What is the difference between mnesia:read/1 and mnesia:wread/1?

Both fetch a record by key inside a transaction, but they take different lock types up front. mnesia:read/1 takes a read lock, which is fine for a plain lookup, but if you intend to immediately write back an updated version of that same record, a read lock can force an extra lock upgrade step. mnesia:wread/1 takes a write lock right away instead.

mnesia:transaction(fun() ->
    [P] = mnesia:wread({person, 1}),
    mnesia:write(P#person{age = P#person.age + 1})
end).

Using wread/1 when you know you're about to write signals your intent up front, which can reduce the chance of a deadlock between two transactions that both read-then-write the same record concurrently — each avoids the read-lock-to-write-lock upgrade dance that could otherwise let two transactions each hold a read lock and wait on each other for the write lock.

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