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

  • Article

Applies to: SQL Server - Linux

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 doesn't 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 isn't 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. Installation guidance for SQL Server on Linux.

    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 for high availability. The examples in this article don't use fencing agents. They are for testing and validation only.

    A Pacemaker 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 isn't 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:

Considerations for multiple Network Interfaces (NICs)

For information on setting up an availability group for servers with multiple NICs, see the relevant sections for:

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.

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.

  1. Update the computer name for each host.

    Each SQL Server instance name must be:

    • 15 characters or fewer.
    • 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 address 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 sample, 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 Always On availability groups

Enable Always On availability groups for each node that hosts a SQL Server instance, and 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 Extended Events (XE) 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 Configure Extended Events for availability groups.

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>'
);

In the previous example, replace <private-key-password> with the same password you used when creating the certificate on the primary replica.

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 DATABASE_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 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.

Important

For SQL Server 2017 (14.x), the only authentication method supported for the database mirroring endpoint is CERTIFICATE. The WINDOWS option isn't available.

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

Create the AG

The examples in this section explain how to create the availability group using Transact-SQL. You can also use the SQL Server Management Studio Availability Group Wizard. When you create an AG with the wizard, it returns an error when you join the replicas to the AG. To fix this, grant ALTER, CONTROL, and VIEW DEFINITIONS to the pacemaker on the AG on all replicas. Once permissions are granted on the primary replica, join the nodes to the AG through the wizard, but for HA to function properly, grant permission on all replicas.

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.

Important

In the current implementation of the SQL Server resource agent, the node name must match the ServerName property from your instance. For example, if your node name is node1, make sure SERVERPROPERTY('ServerName') returns node1 in your SQL Server instance. If there's a mismatch, your replicas will go into a resolving state after the pacemaker resource is created.

A scenario where this rule is important is when using fully qualified domain names. For example, if you use node1.yourdomain.com as the node name during cluster setup, make sure SERVERPROPERTY('ServerName') returns node1.yourdomain.com, and not just node1. The possible workarounds for this problem are:

  • Rename your host name to the FQDN and use sp_dropserver and sp_addserver store procedures to ensure the metadata in SQL Server matches the change.
  • Use the addr option in the pcs cluster auth command to match the node name to the SERVERPROPERTY('ServerName') value and use a static IP as the node address.

Run only one of the following scripts:

Create availability group with three synchronous replicas

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, don't run the following script:

Create availability group with two synchronous replicas and a configuration replica

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 Express Edition. On Express 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 availability group with two synchronous replicas

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. Don't 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 Pacemaker user requires ALTER, CONTROL, and VIEW DEFINITION permissions on the availability group on all replicas. To grant permissions, run the following Transact-SQL script after the availability group is created on the primary replica and each secondary replica immediately after they are added to the availability group. Before you run the script, replace <pacemakerLogin> with the name of the Pacemaker user account. If you don't have a login for Pacemaker, create a sql server login for Pacemaker.

GRANT ALTER, CONTROL, VIEW DEFINITION ON AVAILABILITY GROUP::ag1 TO <pacemakerLogin>
GRANT VIEW SERVER STATE TO <pacemakerLogin>

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 the full recovery model and has a valid log backup. If your database is a test database or a newly created database, take a database backup. On the primary SQL Server, run the following Transact-SQL (T-SQL) script to create and back up a database called db1:

CREATE DATABASE [db1];
GO

ALTER DATABASE [db1]
    SET RECOVERY FULL;
GO

BACKUP DATABASE [db1]
    TO DISK = N'/var/opt/mssql/data/db1.bak';

On the primary SQL Server replica, run the following T-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;
GO

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 isn't required.

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

Remarks

Important

After you configure the cluster and add the AG as a cluster resource, you can't use Transact-SQL to fail over the AG resources. SQL Server cluster resources on Linux aren't coupled as tightly with the operating system as they are on a Windows Server Failover Cluster (WSFC). SQL Server service isn't 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's a known issue in current release where forced failover with data loss to an asynchronous replica doesn't work. This will be fixed in the upcoming release. Manual or automatic failover to a synchronous replica succeeds.

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