Concept. A Transaction Manager is the orchestrator that runs scheduling, locking, write-ahead logging, deadlock detection, and recovery as one cooperating system — making thousands of concurrent transactions look like each ran alone, with the database in a consistent state at every moment.
Intuition. When Mickey buys Premium while Minnie updates her profile and Daffy adds a song, the TM routes each one through the scheduler (interleave for throughput), the lock manager (block them only on rows they share), the WAL (log every change before it touches the data), and — if the server crashes — the recovery manager (replay committed work, roll back the rest). One coherent execution, four cooperating components, ACID preserved.
The Architecture
Why It Scales
The Magic of Independence
Most transactions operate on different data. Consider a bank with 1M accounts:
Transaction on account #47893 is irrelevant to account #82456
Out of 1000 concurrent transactions, only 10-20 might actually conflict
Lock manager utilizes hash tables for O(1) lock checks
WAL ensures sequential writes even amidst random transaction patterns