Configure SQL Server Always On Availability Group for high availability on Linux

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This article describes how to create a SQL Server Always On Availability Group (AG) for high availability on Linux. There are two configuration types for AGs. A high availability configuration uses a cluster manager to provide business continuity. This configuration can also include read-scale replicas. This document explains how to create the AG for high availability.

You can also create an AG without a cluster manager for read-scale. The AG for read scale only provides read-only replicas for performance scale-out. It does not provide high availability. To create an AG for read-scale, see Configure a SQL Server Availability Group for read-scale on Linux.

Configurations that guarantee high availability and data protection require either two or three synchronous commit replicas. With three synchronous replicas, the AG can automatically recover even if one server is not available. For more information, see High availability and data protection for Availability Group configurations.

All servers must be either physical or virtual, and virtual servers must be on the same virtualization platform. This requirement is because the fencing agents are platform specific. See Policies for Guest Clusters.

Roadmap

The steps to create an AG on Linux servers for high availability are different from the steps on a Windows Server failover cluster. The following list describes the high-level steps:

  1. Configure SQL Server on three cluster servers.

    Important

    All three servers in the AG need to be on the same platform - physical or virtual - because Linux high availability uses fencing agents to isolate resources on servers. The fencing agents are specific for each platform.

  2. Create the AG. This step is covered in this current article.

  3. Configure a cluster resource manager, like Pacemaker.

    The way to configure a cluster resource manager depends on the specific Linux distribution. See the following links for distribution specific instructions:

    Important

    Production environments require a fencing agent, like STONITH for high availability. The demonstrations in this documentation do not use fencing agents. The demonstrations are for testing and validation only.

    A Linux cluster uses fencing to return the cluster to a known state. The way to configure fencing depends on the distribution and the environment. Currently, fencing is not available in some cloud environments. For more information, see Support Policies for RHEL High Availability Clusters - Virtualization Platforms.

    For SLES, see SUSE Linux Enterprise High Availability Extension.

  4. Add the AG as a resource in the cluster.

    The way to add the AG as a resource in the cluster depends on the Linux distribution. See the following links for distribution specific instructions:

Prerequisites

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.

Note

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."

  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, 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
    

    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 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 AlwaysOn 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 0.0.0.0.

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;

Note

If you use SQL Server Express Edition on one node to host a configuration-only replica, the only valid value for ROLE is WITNESS. Run the following script on SQL Server Express Edition:

CREATE ENDPOINT [Hadr_endpoint]
    AS TCP (LISTENER_PORT = **<5022>**)
    FOR DATA_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.

Important

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

For more information, see The database mirroring endpoint (SQL Server).

Create the AG

For a high availability configuration that ensures automatic failover, the AG requires at least three replicas. Either of the following configurations can support high availability:

For information, see High availability and data protection for Availability Group configurations.

Note

The availability groups can include additional synchronous or asynchronous replicas.

Create the AG for high availability on Linux. Use the CREATE AVAILABILITY GROUP with CLUSTER_TYPE = EXTERNAL.

  • Availability group - CLUSTER_TYPE = EXTERNAL Specifies that an external cluster entity manages the AG. Pacemaker is an example of an external cluster entity. When the AG cluster type is external,

  • Set Primary and secondary replicas FAILOVER_MODE = EXTERNAL. Specifies that the replica interacts with an external cluster manager, like Pacemaker.

The following Transact-SQL scripts create an AG for high availability 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. Update the following script for your environment. Replace the <node1>, <node2>, or <node3> values with the names of the SQL Server instances that host the replicas. Replace the <5022> with the port you set for the data mirroring endpoint. To create the AG, run the following Transact-SQL on the SQL Server instance that hosts the primary replica.

Run only one of the following scripts:

  • Create AG with three synchronous replicas

    CREATE AVAILABILITY GROUP [ag1]
        WITH (DB_FAILOVER = ON, CLUSTER_TYPE = EXTERNAL)
        FOR REPLICA ON
            N'<node1>' 
             WITH (
                ENDPOINT_URL = N'tcp://<node1>:<5022>',
                AVAILABILITY_MODE = SYNCHRONOUS_COMMIT,
                FAILOVER_MODE = EXTERNAL,
                SEEDING_MODE = AUTOMATIC
                ),
            N'<node2>' 
             WITH ( 
                ENDPOINT_URL = N'tcp://<node2>:<5022>', 
                AVAILABILITY_MODE = SYNCHRONOUS_COMMIT,
                FAILOVER_MODE = EXTERNAL,
                SEEDING_MODE = AUTOMATIC
                ),
            N'<node3>'
            WITH( 
               ENDPOINT_URL = N'tcp://<node3>:<5022>', 
               AVAILABILITY_MODE = SYNCHRONOUS_COMMIT,
               FAILOVER_MODE = EXTERNAL,
               SEEDING_MODE = AUTOMATIC
               );
    
    ALTER AVAILABILITY GROUP [ag1] GRANT CREATE ANY DATABASE;
    

    Important

    After you run the preceding script to create an AG with three synchronous replicas, do not run the following script:

  • Create AG with two synchronous replicas and a configuration replica:

    Important

    This architecture allows any edition of SQL Server to host the third replica. For example, the third replica can be hosted on SQL Server Enterprise Edition. On Enterprise Edition, the only valid endpoint type is WITNESS.

