Before this series, Alternator's CreateTable operation defaults to creating a table replicated with vnodes, not tablets. The reasons for this default included missing support for LWT, Materialized Views, Alternator TTL and Alternator Streams if tablets are used. But today, all of these (except the still-experimental Alternator Streams) are now fully available with tablets, so we are finally ready to switch Alternator to use tablets by default in new tables.
We will use the same configuration parameter that CQL uses, tablets_mode_for_new_keyspaces, to determine whether new keyspaces use tablets by default. If set to `enabled`, tablets are used by default on new tables. If set to `disabled`, tablets will not be used by default (i.e., vnodes will be used, as before). A third value, `enforced` is similar to `enabled` but forbids overriding the default to vnodes when creating a table.
As before, the user can set a tag during the CreateTable operation to override the default choice of tablets or vnodes (unless in `enforced` mode). This tag is now named `system:initial_tablets` - whereas before this patch it was called `experimental:initial_tablets`. The rules stay the same as with the earlier, experimental:initial_tablets tag: when supplied with a numeric value, the table will use tablets. When supplied with something else (like a string "none"), the table will use vnodes.
Fixes https://github.com/scylladb/scylladb/issues/22463
Backport to 2025.4, it's important not to delay phasing out vnodes.
Closesscylladb/scylladb#26836
* github.com:scylladb/scylladb:
test,alternator: use 3-rack clusters in tests
alternator: improve error in tablets_mode_for_new_keyspaces=enforced
config: make tablets_mode_for_new_keyspaces live-updatable
alternator: improve comment about non-hidden system tags
alternator: Fix test_ttl_expiration_streams()
alternator: Fix test_scan_paging_missing_limit()
alternator: Don't require vnodes for TTL tests
alternator: Remove obsolete test from test_table.py
alternator: Fix tag name to request vnodes
alternator: Fix test name clash in test_tablets.py
alternator: test_tablets.py handles new policy reg. tablets
alternator: Update doc regarding tablets support
alternator: Support `tablets_mode_for_new_keyspaces` config flag
Fix incorrect hint for tablets_mode_for_new_keyspaces
Fix comment for tablets_mode_for_new_keyspaces
042303f0c9