"Our domain objects have schema version dependent format, for efficiency
reasons. The data structures which map between columns and values rely on
column ids, which are consecutive integers. For example, we store cells in a
vector where index into the vector is an implicit column id identifying table
column of the cell. When columns are added or removed the column ids may
shift. So, to access mutations or query results one needs to know the version
of the schema corresponding to it.
In case of query results, the schema version to which it conforms will always
be the version which was used to construct the query request. So there's no
change in the way query result consumers operate to handle schema changes. The
interfaces for querying needed to be extended to accept schema version and do
the conversions if necessary.
Shard-local interfaces work with a full definition of schema version,
represented by the schema type (usually passed as schema_ptr). Schema versions
are identified across shards and nodes with a UUID (table_schema_version
type). We maintain schema version registry (schema_registry) to avoid fetching
definitions we already know about. When we get a request using unknown schema,
we need to fetch the definition from the source, which must know it, to obtain
a shard-local schema_ptr for it.
Because mutation representation is schema version dependent, mutations of
different versions don't necessarily commute. When a column is dropped from
schema, the dropped column is no longer representable in the new schema. It is
generally fine to not hold data for dropped columns, the intent behind
dropping a column is to lose the data in that column. However, when merging an
incoming mutation with an existing mutation both of which have different
schema versions, we'd have to choose which schema should be considered
"latest" in order not to loose data. Schema changes can be made concurrently
in the cluster and initiated on different nodes so there is not always a
single notion of latest schema. However, schema changes are commutative and by
merging changes nodes eventually agree on the version. For example adding
column A (version X) on one node and adding column B (version Y) on another
eventually results in a schema version with both A and B (version Z). We
cannot tell which version among X and Y is newer, but we can tell that version
Z is newer than both X and Y. So the solution to the problem of merging
conflicting mutations could be to ensure that such merge is performed using
the schema which is superior to schemas of both mutations.
The approach taken in the series for ensuring this is as follows. When a node
receives a mutation of an unknown schema version it first performs a schema
merge with the source of that mutation. Schema merge makes sure that current
node's version is superior to the schema of incoming mutation. Once the
version is synced with, it is remembered as such and won't be synced with on
later mutations. Because of this bookkeeping, schema versions must be
monotonic; we don't want table altering to result in any earlier version
because that would cause nodes to avoid syncing with them. The version is a
cryptographically-secure hash of schema mutations, which should fulfill this
purpose in practice.
TODO: It's possible that the node is already performing a sync triggered by
broadcasted schema mutations. To avoid triggering a second sync needlessly, the
schema merging should mark incoming versions as being synced with.
Each table shard keeps track of its current schema version, which is
considered to be superior to all versions which are going to be applied to it.
All data sources for given column family within a shard have the same notion
of current schema version. Individual entries in cache and memtables may be at
earlier versions but this is hidden behind the interface. The entries are
upgraded to current version lazily on access. Sstables are immutable, so they
don't need to track current version. Like any other data source, they can be
queried with any schema version.
Note, the series triggered a bug in demangler:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68700"
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