SQL Server is a relational database management system (RDBMS) developed by Microsoft. Its architecture is designed to manage data storage, retrieval, and processing efficiently while ensuring high availability, security, and performance.
Here’s an overview of SQL Server architecture
Main component of SQL Server
SQL server mainly divided into below main parts
- Protocol Layer
- Relational Engine (Query Processor)
- Storage Engine
- SQL OS ( OS Layer)
A. Protocol Layer
This is the entry point for client applications to communicate with SQL Server. It handles communication protocols like:
- TCP/IP
- Named Pipes
- Shared Memory
The protocol layer translates client requests (e.g., T-SQL queries) into commands that the server can understand and process
B. Relational Engine (Query Processor)
The Relational Engine is responsible for processing queries and executing them. It includes the following components:
- Parser: Parses T-SQL statements into a format SQL Server can understand.
- Algebrizer: Analyzes and resolves the objects (tables, columns) in the query.
- Query Optimizer: Generates the most efficient execution plan based on available indexes, statistics, and query structure.
- Query Executor: Executes the query as per the execution plan.
C. Storage Engine
The Storage Engine manages how data is stored, retrieved, and modified. Its components include:
- Access Methods: Determines how to retrieve rows from data pages or indexes.
- Buffer Manager: Manages memory pages for data caching to minimize physical I/O operations.
- Transaction Manager:
- Log Manager: Handles the Write-Ahead Logging (WAL) protocol for transaction durability.
- Lock Manager: Implements locking and concurrency control mechanisms.
- Checkpoint Manager: Writes dirty pages (modified pages in memory) to disk periodically to ensure data durability.
D. SQL OS (Operating System Layer)
The SQL OS abstracts the underlying Windows OS for tasks like:
- Memory management
- I/O management
- Scheduling threads (using cooperative multitasking with “workers”)
Core Architectural Features
A. Database Files
- Primary Data File (.mdf): Contains schema and data.
- Secondary Data Files (.ndf): Optional files for large databases.
- Transaction Log File (.ldf): Records all transactions for recovery purposes.
B. Indexes
SQL Server uses indexes to improve query performance:
- Clustered Index: Data is stored in a sorted order.
- Non-Clustered Index: Pointers to data with additional lookup overhead.
C. Buffer Pool
A shared memory area used to cache:
- Data pages
- Execution plans
- Indexes
D. Transactions
Follows ACID principles to ensure:
- Atomicity: Transactions are all-or-nothing.
- Consistency: Database remains consistent after transactions.
- Isolation: Concurrent transactions do not interfere with each other.
- Durability: Committed changes persist even after failures.
High Availability and Disaster Recovery
SQL Server provides various mechanisms:
- Always On Availability Groups: High availability through replicas.
- Failover Clustering: Shared storage with automatic failover.
- Log Shipping: Periodic backups sent to secondary servers.
- Database Mirroring: Redundant copies of databases.
Security Features
- Authentication Modes:
- Windows Authentication
- SQL Server Authentication
- Encryption:
- Transparent Data Encryption (TDE)
- Always Encrypted
- Row-Level Security (RLS): Restricts access to rows based on user context.
- Dynamic Data Masking (DDM): Masks sensitive data dynamically.
Execution Flow
- Client Request: Sent via the Protocol Layer.
- Query Processing:
- The Relational Engine parses and optimizes the query.
- Data Retrieval:
- The Storage Engine retrieves data from disk or the buffer pool.
- Result: Returned to the client via the Protocol Layer.
This architecture ensures SQL Server’s scalability, robustness, and ability to handle complex workloads efficiently.