Configure a SQL Server Availability Group for read-scale on Linux
You can configure a SQL Server Always On Availability Group (AG) for read-scale workloads on Linux. There are two types of architectures for AGs. A architecture for high availability uses a cluster manager to provide improved business continuity. This architecture also can include read-scale replicas. To create the high-availability architecture, see Configure SQL Server Always On Availability Group for high availability on Linux. The other architecture supports only read-scale workloads. This article explains how to create an AG 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 hosted on different operating system platforms. It cannot support high availability.
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.
On Linux, you must create an availability group before you add it as a cluster resource to be managed by the cluster. This document provides an example that creates the availability group. For distribution-specific instructions to create the cluster and add the availability group as a cluster resource, see the links under "Next steps."
Update the computer name for each host.
Each SQL Server name must be:
- 15 characters or less.
- Unique within the network.
To set the computer name, edit
/etc/hostname. The following script lets you edit
sudo vi /etc/hostname
Configure the hosts file.
If hostnames are registered with their IP in the DNS server, you don't need to do the following steps. Validate that all the nodes intended to be part of the availability group configuration can communicate with each other. (A ping to the hostname should reply with the corresponding IP address.) Also, make sure that the /etc/hosts file doesn't contain a record that maps the localhost IP address 127.0.0.1 with the hostname of the node.
The hosts file on every server contains the IP addresses and names of all servers that will participate in the availability group.
The following command returns the IP address of the current server:
sudo ip addr show
/etc/hosts. The following script lets you edit
sudo vi /etc/hosts
The following example shows
/etc/hostson node1 with additions for node1, node2, and node3. In this document, node1 refers to the server that hosts the primary replica. And node2 and node3 refer to servers that host the secondary replicas.
127.0.0.1 localhost localhost4 localhost4.localdomain4 ::1 localhost localhost6 localhost6.localdomain6 10.128.18.12 node1 10.128.16.77 node2 10.128.15.33 node3
Install SQL Server
Install SQL Server. The following links point to SQL Server installation instructions for various distributions:
Enable AlwaysOn availability groups and restart mssql-server
Enable AlwaysOn availability groups on each node that hosts a SQL Server instance. Then restart
mssql-server. Run the following script:
sudo /opt/mssql/bin/mssql-conf set hadr.hadrenabled 1 sudo systemctl restart mssql-server
Enable an AlwaysOn_health event session
You can optionally enable AlwaysOn availability groups extended events to help with root-cause diagnosis when you troubleshoot an availability group. 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 XE session, see Always On extended events.
Create a certificate
The SQL Server service on Linux uses certificates to authenticate communication 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. Connect to the primary SQL Server instance. To create the certificate, run the following Transact-SQL script:
CREATE MASTER KEY ENCRYPTION BY PASSWORD = '**<Master_Key_Password>**'; CREATE CERTIFICATE dbm_certificate WITH SUBJECT = 'dbm'; BACKUP CERTIFICATE dbm_certificate TO FILE = '/var/opt/mssql/data/dbm_certificate.cer' WITH PRIVATE KEY ( FILE = '/var/opt/mssql/data/dbm_certificate.pvk', ENCRYPTION BY PASSWORD = '**<Private_Key_Password>**' );
At this point, your primary SQL Server replica has a certificate at
/var/opt/mssql/data/dbm_certificate.cer and a private key at
var/opt/mssql/data/dbm_certificate.pvk. Copy these two files to the same location on all servers that will host availability replicas. Use the mssql user, or give permission to the mssql user to access these files.
For example, on the source server, the following command copies the files to the target machine. Replace the
**<node2>** values with the names of the SQL Server instances that will host the replicas.
cd /var/opt/mssql/data scp dbm_certificate.* root@**<node2>**:/var/opt/mssql/data/
On each target server, give permission to the mssql user to access the certificate.
cd /var/opt/mssql/data chown mssql:mssql dbm_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. 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 servers:
CREATE MASTER KEY ENCRYPTION BY PASSWORD = '**<Master_Key_Password>**'; CREATE CERTIFICATE dbm_certificate FROM FILE = '/var/opt/mssql/data/dbm_certificate.cer' WITH PRIVATE KEY ( FILE = '/var/opt/mssql/data/dbm_certificate.pvk', DECRYPTION BY PASSWORD = '**<Private_Key_Password>**' );
Create the 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 certificate that you created. 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 DATABASE_MIRRORING ( ROLE = ALL, AUTHENTICATION = CERTIFICATE dbm_certificate, ENCRYPTION = REQUIRED ALGORITHM AES ); ALTER ENDPOINT [Hadr_endpoint] STATE = STARTED;
If you use SQL Server Express Edition on one node to host a configuration-only replica, the only valid value for
WITNESS. Run the following script on SQL Server Express Edition:
CREATE ENDPOINT [Hadr_endpoint] AS TCP (LISTENER_PORT = **<5022>**) FOR DATABASE_MIRRORING ( ROLE = WITNESS, AUTHENTICATION = CERTIFICATE dbm_certificate, ENCRYPTION = REQUIRED ALGORITHM AES ); ALTER ENDPOINT [Hadr_endpoint] STATE = STARTED;
The TCP port on the firewall must be open for the listener port.
