In this patch we implement the Expected parameter for the UpdateItem,
PutItem and DeleteItem operations. This parameter allows a conditional
update - i.e., do an update only if the existing value of the item
matches some condition.
This is the older form of conditional updates, but is still used by many
applications, including Amazon's Tic-Tac-Toe demo.
As usual, we do not yet provide isolation guarantees for read-modify-write
operations - the item is simply read before the modification, and there is
no protection against concurrent operation. This will of course need to be
addressed in the future.
The Expected parameter has a relatively large number of variations, and most
of them are supported by this code, except that currenly only two comparison
operators are supported (EQ and BEGINS_WITH) out of the 13 listed in the
documentation. The rest will be implemented later.
This patch also includes comprehensive tests for the Expected feature.
These tests are almost exhaustive, except for one missing part (labled FIXME) -
among the 13 comparison operations, the tests only check the EQ and BEGINS_WITH
operators. We'll later need to add checks to the rest of them as well.
As usual, all the tests pass on Amazon DynamoDB, and after this patch all
of them succeed on Alternator too.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20190905125558.29133-1-nyh@scylladb.com>
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