SQL Server vs MySQL/PostgreSQL: Transaction Log Comparison

1. Transaction Log Architecture Comparison

2. Impact on Primary-Secondary Replication and High Availability Design

3. Deep Dive: Why SQL Server Supports Database-Level HA?

✅ ​​SQL Server​​:

• Each database has independent .ldf files, physically stored separately.
• ​​Always On’s log transport unit​​: Log Scanner threads capture and transmit log blocks per database.
• ​​Impact​​: Availability Groups support per-database membership, allowing different DBs to be assigned to distinct HA groups.

🚫 ​​MySQL & PostgreSQL​​:

• binlog/WAL are instance-level logs, centrally managed:
  • MySQL: Uses shared ib_logfile (InnoDB redo log) and binlog.
  • PostgreSQL: Unified WAL (e.g., 0000000100000001000000A2) for all transactions.
• ​​Impact​​: Cannot isolate a single DB for replication/recovery (logs are "mixed"). Replication must be instance-wide.

4. Architecture Scenarios & Capability Comparison

Capability Dimensions:

5. Key Limitations & Considerations

SQL Server Constraints:

• Requires Windows Failover Cluster (WSFC) and domain environment.
• Databases must be in ​​Full Recovery Mode​​ with at least one full backup.
• System databases (master, msdb) cannot join Availability Groups (require separate sync).

MySQL/PostgreSQL Workarounds (with Trade-offs):

• ​​Dedicated Instances​​: Isolate critical DBs at the cost of resource efficiency.
• ​​Logical Replication (PostgreSQL)​​:
  • Pros: Table-level sync, heterogeneous targets.
  • Cons: No auto-failover, weaker transaction consistency than physical replication.
• ​​Middleware Sharding​​: Tools like Vitess (MySQL) or Citus (PostgreSQL) add complexity.

6. Conclusion

🎯 ​​SQL Server’s database-level log independence​​ enables fine-grained HA and replication, while ​​MySQL/PostgreSQL’s instance-level logs​​ restrict synchronization to the entire instance.

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