For the SQL Server 2017 release, the only authentication method supported for the database mirroring endpoint is
WINDOWS option will be enabled in a future release.
For more information, see The database mirroring endpoint (SQL Server).
Create the AG
Create the AG. Set
CLUSTER_TYPE = NONE. In addition, set each replica with
FAILOVER_MODE = MANUAL. Client applications running analytics or reporting workloads can directly connect to the secondary databases. You also can 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 AG named
ag1. The script configures the AG 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 AG. 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 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 Servers to the AG
The following Transact-SQL script joins a server to an AG named
ag1. Update the script for your environment. On each secondary SQL Server replica, run the following Transact-SQL script to join the AG:
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 this is a test database or a newly created database, take a database backup. On the primary SQL Server, run the following Transact-SQL script to create and back up a database called
CREATE DATABASE [db1]; ALTER DATABASE [db1] SET RECOVERY FULL; BACKUP DATABASE [db1] TO DISK = N'/var/opt/mssql/data/db1.bak';
On the primary SQL Server replica, run the following Transact-SQL script to add a database called
db1 to an availability group called
ALTER AVAILABILITY GROUP [ag1] ADD DATABASE [db1];
Verify that the database is created on the secondary servers
On each secondary SQL Server replica, run the following query to see if the
db1 database was created and is synchronized:
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 AG 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. Specifically, create the AG with
CLUSTER_TYPE=WSFC (in Windows) or
CLUSTER_TYPE=EXTERNAL (in Linux). Then integrate with a cluster manager by using either Windows Server failover clustering on Windows or Pacemaker on Linux.
Connect to read-only secondary replicas
There are two ways to connect to read-only secondary replicas. Applications can connect directly to the SQL Server instance that hosts the secondary replica and query the databases. They also can use read-only routing, which requires a listener.
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 a typical availability group, 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:
- Manual failover without data loss
- Forced manual failover with data loss
Manual failover without data loss
Use this method when the primary replica is available, but you need to temporarily or permanently change which instance 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 current primary and target secondary replica
ALTER AVAILABILITY GROUP [AGRScale] 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 [AGRScale] 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.
This setting is not specific to failover and should be set based on the requirements of the environment.
Set the primary replica offline to prepare for the role change:
ALTER AVAILABILITY GROUP [AGRScale] OFFLINE
Promote the target secondary replica to primary.
ALTER AVAILABILITY GROUP AGRScale FORCE_FAILOVER_ALLOW_DATA_LOSS;
Update the role of the old primary to
SECONDARY, run the following command on the SQL Server instance that hosts the old primary replica:
ALTER AVAILABILITY GROUP [AGRScale] SET (ROLE = SECONDARY);
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.
Resume data movement, run the following command for every database in the availability group on the SQL Server instance that hosts the primary replica:
ALTER DATABASE [db1] SET HADR RESUME
Re-create any listener you created for read-scale purposes and that isn't managed by a cluster manager. If the original listener points to the old primary, drop it and re-create it to point to the new primary.
Forced manual failover with data loss
If the primary replica is not available and can't immediately be recovered, then you need to force a failover to the secondary replica with data loss. However, if the original primary replica recovers after failover, it will assume the primary role. To avoid having each replica be in a different state, remove the original primary from the availability group after a forced failover with data loss. Once the original primary comes back online, remove the availability group from it entirely.
To force a manual failover with data loss from primary replica N1 to secondary replica N2, follow these steps:
On the secondary replica (N2), initiate the forced failover:
ALTER AVAILABILITY GROUP [AGRScale] FORCE_FAILOVER_ALLOW_DATA_LOSS;
On the new primary replica (N2), remove the original primary (N1):
ALTER AVAILABILITY GROUP [AGRScale] REMOVE REPLICA ON N'N1';
Validate that all application traffic is pointed to the listener and/or the new primary replica.
If the original primary (N1) comes online, immediately take availability group AGRScale offline on the original primary (N1):
ALTER AVAILABILITY GROUP [AGRScale] OFFLINE
If there is data or unsynchronized changes, preserve this data via backups or other data replicating options that suit your business needs.
Next, remove the availability group from the original primary (N1):
DROP AVAILABILITY GROUP [AGRScale];
Drop the availability group database on original primary replica (N1):
USE [master] GO DROP DATABASE [AGDBRScale] GO
(Optional) If desired, you can now add N1 back as a new secondary replica to the availability group AGRScale.