What is a nested Mnesia transaction and how does it behave differently from a top-level one?

Calling mnesia:transaction/1 from inside code that's already running within another transaction creates a nested transaction. Rather than being a fully independent transaction, it shares the outer transaction's locks and only really commits when the outermost transaction itself commits — an abort of the inner one aborts the whole outer transaction too.

mnesia:transaction(fun() ->
    mnesia:write(#a{id = 1}),
    mnesia:transaction(fun() ->     %% nested
        mnesia:write(#b{id = 1})
    end),
    mnesia:write(#c{id = 1})
end).

This differs from what "nested transaction" might suggest in some other databases (an independently committable sub-unit) — in Mnesia, there's really just one logical transaction underneath, and nesting mainly matters for code organization (letting a helper function wrap its own operations in a transaction without caring whether it's already inside one) rather than providing partial-commit semantics.

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