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DynamoDB API's BatchGetItem invokes a number (up to 25) of read requests in parallel, returning when all results are available. Alternator naively implemented this by sending all read requests in parallel, no matter which requests these were. That implementation was inefficient when all the requests are to different items (clustering rows) of the same partition. In a multi-node setup this will end up sending 25 separate requests to the same remote node(s). Even on a single-node setup, this may result in reading from disk more than once, and even if the partition is cached - doing an O(logN) search in each multiple times. What we do in this patch, instead, is to group all the BatchGetItem requests that aimed at the same partition into a single read request asking for a (sorted) list of clustering keys. This is similar to an "IN" request in CQL. As an example of the performance benefit of this patch, I tried a BatchGetItem request asking for 20 random items from a 10-million item partition. I measured the latency of this request on a single-node Scylla. Before this patch, I saw a latency of 17-21 ms (the lower number is when the request is retried and the requested items are already in the cache). After this patch, the latency is 10-14 ms. The performance improvement on multi-node clusters are expected to be even higher. Unfortunately the patch is less trivial than I hoped it would be, because some of the old code was organized under the assumption that each read request only returned one item (and if it failed, it means only one item failed), so this part of the code had to be reorganized (and, for making the code more readable, coroutinized). An unintended benefit of the code reorganization is that it also gave me an opportunity to fail an attempt to ask BatchGetItem the same item more than once (issue #10757). The patch also adds a few more corner cases in the tests, to be even more sure that the code reorganization doesn't introduce a regression in BatchGetItem. Fixes #10753 Fixes #10757 Signed-off-by: Nadav Har'El <nyh@scylladb.com>