This series implements a new per-row TTL feature for CQL. The per-row TTL feature was requested in issue #13000. It is a feature that does not exist in Cassandra, and was inspired by DynamoDB's TTL feature - and under the hood uses the same implementation that we used in Alternator to implement this DynamoDB feature.
The new per-row TTL feature is completely separate from CQL's existing per-write (and per-cell) TTL, and both will be available to users.
In the per-row TTL feature, one column in the table is designated as the "TTL" column, and its value for a row is the expiration time for that row. The TTL column can be designated at table creation time, e.g.:
```cql
CREATE TABLE tab (
id int PRIMARY KEY,
t text,
expiration timestamp TTL
);
```
Or after the table already exists with:
```cql
ALTER TABLE tab TTL expiration
```
Expiration can also be disabled, with:
```cql
ALTER TABLE tab TTL NULL
```
The new per-row TTL feature has two features that users have been asking for:
1. A user can change the value of just the TTL column - without rewriting the entire row - to change the expiration time of the entire row.
2. When an expired row is finally deleted, a CDC event about this deletion appears in the CDC log (if CDC is enabled), including - if a preimage is enabled - the content of the deleted row.
To achieve the second goal (CDC events), a row is not guaranteed to disappear at exactly its expiration time (as CQL's original TTL feature guarantees). Rather, the row is deleted some time later, depending on `alternator_ttl_period_in_seconds`; Until the actual deletion, the row is still readable (and even writable). But we are guaranteed that when the row is finally deleted, the CDC event will come too.
The implementation uses the same background thread used by Alternator to periodically scan for expired items and delete them.
The expiration thread keeps the same metrics as it did for Alternator:
* `scylla_expiration_scan_passes`
* `scylla_expiration_scan_table`
* `scylla_expiration_items_deleted`
* `scylla_expiration_secondary_ranges_scanned`
The series begins with a few small preparation patches, followed by the main part of the feature (which isn't big, since we are just enabling the pre-existing Alternator expiration machinary for CQL) and finally 30 tests (single-node and multi-node tests) and documentation.
This series is a new feature, so traditionally would not be backported. However, I wouldn't be surprised if we will be requested to backport it so that customers will not need to wait for a new major release.
Fixes#13000Closesscylladb/scylladb#28320
* github.com:scylladb/scylladb:
test/cqlpy: verify that a column can't be both STATIC and PRIMARY KEY
docs/cql: document the new CQL per-row TTL feature
test/cluster: tests for the new CQL per-row TTL feature
test/cqlpy: tests for the new CQL per-row TTL feature
test: set low alternator_ttl_period_in_seconds in CQL tests
cql ttl: fix ALTER TABLE to disable TTL if column is dropped
cql ttl: add setting/unsetting of TTL column to ALTER TABLE
cql ttl: add TTL column support to CREATE TABLE and DESC TABLE
ttl: add CQL support to Alternator's TTL expiration service
alternator ttl: move TTL_TAG_KEY to a header file
alternator ttl: remove unnecessary check of feature flag
cql: add "cql_row_ttl" cluster feature
alternator: fix error message if UpdateTimeToLive is not supported
a03ebe1a29