locked
Mechanism behind High Availability Always On RRS feed

  • Question

  • Hello,

    Please explain the concept as to how the synchronization is achieved between primary and secondary replicas in Availability groups.

    Is it similar to taking transaction backups on primary and pushing them at secondary replica all the time. How will the manual activity impact the setup. For eg. If a user changes few tables and structure in one of the database then it will get replicated to secondary replica automatically?

    Best Regards,


    Vaibhav Antriwale

    Wednesday, March 1, 2017 8:58 AM

All replies

  • Yes, the principal is similar to log backup, but the "delivery mechanism" isn't log backup files, but rather a TCP session from the primary to the secondary which synchronously or asynchronously pushes log records to the secondary.

    Structural changes (like adding a column) is also logged in the transaction log so such will also be reflected o the secondary/secondaries.


    Tibor Karaszi, SQL Server MVP (Web Blog)

    Wednesday, March 1, 2017 9:15 AM
  • Tibor I believe the mechanism is not similar to log backup, log backup is what we do in transaction log shipping in mirroring and AOAG we have transaction log records *block* which are flushed to or sent to secondary it is never the transaction log backup. A more detailed diagram is given in This Blog

    Vaibhav,

    Yes changes when done on primary generate a log record which has information about changes made on current replica this log record block is pushed to secondary and hardened and then replayed to bring changes what is their on primary

    For mode details I suggest you read AlwaysON Learning Series


    Cheers,

    Shashank

    Please mark this reply as answer if it solved your issue or vote as helpful if it helped so that other forum members can benefit from it

    My TechNet Wiki Articles

    MVP

    • Proposed as answer by Teige Gao Sunday, March 12, 2017 1:48 AM
    Wednesday, March 1, 2017 10:36 AM
  • Shanky,

    That is why I said that the delivery mechanism differs. I.e., not a log backup, but rather a stream of log records. But the principals is similar. What you do is reflected in the log, and that information is *somehow" transferred to the replica to be applied there.

    Splitting hairs, I guess. :-)


    Tibor Karaszi, SQL Server MVP (Web Blog)

    Wednesday, March 1, 2017 10:42 AM
  • I found this explanation to be pretty helpful:
    How It Works: Always On - When Is My Secondary Failover Ready?

    Hope this helps,


    Phil Streiff, MCDBA, MCITP, MCSA

    • Proposed as answer by Teige Gao Sunday, March 12, 2017 1:48 AM
    Wednesday, March 1, 2017 4:42 PM