Configure read-scale for an Always On availability group
You can configure a SQL Server Always On availability group for read-scale workloads on Windows. There are two types of architecture for availability groups:
- An architecture for high availability that uses a cluster manager to provide improved business continuity and that can include readable-secondary replicas. To create this high-availability architecture, see Create and configure availability groups on Windows.
- An architecture that supports only read-scale workloads.
This article explains how to create an availability group without a cluster manager for read-scale workloads. This architecture provides read-scale only. It doesn't provide high availability.
An availability group with
CLUSTER_TYPE = NONE can include replicas that are hosted on a variety of operating system platforms. It cannot support high availability. For the Linux operating system, see Configure a SQL Server availability group for read-scale on Linux.
Before you create the availability group, you need to:
- Set your environment so that all the servers that will host availability replicas can communicate.
- Install SQL Server. See Install SQL Server for details.
Enable Always On availability groups and restart mssql-server
The following command utilizes cmdlets from the sqlserver module that's published in the PowerShell Gallery. You can install this module by using the Install-Module command.
Enable Always On availability groups on each replica that hosts a SQL Server instance. Then restart the SQL Server service. Run the following command to enable and then restart the SQL Server services:
Enable-SqlAlwaysOn -ServerInstance <server\instance> -Force
Enable an AlwaysOn_health event session
To help with root-cause diagnosis when you troubleshoot an availability group, you can optionally enable an Always On availability groups extended events (XEvents) session. To do so, run the following command on each instance of SQL Server:
ALTER EVENT SESSION AlwaysOn_health ON SERVER WITH (STARTUP_STATE=ON); GO
For more information about this XEvents session, see Always On availability groups extended events.
Database-mirroring endpoint authentication
For synchronization to function properly, the replicas that are involved in the read-scale availability group need to authenticate over the endpoint. The two main scenarios that you can use for such authentication are covered in the next sections.
In an Active Directory environment where all secondary replicas are joined to the same domain, SQL Server can authenticate by utilizing the service account. You must explicitly create a login for the service account on each SQL Server instance:
CREATE LOGIN [<domain>\service account] FROM WINDOWS;
SQL login authentication
In environments where the secondary replicas might not be joined to an Active Directory domain, you must utilize SQL authentication. The following Transact-SQL script creates a login named
dbm_login and a user named
dbm_user. Update the script with a strong password. To create the database-mirroring endpoint user, run the following command on all SQL Server instances:
CREATE LOGIN dbm_login WITH PASSWORD = '**<1Sample_Strong_Password!@#>**'; CREATE USER dbm_user FOR LOGIN dbm_login;
If you utilize a secondary replica that requires authentication with SQL authentication, use a certificate for authenticating between the mirroring endpoints.
The following Transact-SQL script creates a master key and a certificate. It then backs up the certificate and secures the file with a private key. Update the script with strong passwords. Run the script on the primary SQL Server instance to create the certificate:
CREATE MASTER KEY ENCRYPTION BY PASSWORD = '**<Master_Key_Password>**'; CREATE CERTIFICATE dbm_certificate WITH SUBJECT = 'dbm'; BACKUP CERTIFICATE dbm_certificate TO FILE = 'C:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\DATA\dbm_certificate.cer' WITH PRIVATE KEY ( FILE = 'c:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\DATA\dbm_certificate.pvk', ENCRYPTION BY PASSWORD = '**<Private_Key_Password>**' );
At this point, your primary SQL Server replica has a certificate at
c:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\DATA\dbm_certificate.cer and a private key at
c:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\DATA\dbm_certificate.pvk. Copy these two files to the same location on all servers that will host availability replicas.
On each secondary replica, ensure that the service account for the SQL Server instance has permissions to access the certificate.
