Broadcast tables are tables for which all statements are strongly
consistent (linearizable), replicated to every node in the cluster and
available as long as a majority of the cluster is available. If a user
wants to store a “small” volume of metadata that is not modified “too
often” but provides high resiliency against failures and strong
consistency of operations, they can use broadcast tables.
The main goal of the broadcast tables project is to solve problems which
need to be solved when we eventually implement general-purpose strongly
consistent tables: designing the data structure for the Raft command,
ensuring that the commands are idempotent, handling snapshots correctly,
and so on.
In this MVP (Minimum Viable Product), statements are limited to simple
SELECT and UPDATE operations on the built-in table. In the future, other
statements and data types will be available but with this PR we can
already work on features like idempotent commands or snapshotting.
Snapshotting is not handled yet which means that restarting a node or
performing too many operations (which would cause a snapshot to be
created) will give incorrect results.
In a follow-up, we plan to add end-to-end Jepsen tests
(https://jepsen.io/). With this PR we can already simulate operations on
lists and test linearizability in linear complexity. This can also test
Scylla's implementation of persistent storage, failure detector, RPC,
etc.
Design doc: https://docs.google.com/document/d/1m1IW320hXtsGulzSTSHXkfcBKaG5UlsxOpm6LN7vWOc/edit?usp=sharingCloses#11164
* github.com:scylladb/scylladb:
raft: broadcast_tables: add broadcast_kv_store test
raft: broadcast_tables: add returning query result
raft: broadcast_tables: add execution of intermediate language
raft: broadcast_tables: add compilation of cql to intermediate language
raft: broadcast_tables: add definition of intermediate language
db: system_keyspace: add broadcast_kv_store table
db: config: add BROADCAST_TABLES feature flag
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