How does Mnesia's transaction manager serialize concurrent transactions touching the same records?

Mnesia uses pessimistic locking rather than optimistic concurrency control: when a transaction reads or writes a record, mnesia_tm acquires the appropriate lock (read or write) on that record before proceeding, and a conflicting lock request from another concurrent transaction simply has to wait until the first transaction releases it at commit or abort.

sequenceDiagram
    participant T1
    participant T2
    participant TM as mnesia_tm
    T1->>TM: request write lock on Key
    TM-->>T1: granted
    T2->>TM: request write lock on Key
    Note over TM: T2 blocks, waiting
    T1->>TM: commit, release lock
    TM-->>T2: lock granted, T2 proceeds

This guarantees serializability for conflicting operations without needing to detect and retry after the fact the way optimistic approaches do — the cost is that a transaction can block waiting on a lock held by another, which is exactly the scenario that can escalate into a deadlock if two transactions each hold a lock the other is waiting for, something mnesia_tm detects and resolves by aborting one of them.