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Add comment about what strorage.stream does
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# See the License for the specific language governing permissions and
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# limitations under the License.
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""" This module is responsible for getting events from the DB for pagination
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and event streaming.
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The order it returns events in depend on whether we are streaming forwards or
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are paginating backwards. We do this because we want to handle out of order
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messages nicely, while still returning them in the correct order when we
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paginate bacwards.
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This is implemented by keeping two ordering columns: stream_ordering and
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topological_ordering. Stream ordering is basically insertion/received order
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(except for events from backfill requests). The topolgical_ordering is a
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weak ordering of events based on the pdu graph.
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This means that we have to have two different types of tokens, depending on
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what sort order was used:
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- stream tokens are of the form: "s%d", which maps directly to the column
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- topological tokems: "t%d-%d", where the integers map to the topological
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and stream ordering columns respectively.
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"""
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from twisted.internet import defer
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from ._base import SQLBaseStore
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