Nadav Har'El 13a3aca460 alternator: initial implmentation of TTL expiration service ...

In this patch we add an incomplete implementation of an expiration
service to Alternator, which periodically scans the data in the table,
looking for expired items and deleting them.

This implementation involves a new "expiration service" which runs a
background scan in each shard. Each shard "owns" a subset of the token
ranges - the intersection of the node's primary ranges with this shard's
token ranges - and scans those ranges over and over, deleting any items
which are found expired.

This implementation is good enough to make all existing tests but one
pass, but is still a partial and inefficient implementation littered with
FIXMEs throughout the code. Among other things, this implementation
doesn't do anything reasonable about pacing of the scan or about multiple
tables, it scans entire items instead of only the needed parts, and
if a node goes down, the part of the token range which it "owns" will not
be scanned for expiration (we need living nodes to take over the
background expiration work for dead nodes).

The current tests cannot expose these problems, so we will need to develop
additional tests for them.

Because this implementation is very partial, the Alternator TTL continues
to remain "experimental", cannot be used without explicitly enabling this
experimental feature, and must not be used for any important deployment.
The new TTL expiration service will only run (at the moment) in the
background if the Alternator TTL experimental feature is enabled and
and if Alternator is enabled as well.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

2021-11-25 22:01:37 +02:00

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