Configure read scale-out availability group for SQL Server on Linux

You can configure a read scale-out availability group for SQL Server on Linux. There are two architectures for availability groups. A high availability architecture uses a cluster manager to provide improved business continuity. This architecture can also include read scale-out replicas. To create the high availability architecture, see Configure Always On availability group for SQL Server on Linux.

This document explains how to create a read scale-out availability group without a cluster manager. This architecture only provides read scale-out only. It does not provide high availability.

Prerequisites

Before you create the availability group, you need to:

  • Set your environment so all servers that will host availability replicas can communicate
  • Install SQL Server
Note

On Linux, you must create an availability group before adding 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.

  1. 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 /etc/hostname with vi.

    sudo vi /etc/hostname
    
  2. Configure the hosts file

Note

If hostnames are registered with their IP in the DNS server, there is no need to do the steps below. Validate that all nodes that are going to be part of the availability group configuration can communicate with each other (pinging the hostname should reply with the corresponding IP address). Also, make sure that /etc/hosts file does not contain a record that maps 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

Update /etc/hosts. The following script lets you edit /etc/hosts with vi.

sudo vi /etc/hosts

The following example shows /etc/hosts on node1 with additions for node1, node2, and node3. In this document node1 refers to the server hosting the primary replica. node2, and node3 refer to servers hosting 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 Always On availability groups and restart sqlserver

Enable Always On availability groups on each node hosting 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 AlwaysOn_health event session

You can optionally enable Always On availability groups extended events to help with root-cause diagnosis when you troubleshoot an availability group. Run the following command on each 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 db mirroring endpoint user

The following Transact-SQL script creates a login named dbm_login, and a user named dbm_user. Update the script with a strong password. Run the following command on all SQL Server instances to create the database mirroring endpoint user.

CREATE LOGIN dbm_login WITH PASSWORD = '**<1Sample_Strong_Password!@#>**';
CREATE USER dbm_user FOR LOGIN dbm_login;

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 certificate. It then backs the certificate up and secures the file with a private key. Update the script with strong passwords. Connect to the primary SQL Server instance and run the following Transact-SQL 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 = '/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 mssql user to access these files.

For example on the source server, the following command copies the files to the target machine. Replace the 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 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 certificate from the backup that you created on the primary SQL Server replica. The command also authorizes the user 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. Run the following script on all secondary servers to create the certificate.

CREATE MASTER KEY ENCRYPTION BY PASSWORD = '**<Master_Key_Password>**';
CREATE CERTIFICATE dbm_certificate   
    AUTHORIZATION dbm_user
    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 Transmission Control Protocol (TCP) to send and receive messages between the server instances participating database mirroring sessions or hosting availability replicas. The database mirroring endpoint listens on a unique TCP port number.

The following Transact-SQL creates a listening endpoint named Hadr_endpoint for the availability group. It starts the endpoint, and gives connect permission to the user that you created. Before you run the script, replace the values between **< ... >**.

Note

For this release, do not use a different IP address for the listener IP. We are working on a fix for this issue, but the only acceptable value for now is '0.0.0.0'.

Update the following Transact-SQL for your environment on all SQL Server instances:

CREATE ENDPOINT [Hadr_endpoint]
    AS TCP (LISTENER_IP = (0.0.0.0), 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 [dbm_login];
Important

The TCP port on the firewall needs to be open for the listener port.

Important

For SQL Server 2017 release, the only authentication method supported for database mirroring endpoint is CERTIFICATE. WINDOWS option will be enabled in a future release.

For complete information, see The Database Mirroring Endpoint (SQL Server).

Create the availability group

Create the availability group. Set CLUSTER_TYPE = NONE. In addition, set each replica with FAILOVER_MODE = NONE. Client applications running 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 name 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 **<node1>** and **<node2>** values with the names of the SQL Server instances that host the replicas. Replace the **<5022>** with the port you set for the endpoint. Run the following Transact-SQL 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 availability group

The following Transact-SQL script joins a server to an availability group named ag1. Update the script for your environment. On each secondary SQL Server replica, run the following Transact-SQL to join the availability group.

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 the database you are adding to the Availability group is in full recovery mode and has a valid log backup. If this is a test database or a new database created, take a database backup. On the primary SQL Server, run the following Transact-SQL to create and back up a database called db1.

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 to add a database called db1 to an availability group called ag1.

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 has been 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 is not an high availability configuration, if you need high availability, follow the instructions at Configure Always On Availability Group for SQL Server on Linux. Specifically, create the availability group with CLUSTER_TYPE=WSFC (in Windows) or CLUSTER_TYPE=EXTERNAL (in Linux) and integrate with a cluster manager - either WSFC on Windows or Pacemaker on Linux.

Connect to read-only secondary replicas

There are two ways to connect to the read-only secondary replicas. Applications can connect directly to the SQL Server instance that hosts the secondary replica and query the databases, or they can use read-only routing. read-only routing requires a listener.

Readable secondary replicas

read-only routing

Fail over primary replica on read scale-out availability group

Each availability group has only one primary replica. The primary replica allows reads and writes. To change which replica is the primary, you can fail over. In an availability group for high availability, the cluster manager automates in the failover process. In a read scale-out availability group, the failover process is manual. There are two ways to fail over the primary replica in a read scale availability group.

  • Forced manual fail over with data loss

  • Manual fail over without data loss

Forced fail over with data loss

Use this method when the primary replica is not available and can not be recovered. You can find more information about forced failover with data loss at Perform a Forced Manual Failover.

To force fail over with data loss, connect to the SQL instance hosting the target secondary replica and run:

ALTER AVAILABILITY GROUP [ag1] FORCE_FAILOVER_ALLOW_DATA_LOSS;

Manual fail over 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. Before issuing manual failing over, ensure that the target secondary replica is up to date, so that there is no potential data loss.

The following steps describe how to manually fail over without data loss:

  1. Make the target secondary replica synchronous commit.

    ALTER AVAILABILITY GROUP [ag1] MODIFY REPLICA ON N'**<node2>*' WITH (AVAILABILITY_MODE = SYNCHRONOUS_COMMIT);
    
  2. Update required_synchronized_secondaries_to_committo 1.

    This setting ensures that every active transaction is committed to the primary replica and at least one synchronous secondary. The availability group is ready to fail over when the synchronization_state_desc is SYNCHRONIZED and the sequence_number is the same for both primary and target secondary replica. Run this query to check:

    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; 
    
  3. Demote the primary replica to secondary replica. After the primary replica is demoted, it is read-only. Run this command on the SQL instance hosting the primary replica to update the role to SECONDARY:

    ALTER AVAILABILITY GROUP [ag1] SET (ROLE = SECONDARY); 
    
  4. Promote the target secondary replica to primary.

    ALTER AVAILABILITY GROUP distributedag FORCE_FAILOVER_ALLOW_DATA_LOSS; 
    
    Note

    To delete an availability group use DROP AVAILABILITY GROUP. For an availability group created with CLUSTER_TYPE NONE or EXTERNAL, the command has to be executed on all replicas part of the availability group.

Next steps

Configure distributed availability group

Learn more about availability groups