Today in Alternator vector search, vectors are presented to the API as
lists of numbers. I.e., in JSON a vector is sent in requests and responses
as:
{"L": [{"N": "3.14159"}, {"N":" "6.7"}}
This format is verbose and inefficient for long vectors. Even worse,
because the "N" number format has precision guarantees in DynamoDB,
we cannot optimize the storage of such vectors by, for example, storing
the numbers as 32-bit floats. We actually store these vectors as JSON,
exactly as shown above.
So in this patch we introduce a new DynamoDB type, "FLOAT32VECTOR", for
vectors. The above vector will look like this in JSON:
{"FLOAT32VECTOR": [3.14159, 6.7]}
Note that each number is an unquoted JSON number, not a JSON string.
Importantly, the definition of the "FLOAT32VECTOR" type specifies that
components of the vector only have 32-bit precision. This means that
Scylla may store internally these vectors as lists of 32-bit floats -
not as a JSON. And indeed, this patch includes this optimization:
Top-level vector attributes are now encoded in an optimized way,
as a byte 5 (alternator_type::FLOAT32VECTOR) followed by the elements
of the vector, just 4 bytes each (the 4-byte big-endian IEEE 754
representation of each floating-point component).
This patch also includes documentation, and extensive tests that the
new "FLOAT32VECTOR" type works (which also serves as an example how to
use it in the boto3 SDK), that it is indeed encoded internally as 32-bit
floats and not wasteful JSON strings, and that vector search on such items
work. The last thing requires cooperation from the vector store, of
course - it needs to be able to understand the new optimized encoding
of vector attributes in addition to the old unoptimized one.
Note that the old unoptimized ("list of numbers") vectors are still
supported. Although not recommended for general use, some users might
still want to use the unoptimized type if they have pre-existing data
created on DynamoDB or Alternator without vector search in mind, and
the vectors already exist as lists of numbers.
Although this is less important, the new vector type "FLOAT32VECTOR"
is also allowed in a Query's QueryVector.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
This patch adds a test file, test/alternator/test_encoding.py, testing
how Alternator stores its data in the underlying CQL database. We test
how tables are named, and how attributes of different types are encoded
into CQL.
The test, which begins with a long comment, also doubles as developer-
oriented *documention* on how Alternator encodes its data in the CQL
database. This documentation is not intended for end-users - we do
not want to officially support reading or writing Alternator tables
through CQL. But once in a while, this information can come in handy
for developers.
More importantly, this test will also serve as a regression test,
verifying that Alternator's encoding doesn't change unintentionally.
If we make an unintentional change to the way that Alternator stores
its data, this can break upgrades: The new code might not be able to
read or write the old table with its old encoding. So it's important
to make sure we never make such unintentional changes to the encoding
of Alternator's data. If we ever do make *intentional* changes to
Alternator's data encoding, we will need to fix the relevant test;
But also not forget to make sure that the new code is able to read
the old encoding as well.
The new tests use both "dynamodb" (Alternator) and "cql" fixtures,
to test how CQL sees the Alternator tables. So naturally are these
tests are marked "scylla_only" and skipped on DynamoDB.
Fixes#19770.
Closesscylladb/scylladb#28866