    CREATE AVAILABILITY GROUP [ag1] 
       WITH (CLUSTER_TYPE = EXTERNAL) 
       FOR REPLICA ON 
        N'<node1>' WITH ( 
           ENDPOINT_URL = N'tcp://<node1>:<5022>', 
           AVAILABILITY_MODE = SYNCHRONOUS_COMMIT, 
           FAILOVER_MODE = EXTERNAL, 
           SEEDING_MODE = AUTOMATIC 
           ), 
        N'<node2>' WITH (  
           ENDPOINT_URL = N'tcp://<node2>:<5022>',  
           AVAILABILITY_MODE = SYNCHRONOUS_COMMIT, 
           FAILOVER_MODE = EXTERNAL, 
           SEEDING_MODE = AUTOMATIC 
           ), 
        N'<node3>' WITH ( 
           ENDPOINT_URL = N'tcp://<node3>:<5022>', 
           AVAILABILITY_MODE = CONFIGURATION_ONLY  
           );
    ALTER AVAILABILITY GROUP [ag1] GRANT CREATE ANY DATABASE;
    

  • Create AG with two synchronous replicas

    Include two replicas with synchronous availability mode. For example, the following script creates an AG called ag1. node1 and node2 host replicas in synchronous mode, with automatic seeding and automatic failover.

    Important

    Only run the following script to create an AG with two synchronous replicas. Do not run the following script if you ran either preceding script.

    CREATE AVAILABILITY GROUP [ag1]
       WITH (CLUSTER_TYPE = EXTERNAL)
       FOR REPLICA ON
       N'node1' WITH (
          ENDPOINT_URL = N'tcp://node1:5022',
          AVAILABILITY_MODE = SYNCHRONOUS_COMMIT,
          FAILOVER_MODE = EXTERNAL,
          SEEDING_MODE = AUTOMATIC
       ),
       N'node2' WITH ( 
          ENDPOINT_URL = N'tcp://node2:5022', 
          AVAILABILITY_MODE = SYNCHRONOUS_COMMIT,
          FAILOVER_MODE = EXTERNAL,
          SEEDING_MODE = AUTOMATIC
       );
    
    ALTER AVAILABILITY GROUP [ag1] GRANT CREATE ANY DATABASE;
    

You can also configure an AG with CLUSTER_TYPE=EXTERNAL using SQL Server Management Studio or PowerShell.

Join secondary replicas to the AG

The following Transact-SQL script joins a SQL Server instance to an AG named ag1. Update the script for your environment. On each SQL Server instance that hosts a secondary replica, run the following Transact-SQL to join the AG.

ALTER AVAILABILITY GROUP [ag1] JOIN WITH (CLUSTER_TYPE = EXTERNAL);

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 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 script 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 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;

Important

After you create the AG, you must configure integration with a cluster technology like Pacemaker for high availability. For a read-scale configuration using AGs, starting with SQL Server 2017 (14.x), setting up a cluster is not required.

If you followed the steps in this document, you have an AG that is not yet clustered. The next step is to add the cluster. This configuration is valid for read-scale/load balancing scenarios, it is not complete for high availability. For high availability, you need to add the AG as a cluster resource. See Next steps for instructions.

Notes

Important

After you configure the cluster and add the AG as a cluster resource, you cannot use Transact-SQL to fail over the AG resources. SQL Server cluster resources on Linux are not coupled as tightly with the operating system as they are on a Windows Server Failover Cluster (WSFC). SQL Server service is not aware of the presence of the cluster. All orchestration is done through the cluster management tools. In RHEL or Ubuntu use pcs. In SLES use crm.

Important

If the AG is a cluster resource, there is a known issue in current release where forced failover with data loss to an asynchronous replica does not work. This will be fixed in the upcoming release. Manual or automatic failover to a synchronous replica succeeds.

Next steps

Configure Red Hat Enterprise Linux Cluster for SQL Server Availability Group Cluster Resources

Configure SUSE Linux Enterprise Server Cluster for SQL Server Availability Group Cluster Resources

Configure Ubuntu Cluster for SQL Server Availability Group Cluster Resources