Batching input reader for appengine-mapreduce

batching_reader.py

class KeyRangeBatchKeyIterator(
        datastore_range_iterators.AbstractKeyRangeIterator):
    """Yields datastore.Key type within a key range in batches.

    This allows us to do several RPCs in parallel, hopefully reducing running
    time.

    TODO(colin): the query batch size is the same as the processing batch size.
        Figure out if there's a benefit to changing this.
    """
    _KEYS_ONLY = True

    def __iter__(self):
        """Yield batches of keys for mapping.

        The yielded batch is a generator, so that the query cursor doesn't move
        until the mapper starts processing elements.
        """
        self._query = self._key_range.make_ascending_datastore_query(
            self._query_spec.entity_kind, filters=self._query_spec.filters)

        while True:
            keys = itertools.islice(
                self._query.Run(config=datastore_query.QueryOptions(
                    batch_size=self._query_spec.batch_size,
                    keys_only=True,
                    start_cursor=self._cursor)),
                self._query_spec.batch_size)
            yield keys
            if self._cursor == self._query.GetCursor():
                # We haven't moved since last time, which means we're at the
                # end.
                break
            self._cursor = self._query.GetCursor()

    def _get_cursor(self):
        """Get the current query cursor.

        (AbstractKeyRangeIterator method taken from KeyRangeEntityIterator.)
        """
        if self._query:
            return self._query.GetCursor()
        return self._cursor


# Register the new iterator so that the mapreduce library knows how to create
# one from the JSON mapreduce payload.
datastore_range_iterators._KEY_RANGE_ITERATORS[
    KeyRangeBatchKeyIterator.__name__] = KeyRangeBatchKeyIterator


class DatastoreKeyBatchingInputReader(
        mapreduce.input_readers.RawDatastoreInputReader):
    """Mapreduce input reader that yields datastore keys in batch."""
    _KEY_RANGE_ITER_CLS = KeyRangeBatchKeyIterator

LICENSE

The MIT License (MIT)

Copyright (c) 2015 Khan Academy

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