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Currently the number of database changes inside a transaction (or
snapshot, see snapshot/1)
is limited to 2 ** 32 -1. If this limit is exceeded a
exception is raised.
Transactions may be nested. The above mentioned limitation for the number of database changes applies to the combined number in nested transactions.
If Goal succeeds, the transaction is committed. This implies that (1) any clause that is asserted in the transaction and not retracted in the same transaction is made globally visible and (2) and clause the existed before the transaction and is retracted in the transaction becomes globally invisible. Multiple transactions may retract the same clause and be committed, i.e., committing a retract that was already performed is a no-op. All modifications become atomically visible to other threads. The transaction/3 variation allows for verifying constraints just before the commit takes place.
Clause ordering Inside a transaction clauses can be added using asserta/1 and assertz/1. If only a single transaction is active at any point in time transactions preserve the usual ordering of clauses. However, if multiple transactions manipulate the same predicate(s) concurrently (typically using transaction/3), the final order of the clauses is the order in which the transactions asserted the clauses and not the order in which the transactions are committed.