What is the difference between mnesia:transaction/1 and mnesia:sync_transaction/1?

Both give the same atomicity and locking guarantees, but they differ in when they return control to the caller relative to replication. mnesia:transaction/1 can return as soon as the commit is decided locally, while replication to other nodes finishes asynchronously in the background. mnesia:sync_transaction/1 waits until the commit has actually been acknowledged by every involved replica before returning.

mnesia:sync_transaction(fun() ->
    mnesia:write(#session{id = 1, user = "ada"})
end).

The practical difference shows up under load: with plain transaction/1, a caller could get {atomic, ok} back and, in theory, briefly race ahead of replication actually landing everywhere. sync_transaction/1 removes that gap at the cost of waiting longer per call, which matters when a caller needs a hard guarantee that a subsequent read from any node in the cluster will already see the write it just made.

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