Explain the lifecycle of a Mnesia transaction from start to commit?

Calling mnesia:transaction(Fun) kicks off a well-defined sequence: Mnesia runs your function, tracking every read and write it performs, acquiring the appropriate locks as it goes, before deciding whether the whole thing can be committed.

flowchart TD
    A[mnesia:transaction Fun called] --> B[Fun executes: reads take read locks, writes take write locks]
    B --> C{Fun completes without exception?}
    C -->|Yes| D[Two-phase commit across all replicas of touched tables]
    D --> E{All replicas prepared successfully?}
    E -->|Yes| F["{atomic, Result} returned; locks released"]
    E -->|No| G["{aborted, Reason} returned; no changes applied"]
    C -->|No, exception raised| G

If the function runs to completion without an exception, Mnesia attempts to commit — for a replicated table, this means the two-phase prepare/commit exchange across every node holding a copy. Only if every involved replica confirms it can commit does the transaction actually take effect everywhere, returning {atomic, Result}; any failure along the way aborts the whole thing, applying no changes and returning {aborted, Reason} instead.