SQL Server Cluster, SQL DB Cluster

SQL Server Mirroring is a solution to improve the availability of SQL Server databases. It has three roles of Principal, Mirror, and Witness. The Principal can add, delete, change and check the database and provide read and write services to the outside world. The Mirror can only synchronize the database data from the Principal, and does not provide read and write services to the outside world. The Witness is used to automatically switch the role between Principal and Mirror nodes when Principal or Mirror fails.

Advantages:
Real-time Data Replication: Ensures real-time replication of database changes, ensuring immediate synchronization from the primary database to the mirrored database.
Simple Configuration: Compared to other high availability solutions, mirroring has a relatively simple configuration, making it easy to set up and maintain.
Supports Automatic or Manual Failover: Can automatically or manually switch to the mirrored database when the primary database is unavailable, achieving high availability.
Suitable for Small-Scale Deployments: Ideal for the high availability needs of small-scale databases without requiring complex configurations.

Users:
Small enterprises or projects with relatively low high availability requirements.
Those in need of a simple and easy-to-maintain high availability solution.
Scenarios that do not require support for read-write separation or global load balancing.

Kubernetes-based SQL Cluster leverages containerization within the Kubernetes system, offering a flexible, portable, and efficient database deployment. This integration utilizes Kubernetes' automation and management capabilities, streamlining configuration and maintenance for easier management and operation in modern container environments.

Advantages:
High Availability: Ensures SQL Server cluster's availability via containerization and Kubernetes' fault tolerance.
Elastic Scalability: Dynamically scales the cluster to meet workload demands, optimizing resource use.
Simplified Deployment: Kubernetes streamlines configuration, deployment, and maintenance, reducing complexities.
Flexibility and Portability: Containers allow deployment in diverse environments for broader application reach.
Persistent Storage: Uses Kubernetes' persistent storage for data reliability, even in case of container failures.

Users:
Developers for cloud-native applications.
Operations teams are seeking simplified database management, enhanced availability, and elasticity.
Organizations with dynamic workloads require scalable database resources.
Entities aiming for consistent application deployment across on-premises and cloud environments.

The Always On availability groups feature is a high-availability and disaster-recovery solution that provides an enterprise-level alternative to database mirroring. Introduced in SQL Server 2012 (11.x), Always On availability groups maximizes the availability of a set of user databases for an enterprise. An availability group supports a failover environment for a discrete set of user databases, known as availability databases, that fail over together.

Advantages:
Read/Write Separation: Supports configuring multiple replicas within availability groups, enabling read/write separation to enhance database performance.
Automatic Failover: Automatically switches to an available replica in the event of the primary database's unavailability, ensuring continuous service.
Multi-Database Support: Availability groups can encompass multiple databases, making it suitable for large-scale applications.
Geographical Replication: Configuration of availability groups across different geographical locations enhances business continuity and disaster recovery capabilities.

Users:
Medium to large enterprises with higher demands for high availability and performance.
Scenarios requiring support for complex applications and multiple databases.
Businesses with global load balancing and geographical read/write separation requirements.

SQL Server Master-Slave Replication is a one-way replication (from master to slave). Only the master database is used for the write operations, while read operations may be spread on multiple slave databases. This means that if master-slave replication is used as the scale-out solution, you need to have at least two data sources defined, one for write operations and the second for read operations.

Advantages:
Flexibility: Allows selective replication of specific databases or tables to meet different requirements.
Support for Multiple Subscribers: Capable of replicating changes to multiple subscribers, suitable for distributed systems.
Scalable Write Operations: Ideal for scenarios requiring extensive write operations, as write operations do not block read operations.
Cross-Version Replication: Permits replication between different versions of SQL Server.

Users:
Users requiring high availability for specific databases or tables.
Applications with a need for distributing read and write operations.
Scenarios demanding replication between different versions of SQL Server.