Add more robust cache invalidation to CachingBucket #9575
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This change makes use of `add` operations for special "lock" cache entries to ensure that when items in object storage are mutated, stale results are not immediately stored to cache again. It does this by `set`ing "lock" cache entries with a short TTL when an item in object storage is about to be mutated. This prevents reads of the item from caching the results afterwards. After the item is mutated in object storage, its cache entries (excluding the lock entries) are deleted. After the lock entries expire, reads of the item are allowed to store results in the cache again. Part of #9386 Signed-off-by: Nick Pillitteri <nick.pillitteri@grafana.com>
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This PR addresses a potential race condition with cache invalidation discussed in #9434
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narqo
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Oct 10, 2024
Signed-off-by: Nick Pillitteri <nick.pillitteri@grafana.com>
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What this PR does
This change makes use of
addoperations for special "lock" cache entries to ensure that when items in object storage are mutated, stale results are not immediately stored to cache again.It does this by
seting "lock" cache entries with a short TTL when an item in object storage is about to be mutated. This prevents reads of the item from caching the results afterwards. After the item is mutated in object storage, its cache entries (excluding the lock entries) are deleted. After the lock entries expire, reads of the item are allowed to store results in the cache again.Which issue(s) this PR fixes or relates to
Part of #9386
Follow up to #9387
Notes for reviewers
At a high level the way this works is:
CachingBucket.Uploadstarts a mutation operation on some item in object storage and special "lock" keys get set in Memcached with a short (15s) TTL.CachingBucket.Uploadcompletes the mutation operation on the object storage item.CachingBucket.Uploaddeletes any cache entries associated with the item (contents, exists, attributes), but not the lockCachingBucket.Getthat attempt to cache the item that has just been modified will short-circuit for the next15sbecause they cannot successfullyMemcached.Add()the lock key.CachingBucket.Getwill be able to cache the item again.Checklist
CHANGELOG.mdupdated - the order of entries should be[CHANGE],[FEATURE],[ENHANCEMENT],[BUGFIX].about-versioning.mdupdated with experimental features.