive the log. The primary replica waits for confirmation from the synchronous-commit secondary replica.
  3. The secondary replica hardens the log and returns an acknowledgement to the primary replica.
  4. On receiving the confirmation from the secondary replica, the primary replica finishes the commit processing and sends a confirmation message to the client.


If a synchronous-commit secondary replica times out without confirming that it has hardened the log, the primary marks that secondary replica as failed. The connected state of the secondary replica changes to DISCONNECTED, and the primary replica stops waiting for confirmation from the secondary replica. This behavior ensures that a failed synchronous-commit secondary replica does not prevent hardening of the transaction log on the primary replica.

Synchronous-commit mode protects your data by requiring the data to be synchronized between two places, at the cost of somewhat increasing the latency of the transaction.



Last modified: April 8, 2021



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