Create the certificate on secondary servers
The following Transact-SQL script creates a master key and a certificate from the backup that you created on the primary SQL Server replica. The command also authorizes users to access the certificate. Update the script with strong passwords. The decryption password is the same password that you used to create the .pvk file in a previous step. To create the certificate, run the following script on all secondary replicas:
CREATE MASTER KEY ENCRYPTION BY PASSWORD = '**<Master_Key_Password>**'; CREATE CERTIFICATE dbm_certificate AUTHORIZATION dbm_user FROM FILE = 'C:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\DATA\dbm_certificate.cer' WITH PRIVATE KEY ( FILE = 'c:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\DATA\dbm_certificate.pvk', DECRYPTION BY PASSWORD = '**<Private_Key_Password>**' );
Create database-mirroring endpoints on all replicas
Database-mirroring endpoints use the Transmission Control Protocol (TCP) to send and receive messages between the server instances that participate in database-mirroring sessions or host availability replicas. The database-mirroring endpoint listens on a unique TCP port number.
The following Transact-SQL script creates a listening endpoint named
Hadr_endpoint for the availability group. It starts the endpoint and gives connection permission to the service account or SQL login that you created in a previous step. Before you run the script, replace the values between
**< ... >**. Optionally you can include an IP address,
LISTENER_IP = (0.0.0.0). The listener IP address must be an IPv4 address. You can also use
Update the following Transact-SQL script for your environment on all SQL Server instances:
CREATE ENDPOINT [Hadr_endpoint] AS TCP (LISTENER_PORT = **<5022>**) FOR DATA_MIRRORING ( ROLE = ALL, AUTHENTICATION = CERTIFICATE dbm_certificate, ENCRYPTION = REQUIRED ALGORITHM AES ); ALTER ENDPOINT [Hadr_endpoint] STATE = STARTED; GRANT CONNECT ON ENDPOINT::[Hadr_endpoint] TO [<service account or user>];
The TCP port on the firewall must be open for the listener port.
For more information, see The database-mirroring endpoint (SQL Server).
Create an availability group
Create an availability group. Set
CLUSTER_TYPE = NONE. In addition, set each replica with
FAILOVER_MODE = NONE. Client applications that run analytics or reporting workloads can directly connect to the secondary databases. You can also create a read-only routing list. Connections to the primary replica forward read connection requests to each of the secondary replicas from the routing list in a round-robin fashion.
The following Transact-SQL script creates an availability group named
ag1. The script configures the availability group replicas with
SEEDING_MODE = AUTOMATIC. This setting causes SQL Server to automatically create the database on each secondary server after it is added to the availability group.
Update the following script for your environment. Replace the
<node2> values with the names of the SQL Server instances that host the replicas. Replace the
<5022> value with the port that you set for the endpoint. Run the following Transact-SQL script on the primary SQL Server replica:
CREATE AVAILABILITY GROUP [ag1] WITH (CLUSTER_TYPE = NONE) FOR REPLICA ON N'<node1>' WITH ( ENDPOINT_URL = N'tcp://<node1>:<5022>', AVAILABILITY_MODE = ASYNCHRONOUS_COMMIT, FAILOVER_MODE = MANUAL, SEEDING_MODE = AUTOMATIC, SECONDARY_ROLE (ALLOW_CONNECTIONS = ALL) ), N'<node2>' WITH ( ENDPOINT_URL = N'tcp://<node2>:<5022>', AVAILABILITY_MODE = ASYNCHRONOUS_COMMIT, FAILOVER_MODE = MANUAL, SEEDING_MODE = AUTOMATIC, SECONDARY_ROLE (ALLOW_CONNECTIONS = ALL) ); ALTER AVAILABILITY GROUP [ag1] GRANT CREATE ANY DATABASE;
Join secondary SQL Server instances to the availability group
The following Transact-SQL script joins a server to an availability group named
ag1. Update the script for your environment. To join the availability group, run the following Transact-SQL script on each secondary SQL Server replica:
ALTER AVAILABILITY GROUP [ag1] JOIN WITH (CLUSTER_TYPE = NONE); ALTER AVAILABILITY GROUP [ag1] GRANT CREATE ANY DATABASE;
Add a database to the availability group
Ensure that the database you add to the availability group is in full recovery mode and has a valid log backup. If the database is a test database or a newly created database, take a database backup. To create and back up a database called
db1, run the following Transact-SQL script on the primary SQL Server instance:
CREATE DATABASE [db1]; ALTER DATABASE [db1] SET RECOVERY FULL; BACKUP DATABASE [db1] TO DISK = N'c:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\Backup\db1.bak';
To add a database called
db1 to an availability group called
ag1, run the following Transact-SQL script on the primary SQL Server replica:
ALTER AVAILABILITY GROUP [ag1] ADD DATABASE [db1];
Verify that the database is created on the secondary servers
To see whether the
db1 database was created and is synchronized, run the following query on each secondary SQL Server replica:
SELECT * FROM sys.databases WHERE name = 'db1'; GO SELECT DB_NAME(database_id) AS 'database', synchronization_state_desc FROM sys.dm_hadr_database_replica_states;
This availability group isn't a high-availability configuration. If you need high availability, follow the instructions at Configure an Always On availability group for SQL Server on Linux or Creation and Configuration of availability groups on Windows.
