In today's competitive world, it is very necessary to keep the critical applications available 24*7 to the business as well as to the customer. All the organizations have to design their Infrastructure in such a way, so that they can achieve maximum availability(High Availability, i.e. HA) with minimum outage( achieved using Disaster Recovery, i.e. DR solution). SQL Server provides various High Availability(HA) and Disaster Recovery(DR) solutions to maximize database availability.

HADR solution protects the data by replicating live data changes from primary server to Secondary server, so if anything goes wrong with the primary server, secondary server can be brought online with minimum impact to the business.

Basic Availability Group(BAG) Architecture:

Below are some of the terminologies used in AG HADR solution.

Availability Groups:

are the logical containers that are hosting always on availability group database and replica.

Availability Replicas:

are the instances which are hosting always on availability databases.

Availability Database:

are the databases that are being protected by Always on Availability group feature.

Availability Group Listener:

An availability group listener is a virtual network name that clients can connect to in order to access a database in a primary or secondary replica of an Always On availability group

Recovery Time Objective(RTO):

RTOs are used to measure how much time it takes to recover the data and that particular time period should be acceptable by the business and It is an important consideration in a disaster recovery plan. RTO is measured in seconds, minutes, hours or days depending upon application criticality.

Recovery Point Objective(RPO):

RPO is the maximum acceptable amount of data loss after a planned or unplanned outage. RPO time varies depending upon application criticality. How much data an Organization can lose with minimum business impact, should be defined in the organization's business continuity plan (BCP). so in short, RPOs and RTOs are crucial concepts for business continuity.

Always On Availability Groups(AG) as HADR solution:

Always On Availability Groups is one of the HADR solution available in SQL Server that provides high availability (HA) and/or disaster recovery (DR) solutions to the Organization's critical data. This HADR solution has got many benefits, such as multiple replicas, automatic failover, load balancing and read-only access to secondary replicas, etc. These features were first introduced in SQL Server 2012, that to only Enterprise Edition.

Starting with SQL Server 2016 Standard edition, this HADR solution was introduced with limited feature called as Basic Availability Groups(BAGs) and this is improved version of Database Mirroring.

Basic Availability Group(BAG) is useful for providing disaster recovery and high availability solution to SQL databases.  Cost saving is one of the prime reason for using BAGs, as it is not necessary to purchase SQL Server Enterprise edition license for this HADR solution.

Basic availability groups include the following limitations:

• Limited to two replicas only (primary and secondary).
• No read access on secondary replica.
• No backups on secondary replica.
• No integrity checks on secondary replicas.
• Multiple databases on same SQL instance cannot be part of same availability groups. Each database has its own availability groups.
• Basic availability groups are only supported on Standard Edition servers.
• Basic availability groups cannot be part of a distributed availability group.

Please visit Microsoft Page for more information on Basic Availability Group(BAG)

Below is the table showing features available in Enterprise Vs Standard edition of SQL Server 2019 and for more information, please visit Editions and supported features