Configure Always On Availability Group for SQL Server on Linux

An availability group supports a failover environment for a discrete set of user databases - known as availability databases - that fail over together. An availability group supports one set of read-write primary databases and one to eight sets of corresponding secondary databases. Optionally, secondary databases can be made available for read-only access and/or some backup operations. An availability group defines a set of two or more failover partners, known as availability replicas. Availability replicas are components of the availability group. For details see Overview of Always On Availability Groups (SQL Server).

This document describes the specifics of availability groups on SQL Server on Linux. The document uses the following specific terms:

availability group
A container for a set of databases, availability databases, that fail over together.

availability database
A database that belongs to an availability group. For each availability database, the availability group maintains a single read-write copy (the primary database) and one to eight read-only copies (secondary databases).

primary database
The read-write copy of an availability database.

secondary database
A read-only copy of an availability database.

availability replica
An instantiation of an availability group that is hosted by a specific instance of SQL Server and maintains a local copy of each availability database that belongs to the availability group. Two types of availability replicas exist: a single primary replica and one to eight secondary replicas.

primary replica
The availability replica that makes the primary databases available for read-write connections from clients and, also, sends transaction log records for each primary database to every secondary replica.

secondary replica
An availability replica that maintains a secondary copy of each availability database, and serves as a potential failover targets for the availability group. Optionally, a secondary replica can support read-only access to secondary databases can support creating backups on secondary databases.

availability group listener
A server name to which clients can connect in order to access a database in a primary or secondary replica of an Always On availability group. Availability group listeners direct incoming connections to the primary replica or to a read-only secondary replica.

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 and node2. In this document node1 refers to the primary SQL Server replica. node2 refers to the secondary SQL Server.;

   127.0.0.1   localhost localhost4 localhost4.localdomain4
   ::1       localhost localhost6 localhost6.localdomain6
   10.128.18.128 node1
   10.128.16.77 node2

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 SQL Server service, then restart mssql-server. Run the following script:

sudo /opt/mssql/bin/mssql-conf set hadrenabled 1
sudo systemctl restart mssql-server

Enable AlwaysOn_health event session

You can optionaly enable Always On Availability Groups specific extended events to help with root-cause diagnosis when you troubleshoot an availability group.

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 Servers 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

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 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 the 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.

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

Create the availability group

Create the availability group. 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 will 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 to create the availability group.

CREATE AVAILABILITY GROUP [ag1]
    WITH (DB_FAILOVER = ON, CLUSTER_TYPE = NONE)
    FOR REPLICA ON
        N'**<node1>**' WITH (
            ENDPOINT_URL = N'tcp://**<node1>:**<5022>**',
            AVAILABILITY_MODE = SYNCHRONOUS_COMMIT,
            FAILOVER_MODE = AUTOMATIC,
            SEEDING_MODE = AUTOMATIC,
            SECONDARY_ROLE (ALLOW_CONNECTIONS = ALL)
            ),
        N'**<node2>**' WITH ( 
            ENDPOINT_URL = N'tcp://**<node2>**:**<5022>**', 
            AVAILABILITY_MODE = SYNCHRONOUS_COMMIT,
            FAILOVER_MODE = AUTOMATIC,
            SEEDING_MODE = AUTOMATIC,
            SECONDARY_ROLE (ALLOW_CONNECTIONS = ALL)
            );

ALTER AVAILABILITY GROUP [ag1] GRANT CREATE ANY DATABASE;
Note

CLUSTER_TYPE is a new option for CREATE AVAILABILITY GROUP. An availability group requiresCLUSTER_TYPE = NONE when it is on a SQL Server instance that is not a member of a Windows Server Failover Cluster.

Join secondary SQL Servers to the availability group

The following Transact-SQL script joins a server to an availablity 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'NUL';

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;

Notes

The availability group is not a clustered resource at this point.

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

While the availability group is not in a cluster, note the following behaviors:

  • If the primary replica goes down and comes back up - for example if the SQL Server instance or node restarts the availability group will go in RESOLVING state. A database restart does not trigger the availability group to go into a RESOLVING state. Only an instance restart triggers availability group state evaluation. Because there is no cluster controller to manage the availability group elect one of replicas as primary, you need to manually fail over the availability group. To do this, run ALTER AVAILABILITY GROUP FAILOVER on the replica that you choose as primary. You can run ALTER AVAILABILITY GROUP FAILOVER on any the former primary replica or any secondary replica. Note the following behavior:

    • If you run this on the former primary replica then previous configuration returns.
    • If you run this on a secondary replica then the rest of replicas - including the former PRIMARY - will automatically join the availability group.
  • During failover a cluster manager ensures that the demotion action is completed before the promotion action starts so that there will not be two primary replicas in the configuration. On Windows, WSFC is the cluster manager. On a Linux cluster, Pacemaker can be the cluster manager. Without a cluster manager, you have to manually demote the primary replica and promote the secondary replicas in sequential order. If you initiate a failover without a cluster manager, there is no guarantee the demotion is successful - if the primary replica is unresponsive, for example - but promotion can succeed. To avoid having two primary replicas if the demotion fails, you can complete the manual fail over in two steps. First demote the primary replica, and then promote the secondary replica.

    1. Demote the current primary. On the primary SQL Server replica, run the following query: Transact-SQL ALTER AVAILABILITY GROUP [ag1] SET (ROLE = SECONDARY);
    2. Promote the current secondary to new primary. On the node that you want to promote run the following query: Transact-SQL ALTER AVAILABILITY GROUP [ag1] FAILOVER;
Important

After you configure the cluster and add the availability group as a cluster resource, you cannot use Transact-SQL to fail over the availability group 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 availability group 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 will succeed.

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