This is similar to the timestamp based splitting writer, except
that it splits data based on the shard where the partition key
is supposed to be placed.
It is similar to the multishard_writer, in the sense that it
creates n streams for n shards, but it does not want to process
the streams in the owner shards. We want to use that in processes
like resharding where it is fine for a foreign shard to deal
with a mutation.
One option would be to augment the multishard_writer to optionally
achieve these properties, but having a separate splitter is both
simpler and faster.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
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