I have a distributed SQL Server (Always On) High Availability Group, using SQL Server 2016, with the purpose of disaster recovery. The servers exist in different datacenters, and the primary is doing async commits to the Distributed AG. I want to test a failover to the DAG without disrupting the traffic flow to the primary.
My question is - If I execute the ALTER AVAILABILITY GROUP [DAG_NAME_HERE] FORCE_FAILOVER_ALLOW_DATA_LOSS; command on the DAG, will both servers now be able to handle read/writes, or will the original primary become unavailable, and only the DAG (which is now the primary) can handle read/writes.
Only the secondary (which is not the primary) can do read/writes. The others are suspended.
From MS (https://learn.microsoft.com/en-us/sql/database-engine/availability-groups/windows/perform-a-forced-manual-failover-of-an-availability-group-sql-server?view=sql-server-ver15):
The secondary databases in the remaining secondary replicas are
suspended and must be manually resumed.
Confirmed that running the ALTER AVAILABILITY GROUP [DAG_NAME_HERE] FORCE_FAILOVER_ALLOW_DATA_LOSS; SQL Command against the secondary in a Distributed Availability Group configuration, both SQL Servers essentially become a primary that can handle read/writes, but synchronization between the 2 AG's is halted.
Related
What are the main differences between sql-server replication and availability groups in an Azure Iaas data solution? (sql server instances on a VM)
Are these the same concept?
SQL Server Replication copies data and replays changes using SQL Commands to replicas. Sometimes this is called "Logical Replication". Replication is typically not used for High-Availibility or Disaster Recovery, but can be used to create a readable replica of a production database or to copy selected data from a main database into one or more subscribers.
AlwaysOn Availibility Groups copies and applies the log records to syncronize databases. The changes are applied using the same transaction log redo process that's used in backup/restore and database recovery. Sometimes this is called "Physical Replication", as the database files on every replica are identical. AlwaysOn Availibility Groups can be used for High-Availitbiiy, Disaster Recovery, and to create Readable Replicas of a production database.
I have a cluster of databases, one primary and two secondary. I need to enable CDC on a database, but I want to enable it on one of the secondary databases to eliminate any resource consumption on the primary database (similar to SQL Server secondary database backup). Is this possible to do it and how? If not: can you tell me the best practices for enabling CDC on cluster?
I want to enable it on one of the secondary databases to eliminate any resource consumption on the primary database
This is not possible. CDC writes the changes back to system tables in the target database, so must run against the primary replica. See Replication, change tracking, & change data capture - Always On availability groups
I have two bases in a group on SQL Server.
The first one is primary and the second is the replica.
I want to connect to the replica (from a simple C # application) if the primary one falls.
How can I do that? Thank you!
If your databases are in an Availability Group (AG), you should point your application to the Listener for the AG. This is a network name that will always point to the primary server for the AG.
SQL Server 2008 Enterprise and later versions support backup compression. When creating a log shipping configuration, you can control the backup compression behavior of log backups and if you already have a Replica then you can configure it When an availability failover occurs, existing persistent connections to the availability are terminated and the client must establish a new connection in order to continue working with the same primary database or read-only secondary database. For more information.
Configure Log Shipping
Secondary Database Settings
Application Failover
I tried many things and analyzed lots of documents but I haven't found a solution yet.
I have three virtual machines in VmWare called (DC,SQLServer01,SQLServer02). All of SQL Servers are member of a domain.(DC) I installed failover cluster for SQLServer01 and SQLServer02. I did necessary configurations in SQLServer01. Then I installed SQL Server 2014 for both servers. Now, I created an alwaysOn group. SQLServer01 is a primary and other is secondary. When I cut the connection of SQLServer01, everything is fine (Secondary becomes primary). It is acceptable for other condition.
However, when all servers are online, I can not do any operation (insert,update,delete,alter ,etc) except read operations in my secondary replica. I see always "database is read only" error. In properties of Alwayson group, both primary and secondary replica have all connections and secondary readable is "YES".
I want to make CRUD operations even if all servers are online. (I mean, do everything also for secondary replica. )
So, do you have any suggestion or idea?
Thank your time and consideration.
The error occurs because writing to secondary replicas in sql server is not possible. Only primary replica can host read-write databases, and an availability group can only have one single primary replica. Secondary replicas can host read-only databases only. When both replicas are available, only one of the two can be the primary and therefore support read-write. When only a single replica is available, that replica becomes primary replica because there are no other replicas, and read-write operations against that replica is possible.
What you need to configure instead is replication.
In SQL Server, merge replication allows you to write at multiple nodes, with periodic synchronization that resolves conflicts and pushes changes to all replicas.
Peer to Peer replication is another solution. Application layer must not allow conflicts (update of same row at more than one node), but is much faster.
I would like to know the differences between Failover Clustering and AlwaysOn Availability Groups in SQL Server 2012
AlwaysOn FailOver Clustering (FCI) is associated with Windows Services FailOver Clustering (WSFC). This is a High Availability (HA) scenario where two (or more) servers share a SAN, or WAN, or NAS. The first server is the Active Node (Node1). The second server is the Passive Node (Node2). A 3rd server called the Witness or Quorum server, does a heartbeat check on both servers on a set interval, usually between 1 to 3 seconds. If either of the servers doesn't answer the heartbeat ping, the other server is declared the primary node. If that is Node1, then no changes occur. If that is Node2, then the Witness server automatically fails-over the control of the databases to that node. Failover usually is withing 15 seconds. Any uncommitted transactions are automatically rolled-back.
AlwaysOn Availability Groups is an improvement on Database Mirroring. Database mirroring is for single databases, and also has two or more nodes. In this scenario, each of the nodes has it's own storage. The mirroring mechanism will either synchronously or Asynchronously ship transactions to the mirrored node. Synchronous mode would wait for the validation signal from the mirror before committing the transaction in the primary. Asynchronous mode would ship off the transactions and go straight to a local commit. AlwaysOn Availability Groups improves upon that concept by grouping databases together in Availability Groups. When one database fails-over, they all do. You must have AlwaysOn FCI configured to utilize AlwaysOn Availability Groups, as the have some common libraries.
references:
Adding Always-on Availability Groups to Existing Failover Clusters
Creating an SQL Server w/ Always-on Availability Group
Always-on Availability Groups vs Always-on Failover Clusters