DBProvider.transact()method

Run a callback as a single atomic transaction — every write made through the callback's provider is committed together, or not at all.

• The callback receives a transaction-scoped DBProvider; reads and writes made through it belong to the transaction.
• Reads see a consistent snapshot of the data from before the transaction, and do not see the transaction's own uncommitted writes.
• If the callback throws, nothing is committed and the error is rethrown.
• The callback may run more than once if the backend retries on contention, so it must have no side effects other than through its provider.
• Portable code must not use realtime sequences or nested transact() calls inside a transaction — most backends throw UnsupportedError, though some (e.g. MemoryDBProvider) support them scoped to the transaction.
• Not every provider supports transactions — the base implementation throws UnsupportedError.