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As mentioned above threaded iter provides support for producer consumer model where producer is a standalone thread. The PrefetchingIter uses the threaded iter and provides interface for data parsing and loading from custom format or record io format. This parsing and loading logic may in turn spawn multiple threads. Any exceptions thrown in these these in these threads should be caught and transported to the child thread where it will be rethrown. This rethrown exception will be caught and transported to the main thread.
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