We had Alternator's current compatibility with DynamoDB described in
two places - alternator.md and compatibility.md. This duplication was
not only unnecessary, in some places it led to inconsistent claims.
In general, the better description was in compatibility.md, so in
this patch we remove the compatibility section from alternator.md
and instead link to compatibility.md. There was a bit of information
that was missing in compatibility.md, so this patch adds it.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20210215203057.1132162-1-nyh@scylladb.com>
This patch adds full support for nested attribute paths (e.g., a.b[3].c)
in UpdateExpression. After in previous patches we already added such
support for ProjectionExpression, ConditionExpression and FilterExpression
this means the nested attribute paths feature is now complete, so we
remove the warning from the documents. However, there is one last loose
end to tie and we will do it in the next patch: After this patch, the
combination of UpdateExpression with nested attributes and ReturnValues
is still wrong, and the test for it in test_returnvalues.py still xfails.
Note that previous patches already implemented support for attribute paths
in expression evaluations - i.e., the right-hand side of UpdateExpression
actions, and in this patch we just needed to implement the left hand side:
When an update action is on an attribute a.b we need to read the entire
content of the top-level a (an RWM operation), modify just the b part of
its json with the result of the action, and finally write back the entire
content of a. Of course everything gets complicated by the fact that we
can have multiple actions on multiple pieces of the same JSON, and we also
need to detect overlapping and conflicting actions (we already have this
detection in the attribute_path_map<> class we introduced in a previous
patch).
I decided to leave one small esoteric difference, reproduced by the xfailing
test_update_expression.py::test_nested_attribute_remove_from_missing_item:
As expected, "SET x.y = :val" fails for an item if its attribute x doesn't
exist or the item itself does not exist. For the update expression
"REMOVE x.y", DynamoDB fails if the attribute x doesn't exist, but oddly
silently passes if the entire item doesn't exist. Alternator does not
currently reproduce this oddity - it will fail this write as well.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Add to the DynamoDB compatibility document, docs/alternator/compatibility.md,
a mention that Alternator streams are still an experimental features, and
how to turn it on (at this point CDC is no longer an experimental feature,
but Alternator Streams are).
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20201112184436.940497-1-nyh@scylladb.com>
This patch adds a new document, docs/alternator/compatibility.md,
which focuses on what users switching from DynamoDB to Alternator
need to know about where Alternator differs from DynamoDB and which
features are missing.
The compatibility information in the old alternator.md is not deleted
yet. It probably should.
Fixes#7556
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20201110180242.716295-1-nyh@scylladb.com>