Connect to read-only secondary replicas
You can connect to read-only secondary replicas in either of two ways:
- Applications can connect directly to the SQL Server instance that hosts the secondary replica and query the databases. For more information, see Readable secondary replicas.
- Applications can also use read-only routing, which requires a listener. For more information, see Read-only routing.
Fail over the primary replica on a read-scale availability group
Each availability group has only one primary replica. The primary replica allows reads and writes. To change which replica is primary, you can fail over. In an availability group for high availability, the cluster manager automates the failover process. In an availability group with cluster type NONE, the failover process is manual.
There are two ways to fail over the primary replica in an availability group with cluster type NONE:
- Forced manual failover with data loss
- Manual failover without data loss
Forced manual failover with data loss
Use this method when the primary replica isn't available and can't be recovered.
To force failover with data loss, connect to the SQL Server instance that hosts the target secondary replica and then run the following command:
ALTER AVAILABILITY GROUP [ag1] FORCE_FAILOVER_ALLOW_DATA_LOSS;
When the previous primary replica recovers, it will also assume the primary role. To ensure that the previous primary replica transitions into a secondary role run the following command on the previous primary replica.
ALTER AVAILABILITY GROUP [ag1] SET (ROLE = SECONDARY);
Manual failover without data loss
Use this method when the primary replica is available, but you need to temporarily or permanently change the configuration and change the SQL Server instance that hosts the primary replica. To avoid potential data loss, before you issue the manual failover, ensure that the target secondary replica is up to date.
To manually fail over without data loss:
Make the target secondary replica
ALTER AVAILABILITY GROUP [ag1] MODIFY REPLICA ON N'<node2>' WITH (AVAILABILITY_MODE = SYNCHRONOUS_COMMIT);
To identify that active transactions are committed to the primary replica and at least one synchronous secondary replica, run the following query:
SELECT ag.name, drs.database_id, drs.group_id, drs.replica_id, drs.synchronization_state_desc, ag.sequence_number FROM sys.dm_hadr_database_replica_states drs, sys.availability_groups ag WHERE drs.group_id = ag.group_id;
The secondary replica is synchronized when
The following script sets
REQUIRED_SYNCHRONIZED_SECONDARIES_TO_COMMITto 1 on an availability group named
ag1. Before you run the following script, replace
ag1with the name of your availability group:
ALTER AVAILABILITY GROUP [ag1] SET (REQUIRED_SYNCHRONIZED_SECONDARIES_TO_COMMIT = 1);
This setting ensures that every active transaction is committed to the primary replica and at least one synchronous secondary replica.
Demote the primary replica to a secondary replica. After the primary replica is demoted, it's read-only. To update the role to
SECONDARY, run the following command on the SQL Server instance that hosts the primary replica:
ALTER AVAILABILITY GROUP [ag1] SET (ROLE = SECONDARY);
Promote the target secondary replica to primary.
ALTER AVAILABILITY GROUP ag1 FORCE_FAILOVER_ALLOW_DATA_LOSS;
To delete an availability group, use DROP AVAILABILITY GROUP. For an availability group that's created with cluster type NONE or EXTERNAL, execute the command on all replicas that are part of the availability group.