What is SQL Server Transaction Log architecture and how does write-ahead logging work?

SQL Server uses a transaction log to guarantee durability using write-ahead logging (WAL).

The transaction log records every data modification before the changes are written to data files. This ensures durability (ACID). Write-Ahead Logging guarantees that log records are hardened to disk before corresponding data pages are flushed. In case of crash, SQL Server replays the log (redo) or rolls back incomplete transactions (undo). The log is logically divided into VLFs (Virtual Log Files), and improper sizing leads to fragmentation and recovery delays.