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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=sharing Closes #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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