Nadav Har'El e8cbbba653 alternator: partial implementation of ReturnValues parameter ...

Before this patch, we only supported the ReturnValues=NONE setting of the
PutItem, UpdateItem and DeleteItem operations.

This patch also adds full support for the ReturnValues=ALL_OLD option
in all three operation. This option directs Alternator to return the full
old (i.e., pre-modification) contents of the item.

We implement this as a RMW (read-modify-write) operation just as we do
other RMW operations - i.e., by default we use LWT, to ensure that we really
return the value of the item directly before the modification, the same
value that would have been used in a conditional expression if there was one.

NOTE: This implementation means one cannot use ReturnValues=ALL_OLD in
forbid_rmw write isolation mode. One may theorize that if we only need the
read-before-write for ReturnValues and not for a conditional expression,
it should have been enough to use a separate read (as we do in unsafe_rmw
isolation mode) before the write. But we don't have this "optimization" yet
and I'm not sure it's a valid optimization at all - see discussion in
a new issue #5851.

This patch completes the ReturnValues support for the PutItem and DeleteItem
operations. However, the third operation, UpdateItem, supports three more
ReturnValues modes: UPDATED_OLD, ALL_NEW and UPDATED_NEW. We do not yet
support those in this patch. If a user tries to use one of these three modes,
an informative error message will be returned. The three tests for these
three unimplemented settings continue to xfail, but the rest of the tests
in test_returnvalues.py (except one test of nested attribute paths) now
pass so their xfail flag is dropped.

Refs #5053

Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20200219135658.7158-1-nyh@scylladb.com>

2020-02-21 08:32:47 +01:00

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